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1
Garmann, D. J., and M. R. Visbal. "Interactions of a streamwise-oriented vortex with a finite wing." Journal of Fluid Mechanics 767 (February 24, 2015): 782–810. http://dx.doi.org/10.1017/jfm.2015.51.
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AbstractA canonical study is developed to investigate the unsteady interactions of a streamwise-oriented vortex impinging upon a finite surface using high-fidelity simulation. As a model problem, an analytically defined vortex superimposed on a free stream is convected towards an aspect-ratio-six ($\mathit{AR}=6$) plate oriented at an angle of ${\it\alpha}=4^{\circ }$ and Reynolds number of $\mathit{Re}=20\,000$ in order to characterize the unsteady modes of interaction resulting from different spanwise positions of the incoming vortex. Outboard, tip-aligned and inboard positioning are shown to produce three distinct flow regimes: when the vortex is positioned outboard of, but in close proximity to, the wingtip, it pairs with the tip vortex to form a dipole that propels itself away from the plate through mutual induction, and also leads to an enhancement of the tip vortex. When the incoming vortex is aligned with the wingtip, the tip vortex is initially strengthened by the proximity of the incident vortex, but both structures attenuate into the wake as instabilities arise in the pair’s feeding sheets from the entrainment of opposite-signed vorticity into either structure. Finally, when the incident vortex is positioned inboard of the wingtip, the vortex bifurcates in the time-mean sense with portions convecting above and below the wing, and the tip vortex is mostly suppressed. The time-mean bifurcation is actually a result of an unsteady spiralling instability in the vortex core that reorients the vortex as it impacts the leading edge, pinches off, and alternately attaches to either side of the wing. The increased effective angle of attack inboard of impingement enhances the three-dimensional recirculation region created by the separated boundary layer off the leading edge which draws fluid from the incident vortex inboard and diminishes its impact on the outboard section of the wing. The slight but remaining downwash present outboard of impingement reduces the effective angle of attack in that region, resulting in a small separation bubble on either side of the wing in the time-mean solution, effectively unloading the tip outboard of impingement and suppressing the tip vortex. All incident vortex positions provide substantial increases in the wing’s lift-to-drag ratio; however, significant sustained rolling moments also result. As the vortex is brought inboard, the rolling moment diminishes and eventually switches sign as the reduced outboard loading balances the augmented sectional lift inboard of impingement.
2
Kislitsyn, S. A., and V. S. Berdnikov. "Numerical studies of the advective flow of heptadecane in a horizontal layer with a longitudinal temperature gradient at the lower boundary." Journal of Physics: Conference Series 2119, no. 1 (2021): 012085. http://dx.doi.org/10.1088/1742-6596/2119/1/012085.
Texte intégralRésumé :
Abstract Numerical studies of the convective flow of heptadecane in a horizontal layer with a suddenly applied longitudinal temperature gradient at the lower high-thermal conductivity boundary have been carried out by the finite element method. A system of nonstationary dimensionless equations of free convection containing stream function, velocity vortex, and temperature as variables was solved. The calculations were carried out with a free upper boundary with and without taking into account the influence of the thermocapillary effect.
3
Radomsky, R. W., and K. A. Thole. "High Free-Steam Turbulence Effects on Endwall Heat Transfer for a Gas Turbine Stator Vane." Journal of Turbomachinery 122, no. 4 (2000): 699–708. http://dx.doi.org/10.1115/1.1312807.
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High free-stream turbulence along a gas turbine airfoil and strong secondary flows along the endwall have both been reported to increase convective heat transfer significantly. This study superimposes high free-stream turbulence on the naturally occurring secondary flow vortices to determine the effects on the flowfield and the endwall convective heat transfer. Measured flowfield and heat transfer data were compared between low free-stream turbulence levels (0.6 percent) and combustor simulated turbulence levels (19.5 percent) that were generated using an active grid. These experiments were conducted using a scaled-up, first-stage stator vane geometry. Infrared thermography was used to measure surface temperatures on a constant heat flux plate placed on the endwall surface. Laser-Doppler Velocimetry (LDV) measurements were performed of all three components of the mean and fluctuating velocities of the leading edge horseshoe vortex. The results indicate that the mean flowfields for the leading edge horseshoe vortex were similar between the low and high free-stream turbulence cases. High turbulence levels in the leading edge–endwall juncture were attributed to a vortex unsteadiness for both the low and high free-stream turbulence cases. While, in general, the high free-stream turbulence increased the endwall heat transfer, low augmentations were found to coincide with the regions having the most intense vortex motions. [S0889-504X(00)00704-2]
4
Stechman, Daniel M., Robert M. Rauber, Greg M. McFarquhar, Brian F. Jewett, and David P. Jorgensen. "Interaction of an Upper-Tropospheric Jet with a Squall Line Originating along a Cold Frontal Boundary." Monthly Weather Review 144, no. 11 (2016): 4197–219. http://dx.doi.org/10.1175/mwr-d-16-0044.1.
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Abstract On 8 June 2003, an expansive squall line along a surface cold frontal boundary was sampled during the Bow Echo and Mesoscale Convective Vortex Experiment. The Naval Research Laboratory P-3 aircraft and the National Oceanic and Atmospheric Administration P-3 aircraft simultaneously sampled the leading and trailing edge of this squall line, respectively, with X-band Doppler radars. Data from these two airborne radar systems have been synthesized to produce a pseudo-quad-Doppler analysis of the squall line, yielding a detailed three-dimensional kinematic analysis of its structure. A simulation of the squall line was carried out using the Weather Research and Forecasting Model to complement the pseudo-quad-Doppler analysis. The simulation employed a 3-km, convection-allowing, nested domain centered over the pseudo-quad-Doppler domain, along with a 9-km parent domain to capture the larger synoptic-scale cyclone. The pseudo-quad-Doppler analysis reveals that the convective line was embedded within the upper-tropospheric jet stream, causing local decelerations and deviations in the jet-level flow. The vertical transport of low momentum air from the boundary layer via convective updrafts is shown to significantly decelerate jet-level flow. Pressure perturbations associated with the intrusion of low momentum air into the jet stream–level flow led to deviation of the jet stream flow around the squall line that resulted in counter-rotating ribbons of vertical vorticity parallel to the squall line. Model results indicate that disturbances in the jet stream structure persisted downwind of the squall line for several hours.
5
Kumar, Bhaskar, and Sanjay Mittal. "On the origin of the secondary vortex street." Journal of Fluid Mechanics 711 (September 24, 2012): 641–66. http://dx.doi.org/10.1017/jfm.2012.421.
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AbstractThe origin of the secondary vortex street, observed in the far wake in the flow past a circular cylinder, is investigated. The Reynolds number, based on the diameter of the cylinder, is 150. The von Kármán vortex street, which originates in the near wake, decays exponentially downstream of the cylinder. Beyond the region of decay, a broad band of frequencies are selectively amplified, leading to the formation of a secondary vortex street consisting of packets of large-scale vortex structures. The streamwise location of the onset of the instability, frequency of the generation of packets and their convection speed are estimated via direct numerical simulation (DNS). Global linear stability analysis of the time-averaged flow reveals the presence of unstable convective modes that travel at almost the same speed and have a structure similar to the packet-like disturbances as observed in the DNS. Sensitivity analysis of the global convective modes to structural perturbations is carried out to locate the region of the wake that is most significant in generating the modes responsible for the appearance of the secondary vortex street. This information is utilized to control the flow. By placing a ‘slip’ splitter plate along the wake centre line, in the overlap region of the direct and the adjoint modes, the oscillations in the far wake are significantly reduced, though the oscillations related to the primary vortex shedding in the near wake are not. It is also found that suppression of the primary vortex shedding leads to annihilation of the secondary vortex street as well. Linear stability analysis of the steady-state flow does not yield any modes that can explain the appearance of the secondary vortex street. The steady and time-averaged wake profiles, for the $\mathit{Re}= 150$ flow, are compared to bring out the differences in the two. The effect of free-stream oscillations on the evolution of the secondary vortex street is investigated. By reducing the amplitude of inlet excitation, a gradual transition from ordered shedding in the far wake to the appearance of a broad-band spectrum of frequencies, as in the unforced wake, is observed. All the computations have been carried out using a stabilized finite element method.
6
Mandal, A. C., and J. Dey. "An experimental study of boundary layer transition induced by a cylinder wake." Journal of Fluid Mechanics 684 (September 1, 2011): 60–84. http://dx.doi.org/10.1017/jfm.2011.270.
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AbstractBoundary layer transition induced by the wake of a circular cylinder in the free stream has been investigated using the particle image velocimetry technique. Some differences between simulation and experimental studies have been reported in the literature, and these have motivated the present study. The appearance of spanwise vortices in the early stage is further confirmed here. A spanwise vortex appears to evolve into a $ \mrm{\Lambda} $/hairpin vortex; the flow statistics also confirm such vortices. With increasing Reynolds number, based on the cylinder diameter, and with decreasing cylinder height from the plate, the physical size of these hairpin-like structures is found to decrease. Some mean flow characteristics, including the streamwise growth of the disturbance energy, in a wake-induced transition resemble those in bypass transition induced by free stream turbulence. Streamwise velocity streaks that are eventually generated in the late stage often undergo sinuous-type oscillations. Similar to other transitional flows, an inclined shear layer in the wall-normal plane is often seen to oscillate and shed vortices. The normalized shedding frequency of these vortices, estimated from the spatial spacing and the convection velocity of these vortices, is found to be independent of the Reynolds number, similar to that in ribbon-induced transition. Although the nature of free stream disturbance in a wake-induced transition and that in a bypass transition are different, the late-stage features including the flow breakdown characteristics of these two transitions appear to be similar.
7
Chang, Keun-Shik, and Jong-Youb Sa. "The effect of buoyancy on vortex shedding in the near wake of a circular cylinder." Journal of Fluid Mechanics 220 (November 1990): 253–66. http://dx.doi.org/10.1017/s002211209000324x.
Texte intégralRésumé :
The phenomenon of vortex shedding from a heated/cooled circular cylinder has been investigated numerically in the mixed natural and forced convection regimes. Accuracy of the computation was achieved by the fourth-order Hermitian relation applied to the contravariant velocity components in the convection terms of the vorticity transport equation, and by the far-boundary stream-function condition of an integral-series form developed by the authors. Purely periodic flows at Re = 100, efficiently established through the use of a direct elliptic solver called the SEVP, was found to degenerate into a steady twin-vortex pattern at the critical Grashof number 1500, confirming an earlier experimental observation identified as ‘breakdown of the Kármán vortex street’. Various other buoyancy effects about the heated/cooled cylinder are discussed by means of the flow patterns, the Nusselt number and the drag coefficient curves.
8
Moharreri, S. S., B. F. Armaly, and T. S. Chen. "Measurements in the Transition Vortex Flow Regime of Mixed Convection Above a Horizontal Heated Plate." Journal of Heat Transfer 110, no. 2 (1988): 358–65. http://dx.doi.org/10.1115/1.3250492.
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Experimental results covering the transition vortex flow regime of mixed convection over a heated, horizontal flat plate are presented. A criterion for the onset of vortex instability as a function of critical Reynolds and Grashof numbers was established with the aid of a flow visualization technique. The three-dimensional nature of this flow regime was documented through both velocity and temperature measurements using laser-Doppler and hot/cold-wire anemometers, respectively. A higher buoyancy force, through a higher plate temperature or a larger downstream distance, and/or a lower free-stream velocity, intensifies the strength of the vortices. Velocity and temperature profiles through vortex peaks and valleys are reported to quantify the behavior of these vortices. It has been found from these measurements that the two-dimensional laminar mixed convection flow changes into a transitional three-dimensional vortex flow in a relatively short distance from the leading edge of the plate. The vortex three-dimensional flow continues to intensify as the buoyancy force increases and then develops into a two-dimensional fully turbulent flow at the end of the transition regime. These findings place an upper limit on the applicability of the two-dimensional, laminar boundary layer flow analysis for mixed convection over a heated horizontal flat plate.
9
Heidarzadeh, Habibollah, Mousa Farhadi, and Kurosh Sedighi. "Convective heat transfer over a wall mounted cube at different angle of attack using large eddy simulation." Thermal Science 18, suppl.2 (2014): 301–15. http://dx.doi.org/10.2298/tsci110614088h.
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Turbulent fluid flow and convective heat transfer over the wall mounted cube in different flow angle of attack have been studied numerically using Large Eddy Simulation. Cube faces and plate have a constant heat flux. Dynamic Smagorinsky (DS) subgrid scale model were used in this study. Angles were in the range 0???45 and Reynolds number based on the cube height and free stream velocity was 4200. The numerical simulation results were compared with the experimental data of Nakamura et al [6, 7]. Characteristics of fluid flow field and heat transfer compared for four angles of attack. Flow around the cube was classified to four regimes. Results was represented in the form of time averaged normalized streamwise velocity and Reynolds stress in different positions, temperature contours, local and average Nusselt number over the faces of cube. Local convective heat transfer on cube faces was affected by flow pattern around the cube. The local convective heat transfer from the faces of the cube and plate are directly related to the complex phenomena such as horse shoe vortex, arch vortexes in behind the cube, separation and reattachment. Results show that overall convective heat transfer of cube and mean drag coefficient have maximum and minimum value at ?=0 deg and ?=25 deg respectively.
10
Hall, Philip. "Vortex–wave interactions: long-wavelength streaks and spatial localization in natural convection." Journal of Fluid Mechanics 703 (June 12, 2012): 99–110. http://dx.doi.org/10.1017/jfm.2012.196.
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AbstractThe unidirectional shear flow driven by buoyancy effects in a vertical channel when a temperature difference is maintained between the walls of the channel is considered. The flow is unstable to waves which interact to reinforce the original flow and make it ‘streaky’. Such ‘vortex–wave’ interactions have been the subject of much recent research but little is yet known about what happens when the wavelength of the roll/streak flow becomes large. An asymptotic structure for long-wavelength interactions is derived and the tendency of the fluid to resist this state and the flow to become localized is revealed. Here the high-Grashof-number limit is considered and it is shown how a self-sustained process can occur with vortices interacting with a wave system in a manner similar to that discussed by Hall & Smith (J. Fluid Mech., vol. 227, 1991, pp. 641–666) and Hall & Sherwin (J. Fluid Mech., vol. 661, 2010, pp. 178–205). The work is closely related to numerical simulations of self-sustained processes in for example Couette flow but the fact that the basic flow here is generated by buoyancy effects enables us to make analytical progress. It is shown that the wave part of the interaction process has a flat critical layer and its wavelength is twice that of the streaky flow which supports it. Such subharmonic vortex–wave/self-sustained process interactions have not been previously identified.
11
Kumar De, Arnab, and Amaresh Dalal. "Numerical Study of Laminar Forced Convection Fluid Flow and Heat Transfer From a Triangular Cylinder Placed in a Channel." Journal of Heat Transfer 129, no. 5 (2006): 646–56. http://dx.doi.org/10.1115/1.2712848.
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Computational study of two-dimensional laminar flow and heat transfer past a triangular cylinder placed in a horizontal channel is presented for the range 80≤Re≤200 and blockage ratio 1/12≤β≤1/3. A second-order accurate finite volume code with nonstaggered arrangement of variables is developed employing momentum interpolation for the pressure-velocity coupling. Global mode of cross-stream velocity oscillations predict the first bifurcation point increases linearly with blockage ratio with no second bifurcation found in the range of Re studied. The Strouhal number and rms of lift coefficient increase significantly with blockage ratio and Reynolds number while overall Nusselt number remains almost unchanged for different blockage ratios. At lower blockage ratios, flow is found to be similar to the unconfined flow and is more prone to wake instability. Instantaneous streak lines provide an excellent means of visualizing the von Kármán vortex street.
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Kanna, R., Sayed Sayeed Ahmad, P. Venkata Reddy, et al. "Investigation Of Forced Convection Heat Transfer From A Heater Mounted Ina Cavity Wall Using Various Nanofluids." E3S Web of Conferences 128 (2019): 07002. http://dx.doi.org/10.1051/e3sconf/201912807002.
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Forced convection heat transfer from heater mounted in a cavity wall is investigated to reveal the relation among nanofluid properties. The base fluid is considered as water. The present study is focused on forced convection heat transfer from square heater subject to inflow and outflow inside a squarecavity. The interesting physics will be reported in connection with volume fraction, Reynolds number and nanomaterial properties. It is found that for a particular Reynolds number when nanomaterial is introduced the local heat transfer is increased. The wall attached vortex attributes a constant Nusselt number. It is also noticed that when the heater wall is subject to combination of vortex and main stream fluid results high Nusselt number than heat transfer due to wall attached vortices. Nanofluid results high Nusselt number for the same Reynolds number.
13
Dunkerton, T. J., M. T. Montgomery, and Z. Wang. "Tropical cyclogenesis in a tropical wave critical layer: easterly waves." Atmospheric Chemistry and Physics Discussions 8, no. 3 (2008): 11149–292. http://dx.doi.org/10.5194/acpd-8-11149-2008.
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Abstract. The development of tropical depressions within tropical waves over the Atlantic and eastern Pacific is usually preceded by a "surface low along the wave" as if to suggest a hybrid wave-vortex structure in which flow streamlines not only undulate with the waves, but form a closed circulation in the lower troposphere surrounding the low. This structure, equatorward of the easterly jet axis, resembles the familiar critical layer of waves in shear flow, a flow configuration which arguably provides the simplest conceptual framework for tropical cyclogenesis resulting from tropical waves, their interaction with the mean flow, and with diabatic processes associated with deep moist convection. The critical layer represents a sweet spot for tropical cyclogenesis in which a proto-vortex may form and grow within its parent wave. A common location for storm development within the critical layer is given by the intersection of the wave's critical latitude and trough axis, with analyzed vorticity centroid nearby. The wave and vortex live together for a time, and initially propagate at approximately the same speed. In most cases this coupled propagation continues for a few days after a tropical depression is identified. For easterly waves, as the name suggests, the propagation is westward. It is shown that in order to visualize optimally this "marsupial paradigm" one should view the flow streamlines, or stream function, in a frame of reference translating horizontally with the phase propagation of the parent wave. This translation requires an appropriate "gauge" that renders translating streamlines and isopleths of translating stream function approximately equivalent to flow trajectories. In the translating frame, the closed circulation is stationary, and a dividing streamline effectively separates air within the critical layer from air outside. The critical layer equatorward of the easterly jet axis is important to tropical cyclogenesis because it provides (i) a region of cyclonic vorticity and weak deformation by the resolved flow, (ii) containment of moisture entrained by the gyre and/or lofted by deep convection therein, (iii) confinement of mesoscale vortex aggregation, (iv) a predominantly convective type of heating profile, and (v) maintenance or enhancement of the parent wave until the vortex becomes a self-sustaining entity and emerges from the wave as a tropical depression. These ideas are formulated in three new hypotheses describing the flow kinematics and dynamics, moist thermodynamics and wave/vortex interactions comprising the marsupial paradigm. A survey of 55 named tropical storms in 1998–2001 reveals that actual critical layers sometimes resemble the ideal east-west train of cat's eyes, but are usually less regular, with one or more recirculation regions in the translating frame. It is shown that a "wave gauge" given by the translation speed of the parent wave is the appropriate choice, as well, for isolated proto-vortices carried by the wave. Some implications for entrainment/containment of vorticity and moisture in the cat's eye are discussed from this perspective, based on the observational survey.
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CAULFIELD, C. P., and W. R. PELTIER. "The anatomy of the mixing transition in homogeneous and stratified free shear layers." Journal of Fluid Mechanics 413 (June 25, 2000): 1–47. http://dx.doi.org/10.1017/s0022112000008284.
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We investigate the detailed nature of the ‘mixing transition’ through which turbulence may develop in both homogeneous and stratified free shear layers. Our focus is upon the fundamental role in transition, and in particular the associated ‘mixing’ (i.e. small-scale motions which lead to an irreversible increase in the total potential energy of the flow) that is played by streamwise vortex streaks, which develop once the primary and typically two-dimensional Kelvin–Helmholtz (KH) billow saturates at finite amplitude.Saturated KH billows are susceptible to a family of three-dimensional secondary instabilities. In homogeneous fluid, secondary stability analyses predict that the stream-wise vortex streaks originate through a ‘hyperbolic’ instability that is localized in the vorticity braids that develop between billow cores. In sufficiently strongly stratified fluid, the secondary instability mechanism is fundamentally different, and is associated with convective destabilization of the statically unstable sublayers that are created as the KH billows roll up.We test the validity of these theoretical predictions by performing a sequence of three-dimensional direct numerical simulations of shear layer evolution, with the flow Reynolds number (defined on the basis of shear layer half-depth and half the velocity difference) Re = 750, the Prandtl number of the fluid Pr = 1, and the minimum gradient Richardson number Ri(0) varying between 0 and 0.1. These simulations quantitatively verify the predictions of our stability analysis, both as to the spanwise wavelength and the spatial localization of the streamwise vortex streaks. We track the nonlinear amplification of these secondary coherent structures, and investigate the nature of the process which actually triggers mixing. Both in stratified and unstratified shear layers, the subsequent nonlinear amplification of the initially localized streamwise vortex streaks is driven by the vertical shear in the evolving mean flow. The two-dimensional flow associated with the primary KH billow plays an essentially catalytic role. Vortex stretching causes the streamwise vortices to extend beyond their initially localized regions, and leads eventually to a streamwise-aligned collision between the streamwise vortices that are initially associated with adjacent cores.It is through this collision of neighbouring streamwise vortex streaks that a final and violent finite-amplitude subcritical transition occurs in both stratified and unstratified shear layers, which drives the mixing process. In a stratified flow with appropriate initial characteristics, the irreversible small-scale mixing of the density which is triggered by this transition leads to the development of a third layer within the flow of relatively well-mixed fluid that is of an intermediate density, bounded by narrow regions of strong density gradient.
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Zhu, Baoshan, Jun Lei, and Shuliang Cao. "Numerical Simulation of Vortex Shedding and Lock-in Characteristics for a Thin Cambered Blade." Journal of Fluids Engineering 129, no. 10 (2007): 1297–305. http://dx.doi.org/10.1115/1.2776964.
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In this paper, vortex-shedding patterns and lock-in characteristics that vortex-shedding frequency synchronizes with the natural frequency of a thin cambered blade were numerically investigated. The numerical simulation was based on solving the vorticity-stream function equations with the fourth-order Runge–Kutta scheme in time and the Chakravaythy–Oscher total variation diminishing (TVD) scheme was used to discretize the convective term. The vortex-shedding patterns for different blade attack angles were simulated. In order to confirm whether the vortex shedding would induce blade self-oscillation, numerical simulation was also carried out for blade in a forced oscillation. By changing the pitching frequency and amplitude, the occurrence of lock-in at certain attack angles was determined. Inside the lock-in zone, phase differences between the blade’s pitching displacement and the torque acting on the blade were used to infer the probability of the blade self-oscillation.
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Pasipoularides, Ares, Ming Shu, Ashish Shah, Michael S. Womack, and Donald D. Glower. "Diastolic right ventricular filling vortex in normal and volume overload states." American Journal of Physiology-Heart and Circulatory Physiology 284, no. 4 (2003): H1064—H1072. http://dx.doi.org/10.1152/ajpheart.00804.2002.
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Functional imaging computational fluid dynamics simulations of right ventricular (RV) inflow fields were obtained by comprehensive software using individual animal-specific dynamic imaging data input from three-dimensional (3-D) real-time echocardiography (RT3D) on a CRAY T-90 supercomputer. Chronically instrumented, lightly sedated awake dogs ( n = 7) with normal wall motion (NWM) at control and normal or diastolic paradoxical septal motion (PSM) during RV volume overload were investigated. Up to the E-wave peak, instantaneous inflow streamlines extended from the tricuspid orifice to the RV endocardial surface in an expanding fanlike pattern. During the descending limb of the E-wave, large-scale (macroscopic or global) vortical motions ensued within the filling RV chamber. Both at control and during RV volume overload (with or without PSM), blood streams rolled up from regions near the walls toward the base. The extent and strength of the ring vortex surrounding the main stream were reduced with chamber dilatation. A hypothesis is proposed for a facilitatory role of the diastolic vortex for ventricular filling. The filling vortex supports filling by shunting inflow kinetic energy, which would otherwise contribute to an inflow-impeding convective pressure rise between inflow orifice and the large endocardial surface of the expanding chamber, into the rotational kinetic energy of the vortical motion that is destined to be dissipated as heat. The basic information presented should improve application and interpretation of noninvasive (Doppler color flow mapping, velocity-encoded cine magnetic resonance imaging, etc.) diastolic diagnostic studies and lead to improved understanding and recognition of subtle, flow-associated abnormalities in ventricular dilatation and remodeling.
17
Zhang, X. "Turbulence Measurements of a Longitudinal Vortex Generated by an Inclined Jet in a Turbulent Boundary Layer." Journal of Fluids Engineering 120, no. 4 (1998): 765–71. http://dx.doi.org/10.1115/1.2820736.
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A longitudinal vortex in a flat-plate turbulent boundary layer was examined in a wind tunnel experiment using Laser Doppler Anemometry. The vortex was produced by an inclined round jet (D = 14 mm) in the turbulent boundary layer (δ0.99 ≈ 25 mm). The jet nozzle was positioned at pitch and skew angles of 45 deg to the oncoming stream, and the jet speed ratios (jet speed/freestream flow speed) were 0.5, 1.0, and 1.5. The flow was characterized by embedded vortices, induced high turbulent kinetic energy peak, local areas of high primary shear stress, and negative shear stress. Two types of normal stress evolution were observed: (a) low normal stress beneath the vortex on the upwash side and high normal stress above the center of the vortex, caused by spanwise momentum transfer and local turbulent production; (b) high normal stress beneath the vortex on the upwash side and high normal stress coinciding with the center of the vortex, produced by spanwise and transverse momentum transfer of a vortex away from the wall with turbulent convection playing an important role. The study provided a database for numerical modeling effort.
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Chen, C. C., A. Labhabi, H. C. Chang, and R. E. Kelly. "Spanwise pairing of finite-amplitude longitudinal vortex rolls in inclined free-convection boundary layers." Journal of Fluid Mechanics 231 (October 1991): 73–111. http://dx.doi.org/10.1017/s0022112091003324.
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Buoyancy-driven flow on a heated inclined plate can become unstable to static longitudinal roil instability at a critical distance, measured by $\tilde{R}_{\rm c}$, from the leading edge. Experiments in water by Sparrow & Husar (1969) indicate that these rolls undergo a second transition further downstream such that adjacent rolls merge and their spanwise wavelength is doubled. We study this secondary bifurcation phenomenon here with a set of model equations by first constructing the full eigenspectrum and eigenfunctions with a Chebyshev–Tau spectral method and then deriving the pertinent amplitude equations. By stipulating that the dimensional cross-stream wavelength of the rolls remains constant beyond $\tilde{R}_{\rm c}$, which is consistent with experimental observation, we show that the finite-amplitude primary rolls are destabilized by the ½ subharmonic mode at another critical distance $\tilde{R}_{\frac{1}{2}}$ from the edge. This ½ mode is shown to have an asymmetric spatial phase shift of ½π relative to the original 1 mode of the primary rolls, thus explaining the unique dislocation of tracer streaks after the rolls coalesce in the experiments. Also consistent with experimental observation is the theoretical result that the merged rolls are annihilated downstream by a saddle-node bifurcation before further wavclength doubling can occur. Simple amplitude criteria and critical distances from the leading edge for the various transitions are derived and compared to experimental values.
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Nikulina, S. A., A. V. Perminov, and T. P. Lyubimova. "Thermal vibrational convection of a pseudoplastic fluid in a rectangular cavity." Вестник Пермского университета. Физика, no. 3 (2020): 14–23. http://dx.doi.org/10.17072/1994-3598-2020-3-14-23.
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Thermal vibrational convection of a pseudoplastic fluid in a closed rectangular cavity, which is in zero gravity and performing longitudinal high-frequency linearly polarized vibrations, is studied. The temperature gradient is perpendicular to the direction of vibration. The system of equations of thermovibrational convection of a Williamson pseudoplastic fluid is given. The problem was solved by the finite difference method. The effect of vibrations on the structure and intensity of flows is investigated. The magnitude of the vibrational effect on the liquid was determined by the vibrational Grashof number. The dependences of the maximum of the stream function and the Nusselt number, which determines the heat flux through the boundary of the cavity, on the vibrational Grashof number are obtained. The threshold values of the vibrational Grashof number and the Nusselt number corresponding to a change in the flow regime are determined. At small values of the Grashof vibration number in the cavity, a slow four-vortex symmetric flow is observed. With an increase in the vibrational impact, an intense three-vortex motion arises in the cavity, which transforms into five vortex-like motion. For the five vortex flows, there exists the region of Grashof vibration numbers, where this flow is oscillatory in nature. With increasing degree of non-Newtonian fluid, initially periodic oscillations become chaotic.
20
Chang, Chih-Pei, Mong-Ming Lu, and Hock Lim. "Monsoon Convection in the Maritime Continent: Interaction of Large-Scale Motion and Complex Terrain." Meteorological Monographs 56 (April 1, 2016): 6.1–6.29. http://dx.doi.org/10.1175/amsmonographs-d-15-0011.1.
Texte intégralRésumé :
Abstract The Asian monsoon is a planetary-scale circulation system powered by the release of latent heat, but important features of deep convection and rainfall distribution cannot be adequately represented by the large-scale patterns. This is mainly due to the strong influences of terrain that are important across a wide range of horizontal scales, especially over the Maritime Continent where the complex terrain has a dominant effect on the behavior of convective rainfall during the boreal winter monsoon. This chapter is a review and summary of published results on the effects on monsoon convection due to interactions between the Maritime Continent terrain and large-scale transient systems. The Maritime Continent topographic features strongly affect both the demarcation of the boreal summer and winter monsoon regimes and the asymmetric seasonal marches during the transition seasons. In the western part of the region, the complex interactions that lead to variability in deep convection are primarily controlled by the cold surges and the synoptic-scale Borneo vortex. The Madden–Julian oscillation (MJO) reduces the frequency of weaker surges through an interference with their structure. It also influences convection, particularly on the diurnal cycle and when synoptic activities are weak. When both surges and the Borneo vortex are present, interactions between these circulations with the terrain can cause the strongest convection, which has included Typhoon Vamei (2001), which is the only observed tropical cyclone that developed within 1.5° of the equator. The cold surges are driven by midlatitude pressure rises associated with the movement of the Siberian high. Rapid strengthening of surge northeasterly winds can be explained as the tropical response via a geostrophic adjustment process to the pressure forcing in the form of an equatorial Rossby wave group. Dispersion of meridional modes leads to a northeast–southwest orientation that allows the surge to stream downstream through the similarly oriented South China Sea. This evolution leads to a cross-equatorial return flow and a cyclonic circulation at the equator, and thus a mechanism for equatorial cyclogenesis. Although the narrow width of the southern South China Sea facilitates strengthening of the cold surge, it also severely restricts the likelihood of cyclone development so that Vamei remains to be the only typhoon observed in the equatorial South China Sea. Climate variations from El Niño–Southern Oscillation to climate change may impact the interactions between the large-scale motion and Maritime Continent terrain because they lead to changes in the mean flow. The thermodynamic effects on the interaction between MJO and the monsoon surges and Borneo vortex over the complex terrain also need to be addressed. These and other questions such as any possible changes in the likelihood of equatorial tropical cyclogenesis as a result of climate change are all important areas for future research.
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Hossain, Mosharf, Nepal Roy, and Anwar Hossain. "Boundary layer flow and heat transfer in a micropolar fluid past a permeable at plate." Theoretical and Applied Mechanics 40, no. 3 (2013): 403–25. http://dx.doi.org/10.2298/tam1303403h.
Texte intégralRésumé :
An analysis is performed to study the shear stress, the couple-stress and heat transfer characteristics of a laminar mixed convection boundary layer flow of a micropolar fluid past an isothermal permeable plate. The governing nonsimilar boundary layer equations are analyzed using the (i) series solution for small ?, (ii) asymptotic solution for large ? and (iii) primitive-variable formulation and the stream function formulation are being used for all ?. The effects of the material parameters, such as, the vortex viscosity parameter, K, and the transpiration parameter, s, on the shear stress, the couple-stress and heat transfer have been investigated. The agreement between the solutions obtained from the stream-function formulation and the primitive-variable formulation is found to be excellent.
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Martins, Luis-Filipe, and Ahmed F. Ghoniem. "Simulation of the Nonreacting Flow in a Bluff-Body Burner; Effect of the Diameter Ratio." Journal of Fluids Engineering 115, no. 3 (1993): 474–84. http://dx.doi.org/10.1115/1.2910163.
Texte intégralRésumé :
Axisymmetric vortex simulation is used to study the unsteady dynamics of the flowfield generated by the interaction between two concentric jets initially separated by a thick bluff-body. The computational scheme treats convective transport in a Lagrangian sense by discretizing the vorticity into a number of finite-area vortex ring elements which move along particle trajectories during each convective substep, thus reducing the numerical diffusion and allowing simulations at high Reynolds number. In this paper, investigation is focused on the time-dependent dynamics and the effect of the diameter ratio across the bluff-body on the wake flow. In both cases simulated, the dynamics is governed by the shedding of large vortex eddies from the inner and outer sides of the bluff-body. Mixing between the two streams is enhanced by the merging of these eddies downstream the bluff-body and the formation of composite structures. We find that the frequency of shedding, the level of fluctuations and the degree of organization are strongly dependent on the diameter ratio. The fluctuation associated with this shedding increases as the diameter ratio becomes larger. The origin and mechanism of shedding in each case are determined from the results.
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Nguyen, Leon T., and John Molinari. "Rapid Intensification of a Sheared, Fast-Moving Hurricane over the Gulf Stream." Monthly Weather Review 140, no. 10 (2012): 3361–78. http://dx.doi.org/10.1175/mwr-d-11-00293.1.
Texte intégralRésumé :
Abstract Hurricane Irene (1999) rapidly intensified from 65 to 95 kt (~33.4 to 48.9 m s−1) in 18 h. During the rapid intensification (RI) period, the northeastward storm motion increased from 10 to 18 m s−1, the ambient southwesterly vertical wind shear increased from 6–7 to 10–13 m s −1, and the downshear tilt of the inner core vortex increased. The azimuthal wavenumber-1 asymmetric convection that developed was consistent with a superposition of shear-induced and storm motion–induced forcing for vertical motion downshear and ahead of the center. Although the diabatic heating remained strongly asymmetric, it was of sufficient intensity to dramatically increase the azimuthally averaged heating. This heating occurred almost entirely inside the radius of maximum winds, a region known to favor rapid warm core development and spinup of the vortex. It is hypothesized that asymmetric forcing from the large vertical wind shear and rapid storm motion were responsible for RI. An unanswered question is what determines whether the heating will develop within the radius of maximum winds. Extraordinarily deep cells developed in the inner core toward the end of the RI period. Rather than causing RI, these cells appeared to be an outcome of the same processes noted above.
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Dunkerton, T. J., M. T. Montgomery, and Z. Wang. "Tropical cyclogenesis in a tropical wave critical layer: easterly waves." Atmospheric Chemistry and Physics 9, no. 15 (2009): 5587–646. http://dx.doi.org/10.5194/acp-9-5587-2009.
Texte intégralRésumé :
Abstract. The development of tropical depressions within tropical waves over the Atlantic and eastern Pacific is usually preceded by a "surface low along the wave" as if to suggest a hybrid wave-vortex structure in which flow streamlines not only undulate with the waves, but form a closed circulation in the lower troposphere surrounding the low. This structure, equatorward of the easterly jet axis, is identified herein as the familiar critical layer of waves in shear flow, a flow configuration which arguably provides the simplest conceptual framework for tropical cyclogenesis resulting from tropical waves, their interaction with the mean flow, and with diabatic processes associated with deep moist convection. The recirculating Kelvin cat's eye within the critical layer represents a sweet spot for tropical cyclogenesis in which a proto-vortex may form and grow within its parent wave. A common location for storm development is given by the intersection of the wave's critical latitude and trough axis at the center of the cat's eye, with analyzed vorticity centroid nearby. The wave and vortex live together for a time, and initially propagate at approximately the same speed. In most cases this coupled propagation continues for a few days after a tropical depression is identified. For easterly waves, as the name suggests, the propagation is westward. It is shown that in order to visualize optimally the associated Lagrangian motions, one should view the flow streamlines, or stream function, in a frame of reference translating horizontally with the phase propagation of the parent wave. In this co-moving frame, streamlines are approximately equivalent to particle trajectories. The closed circulation is quasi-stationary, and a dividing streamline separates air within the cat's eye from air outside. The critical layer equatorward of the easterly jet axis is important to tropical cyclogenesis because its cat's eye provides (i) a region of cyclonic vorticity and weak deformation by the resolved flow, (ii) containment of moisture entrained by the developing gyre and/or lofted by deep convection therein, (iii) confinement of mesoscale vortex aggregation, (iv) a predominantly convective type of heating profile, and (v) maintenance or enhancement of the parent wave until the vortex becomes a self-sustaining entity and emerges from the wave as a tropical depression. The entire sequence is likened to the development of a marsupial infant in its mother's pouch. These ideas are formulated in three new hypotheses describing the flow kinematics and dynamics, moist thermodynamics and wave/vortex interactions comprising the "marsupial paradigm". A survey of 55 named tropical storms in 1998–2001 reveals that actual critical layers sometimes resemble the ideal east-west train of cat's eyes, but are usually less regular, with one or more recirculation regions in the co-moving frame. It is shown that the kinematics of isolated proto-vortices carried by the wave also can be visualized in a frame of reference translating at or near the phase speed of the parent wave. The proper translation speeds for wave and vortex may vary with height owing to vertical shear and wave-vortex interaction. Some implications for entrainment/containment of vorticity and moisture in the cat's eye are discussed from this perspective, based on the observational survey.
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Lane, Jeremiah S., and Benjamin F. Akers. "Two-Dimensional Steady Boussinesq Convection: Existence, Computation and Scaling." Fluids 6, no. 12 (2021): 425. http://dx.doi.org/10.3390/fluids6120425.
Texte intégralRésumé :
This research investigates laser-induced convection through a stream function-vorticity formulation. Specifically, this paper considers a solution to the steady Boussinesq Navier–Stokes equations in two dimensions with a slip boundary condition on a finite box. A fixed-point algorithm is introduced in stream function-vorticity variables, followed by a proof of the existence of steady solutions for small laser amplitudes. From this analysis, an asymptotic relationship is demonstrated between the nondimensional fluid parameters and least upper bounds for laser amplitudes that guarantee existence, which accords with numerical results implementing the algorithm in a finite difference scheme. The findings indicate that the upper bound for laser amplitude scales by O(Re−2Pe−1Ri−1) when Re≫Pe, and by O(Re−1Pe−2Ri−1) when Pe≫Re. These results suggest that the existence of steady solutions is heavily dependent on the size of the Reynolds (Re) and Peclet (Pe) numbers, as noted in previous studies. The simulations of steady solutions indicate the presence of symmetric vortex rings, which agrees with experimental results described in the literature. From these results, relevant implications to thermal blooming in laser propagation simulations are discussed.
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Shangguan, Yanqin, Xian Wang, Fei Cao, and Yandan Zhu. "High-resolution simulation of film cooling with blowing ratio and inclination angle effects based on hybrid thermal lattice Boltzmann method." Thermal Science, no. 00 (2021): 286. http://dx.doi.org/10.2298/tsci210424286s.
Texte intégralRésumé :
A series of high-resolution simulations on film cooling with varying blowing ratios and inclination angles are carried out by using in-house code based on hybrid thermal lattice Boltzmann method. Three blowing ratios ranging from 0.2 to 0.8 and four inclination angles from 15? to 60? are chosen for the simulations. The evolutionary mechanism of coherent structure in three domains of film-covering region is studied from the view of space and time. Besides, the influencing mechanism of blowing ratio and inclination angle on flow and heat-transfer features of film cooling is uncovered. Results show that hairpin vortex, hairpin packet and quasi-stream-wise vortex appearing in rotating domain play a key role in heat-transfer process of film cooling. The strong ejection, sweep and vortex rotation resulted from these vortices enhance the convective heat transfer. It is also found that the size of coherent structure varies significantly with blowing ratio and its integral form shows a strong dependence on inclination angle. Moreover, inclination angle of coolant jet has a significant impact on turbulence fluctuation intensity. The influence of blowing ratio on the attachment of coolant film and film-cooling performance is more obvious than that of inclination angle. It is believed that all of these are related closely to the variation of stream-wise and wall-normal jet velocity in the case of various blowing ratios and inclination angles.
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Wolf, William R., João Luiz F. Azevedo, and Sanjiva K. Lele. "Convective effects and the role of quadrupole sources for aerofoil aeroacoustics." Journal of Fluid Mechanics 708 (August 10, 2012): 502–38. http://dx.doi.org/10.1017/jfm.2012.327.
Texte intégralRésumé :
AbstractThe present investigation of aerofoil self-noise generation and propagation concerns the effects of mean flow and quadrupole sources on the broadband noise that arises from the interaction of turbulent boundary layers with the aerofoil trailing edge and the tonal noise that arises from vortex shedding generated by laminar boundary layers and trailing-edge bluntness. Compressible large-eddy simulations (LES) are conducted for a NACA0012 aerofoil with rounded trailing edge for four flow configurations with different angles of incidence, boundary layer tripping configurations and free-stream Mach numbers. The Reynolds number based on the aerofoil chord is fixed at ${\mathit{Re}}_{c} = 408\hspace{0.167em} 000$. The acoustic predictions are performed by the Ffowcs Williams & Hawkings (FWH) acoustic analogy formulation and incorporate convective effects. Surface and volume integrations of dipole and quadrupole source terms appearing in the FWH equation are performed using a three-dimensional wideband multi-level adaptive fast multipole method (FMM) in order to accelerate the calculations of aeroacoustic integrals. In order to validate the numerical solutions, flow simulation and acoustic prediction results are compared to experimental data available in the literature and good agreement is observed in terms of both aerodynamic and aeroacoustic results. For low-Mach-number flows, quadrupole sources can be neglected in the FWH equation and mean flow effects appear only for high frequencies. However, for higher speeds, convection effects are relevant for all frequencies and quadrupole sources have a more pronounced effect for medium and high frequencies. The convective effects are most readily observed in the upstream direction.
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Majdi, Hasan Shakir, Mahmoud A. Mashkour, Laith Jaafer Habeeb, and Marko Ilic. "Mixed convection around a circular cylinder in a buoyancy-assisting flow." Curved and Layered Structures 9, no. 1 (2022): 81–95. http://dx.doi.org/10.1515/cls-2022-0008.
Texte intégralRésumé :
Abstract In this paper, the effect of mixed convection on the flow behavior and heat transfer around a circular cylinder disclosed to a vertically upward laminar air stream is numerically examine. The buoyancy-aided flow is utilized to eliminate and control the vortex shedding of the cylinder. The influence of the Grashof number, 0 ≤ Gr ≤ 6000, the flow and thermal patterns, as well as the local and mean Nusselt number, is investigated at a constant Reynolds number of 100. The unsteady Navier-Stokes’s equations are solved employing a finite-volume method to simulate numerically the velocity and temperature fields in time and space. The results showed periodic instability in the flow and thermal fields for a range of Grashof number Gr ≤ 1300. Also, there is critical value of Grashof number for stopping this instability and the vortex shedding formed behind the cylinder, by the effect of heating. Thus, by increasing Grashof number between 1400 ≤ Gr ≤ 4000, the periodic flow vanishes and converts into steady flow with twin eddies attached to the cylinder from the back. Furthermore, as Grashof number increases behind Gr ≥ 5000, the flow becomes completely attached to the cylinder surface without any separation.
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Fořt, Ivan, Miloslav Hošťálek, Hans Dietrich Laufhütte, and Alfons Bertram Mersmann. "Description of the flow of mechanically agitated liquid in a system with cylindrical draft-tube and radial baffles." Collection of Czechoslovak Chemical Communications 52, no. 6 (1987): 1416–29. http://dx.doi.org/10.1135/cccc19871416.
Texte intégralRésumé :
A model is described of two-dimensional vortex turbulent flow of homogeneous liquid in a cylindrical tank with flat bottom and radial baffles at its walls agitated with an inclined plane blade impeller rotating in a cylindrical draft-tube. The obtained field of the mean Stokes stream function expresses the streamline distribution in the system. As the boundary conditions of the used solution of stream equation serve partly the values of the mean Stokes stream function on the system boundaries (bottom, liquid level, walls of tank and draft-tube, tank axis), partly the radial profiles of axial and radial components of mean velocity on the level of draft-tube lower base obtained by the laser-doppler anemometry. It follows from the comparison with results of previously published studies that in systems with cylindrical draft-tube and axial high-speed impeller, the convective flow intensity of agitated liquid is higher and the streamline distribution in system is more uniform providing that the conical bottom with 120° vertex angle is used instead of the flat bottom.
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Rani, H. P., V. Narayana, and Y. Rameshwar. "Analysis Of Field Synergy In Bottom Heated Lid Driven Cubical Cavity." E3S Web of Conferences 128 (2019): 07007. http://dx.doi.org/10.1051/e3sconf/201912807007.
Texte intégralRésumé :
This study presents an innovative visualization tool for the analysis of the mixed convection in a lid-driven air filled cubical cavity heated from below. The total energy of the flow in the cavity isvisualized based on the energy stream functions or energy streamlines. Also the heat transfer enhancement in the cavity is presented with an analogy between conduction and convection, namely, the field synergy principle. Flow is assumed to be driven by the vertical temperature gradient and by the top lid of the cavity, which is assumed to slide on its own plane at a uniform speed. The top and bottom walls are assumed to be isothermal and all other walls are thermally insulated. Non dimensional governing equations of this problem are solved by using the finite volume method. Established open source CFD package OpenFOAM is utilized to investigate the flow with respect to the control parameters arising in the system. The nonlinear terms arising in the governing equations are discretized with the NVD schemes. The convection differencing schemes namely, UPWIND, QUICK, SUPERBEE and SFCD discussed and are used to simulate the flow using MPI code. It is observed that the computational cost for all the differencing schemes get reduced tremendously when the MPI code is implemented. Also SFCD scheme gave the Nuseelt number values close to those available in the literature. Extensive numerical flow visualization is conducted for the Reynolds number (Re = 100, 400, 1000) and the Richardson number (Ri = 0.001, 1, 10), which categorize the free and forced convective flow, respectively. It is observed that for a fixed value of Re, as Ri increases, the average Nusselt number (Nu¯), decreases. This shows that the natural convection starts to prevail with an increasing of Ri. But, for a fixed Ri, as Re increases (Nu¯) increases and the forced convection mode becomes dominant, leading to a chaotic flow. Plots demonstrating the influences of Re and Ri in termsof the contours of the fluid streamlines, isotherms, vortex corelines, and field synergy principle. The synergy angle of buoyant-aiding flow is high while the buoyant-opposing flow is significantly less than that of forced convection flow.
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Sun, Meng, Jieyu Jiang, Yongzhe Yu, Canxing He, Kun Liu, and Bin Zhang. "The impinging wall effect on flame dynamics and heat transfer in non-premixed jet flames." Thermal Science, no. 00 (2022): 76. http://dx.doi.org/10.2298/tsci220126076s.
Texte intégralRésumé :
The impinging jet flame is studied experimentally and numerically accounting for the complex flame-wall interactions in practical combustion devices. Flame dynamics and heat transfer with the effect of impinging wall are analyzed. 3D large eddy simulation coupled with detailed chemical reaction mechanism and particle image velocimetry experiment based on cross-correlation measurement principle are performed for verification and further analysis. Results show that vortices are generated due to the Kelvin-Helmholtz instability originated from velocity gradient. 3D vortex interactions involving vortex rings and spirals are also indicated by vorticity and the convection of stream wise vorticity is responsible for the effect of vortex spirals associated with turbulent flow transition. In addition, results calculated from four wall thermal conditions are compared and analyzed. Dirichlet condition is inferred to be more suitable for the case of wall materials with higher thermal conductivity. It is indicated that wall thermal condition mainly affects the heat transfer in the near-wall region, but has little effect on the momentum transfer. This study provides references for the adoption of wall conditions in numerical simulation and near-wall treatment in combustion systems.
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KURDYUMOV, VADIM N., and AMABLE LIÑÁN. "Free and forced convection around line sources of heat and heated cylinders in porous media." Journal of Fluid Mechanics 427 (January 25, 2001): 389–409. http://dx.doi.org/10.1017/s0022112000002482.
Texte intégralRésumé :
An analysis is presented for the steady, two-dimensional, free convection around line sources of heat and heated cylinders in unbounded saturated porous media. It is extended to account also for the effects of forced convection. The study is based on the Boussinesq equations, with the velocities calculated using Darcy's law.The analysis begins with the non-dimensional formulation and numerical solution of the problem of pure free convection around a line source of heat. When this analysis is extended to include the effects of forced convection, two parameters appear in the non-dimensional formulation: the non-dimensional value, V∞, of the free-stream velocity and its angle γ of inclination with respect to the vertical. We first describe the asymptotic form of the solution for large and small values of the distance to the source. The far-field description, which is also applicable to the flow around heated cylinders, is needed to facilitate the numerical solution of the problem. It includes a thermal wake, aligned with the free stream, and an outer irrotational flow with a sink and a vortex at the line source. The temperature distribution near the source involves a constant A0(V∞, γ), to be calculated with the numerical solution of the complete problem, which is used in the evaluation of the heat transfer from heated cylinders when the Rayleigh and Péclet numbers are small compared with unity. In this case we find an inner region where heat conduction is dominant, and an outer region where the cylinder appears as a line source of heat. The asymptotic analysis is complemented with the numerical solution of the general problem for circular cylinders with a wide range of Rayleigh numbers and some representative values of V∞ and γ. We give correlations for the Nusselt number in the limiting cases of pure free convection and pure forced convection.
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Roy, NC, P. Akther, and AK Halder. "MHD Mixed Convection Flow of a Micropolar Fluid Past a Wedge Fixed in a Fluctuating Free Stream and Surface Temperature." Dhaka University Journal of Science 64, no. 1 (2016): 65–70. http://dx.doi.org/10.3329/dujs.v64i1.28526.
Texte intégralRésumé :
The flow an heat transfer on the unstea y laminar mixe convection boun ary layer in a micropolar flui past a vertical we ge have been stu ie taking into account the effect of magnetic fiel . We assume that the free stream velocity an surface temperature oscillate in magnitu e but not in the irection of the oncoming flow velocity. The governing equations have been solve numerically by using the straight forwar finite ifference metho . The amplitu es of skin friction an couple stress are foun to be significantly epen ent on the Richar sons number, Ri, the magnetic parameter, M, an the vortex viscosity parameter, K. We observe that the amplitu es of skin friction an couple stress increases owing to an increase of the Richar sons number, Ri, while these become lower for the higher value of the magnetic parameter, M, an the vortex viscosity parameter, K. Also the results emonstrate that the effects of the parameters on the amplitu es of heat transfer are rather weak. Dhaka Univ. J. Sci. 64(1): 65-70, 2016 (January)
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Metz, Nicholas D., and Lance F. Bosart. "Derecho and MCS Development, Evolution, and Multiscale Interactions during 3–5 July 2003." Monthly Weather Review 138, no. 8 (2010): 3048–70. http://dx.doi.org/10.1175/2010mwr3218.1.
Texte intégralRésumé :
Abstract From 3 to 5 July 2003 during the Bow Echo and Mesoscale Convective Vortex Experiment (BAMEX), multiple mesoscale convective systems (MCSs 1 and 2) and derechos (derechos AN, AS, A, BW, and BE) progressed across a preferred upper Midwest corridor. The derechos evolved in a favorable synoptic-scale environment. However, the environmental details associated with each derecho, such as the characteristics of the initial surface boundary, the formation position relative to the upper-level jet stream, the presence of an upper-level mesoscale disturbance, and the CAPE/shear environment varied from derecho to derecho. The MCSs and derechos composed three distinct convective episodes. Multiple mesoscale interactions between the MCSs and derechos and the environment altered the character and longevity of these episodes. The first convective episode consisted of derecho A, which formed from merging derechos AN and AS (northern and southern systems, respectively). The ∼200-hPa-deep cold pool associated with derecho A decreased surface potential temperatures by 4–8 K. MCS 1 dissipated upon entering this cold pool and an inertia–gravity wave was emitted that helped to spawn MCS 2. This inertia–gravity wave connected MCSs 1 and 2 into a compound convective episode. As derecho BW (western system) approached a strong surface boundary across Iowa created by the cold pools of derecho A and MCS 1, derecho BE (eastern system) formed. The remnants of derecho BW merged with derecho BE creating another compound convective episode. The upscale effects resulting from this active convective period directly affected subsequent convective development. Upper-level diabatic heating associated with derecho A resulted in NCEP GFS 66-h negative 1000–500-hPa thickness errors of 4–8 dam (forecast too cold) and negative 200-hPa wind errors of 10–20 m s−1 (forecast too weak). The resulting stronger than forecast 200-hPa jet stream likely increased synoptic-scale forcing for the formation and evolution of derecho BW.
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Roy, N. C., and R. S. R. Gorla. "Unsteady MHD Mixed Convection Flow of a Micropolar Fluid Over a Vertical Wedge." International Journal of Applied Mechanics and Engineering 22, no. 2 (2017): 363–91. http://dx.doi.org/10.1515/ijame-2017-0022.
Texte intégralRésumé :
AbstractAn analysis is presented to investigate the unsteady magnetohydrodynamic (MHD) mixed convection boundary-layer flow of a micropolar fluid over a vertical wedge in the presence of thermal radiation and heat generation or absorption. The free-stream velocity and surface temperature are assumed to be oscillating in magnitude but not in the direction of the oncoming flow velocity. The governing equations have been solved by two distinct methods, namely, the finite difference method for the entire frequency range, and the series solution for low frequency range and the asymptotic series expansion method for the high frequency range. Numerical solutions provide a good agreement with the series solutions. The amplitudes of skin friction and couple stress coefficients are found to be strongly dependent on the Richardson number and the vortex viscosity parameter. The Prandtl number, the conduction-radiation parameter, the surface temperature parameter and the pressure gradient parameter significantly affect the amplitudes of skin friction, couple stress and surface heat transfer rates. However, the amplitudes of skin friction coefficient are considerably affected by the magnetic field parameter, whereas the amplitudes of heat transfer rate are appreciably changed with the heat generation or absorption parameter. In addition, results are presented for the transient skin friction, couple stress and heat transfer rate with the variations of the Richardson number, the vortex viscosity parameter, the pressure gradient parameter and the magnetic field parameter.
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Mathelin, Lionel, Franc¸oise Bataille, and Andre´ Lallemand. "The Effect of Uniform Blowing on the Flow Past a Circular Cylinder." Journal of Fluids Engineering 124, no. 2 (2002): 452–64. http://dx.doi.org/10.1115/1.1467919.
Texte intégralRésumé :
This work describes blowing through the whole surface of a porous circular cylinder for the control of the near wake dynamics and the thermal protection of the surface. The flow past the cylinder is numerically studied and the blowing is modeled. Comparisons with experimental data are used for validation. It is shown that the blowing tends to increase the boundary layer thickness, to promote its separation and to decrease the viscous drag induced. Similarly, the convective heat transfer is lowered, and in the case of a nonisothermal blowing, the surface is very effectively protected from the hot free stream flow. The near wake is also affected. The vortex shedding frequency is shown to decrease when blowing occurs and a qualitative model is presented to identify the different mechanisms.
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Natarajan, V., and M. K. Chyu. "Effect of Flow Angle-of-Attack on the Local Heat/Mass Transfer From a Wall-Mounted Cube." Journal of Heat Transfer 116, no. 3 (1994): 552–60. http://dx.doi.org/10.1115/1.2910906.
Texte intégralRésumé :
An experimental study of the local mass transfer over the entire surface of a wall-mounted cube is performed with a particular emphasis on the effects of flow angles-of-attack (0 deg ≤ α ≤ 45 deg). Invoking an analogy between heat transfer and mass transfer, the presently obtained mass transfer results can be transformed into their heat transfer counterparts. Reynolds number based on the cube height and mean free-stream velocity varies between 3.1 × 104 and 1.1 × 105. To substantiate the mass transfer results, streakline patterns are visualized on the cube surfaces as well as the endwall using the oil-graphite technique. Significantly different flow regimes and local mass transfer characteristics are identified as the angle-of-attack varies. The overall convective transport is dominated by three-dimensional flow separation that includes multiple horseshoe vortex systems and an arch-shaped vortex wrapping around the rear portion of the cube. In addition to the local study, power correlations between the surface-resolved mass transfer Sherwood number and the Reynolds number are presented for all α values studied. Mass transfer averaged over the entire cube is compared with that of its two-dimensional counterpart with crossflow around a tall prism.
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Diez, F. J., L. P. Bernal, and G. M. Faeth. "Self-Preserving Mixing Properties of Steady Round Nonbuoyant Turbulent Jets in Uniform Crossflows." Journal of Heat Transfer 127, no. 8 (2005): 877–87. http://dx.doi.org/10.1115/1.1991868.
Texte intégralRésumé :
The self-preserving mixing properties of steady round nonbuoyant turbulent jets in uniform crossflows were investigated experimentally. The experiments involved steady round nonbuoyant fresh water jet sources injected into uniform and steady fresh water crossflows within the windowed test section of a water channel facility. Mean and fluctuating concentrations of source fluid were measured over cross sections of the flow using planar-laser-induced-fluorescence (PLIF). The self-preserving penetration properties of the flow were correlated successfully similar to Diez et al. [ASME J. Heat Transfer, 125, pp. 1046–1057 (2003)] whereas the self-preserving structure properties of the flow were correlated successfully based on scaling analysis due to Fischer et al. [Academic Press, New York, pp. 315–389 (1979)]; both approaches involve assumptions of no-slip convection in the cross stream direction (parallel to the crossflow) and a self-preserving nonbuoyant line puff having a conserved momentum force per unit length that moves in the streamwise direction (parallel to the initial source flow). The self-preserving flow structure consisted of two counter-rotating vortices, with their axes nearly aligned with the crossflow (horizontal) direction, that move away from the source in the streamwise direction due to the action of source momentum. Present measurements extended up to 260 and 440 source diameters from the source in the streamwise and cross stream directions, respectively, and yielded the following results: jet motion in the cross stream direction satisfied the no-slip convection approximation; geometrical features, such as the penetration of flow boundaries and the trajectories of the axes of the counter-rotating vortices, reached self-preserving behavior at streamwise distances greater than 40–50 source diameters from the source; and parameters associated with the structure of the flow, e.g., contours and profiles of mean and fluctuating concentrations of source fluid, reached self-preserving behavior at streamwise (vertical) distances from the source greater than 80 source diameters from the source. The counter-rotating vortex structure of the self-preserving flow was responsible for substantial increases in the rate of mixing of the source fluid with the ambient fluid compared to corresponding axisymmetric flows in still environments, e.g., transverse dimensions in the presence of the self-preserving counter-rotating vortex structure were 2–3 times larger than transverse dimensions in self-preserving axisymmetric flows at comparable conditions.
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Yilmaz, T. O., and D. Rockwell. "Flow structure on finite-span wings due to pitch-up motion." Journal of Fluid Mechanics 691 (December 5, 2011): 518–45. http://dx.doi.org/10.1017/jfm.2011.490.
Texte intégralRésumé :
AbstractThe flow structure on low-aspect-ratio wings arising from pitch-up motion is addressed via a technique of particle image velocimetry. The objectives are to: determine the onset and evolution of the three-dimensional leading-edge vortex; provide complementary interpretations of the vortex structure in terms of streamlines, projections of spanwise and surface-normal vorticity, and surfaces of constant values of the second invariant of the velocity gradient tensor (iso-$Q$ surfaces); and to characterize the effect of wing planform (rectangular versus elliptical) on this vortex structure. The pitch-up motion of the wing (plate) is from 0 to $4{5}^{\ensuremath{\circ} } $ over a time span corresponding to four convective time scales, and the Reynolds number based on chord is 10 000. Volumes of constant magnitude of the second invariant of the velocity gradient tensor are interpreted in conjunction with three-dimensional streamline patterns and vorticity projections in orthogonal directions. The wing motion gives rise to ordered vortical structures along its wing surface. In contrast to development of the classical two-dimensional leading-edge vortex, the flow pattern evolves to a strongly three-dimensional form at high angle of attack. The state of the vortex system, after attainment of maximum angle of attack, has a similar form for extreme configurations of wing planform. Near the plane of symmetry, a large-scale region of predominantly spanwise vorticity dominates. Away from the plane of symmetry, the flow is dominated by two extensive regions of surface-normal vorticity, i.e. swirl patterns parallel to the wing surface. This similar state of the vortex structure is, however, preceded by different sequences of events that depend on the magnitude of the spanwise velocity within the developing vortex from the leading edge of the wing. Spanwise velocity of the order of one-half the free stream velocity, which is oriented towards the plane of symmetry of the wing, results in regions of surface-normal vorticity. In contrast, if negligible spanwise velocity occurs within the developing leading-edge vortex, onset of the regions of surface-normal vorticity occurs near the tips of the wing. These extremes of large and insignificant spanwise velocity within the leading-edge vortex are induced respectively on rectangular and elliptical planforms.
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Lin, W. L., and T. F. Lin. "Observation and Computation of Vortex and/or Reverse Flow Development in Mixed Convection of Air in a Slightly Inclined Rectangular Duct." Journal of Heat Transfer 119, no. 4 (1997): 691–99. http://dx.doi.org/10.1115/1.2824173.
Texte intégralRésumé :
Combined flow visualization and conjugated numerical heat transfer analysis were carried out to study the axial evolution of the buoyancy induced secondary vortex and reverse flow in a mixed convective air flow through a bottom heated, slightly inclined rectangular duct. Results were obtained for the Grashof number Gr ranging from 1.6 × 103 to 2.8 × 105, inclined angle φ from −20 deg to 26 deg and the Reynolds number Re below 102 covering the steady and time dependent flows. For the buoyancy-opposing case, at a certain critical buoyancy-to-inertia ratio depending on the Re and φ both the experimental and numerical results clearly showed the generation of the longitudinal vortex rolls in the entry half of the duct and a slender reverse flow zone was induced near the exit end of the duct. At a higher buoyancy-to-inertia ratio the stronger reverse flow moves upstream and is in a time periodic snaking motion which is considered to result from the Kelvin-Helmholtz instability associated with the two counter flow streams, namely, the downstream moving longitudinal vortex rolls and the upstream moving reverse flow. Through the viscous shearing effects the strong snaking reverse flow induces a number of eddies moving along it and the longitudinal rolls are pushed towards the duct sides. This strong interaction between the vortex flow and reverse flow leads to an earlier transition to turbulence. A correlation equation was proposed for the penetration length of the reverse flow. However, for buoyancy-assisting flow no reverse flow is induced and the longitudinal vortex rolls prevail for the buoyancy-to-inertia ratio up to 2.8 × 105. Significant conjugated heat transfer effects were noted from the numerical results.
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Goldstein, R. J., and R. A. Spores. "Turbulent Transport on the Endwall in the Region Between Adjacent Turbine Blades." Journal of Heat Transfer 110, no. 4a (1988): 862–69. http://dx.doi.org/10.1115/1.3250586.
Texte intégralRésumé :
The complex three-dimensional flow in the endwall region near the base of a turbine blade has an important impact on the local heat transfer. The initial horseshoe vortex, the passage vortex, and resulting corner vortices cause large variations in heat transfer over the entire endwall region. Due to these large surface gradients in heat transfer, conventional measurement techniques generally do not provide an accurate determination of the local heat transfer coefficients. In the present study, the heat/mass transfer analogy is used to examine the local transport coefficients for two different endwall boundary layer thicknesses and two free-stream Reynolds numbers. A linear turbine blade cascade is used in conjunction with a removable endwall plate. Naphthalene (C10H8) is cast into a mold on the plate and the rate of naphthalene sublimation is determined at 6000 + locations on the simulated endwall by employing a computer-aided data acquisition system. This technique allows one to obtain detailed contour plots of the local convection coefficient over the entire endwall. By examining the mass transfer contours, it is possible to infer information on the three-dimensional flow in the passage between the blades. Extremely high transport coefficients on the endwall indicate locations of potential overheating and failure in an actual turbine.
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Poonia, Minakshi. "Computational study on MHD power-law fluid in tilted enclosure having sinusoidal heated sidewall." Multidiscipline Modeling in Materials and Structures 16, no. 5 (2020): 1041–59. http://dx.doi.org/10.1108/mmms-08-2019-0154.
Texte intégralRésumé :
PurposeIn the present computational study, the heat transfer and two-dimensional natural convection flow of non-Newtonian power-law fluid in a tilted rectangular enclosure is examined. The left wall of enclosure is subjected to spatially varying sinusoidal temperature distribution and right wall is cooled isothermally while the upper and lower walls are retained to be adiabatic. The flow is considered to be laminar, steady and incompressible under the influence of magnetic field. The governing mass, momentum and energy equations are transformed into dimensionless form in terms of stream function, vorticity and temperature.Design/methodology/approachThen resulted highly non-linear partial differential equations are solved computationally using Galerkin finite element method.FindingsThe exhaustive flow pattern and temperature fields are displayed through streamlines and isotherm contours for various parameters, namely, Prandtl number, Rayleigh number, Hartmann number by considering different power-law index and inclination angle. The effect of inclination angle on average Nusselt number is also shown graphically. This problem observes the potential vortex flow with elliptical core. The results show that the circular strength of the vortex formed reduces as the magnetic field strength grows. As the inclination angle increases the intensity of flow field decreases while the value of average Nusselt number increases.Originality/valueThis study has important applications in thermal management such as cooling techniques used in buildings, nuclear reactors, heat exchangers and power generators.
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KUMAR, BHASKAR, JACOB JOHN KOTTARAM, AMIT KUMAR SINGH, and SANJAY MITTAL. "Global stability of flow past a cylinder with centreline symmetry." Journal of Fluid Mechanics 632 (July 27, 2009): 273–300. http://dx.doi.org/10.1017/s0022112009007241.
Texte intégralRésumé :
Global absolute and convective stability analysis of flow past a circular cylinder with symmetry conditions imposed along the centreline of the flow field is carried out. A stabilized finite element formulation is used to solve the eigenvalue problem resulting from the linearized perturbation equation. All the computations carried out are in two dimensions. It is found that, compared to the unrestricted flow, the symmetry conditions lead to a significant delay in the onset of absolute as well as convective instability. In addition, the onset of absolute instability is greatly affected by the location of the lateral boundaries and shows a non-monotonic variation. Unlike the unrestricted flow, which is associated with von Kármán vortex shedding, the flow with centreline symmetry becomes unstable via modes that are associated with low-frequency large-scale structures. These lead to expansion and contraction of the wake bubble and are similar in characteristics to the low-frequency oscillations reported earlier in the literature. A global linear convective stability analysis is utilized to find the most unstable modes for different speeds of the disturbance. Three kinds of convectively unstable modes are identified. The ones travelling at very low streamwise speed are associated with large-scale structures and relatively low frequency. Shear layer instability, with relatively smaller scale flow structures and higher frequency, is encountered for disturbances travelling at relatively larger speed. For low blockage a new type of instability is found. It travels at relatively high speed and resembles a swirling flow structure. As opposed to the absolute instability, the convective instability appears at much lower Re and its onset is affected very little by the location of the lateral boundaries. Analysis is also carried out for determining the convective stability of disturbances that travel in directions other than along the free stream. It is found that the most unstable disturbances are not necessarily the purely streamwise travelling ones. Disturbances that move purely in the cross-stream direction can also be convectively unstable. The results from the linear stability analysis are confirmed by carrying out direct time integration of the linearized disturbance equations. The disturbance field shows transient growth by several orders of magnitude confirming that such flows act as amplifiers. Direct time integration of the Navier–Stokes equation is carried out to track the time evolution of both the large-scale low-frequency oscillations and small-scale shear layer instabilities. The critical Re for the onset of convective instability is compared with earlier results from local analysis. Good agreement is found.
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Coelho, Sergio L. V., and J. C. R. Hunt. "The dynamics of the near field of strong jets in crossflows." Journal of Fluid Mechanics 200 (March 1989): 95–120. http://dx.doi.org/10.1017/s0022112089000583.
Texte intégralRésumé :
An inviscid three-dimensional vortex-sheet model for the near field of a strong jet issuing from a pipe into a crossflow is derived. The solution for this model shows that the essential mechanisms governing this idealized flow are the distortion of the main transverse vorticity by the generation of additional axial and transverse vorticity within the pipe owing to the pressure gradients induced by the external flow, and the convection of both components of vorticity from the upwind side of the jet to its downwind side.The deformation of the cross-section of the jet which is predicted by this model is compared with the deformation predicted by the commonly used time-dependent two-dimensional vortex-sheet model. Differences arise because the latter model does not take into account the effects of the transport of the transverse component of vorticity. The complete three-dimensional vortex-sheet model leads to a symmetrical deformation of the jet cross-section and no overall deflection of the jet in the direction of the stream.To account for viscous effects, the initial region of a strong jet issuing into a uniform crossflow is modelled as an entraining three-dimensional vortex sheet, which acts like a sheet of vortices and sinks, redistributing the vorticity in the bounding shear layer and inducing non-symmetrical deformations of the cross-section of the jet. This leads to a deflection of the jet in the direction of the stream, and the loci of the centroids of the cross-sections of the jet describe a quadratic curve.Deformations predicted by each of the three models are compared with measurements obtained from photographs of the cross-sections of a jet of air emerging into a uniform crossflow in a wind tunnel. Mean velocity measurements around the jet made with a hot-wire anemometer agree with the theory; they clearly invalidate models of jets based on ‘pressure drag’.
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Zülicke, Christoph, and Dieter Peters. "Simulation of Inertia–Gravity Waves in a Poleward-Breaking Rossby Wave." Journal of the Atmospheric Sciences 63, no. 12 (2006): 3253–76. http://dx.doi.org/10.1175/jas3805.1.
Texte intégralRésumé :
Poleward-breaking Rossby waves often induce an upper-level jet streak over northern Europe. Dominant inertia–gravity wave packets are observed downstream of this jet. The physical processes of their generation and propagation, in such a configuration, are investigated with a mesoscale model. The study is focused on an observational campaign from 17 to 19 December 1999 over northern Germany. Different simulations with the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) have been performed. For a high-resolution process study, three domains were set up that encompass the evolution of Rossby waves and that of inertia–gravity waves. To minimize the impact of model damping, the horizontal and vertical resolution has been adjusted appropriately. With a novel statistical approach, the properties of inertia–gravity wave packets have been estimated. This method uses the horizontal divergence field and takes into account the spatial extension of a wave packet. It avoids the explicit treatment of the background field and works for arbitrary wavelength. Two classes of inertia–gravity waves were found: subsynoptic waves with a horizontal wavelength of about 500 km and mesoscale waves with a horizontal wavelength of about 200 km. The subsynoptic structures were also detected in radiosonde observations during this campaign. The similarity between simulated and observed wavelengths and amplitudes suggests that the simulations can be considered as near realistic. Spontaneous radiation from unbalanced flow is an important process of inertia–gravity wave generation. Synoptic-scale imbalances in the exit region of the upper-tropospheric jet streak were identified with the smoothed cross-stream Lagrangian Rossby number. In a number of simulations with different physics, it was found that the inertia–gravity wave activity was related to the tropospheric jet, orography, and moist convection. The upward propagation of inertia–gravity waves was favored during this event of a poleward-breaking Rossby wave. The presence of the polar vortex induced background winds exceeding the critical line. Consequently, the activity of inertia–gravity waves in the lower stratosphere increased by an order of magnitude during the case study. The successful simulation of the complex processes of generation and propagation showed the important role of poleward Rossby wave breaking for the appearance of inertia–gravity waves in the midlatitudes.
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Ur Rehman, Asif, Muhammad Arif Mahmood, Fatih Pitir, Metin Uymaz Salamci, Andrei C. Popescu, and Ion N. Mihailescu. "Mesoscopic Computational Fluid Dynamics Modelling for the Laser-Melting Deposition of AISI 304 Stainless Steel Single Tracks with Experimental Correlation: A Novel Study." Metals 11, no. 10 (2021): 1569. http://dx.doi.org/10.3390/met11101569.
Texte intégralRésumé :
For laser-melting deposition (LMD), a computational fluid dynamics (CFD) model was developed using the volume of fluid and discrete element modeling techniques. A method was developed to track the flow behavior, flow pattern, and driving forces of liquid flow. The developed model was compared with experimental results in the case of AISI 304 stainless steel single-track depositions on AISI 304 stainless steel substrate. A close correlation was found between experiments and modeling, with a deviation of 1–3%. It was found that the LMD involves the simultaneous addition of powder particles that absorb a significant amount of laser energy to transform their phase from solid to liquid, resulting in conduction-mode melt flow. The bubbles within the melt pool float at a specific velocity and escape from the melt pool throughout the deposition process. The pores are generated if the solid front hits the bubble before escaping the melt pool. Based on the simulations, it was discovered that the deposited layer’s counters took the longest time to solidify compared to the overall deposition. The bubbles strived to leave through the contours in an excess quantity, but became stuck during solidification, resulting in a large degree of porosity near the contours. The stream traces showed that the melt flow adopted a clockwise vortex in front of the laser beam and an anti-clockwise vortex behind the laser beam. The difference in the surface tension between the two ends of the melt pool induces “thermocapillary or Benard–Marangoni convection” force, which is insignificant compared to the selective laser melting process. After layer deposition, the melt region, mushy zone, and solidified region were identified. When the laser beam irradiates the substrate and powder particles are added simultaneously, the melt adopts a backwards flow due to the recoil pressure and thermocapillary or Benard–Marangoni convection effect, resulting in a negative mass flow rate. This study provides an in-depth understanding of melt pool dynamics and flow pattern in the case of LMD additive manufacturing technique.
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TRAUTMAN, MARK A., and ARI GLEZER. "The manipulation of the streamwise vortex instability in a natural convection boundary layer along a heated inclined flat plate." Journal of Fluid Mechanics 470 (October 31, 2002): 31–61. http://dx.doi.org/10.1017/s002211200200839x.
Texte intégralRésumé :
Flow instabilities leading to the formation of streamwise vortices in a natural convection boundary layer over a heated inclined plate submerged in a water tank are manipulated using spanwise arrays of surface-mounted heating elements. The flow over the plate is driven by a two-ply surface heater comprised of a uniform, constant- heat flux heater and a mosaic of 32 × 12 individually controlled heating elements that are used as control actuators. Surface temperature distributions are measured using liquid crystal thermography and the fluid velocity in cross-stream planes is measured using particle image velocimetry (PIV). Time-invariant spanwise-periodic excitation over a range of spanwise wavelengths leads to the formation of arrays of counter-rotating streamwise vortex pairs and to substantial modification of the surface temperature and heat transfer. The increase in surface heat transfer is accompanied by increased entrainment of ambient fluid and, as a consequence, higher streamwise flowrate. Subsequent spanwise-periodic merging of groups of vortices farther downstream retards the streamwise increase of the surface heat transfer rate. Finally, the suppression of small-amplitude spanwise disturbances by linear cancellation is demonstrated.
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Diez, F. J., L. P. Bernal, and G. M. Faeth. "Self-Preserving Mixing Properties of Steady Round Buoyant Turbulent Plumes in Uniform Crossflows." Journal of Heat Transfer 128, no. 10 (2006): 1001–11. http://dx.doi.org/10.1115/1.2345424.
Texte intégralRésumé :
The self-preserving mixing properties of steady round buoyant turbulent plumes in uniform crossflows were investigated experimentally. The experiments involved salt water sources injected into fresh water crossflows within the windowed test section of a water channel. Mean and fluctuating concentrations of source fluid were measured over cross sections of the flow using planar-laser-induced fluorescence which involved seeding the source fluid with Rhodamine 6G dye and adding small concentrations of ethanol to the crossflowing fluid in order to match the refractive indices of the source flow and the crossflow. The self-preserving penetration properties of the flow were correlated successfully based on the scaling analysis of Diez, Bernal, and Faeth (2003, ASME J. Heat Transfer, 125, pp. 1046–1057) whereas the self-preserving structure properties of the flow were correlated successfully based on the scaling analysis of Fischer et al. (1979, Mixing in Inland and Coastal Waters, Academic Press, New York, pp. 315–389); both approaches involved assumptions of no-slip convection in the cross stream (horizontal) direction (parallel to the crossflow) and a self-preserving line thermal having a conserved source specific buoyancy flux per unit length that moves in the streamwise (vertical) direction (parallel to the direction of both the initial source flow and the gravity vector). The resulting self-preserving structure consisted of two counter-rotating vortices having their axes nearly aligned with the crossflow direction that move away from the source in the streamwise (vertical) direction due to the action of buoyancy. Present measurements extended up to 202 and 620 source diameters from the source in the streamwise and cross stream directions, respectively. The onset of self-preserving behavior required that the axes of the counter-rotating vortex system be nearly aligned with the crossflow direction. This alignment, in turn, was a strong function of the source/crossflow velocity ratio, uo∕v∞. The net result was that the onset of self-preserving behavior was observed at streamwise distances of 10–20 source diameters from the source for uo∕v∞=4 (the smallest value of uo∕v∞ considered), increasing to streamwise distances of 160–170 source diameters from the source for uo∕v∞=100 (the largest value of uo∕v∞ considered). Finally, the counter-rotating vortex system was responsible for substantial increases in the rate of mixing of the source fluid with the ambient fluid compared to axisymmetric round buoyant turbulent plumes in still environments, e.g., transverse dimensions in the presence of the self-preserving counter-rotating vortex system were 2–3 times larger than the transverse dimensions of self-preserving axisymmetric plumes at similar streamwise distances from the source.
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Mityakov, Vladimir, Vladimir Seroshtanov, Alexey Vlasov, Vasily Suchok, Pavel Bobylev, and Nikita Zhidkov. "Heat transfer and air flow near a pair of circular cylinders." E3S Web of Conferences 140 (2019): 06012. http://dx.doi.org/10.1051/e3sconf/201914006012.
Texte intégralRésumé :
Convective heat transfer is associated with the nature of the flow near the streamlined solid. Since flow properties vary quickly, it is important to fix their momentary values. We propose a technique for studying heat transfer and air flow based on combined use of gradient heat flux measurement and PIV. The paper presents velocity fields near a pair of circular cylinders and distribution of heat flux per unit area on the surface of the second cylinder. Both cylinders were heated with saturated water steam at atmospheric pressure, thereby keeping temperature of the cylinders constant. The experiments were carried out in the range of Reynolds numbers from 480 to 29800. Differences in vortex structure, dead-air region’s length etc., and heat transfer are revealed depending on the velocity of free stream and the distance between the cylinders. Use of gradient heat flux sensors allows us to estimate pulsations of heat flux at various points of the second cylinder and compare them with the pictures of instantaneous velocity fields. The results are consistent with data from other authors and show the prospects of the proposed methodology.
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MEIBURG, E., E. WALLNER, A. PAGELLA, A. RIAZ, C. HÄRTEL, and F. NECKER. "Vorticity dynamics of dilute two-way-coupled particle-laden mixing layers." Journal of Fluid Mechanics 421 (October 25, 2000): 185–227. http://dx.doi.org/10.1017/s0022112000001737.
Texte intégralRésumé :
The two-way coupling mechanisms in particle-laden mixing layers are investigated, with and without particle settling, and with an emphasis on the resulting modifications to the fluid vorticity field. The governing equations are interpreted with respect to the production and cancellation of vorticity. These mechanisms are shown to be related to the misalignment of the concentration gradient and the slip velocity, as well as to the difference in fluid and particle vorticities. Preliminary insight into the physics is obtained from an analysis of the unidirectional base flow. For this model problem, the conditions are established under which the particle velocity remains a single-valued function of space for all times. The resulting simplified set of two-way-coupled equations governing the vorticity of the fluid and particulate phases, respectively, is solved numerically. The formation of a decaying travelling wave solution is demonstrated over a wide range of parameters. Interestingly, the downward propagation of the fluid vorticity field is not accomplished through convection, but rather by the production and loss of vorticity on opposite sides of the mixing layer. For moderate settling velocities, the simulation results reveal an optimal coupling mechanism between the fluid and particle vorticities at intermediate values of the mass loading parameter. For large settling velocities and intermediate mass loadings, more than one local maximum is seen to evolve in the vorticity field. A scaling law for the downward propagation rates of the vorticity fronts is derived.Two-dimensional particle-laden mixing layers are investigated by means of a mixed Lagrangian–Eulerian approach which is based on the vorticity variable. For uniformly seeded mixing layers, the simulations confirm some of the features observed by Druzhinin (1995b) for the model problem of a two-way-coupled particle-laden Stuart vortex, as well as by Dimas & Kiger (1998) in a linear stability analysis. For small values of the Stokes number, a mild destabilization of the mixing layer is observed. At moderate and large Stokes numbers, on the other hand, the transport of vorticity from the braids into the core of the evolving Kelvin–Helmholtz vortices is seen to be slowed by the two-way coupling effects. As a result, the particle ejection from the vortex cores is weakened. For constant mass loadings, the two-way coupling effects are strongest at intermediate Stokes number values. For moderately large Stokes numbers, the formation of two bands of high particle concentration is observed in the braids, which reflects the multi-valued nature of the particle velocity field. For mixing layers in which only one stream is seeded, the particle concentration gradient across the mixing layer leads to strong vorticity production and loss, which results in an effective net motion of the vortex in the flow direction of the seeded stream. Under particle settling, the vortex propagates downward as well. For the parameter range explored here, its settling velocity agrees well with the scaling law derived from the unidirectional flow analysis.