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1
Khorsandi, B., S. Gaskin e L. Mydlarski. "Effect of background turbulence on an axisymmetric turbulent jet". Journal of Fluid Mechanics 736 (4 novembre 2013): 250–86. http://dx.doi.org/10.1017/jfm.2013.465.
Testo completoAbstract (sommario):
AbstractThe effect of different levels of background turbulence on the dynamics and mixing of an axisymmetric turbulent jet at different Reynolds numbers has been investigated. Approximately homogeneous and isotropic background turbulence was generated by a random jet array and had a negligible mean flow (${\langle {U}_{\alpha } \rangle }/ {u}_{\alpha \mathit{rms}} \ll 1$). Velocity measurements of a jet issuing into two different levels of background turbulence were conducted for three different jet Reynolds numbers. The results showed that the mean axial velocities decay faster with increasing level of background turbulence (compared with a jet in quiescent surroundings), while the mean radial velocities increase, especially close to the edges of the jet. Furthermore, the axial root-mean-square velocities of the jet increased in the presence of background turbulence, as did the jet’s width. However, the mass flow rate of the jet decreased, from which it can be inferred that the entrainment into the jet is reduced in a turbulent background. The effect of background turbulence on the entrainment mechanisms is discussed.
2
Sherif, S. A., e R. H. Pletcher. "Measurements of the Flow and Turbulence Characteristics of Round Jets in Crossflow". Journal of Fluids Engineering 111, n. 2 (1 giugno 1989): 165–71. http://dx.doi.org/10.1115/1.3243618.
Testo completoAbstract (sommario):
Measurements of the velocity and turbulence characteristics of a round turbulent jet in crossflow are reported. The experiments were conducted in a water channel, 8.53 m long, 0.61 m wide, and 1.067 m deep, of the recirculation type. Water was injected vertically upward from a circular pipe located near the channel bottom to simulate the turbulent jet. Normal and 45 deg-slanted fiber-film probes along with appropriate anemomenters and bridges were operated in the constant temperature mode to measure mean velocities, turbulence intensities, Reynolds stresses, structural parameters, correlation coefficients, and the turbulent kinetic energy. The measurements were carried out in the jet and its wake both in and outside the jet plane of symmetry. Details of the jet-wake cross section (including the vortex region) were revealed at a number of downstream locations using constant velocity and turbulence intensity contours.
3
Kamm, R. D., E. T. Bullister e C. Keramidas. "The Effect of a Turbulent Jet on Gas Transport During Oscillatory Flow". Journal of Biomechanical Engineering 108, n. 3 (1 agosto 1986): 266–72. http://dx.doi.org/10.1115/1.3138613.
Testo completoAbstract (sommario):
Axial mass transport due to the combined effects of flow oscillation and a turbulent jet was studied both experimentally and with a simple theoretical model. The experiments show that the distance over which turbulence enhances transport is greatly increased by flow oscillation, and is particularly sensitive to tidal volume. The jet flow rate and jet configuration are relatively less important. To analyze the results, the region influenced by the jet is divided into two zones: a near field in which the time-mean flow velocities are larger than the turbulent fluctuations, and a far field where the time-mean flow is essentially zero. In the far field, axial mass transport is increased due to the turbulence which decays in strength away from the jet. When oscillatory flow is superimposed upon the steady jet flow, the turbulence in the far field interacts with the flow oscillations to augment the transport of turbulence energy and of mass. This transport enhancement is modeled by introducing an effective axial diffusivity analogous to that used in laminar oscillatory flow.
4
SEO, YONGWON, HAENG SIK KO e SANGYOUNG SON. "MULTIFRACTAL CHARACTERISTICS OF AXISYMMETRIC JET TURBULENCE INTENSITY FROM RANS NUMERICAL SIMULATION". Fractals 26, n. 01 (febbraio 2018): 1850008. http://dx.doi.org/10.1142/s0218348x18500081.
Testo completoAbstract (sommario):
A turbulent jet bears diverse physical characteristics that have been unveiled yet. Of particular interest is to analyze the turbulent intensity, which has been a key factor to assess and determine turbulent jet performance since diffusive and mixing conditions are largely dependent on it. Multifractal measures are useful in terms of identifying characteristics of a physical quantity distributed over a spatial domain. This study examines the multifractal exponents of jet turbulence intensities obtained through numerical simulation. We acquired the turbulence intensities from numerical jet discharge experiments, where two types of nozzle geometry were tested based on a Reynolds-Averaged Navier–Stokes (RANS) equations. The [Formula: see text]-[Formula: see text] model and [Formula: see text]-[Formula: see text] model were used for turbulence closure models. The results showed that the RANS model successfully regenerates transversal velocity profile, which is almost identical to an analytical solution. The RANS model also shows the decay of turbulence intensity in the longitudinal direction but it depends on the outfall nozzle lengths. The result indicates the existence of a common multifractal spectrum for turbulence intensity obtained from numerical simulation. Although the transverse velocity profiles are similar for two different turbulence models, the minimum Lipschitz–Hölder exponent [Formula: see text] and entropy dimension [Formula: see text] are different. These results suggest that the multifractal exponents capture the difference in turbulence structures of hierarchical turbulence intensities produced by different turbulence models.
5
VARIANO, EVAN A., e EDWIN A. COWEN. "A random-jet-stirred turbulence tank". Journal of Fluid Mechanics 604 (14 maggio 2008): 1–32. http://dx.doi.org/10.1017/s0022112008000645.
Testo completoAbstract (sommario):
We report measurements of the flow above a planar array of synthetic jets, firing upwards in a spatiotemporally random pattern to create turbulence at an air–water interface. The flow generated by this randomly actuated synthetic jet array (RASJA) is turbulent, with a large Reynolds number and a weak secondary (mean) flow. The turbulence is homogeneous over a large region and has similar isotropy characteristics to those of grid turbulence. These properties make the RASJA an ideal facility for studying the behaviour of turbulence at boundaries, which we do by measuring one-point statistics approaching the air–water interface (via particle image velocimetry). We explore the effects of different spatiotemporally random driving patterns, highlighting design conditions relevant to all randomly forced facilities. We find that the number of jets firing at a given instant, and the distribution of the duration for which each jet fires, greatly affect the resulting flow. We identify and study the driving pattern that is optimal given our tank geometry. In this optimal configuration, the flow is statistically highly repeatable and rapidly reaches steady state. With increasing distance from the jets, there is a jet merging region followed by a planar homogeneous region with a power-law decay of turbulent kinetic energy. In this homogeneous region, we find a Reynolds number of 314 based on the Taylor microscale. We measure all components of mean flow velocity to be less than 10% of the turbulent velocity fluctuation magnitude. The tank width includes roughly 10 integral length scales, and because wall effects persist for one to two integral length scales, there is sizable core region in which turbulent flow is unaffected by the walls. We determine the dissipation rate of turbulent kinetic energy via three methods, the most robust using the velocity structure function. Having a precise value of dissipation and low mean flow allows us to measure the empirical constant in an existing model of the Eulerian velocity power spectrum. This model provides a method for determining the dissipation rate from velocity time series recorded at a single point, even when Taylor's frozen turbulence hypothesis does not hold. Because the jet array offers a high degree of flow control, we can quantify the effects of the mean flow in stirred tanks by intentionally forcing a mean flow and varying its strength. We demonstrate this technique with measurements of gas transfer across the free surface, and find a threshold below which mean flow no longer contributes significantly to the gas transfer velocity.
6
Sato, Hiroshi, Hirofumi Hattori e Yasutaka Nagano. "TURBULENCE MODEL FOR PREDICTING HEAT TRANSFER IN IMPINGING JET(Impinging Jet)". Proceedings of the International Conference on Jets, Wakes and Separated Flows (ICJWSF) 2005 (2005): 117–22. http://dx.doi.org/10.1299/jsmeicjwsf.2005.117.
Testo completo
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Tam, Christopher K. W. "Physics and Prediction of Supersonic Jet Noise". Applied Mechanics Reviews 47, n. 6S (1 giugno 1994): S184—S187. http://dx.doi.org/10.1115/1.3124402.
Testo completoAbstract (sommario):
Both the large turbulence structures and the fine scale turbulence of the flows of supersonic jets are sources of turbulent mixing noise. At moderately high supersonic Mach numbers especially for hot jets, the dominant part of the noise is generated directly by the large turbulence structures. The large turbulence structures propagate downstream at supersonic velocities relative to the ambient sound speed. They generate strong Mach wave radiation analogous to a supersonically travelling wavy wall. A stochastic instability wave model theory of the large turbulence structures and noise of supersonic jets has recently been developed. The theory can predict both the spectrum and directivity of the dominant part of supersonic jet noise up to a multiplicative empirical constant. Calculated results agree well with measurements.
8
Polezhaev, Yu V., A. V. Korshunov e G. V. Gabbasova. "Turbulence and turbulent viscosity in jet flows". High Temperature 45, n. 3 (giugno 2007): 334–38. http://dx.doi.org/10.1134/s0018151x07030091.
Testo completo
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Farrell, Brian F., e Petros J. Ioannou. "Formation of Jets by Baroclinic Turbulence". Journal of the Atmospheric Sciences 65, n. 11 (1 novembre 2008): 3353–75. http://dx.doi.org/10.1175/2008jas2611.1.
Testo completoAbstract (sommario):
Abstract Turbulent fluids are frequently observed to spontaneously self-organize into large spatial-scale jets; geophysical examples of this phenomenon include the Jovian banded winds and the earth’s polar-front jet. These relatively steady large-scale jets arise from and are maintained by the smaller spatial- and temporal-scale turbulence with which they coexist. Frequently these jets are found to be adjusted into marginally stable states that support large transient growth. In this work, a comprehensive theory for the interaction of jets with turbulence, stochastic structural stability theory (SSST), is applied to the two-layer baroclinic model with the object of elucidating the physical mechanism producing and maintaining baroclinic jets, understanding how jet amplitude, structure, and spacing is controlled, understanding the role of parameters such as the temperature gradient and static stability in determining jet structure, understanding the phenomenon of abrupt reorganization of jet structure as a function of parameter change, and understanding the general mechanism by which turbulent jets adjust to marginally stable states supporting large transient growth. When the mean thermal forcing is weak so that the mean jet is stable in the absence of turbulence, jets emerge as an instability of the coupled system consisting of the mean jet dynamics and the ensemble mean eddy dynamics. Destabilization of this SSST coupled system occurs as a critical turbulence level is exceeded. At supercritical turbulence levels the unstable jet grows, at first exponentially, but eventually equilibrates nonlinearly into stable states of mutual adjustment between the mean flow and turbulence. The jet structure, amplitude, and spacing can be inferred from these equilibria. With weak mean thermal forcing and weak but supercritical turbulence levels, the equilibrium jet structure is nearly barotropic. Under strong mean thermal forcing, so that the mean jet is unstable in the absence of turbulence, marginally stable highly nonnormal equilibria emerge that support high transient growth and produce power-law relations between, for example, heat flux and temperature gradient. The origin of this power-law behavior can be traced to the nonnormality of the adjusted states. As the stochastic excitation, mean baroclinic forcing, or the static stability are changed, meridionally confined jets that are in equilibrium at a given meridional wavenumber abruptly reorganize to another meridional wavenumber at critical values of these parameters. The equilibrium jets obtained with this theory are in remarkable agreement with equilibrium jets obtained in simulations of baroclinic turbulence, and the phenomenon of discontinuous reorganization of confined jets has important implications for storm-track reorganization and abrupt climate change.
10
Herricos, Stapountzis, Charalampous Georgios, Tziourtzioumis Dimitrios e Stamatelos Anastasios. "1202 DIFFUSION IN SYNTHETIC JET GENERATED TURBULENCE". Proceedings of the International Conference on Jets, Wakes and Separated Flows (ICJWSF) 2013.4 (2013): _1202–1_—_1202–6_. http://dx.doi.org/10.1299/jsmeicjwsf.2013.4._1202-1_.
Testo completo
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Kukulka, Tobias. "Horizontal Transport of Buoyant Material by Turbulent Jets in the Upper Ocean". Journal of Physical Oceanography 50, n. 3 (marzo 2020): 827–43. http://dx.doi.org/10.1175/jpo-d-19-0276.1.
Testo completoAbstract (sommario):
AbstractCurrents in the ocean surface boundary layer (OSBL) determine the horizontal transport of submerged buoyant material, such as pollutants, plankton, and bubbles. Commonly, the mean horizontal transport, that is, the transport that changes the horizontal position of the material’s center of mass, is assumed to be accomplished by horizontal mean currents. However, surface convergence zones due to OSBL turbulence organize both wind-driven horizontal currents and near-surface concentrated buoyant material. In such surface convergence zones, concentrations of buoyant material are enhanced (e.g., apparent as windrows) and collocate with increased horizontal turbulent currents, here referred to as turbulent jets. In turn, the correlation of turbulent jet flow and material concentrations leads to a net mean horizontal transport due to turbulent motion. To examine this turbulent jet transport, an idealized model is devised for a wind-driven flow that is perturbed by prescribed cellular flow structures with crosswind surface convergence zones. Model solutions of the jet flow and material concentrations reveal that turbulent jet transport is comparable to the transport by horizontal mean currents for sufficiently strong cellular flow and material buoyancy. To test this model, we also perform more realistic turbulence-resolving large-eddy simulations (LESs) of wind and wave-driven OSBL turbulence. LES results are consistent with many features of the idealized model and suggest that the commonly overlooked turbulent jet transport is about 20%–50% of the traditional transport by horizontal mean currents. Thus, turbulent jet transport should be taken into account for accurate transport models of buoyant material in the OSBL.
12
Farrell, Brian F., e Petros J. Ioannou. "Emergence of Jets from Turbulence in the Shallow-Water Equations on an Equatorial Beta Plane". Journal of the Atmospheric Sciences 66, n. 10 (1 ottobre 2009): 3197–207. http://dx.doi.org/10.1175/2009jas2941.1.
Testo completoAbstract (sommario):
Abstract Coherent jets, such as the Jovian banded winds, are a prominent feature of rotating turbulence. Shallow-water turbulence models capture the essential mechanism of jet formation, which is systematic eddy momentum flux directed up the mean velocity gradient. Understanding how this systematic eddy flux convergence is maintained and how the mean zonal flow and the eddy field mutually adjust to produce the observed jet structure constitutes a fundamental theoretical problem. In this work a shallow-water equatorial beta-plane model implementation of stochastic structural stability theory (SSST) is used to study the mechanism of zonal jet formation. In SSST a stochastic model for the ensemble-mean turbulent eddy fluxes is coupled with an equation for the mean jet dynamics to produce a nonlinear model of the mutual adjustment between the field of turbulent eddies and the zonal jets. In weak turbulence, and for parameters appropriate to Jupiter, both prograde and retrograde equatorial jets are found to be stable solutions of the SSST system, but only the prograde equatorial jet remains stable in strong turbulence. In addition to the equatorial jet, multiple midlatitude zonal jets are also maintained in these stable SSST equilibria. These midlatitude jets have structure and spacing in agreement with observed zonal jets and exhibit the observed robust reversals in sign of both absolute and potential vorticity gradient.
13
Young, C. D., J. C. Han, Y. Huang e R. B. Rivir. "Influence of Jet-Grid Turbulence on Flat Plate Turbulent Boundary Layer Flow and Heat Transfer". Journal of Heat Transfer 114, n. 1 (1 febbraio 1992): 65–72. http://dx.doi.org/10.1115/1.2911269.
Testo completoAbstract (sommario):
The influence of high mainstream turbulence on turbulent boundary layer flow and heat transfer is experimentally investigated for length Reynolds numbers between 4 × 104 and 1.5 × 106. The high mainstream turbulence is produced by a round tube grid with uniform jet injection. Injected air is blown in either an upwind or downwind direction at a controllable flow rate. A flat plate test section instrumented with foil thermocouples is located downstream from the jet grid. The turbulence intensity decay and length scale growth along the test plate, the mean velocity and temperature profiles across the boundary layer, and surface heat transfer distribution are measured. The results show that the grid with downwind injection produces a slightly higher turbulence intensity and a smaller length scale than the grid with upwind injection. A higher turbulence intensity and a smaller length scale further enhance the surface heat transfer coefficient. The jet-induced high turbulence does not alter the downstream velocity and temperature profiles in their logarithmic regions, but the wake regions are lower than the zero turbulence profiles. The Reynolds analogy factor, the augmented friction factor, and the augmented Stanton number are higher than those from existing correlations when the jet grid turbulence intensity is greater than 6 percent.
14
RISSO, FRÉDÉRIC, e JEAN FABRE. "Diffusive turbulence in a confined jet experiment". Journal of Fluid Mechanics 337 (25 aprile 1997): 233–61. http://dx.doi.org/10.1017/s0022112097004965.
Testo completoAbstract (sommario):
An experimental analysis of the turbulence in an axisymmetrical jet within a closed tube is presented. At some distance from the nozzle, a turbulent region develops, where the kinetic energy of the mean flow almost vanishes. In this region, the turbulence is transported by turbulent diffusion and its energy decreases with the distance from the inlet. A complete description of the flow field has been achieved using laser Doppler anemometry. Some unusual features are highlighted: the statistical moments of the velocity decay exponentially, the integral length scales remain constant, the radial profiles are self-similar and the Reynolds stress tensor is isotropic and homogeneous in the radial direction. These results highlight the roles of pressure fluctuations and any mean residual motion in the return to isotropy.
15
Spall, Robert E., Elgin A. Anderson e Jeffrey Allen. "Momentum Flux in Plane, Parallel Jets". Journal of Fluids Engineering 126, n. 4 (1 luglio 2004): 665–70. http://dx.doi.org/10.1115/1.1778717.
Testo completoAbstract (sommario):
The evolution of the streamwise momentum flux for two turbulent, plane, parallel jets discharging through slots in a direction normal to a wall was studied both numerically and experimentally. The numerical results, obtained by solving the Reynolds-averaged Navier-Stokes equations employing a standard k−ε turbulence model, predicted to within experimental error measured integrals of the momentum flux downstream of the merge point for jet spacing S/d=5. Integration of the streamwise component of the Reynolds-averaged Navier-Stokes equations over a control volume results in an integral constant that was evaluated numerically for jet spacings S/d=3, 5, 7, 9, and 11, and for different levels of turbulence kinetic energy and dissipation rate at the jet inlet boundaries. Results revealed that the integral constant is decreased as the jet spacing increases, and is also decreased as jet entrainment rates are increased due to higher levels of inlet turbulence kinetic energy, or alternatively, decreased levels of dissipation rate. Streamwise distance to the merge point was also found to decrease for increased levels of turbulence kinetic energy or decreased levels of dissipation rate at the jet inlet.
16
Wolf, D. H., R. Viskanta e F. P. Incropera. "Turbulence Dissipation in a Free-Surface Jet of Water and Its Effect on Local Impingement Heat Transfer From a Heated Surface: Part 2—Local Heat Transfer". Journal of Heat Transfer 117, n. 1 (1 febbraio 1995): 95–103. http://dx.doi.org/10.1115/1.2822330.
Testo completoAbstract (sommario):
This paper presents local heat transfer data for a planar, free-surface jet of water impinging normal on a uniformly heated surface. The hydrodynamic conditions of the jet were altered through the use of different nozzle types (parallel-plate and converging) and flow manipulators (wire grid and screens) to investigate the relationship between jet turbulence and local impingement heat transfer. The flow structures for each of the various nozzle conditions are reported in a companion paper (Wolf et al., 1995), and results are used in this paper to interpret their effect on local heat transfer. In addition to qualitative interpretations, correlations are developed for both the onset of transition to turbulence and the dimensionless convection coefficient at the stagnation point. Higher levels of jet turbulence are shown to induce transition to a turbulent boundary layer at smaller streamwise distances from the stagnation point. The effect of stream-wise turbulence intensity on the convection coefficient is shown to scale approximately as the one-quarter power.
17
Lai, Adrian C. H., Adrian Wing-Keung Law e E. Eric Adams. "A second-order integral model for buoyant jets with background homogeneous and isotropic turbulence". Journal of Fluid Mechanics 871 (20 maggio 2019): 271–304. http://dx.doi.org/10.1017/jfm.2019.269.
Testo completoAbstract (sommario):
Buoyant jets or forced plumes are discharged into a turbulent ambient in many natural and engineering applications. The background turbulence generally affects the mixing characteristics of the buoyant jet, and the extent of the influence depends on the characteristics of both the jet discharge and ambient. Previous studies focused on the experimental investigation of the problem (for pure jets or plumes), but the findings were difficult to generalize because suitable scales for normalization of results were not known. A model to predict the buoyant jet mixing in the presence of background turbulence, which is essential in many applications, is also hitherto not available even for a background of homogeneous and isotropic turbulence (HIT). We carried out experimental and theoretical investigations of a buoyant jet discharging into background HIT. Buoyant jets were designed to be in the range of $1<z/l_{M}<5$, where $l_{M}=M_{o}^{3/4}/F_{o}^{1/2}$ is the momentum length scale, with $z/l_{M}<\sim 1$ and $z/l_{M}>\sim 6$ representing the asymptotic cases of pure jets and plumes, respectively. The background turbulence was generated using a random synthetic jet array, which produced a region of approximately isotropic and homogeneous field of turbulence to be used in the experiments. The velocity scale of the jet was initially much higher, and the length scale smaller, than that of the background turbulence, which is typical in most applications. Comprehensive measurements of the buoyant jet mixing characteristics were performed up to the distance where jet breakup occurred. Based on the experimental findings, a critical length scale $l_{c}$ was identified to be an appropriate normalizing scale. The momentum flux of the buoyant jet in background HIT was found to be conserved only if the second-order turbulence statistics of the jet were accounted for. A general integral jet model including the background HIT was then proposed based on the conservation of mass (using the entrainment assumption), total momentum and buoyancy fluxes, and the decay function of the jet mean momentum downstream. Predictions of jet mixing characteristics from the new model were compared with experimental observation, and found to be generally in agreement with each other.
18
Dahia, Ahmed, Faiza Zidouni, Amine Boualouache, Amina Lyria Cheridi e Amel Dadda. "Numerical Study of Free Liquid Jet Primary Breakup Phenomenon in Still Gases". Algerian Journal of Renewable Energy and Sustainable Development 4, n. 01 (15 giugno 2022): 24–37. http://dx.doi.org/10.46657/ajresd.2022.4.1.3.
Testo completoAbstract (sommario):
The present paper consists of a numerical investigation carried out for primary break up analysis of a vertical water jet. Many parameters impact the flow development such as velocity, turbulence and nozzle shape. In this work, two types of nozzle geometries have been performed, the first is a capillary circular and the second is conical. The calculations have been performed using the CFD Code Fluent of ANSYS, considering laminar and turbulent flow regimes. While turbulence was modelled using RNG k-ε of RANS approach. The main results show that the jet evolves differently in the two considered nozzle geometries comparing the jet intact lengths, drop sizes and distance between successive drops. It is observed that the turbulence increases substantially the jet intact length and enables the jet breakup at the lower part of the water column. For the conical nozzle case, the jet instabilities grow quickly resulting a drop size in the same order of the jet diameter and an intact length larger in comparison with the circular nozzle case.
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Farrell, Brian F., e Petros J. Ioannou. "Structure and Spacing of Jets in Barotropic Turbulence". Journal of the Atmospheric Sciences 64, n. 10 (1 ottobre 2007): 3652–65. http://dx.doi.org/10.1175/jas4016.1.
Testo completoAbstract (sommario):
Abstract Turbulent flows are often observed to be organized into large-spatial-scale jets such as the familiar zonal jets in the upper levels of the Jovian atmosphere. These relatively steady large-scale jets are not forced coherently but are maintained by the much smaller spatial- and temporal-scale turbulence with which they coexist. The turbulence maintaining the jets may arise from exogenous sources such as small-scale convection or from endogenous sources such as eddy generation associated with baroclinic development processes within the jet itself. Recently a comprehensive theory for the interaction of jets with turbulence has been developed called stochastic structural stability theory (SSST). In this work SSST is used to study the formation of multiple jets in barotropic turbulence in order to understand the physical mechanism producing and maintaining these jets and, specifically, to predict the jet amplitude, structure, and spacing. These jets are shown to be maintained by the continuous spectrum of shear waves and to be organized into stable attracting states in the mutually adjusted mean flow and turbulence fields. The jet structure, amplitude, and spacing and the turbulence level required for emergence of jets can be inferred from these equilibria. For weak but supercritical turbulence levels the jet scale is determined by the most unstable mode of the SSST system and the amplitude of the jets at equilibrium is determined by the balance between eddy forcing and mean flow dissipation. At stronger turbulence levels the jet amplitude saturates with jet spacing and amplitude satisfying the Rayleigh–Kuo stability condition that implies the Rhines scale. Equilibrium jets obtained with the SSST system are in remarkable agreement with equilibrium jets obtained in simulations of fully developed β-plane turbulence.
20
Lilley, G. M. "The generation of sound in turbulent motion". Aeronautical Journal 112, n. 1133 (luglio 2008): 381–94. http://dx.doi.org/10.1017/s0001924000002347.
Testo completoAbstract (sommario):
Abstract The present paper reviews and discusses the physical mechanisms of noise generation and reduction in turbulent flows with their applications towards aircraft noise reduction at takeoff and on the approach. This work began in 1948 when Lilley undertook an experimental investigation into the source of jet noise as a necessary precursor to finding methods for the reduction of high speed jet engine noise on civil jet airliners. Westley and Lilley completed this experimental programme in 1951, which included the design of a range of devices for high speed jet noise reduction. It was about this time that similar studies on jet noise were being started elsewhere and in particular by Lassiter and Hubbard in USA. The major contribution to the subject of turbulence as a source of noise came from Sir James Lighthill’s remarkable theory in 1952. In spite of the difficulties attached to theoretical and experimental studies on noise from turbulence, it is shown that with the accumulated knowledge on aerodynamic noise over the past 50 years, together with an optimisation of aircraft operations including flight trajectories, we are today on the threshold of approaching the design of commercial aircraft with turbofan propulsion engines that will not be heard above the background noise of the airport at takeoff and landing beyond 1-2km, from the airport boundary fence. It is evident that in the application of this work, which centres on the physical mechanisms relating to the generation of noise from turbulence and turbulent shear flows, to jet noise, there is not one unique mechanism of jet noise generation for all jet Mach numbers. This author in this publication has concentrated on what appears to be the dominant mechanism of noise generation from turbulence, where the mean convection speeds of the turbulence are subsonic. The noise generated at transonic and supersonic jet speeds invariably involves extra mechanisms, which are only briefly referred to here.
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Broumand, M., M. Birouk e S. Vahid Mahmoodi J. "Liquid jet primary breakup in a turbulent cross-airflow at low Weber number". Journal of Fluid Mechanics 879 (1 ottobre 2019): 775–92. http://dx.doi.org/10.1017/jfm.2019.704.
Testo completoAbstract (sommario):
The influence of turbulence characteristics of a cross-airflow including its velocity fluctuations and integral length and time scales on the primary breakup regime, trajectory and breakup height and time of a transversely injected liquid jet was investigated experimentally. Turbulence intensity of the incoming airflow was varied from $u_{rms}/u_{g}=1.5\,\%$ to 5.5 % (where $u_{g}$ is cross-airflow streamwise mean velocity and $u_{rms}$ is the r.m.s. of the corresponding cross-airflow streamwise mean velocity fluctuation) by placing at the inlet of the test section a perforated plate/grid with a solidity ratio of $S=50\,\%$. Over the range of gas Weber number, $3.1<We_{g}<7.14$, the ensuing liquid jet exhibited more fluctuations and late breakup transitional behaviour under turbulent airflow conditions than in a uniform cross-airflow. Proper orthogonal decomposition of the liquid jet dynamics revealed that the use of grid caused a rise in the wavelength of travelling waves along the liquid jet, which hindered the transition of the liquid jet primary breakup regime from enhanced capillary breakup to the bag breakup mode. The quantitative results demonstrated that, at a constant airflow mean velocity, turbulent cross-airflow caused the liquid jet to bend earlier compared with its uniform counterpart. A power-law empirical correlation was proposed for the prediction of the liquid jet trajectory which takes into account the effect of turbulent Reynolds number. The liquid jet breakup height (in the $y$-axis direction) normalized by the jet diameter, and accordingly the liquid jet breakup time normalized by the airflow integral time scale, were found to decrease with increasing the airflow turbulence intensity. Two power-law empirical correlations were proposed to predict the liquid jet breakup height and time.
22
Wu, Jinxin, Li Cheng, Can Luo e Chuan Wang. "Influence of External Jet on Hydraulic Performance and Flow Field Characteristics of Water Jet Propulsion Pump Device". Shock and Vibration 2021 (24 maggio 2021): 1–15. http://dx.doi.org/10.1155/2021/6690910.
Testo completoAbstract (sommario):
Water jet propulsion technology has broad application prospects in the field of ships, and water jet technology is a kind of high and new technology that is booming and has a wide range of applications. However, there are a few studies on the effect of the external jet on the performance of the water jet propulsion pump, and it is urgent to carry out this research. In this paper, the standard k-ε turbulence model is used to carry out the numerical simulation study of the influence of the external jet on the hydraulic performance and flow field characteristics of the water jet propulsion pump device. This paper discusses the selection of calculation models, the division of grids and the setting of turbulence models, and an in-depth analysis of the calculation results. The research results show that when a high-speed water jet enters a moving water body, it will cause turbulence in the moving water body. With the increase of jet flow, the turbulence phenomenon will be improved. The average velocity of the outlet section of the nozzle is consistent with the change of the total pressure. The average vortex gradually decreases, the turbulent kinetic energy changes little, the turbulence dissipation first decreases and then increases, and the nozzle axial force changes more and more. The axial force and thrust of the device will obviously increase when the two water streams merge and spray, and they will increase with the increase of the jet flow rate. By revealing the influence mechanism of the external jet on the water jet propulsion pump device, it can provide a theoretical basis and guiding direction for further optimizing the hydraulic performance of the entire device.
23
Alhumairi, Mohammed, e Özgür Ertunç. "Active-grid turbulence effect on the topology and the flame location of a lean premixed combustion". Thermal Science 22, n. 6 Part A (2018): 2425–38. http://dx.doi.org/10.2298/tsci170503100a.
Testo completoAbstract (sommario):
Lean premixed combustion under the influence of active-grid turbulence was computationally investigated, and the results were compared with experimental data. The experiments were carried out to generate a premixed flame at a thermal load of 9 kW from a single jet flow combustor. Turbulent combustion models, such as the coherent flame model and turbulent flame speed closure model were implemented for the simulations performed under different turbulent flow conditions, which were specified by the Reynolds number based on Taylor?s microscale, the dissipation rate of turbulence, and turbulent kinetic energy. This study shows that the applied turbulent combustion models differently predict the flame topology and location. However, similar to the experiments, simulations with both models revealed that the flame moves toward the inlet when turbulence becomes strong at the inlet, that is, when Re? at the inlet increases. The results indicated that the flame topology and location in the coherent flame model were more sensitive to turbulence than those in the turbulent flame speed closure model. The flame location behavior on the jet flow combustor significantly changed with the increase of Re?.
24
Bazdidi-Tehrani, Farzad, e Mehdi Jahromi. "ANALYSIS OF SYNTHETIC JET FLOW FIELD: APPLICATION OF URANS APPROACH". Transactions of the Canadian Society for Mechanical Engineering 35, n. 3 (settembre 2011): 337–53. http://dx.doi.org/10.1139/tcsme-2011-0019.
Testo completoAbstract (sommario):
The present paper reports the time dependent simulation of a turbulent plane synthetic jet using an unsteady Reynolds averaged Navier-Stokes approach on the basis of the first and second moment closure turbulence models. All the applied turbulence models can capture a global feature of the long time averaged flow field quite well. However, the standard k – ε model yields a disappointing prediction of the turbulence field with inaccurately high levels of turbulence kinetic energy and normal Reynolds stress distributions. The second moment closure model with quadratic nonlinear pressure strain approximation shows the most reasonable prediction of the phase averaged flow and turbulence fields.
25
Banakh, Viktor, e Igor Smalikho. "Lidar Studies of Wind Turbulence in the Stable Atmospheric Boundary Layer". Remote Sensing 10, n. 8 (3 agosto 2018): 1219. http://dx.doi.org/10.3390/rs10081219.
Testo completoAbstract (sommario):
The kinetic energy of turbulence, the dissipation rate of turbulent energy, and the integral scale of turbulence in the stable atmospheric boundary layer at the location heights of low-level jets (LLJs) have been measured with a coherent Doppler light detection and ranging (lidar) system. The turbulence is shown to be weak in the central part of LLJs. The kinetic energy of turbulence at the maximum velocity heights of the jet does not exceed 0.1 (m/s)2, while the dissipation rate is about 10−5 m2/s3. On average, the integral scale of turbulence in the central part of the jet is about 100 m, which is two to three times less than the effective vertical size of the LLJ.
26
Karabasov, S. A. "Understanding jet noise". Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 368, n. 1924 (13 agosto 2010): 3593–608. http://dx.doi.org/10.1098/rsta.2010.0086.
Testo completoAbstract (sommario):
Jets are one of the most fascinating topics in fluid mechanics. For aeronautics, turbulent jet-noise modelling is particularly challenging, not only because of the poor understanding of high Reynolds number turbulence, but also because of the extremely low acoustic efficiency of high-speed jets. Turbulent jet-noise models starting from the classical Lighthill acoustic analogy to state-of-the art models were considered. No attempt was made to present any complete overview of jet-noise theories. Instead, the aim was to emphasize the importance of sound generation and mean-flow propagation effects, as well as their interference, for the understanding and prediction of jet noise.
27
Thomas, F. O., e V. W. Goldschmidt. "Acoustically Induced Enhancement of Widening and Fluctuation Intensity in a Two-Dimensional Turbulent Jet". Journal of Fluids Engineering 108, n. 3 (1 settembre 1986): 331–37. http://dx.doi.org/10.1115/1.3242582.
Testo completoAbstract (sommario):
The enhancement of widening rate and turbulence intensity in a turbulent plane jet, due to an acoustic disturbance are considered. Detailed data at a representative Strouhal number suggest a well organized symmetric structural array in the initial region of the flow. These highly organized flow structures act as efficient agents in the transport of energy to the fine-grained turbulence, leading to greater diffusivity, enhanced turbulence and an increase in widening. The data also suggest significant differences in the underlying structure of the natural and excited jet flows, hence putting in jeopardy any generalization of coherent motions especially excited to facilitate their study.
28
Alipour, Fariborz, e Ronald C. Scherer. "Characterizing glottal jet turbulence". Journal of the Acoustical Society of America 119, n. 2 (2006): 1063. http://dx.doi.org/10.1121/1.2151809.
Testo completo
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Laribi, Boualem, Djelloul Belkacemi e Hadj Abdellah. "Numerical Investigation of Turbulence Models for Free Jet Flow". Applied Mechanics and Materials 229-231 (novembre 2012): 2082–85. http://dx.doi.org/10.4028/www.scientific.net/amm.229-231.2082.
Testo completoAbstract (sommario):
The turbulent jets with swirl are of particularly significant and practical interest in various fields of industry. The present numerical study concerns the comparison between three models of turbulence for the forecast of a turbulent free axi-symmetric jet with turbulence and his development. The software used was CFD code Fluent with his three turbulence models namely k-, k- and RSM. This different turbulence models are tested to better simulate and view the effectiveness of models in the description of the jet behaviour. The parameters of flow examined are the velocity profile of jet at Reynolds number of 22000. Comparisons between numerical predictions and experimental measurements taken from the technical literature for the case of natural jet development show that the numerical techniques of Computational Fluid Dynamics are an important tool for studying the jet’s behaviour. The results indicate that the RSM models is higher than the k- and k- models when looking at changes in velocity profile behaviour towards experimental profile, the k- model can be used for predicting re-circulating flows if we are interested in global settings only. The prediction obtained by k- model is far from experimental results. It is misadvised to use k- model for the jets analysis. The better numerical results were obtained by CFD code CFX, where results are in good agreement with the experimental results.
30
Lin, Chang, S. C. Hsieh, W. J. Lin, S. H. Chou e R. V. Raikar. "Flow Field in a Skimming Flow Over a Vertical Drop Without End-Sill". Journal of Mechanics 28, n. 4 (16 ottobre 2012): 607–26. http://dx.doi.org/10.1017/jmech.2012.115.
Testo completoAbstract (sommario):
ABSTRACTThe flow structure in the shear layer and in the recirculation zone of a skimming flow downstream of a vertical drop without end-sill measured using high speed particle image velocimetry (HSPIV) and flow visualization method is presented. The interface between the sliding jet and the recirculation zone (zone below sliding jet) was enhanced through non-ventilation condition between the drop structure and the jet. The flow field measured through HSPIV was used to represent the characteristics of mean streamwise velocity in the shear layer and mean horizontal velocity in the recirculation zone. With the growth of shear layer as the jet slides down over the recirculation zone, the momentum exchange from the sliding jet into the recirculation zone via the shear layer increases along with energy loss. Hence, it was observed that the amount of energy dissipated in the skimming flow at the drop structure without ventilation is greater than that with ventilation by an average value of 50% for Yc / H ≥ 0.2 (where Yc = critical depth and H = drop height). However, the flow structure in the recirculation zone is found to be analogous to that of turbulent plane wall jet. The nonlinear regression analysis is used to fit the regressed velocity profiles to the measured HSPIV mean velocity distributions. Further, the appropriate characteristic velocity and length scales are selected to attain the unique similarity profiles both in the shear layer and in the recirculation zone of the skimming flow. The selection of the characteristic scales is also discussed. The similarity profiles are well comparable with those of napped flow without end-sill and with ventilation as well as of skimming flow with end-sill and without ventilation. It is interesting to observe that, the proposed similarity profiles for the shear layer also map the data of backward-facing step flow and cavity shear flow. In addition, the turbulence characteristics in the shear layer, including turbulence intensities, turbulent kinetic energy, viscous and Reynolds shear stresses, and turbulence energy-budget balance, are illustrated in detail. From the variation of turbulence production it is observed that near the drop structure the energy exchange is from the chaotic recirculation zone to the sliding-jet flow, while in the later part it is reversed. Furthermore, the analysis of turbulence energy-budget balance indicates very significant role of turbulence production, pressure diffusion and turbulence diffusion as compared with turbulence advection that has very minor role in turbulence energy-budget balance for the central part of the shear layer.
31
van Reeuwijk, Maarten, e Markus Holzner. "The turbulence boundary of a temporal jet". Journal of Fluid Mechanics 739 (18 dicembre 2013): 254–75. http://dx.doi.org/10.1017/jfm.2013.613.
Testo completoAbstract (sommario):
AbstractWe examine the structure of the turbulence boundary of a temporal plane jet at$\mathit{Re}= 5000$using statistics conditioned on the enstrophy. The data is obtained by direct numerical simulation and threshold values span 24 orders of magnitude, ranging from essentially irrotational fluid outside the jet to fully turbulent fluid in the jet core. We use two independent estimators for the local entrainment velocity${v}_{n} $based on the enstrophy budget. The data show clear evidence for the existence of a viscous superlayer (VSL) that envelopes the turbulence. The VSL is a nearly one-dimensional layer with low surface curvature. We find that both its area and viscous transport velocity adjust to the imposed rate of entrainment so that the integral entrainment flux is independent of threshold, although low-Reynolds-number effects play a role for the case under consideration. This threshold independence is consistent with the inviscid nature of the integral rate of entrainment. A theoretical model of the VSL is developed that is in reasonably good agreement with the data and predicts that the contribution of viscous transport and dissipation to interface propagation have magnitude$2{v}_{n} $and$- {v}_{n} $, respectively. We further identify a turbulent core region (TC) and a buffer region (BR) connecting the VSL and the TC. The BR grows in time and inviscid enstrophy production is important in this region. The BR shows many similarities with the turbulent–non-turbulent interface (TNTI), although the TNTI seems to extend into the TC. The average distance between the TC and the VSL, i.e. the BR thickness is about 10 Kolmogorov length scales or half a Taylor length scale, indicating that intense turbulent flow regions and viscosity-dominated regions are in close proximity.
32
Yamamoto, K., T. Ishida, T. Watanabe e K. Nagata. "Experimental and numerical investigation of compressibility effects on velocity derivative flatness in turbulence". Physics of Fluids 34, n. 5 (maggio 2022): 055101. http://dx.doi.org/10.1063/5.0085423.
Testo completoAbstract (sommario):
Compressibility effects on the velocity derivative flatness [Formula: see text] are investigated by experiments with opposing arrays of piston-driven synthetic jet actuators (PSJAs) and direct numerical simulations (DNS) of statistically steady compressible isotropic turbulence and temporally evolving turbulent planar jets with subsonic or supersonic jet velocities. Experiments using particle image velocimetry show that nearly homogeneous isotropic turbulence is generated at the center of a closed box from interactions between supersonic synthetic jets. The dependencies of [Formula: see text] on the turbulent Reynolds number [Formula: see text] and the turbulent Mach number MT are examined both experimentally and using DNS. Previous studies of incompressible turbulence indicate a universal relationship between [Formula: see text] and [Formula: see text]. However, both experiments and DNS confirm that [Formula: see text] increases relative to the incompressible turbulence via compressibility effects. Although [Formula: see text] tends to be larger with MT in each flow, the [Formula: see text] in the turbulent jets and the turbulence generated from PSJAs deviate from those in incompressible turbulence at lower MT compared with isotropic turbulence sustained by a solenoidal forcing. The PSJAs and supersonic planar jets generate strong pressure waves, and the wave propagation can cause an increased [Formula: see text], even at low MT. These results suggest that the compressibility effects on [Formula: see text] are not solely determined from a local value of MT and depend on the turbulence generation process.
33
Martinuzzi, R., A. M. Zaghloul, W. Al-Qaraguli e R. E. Baddour. "Turbulence Structure of Plane Surface-Jets in a Weak Coflowing Stream for Different Initial Wake Conditions". Journal of Fluids Engineering 120, n. 1 (1 marzo 1998): 76–82. http://dx.doi.org/10.1115/1.2819666.
Testo completoAbstract (sommario):
Plane turbulent surface-jets, in the presence of a weak coflowing ambient current, were experimentally investigated using a single-component Laser Doppler Velocimeter. This study is concerned with the influence on the turbulent mixing characteristics of the relative speed ratio, U′a, and the wake generated behind the plate separating the jet from the ambient stream at the exit. Data were analyzed for similarity characteristics, surface speed decay, jet growth rate, jet momentum flux, jet volume flux, and turbulence decay. An integral analysis of the governing equations was also conducted to examine the turbulent entrainment properties of the surface jets.
34
Zhang, Huiyu, Georg Mauer, Senhui Liu, Meng Liu, Yunjie Jia, Changjiu Li, Chengxin Li e Robert Vaßen. "Modeling of the Effect of Carrier Gas Injection on the Laminarity of the Plasma Jet Generated by a Cascaded Spray Gun". Coatings 12, n. 10 (27 settembre 2022): 1416. http://dx.doi.org/10.3390/coatings12101416.
Testo completoAbstract (sommario):
In this work, the plasma generated by the cascaded SinplexProTM plasma spray gun was studied by means of numerical simulation. Special attention is given to the laminarity of the plasma flow. The simulation part is divided into two parts: arcing simulation inside the spray gun and plasma jet simulation outside the spray gun. A laminar as well as a turbulent model is used in each case. The results show that, under the investigated conditions, the internal flow of the plasma torch can be considered as laminar with low turbulence and can, hence, be regarded as quasi-laminar flow. If carrier gas is injected into the plasma jet, the ideal laminar plasma jet is often greatly affected. However, the turbulent plasma jet with low turbulence intensity generated by the cascaded SinplexProTM plasma spray gun is less affected and can remain stable, which is beneficial to the plasma-spraying process.
35
Madi, Arous. "Heat transfer prediction in a shallow cavity effect of incoming flow characteristics". Thermal Science 20, n. 5 (2016): 1519–32. http://dx.doi.org/10.2298/tsci140119093m.
Testo completoAbstract (sommario):
This study deals numerically with a heat transfer in a turbulent flow over a shallow cavity. Two different configurations of the incoming flow are considered: a boundary layer flow and a plane wall jet flow, in order to examine the wall jet outer layer effect on the heat transfer. This layer is an important additional turbulence source in the wall jet flow. Reynolds number and turbulence intensity effects were investigated in the boundary layer incoming flow case. The cavity depth to nozzle height ratio effect was examined in the wall jet incoming flow case. The numerical approach is based on k-? standard turbulence model. This study reveals that the heat transfer is very sensitive to the incoming flow characteristics. The turbulence intensity increase accelerates the reattachment of the shear layer at the cavity floor and enhances the heat transfer. The reattachment phenomenon seems to be less affected by the Reynolds number. However, an increase in this parameter ameliorates the heat transfer. It was also observed a heat transfer enhancement in the wall jet incoming flow case as compared to that of a boundary layer. Likewise, it was found that the augmentation of the cavity depth to the jet nozzle height ratio improves even more the heat transfer. The maximum heat transfer occurs upstream of the reattachment.
36
Nadarajah, S., S. Balabani, M. J. Tindal e M. Yianneskis. "The turbulence structure of the annular non-swirling flow past an axisymmetric poppet valve". Proceedings of the Institution of Mechanical Engineers, Part C: Journal of Mechanical Engineering Science 212, n. 6 (1 giugno 1998): 455–71. http://dx.doi.org/10.1243/0954406981521367.
Testo completoAbstract (sommario):
This paper describes an experimental investigation of the non-swirling flow through an axisymmetric port and poppet valve assembly under steady flow conditions using laser Doppler anemometry. The three velocity components and the associated Reynolds stresses were measured by ensemble-averaged techniques and the turbulence kinetic energy and its production rate were determined. Time-resolved measurements were also taken in order to determine turbulence time and length scales and the dissipation rate of the turbulence kinetic energy. The Reynolds number, based on the minimum cross-sectional area of the port, was 25000. The flow is characterized by an annular jet which forms two vortices, one on either side of the jet. A jet flapping instability is also evident since the skewness and kurtosis of the velocity probability distribution function depart from the Gaussian form. This instability causes an intermittent mixing between eddies in the jet region and the vortices which introduces a non-turbulent contribution to the measured quantities. The production rates of the turbulence kinetic energy were found to be negative in some regions of the flow, indicating counter-gradient transport of momentum by turbulence; according to the coherent structures approach, the distribution of the Reynolds shear stresses and the length scales in these regions imply possible changes in the orientation of eddies.
37
Chang, Kuo T., e Rong F. Huang. "Development and Characterization of Jet-Injected Vee-Gutter". Journal of Mechanics 20, n. 1 (marzo 2004): 77–83. http://dx.doi.org/10.1017/s1727719100004068.
Testo completoAbstract (sommario):
ABSTRACTThe vee-gutter, which was conventionally used in a combustor for flame holding, was re-designed by employing the unsteady Coanda effect to inject fluids periodically into near wake of the vee-gutter. Fluidic targets were developed to induce self-sustained transverse oscillation of slit-jet. The self-sustained oscillating jet was conducted through passages and injected into the near wake of the vee-gutter. The behaviors and frequency characteristics of the slit-jet in the oscillation cavity and the turbulence properties in the wake were studied experimentally in a wind-tunnel by using the smoke-wire flow visualization technique and the hot-wire anemometer. The oscillation frequencies of the presently developed jet-injection vee-gutter were about 25 to 40 times higher than that of the conventionally used fluidic flowmeter. By estimating the Lagrangian integral time scale and employing the Taylor's frozen flow hypothesis, the integral length scales of turbulence fluctuations were calculated. The results showed that the integral length scales of turbulences of the jet-injected vee-gutter were significantly smaller than their counter parts of the conventional vee-gutter, which indicated the effects of vortex stretching induced by the periodic jet injection. The modifications of turbulence properties were presented and discussed.
38
Antošová, Zuzana, e Zdeněk Trávníček. "Control of a Round Jet Intermittency and Transition to Turbulence by Means of an Annular Synthetic Jet". Actuators 10, n. 8 (5 agosto 2021): 185. http://dx.doi.org/10.3390/act10080185.
Testo completoAbstract (sommario):
This paper deals with active control of a continuous jet issuing from a long pipe nozzle by means of a concentrically placed annular synthetic jet. The experiments in air cover regimes of laminar, transitional, and turbulent main jet flows (Reynolds number ranges 1082–5181). The velocity profiles (time-mean and fluctuation components) of unforced and forced jets were measured using hot-wire anemometry. Six flow regimes are distinguished, and their parameter map is proposed. The possibility of turbulence reduction by forcing in transitional jets is demonstrated, and the maximal effect is revealed at Re = 2555, where the ratio of the turbulence intensities of the forced and unforced jets is decreased up to 0.45.
39
Kohli, A., e D. G. Bogard. "Effects of Very High Free-Stream Turbulence on the Jet–Mainstream Interaction in a Film Cooling Flow". Journal of Turbomachinery 120, n. 4 (1 ottobre 1998): 785–90. http://dx.doi.org/10.1115/1.2841790.
Testo completoAbstract (sommario):
Dispersion of coolant jets in a film cooling flow field is the result of a highly complex interaction between the film cooling jets and the mainstream. Understanding this complex interaction, particularly near the injection location, is critical for improving the predictive capabilities of existing film cooling models, especially when very high free-stream turbulence levels exist. This study uses a high-Frequency-response temperature sensor to investigate the mean and fluctuating thermal field of a film cooling flow for two vastly different free-stream turbulence levels (0.5 and 20 percent). The high-frequency-response temperature sensor provides new information about the film cooling flow in terms of actual rms levels (Θ′), probability density functions (pdf’s), and frequency spectra of the thermal field. Results are presented for both free-stream conditions using round hosed inclined at 35 deg, at a momentum flux ration of I = 0.156 and density ratio of DR = 1.05. The mean thermal field results show severe degradation of the film cooling jet occurs with very high free-stream turbulence levels. Temperature rms results indicate levels as high as Θ′ = 0.25 exist at the jet-mainstream interface. More information is provided by the temperature pdf’s, which are able to identify differences in the jet-mainstream interaction for the two free stream conditions. With small free-stream turbulence, strong intermittent flow structures generated at the jet-mainstream interface disperse the jet by moving hot main stream fluid into the coolant core, and ejecting coolant fluid into the mainstream. When the free stream has large scales and very high turbulence levels, the jet-mainstream interface is obliterated by large-scale turbulent structures originating from the free stream, which completely penetrate the coolant jet, causing very rapid dispersion of the film cooling jet.
40
O¨tu¨gen, M. V., F. Girlea e P. M. Sforza. "The Turbulent Incompressible Jet in a Curved Coflow". Journal of Fluids Engineering 118, n. 2 (1 giugno 1996): 300–306. http://dx.doi.org/10.1115/1.2817377.
Testo completoAbstract (sommario):
The effects of small streamline curvature on the growth and axial flow development of a turbulent incompressible jet in a curved coflow was investigated experimentally. The jet streamline curvature was achieved by introducing the initially round jet tangentially into a stream flowing through a curved channel of square cross-section. The jet issued from a straight pipe and had a fully developed velocity profile at the exit plane. The jet Reynolds number and the coflow-to-jet-velocity ratio were 4300 and 0.11, respectively. A single component laser Doppler anemometer was used to measure the streamwise velocity. Axial mean velocity and turbulence intensity profiles were measured at various streamwise locations in both the plane of curvature and the surface perpendicular to the plane of curvature. The results indicate that the jet growth and turbulence intensity are influenced by the small streamline curvature. The growth rate of the curved jet in the plane of curvature is slightly increased compared to that of a straight jet. However, the growth of the same curved jet is suppressed in the plane perpendicular to the plane of curvature. In the plane of curvature, the inner jet half-width is larger than the outer jet half-width. The mean velocity profiles in this plane are nearly Gaussian when the lateral distance is normalized by the respective inner and outer side jet half-widths. The axial turbulence intensity profiles show asymmetry in the plane of curvature with a pronounced peak on the outer side of the jet.
41
Hromisin, Scott M., Russell W. Powers e Leighton M. Myers. "Unsteady velocity measurements of model-scale supersonic exhaust jets in military-relevant configurations". International Journal of Aeroacoustics 17, n. 1-2 (24 febbraio 2018): 184–215. http://dx.doi.org/10.1177/1475472x17743634.
Testo completoAbstract (sommario):
Of the utmost importance is the need to better understand the high temperature, high velocity flow fields generated by military tactical aircraft during “run up” and take-off that gives rise to extremely hazardous conditions for personnel and equipment within the vicinity of the aircraft. The present study aims to fill the need for high frequency, two velocity component measurements throughout the flow fields produced by university-scale supersonic jets exhausting from nozzles in configurations relevant to practical, full-scale application. Specifically, this work focuses on studying the supersonic jets operating in two basic configurations: horizontal, free jets and jets impinging normal to a ground plane reminiscent of current short-takeoff and vertical landing aircraft. Experiments are conducted at nozzle operating conditions similar to those of full-scale aircraft. Both mean velocities and turbulence components are measured in both flow fields using a laser Doppler velocimeter. Axial components of the mean flow and turbulence are measured in the free jet. In the single impinging jet flow field two-component mean velocity and turbulence components are measured in the jet plume, impingement region, and outwash flow. Free jet velocity measurements show good consistency with 50% increase in jet Reynolds number. Turbulence intensities up to 15% of the mean jet exit velocity are observed at the nozzle exit plane. Laser Doppler measurements in the outwash of an impinging jet show turbulent fluctuations produce unsteady velocities well above the mean value. Two-component impinging jet unsteady velocity spectra show a distinct peak at the same frequency as the impingement tone observed in prior impinging jet acoustic field measurements.
42
Barata, J. M. M., D. F. G. Dura˜o e M. V. Heitor. "Velocity Characteristics of Multiple Impinging Jets Through a Crossflow". Journal of Fluids Engineering 114, n. 2 (1 giugno 1992): 231–39. http://dx.doi.org/10.1115/1.2910020.
Testo completoAbstract (sommario):
The mean and turbulent velocity characteristics of the flowfield resulting from the impingement of two and three jets against a wall through a low-velocity crossflow are quantified in detail making use of a laser-Doppler velocimeter and are discussed together with the visualization of the flow. The experiments have been carried out for a velocity ratio between the jets and the crossflow of 30, for a Reynolds number based on the jet exit of 105,000, and for the jet exit five jet-diameters above the ground plate, and provided a basis to improve knowledge of several related complex flow fields in engineering applications. The results characterize the turbulent transport typical of multiple impinging flows associated with a large penetration of the impinging jets through a crossflow, and quantify the formation of fountain flows due to collision of consecutive wall jets. The turbulence measurements include those of Reynolds shear stress and identify large effects of flow distortion on the turbulence structure parameters that determine the empirical constants in engineering models of turbulence.
43
Constantinou, Navid C., Brian F. Farrell e Petros J. Ioannou. "Statistical State Dynamics of Jet–Wave Coexistence in Barotropic Beta-Plane Turbulence". Journal of the Atmospheric Sciences 73, n. 5 (1 maggio 2016): 2229–53. http://dx.doi.org/10.1175/jas-d-15-0288.1.
Testo completoAbstract (sommario):
Abstract Jets coexist with planetary-scale waves in the turbulence of planetary atmospheres. The coherent component of these structures arises from cooperative interaction between the coherent structures and the incoherent small-scale turbulence in which they are embedded. It follows that theoretical understanding of the dynamics of jets and planetary-scale waves requires adopting the perspective of statistical state dynamics (SSD), which comprises the dynamics of the interaction between coherent and incoherent components in the turbulent state. In this work, the stochastic structural stability theory (S3T) implementation of SSD for barotropic beta-plane turbulence is used to develop a theory for the jet–wave coexistence regime by separating the coherent motions consisting of the zonal jets together with a selection of large-scale waves from the smaller-scale motions that constitute the incoherent component. It is found that mean flow–turbulence interaction gives rise to jets that coexist with large-scale coherent waves in a synergistic manner. Large-scale waves that would exist only as damped modes in the laminar jet are found to be transformed into exponentially growing waves by interaction with the incoherent small-scale turbulence, which results in a change in the mode structure, allowing the mode to tap the energy of the mean jet. This mechanism of destabilization differs fundamentally and serves to augment the more familiar S3T instabilities in which jets and waves arise from homogeneous turbulence with the energy source exclusively from the incoherent eddy field and provides further insight into the cooperative dynamics of the jet–wave coexistence regime in planetary turbulence.
44
Shi, Shuaikang, Huang Xiuchang, Rao zhiqiang e Hua hongxing. "Broadband force spectrum of a pump-jet under inflow turbulence". INTER-NOISE and NOISE-CON Congress and Conference Proceedings 263, n. 6 (1 agosto 2021): 112–22. http://dx.doi.org/10.3397/in-2021-1290.
Testo completoAbstract (sommario):
To clarify the characteristics of unsteady force spectrum of a pump-jet running under inflow turbulent,the turbulence grid and Fourier synthesis method is employed to produce incoming turbulence with spatial flow structure and temporal fluctuation, which is combined with LES (large eddy simulation) to obtain broadband unsteady force spectrum of the pump-jet. The results show that the proposed method could obtain the unsteady force broadband spectrum for duct, stator and rotor. The unsteady force broadband spectrum of the pump-jet is composed of the "hump" around the blade passing frequency and its multiples, the characteristic line spectrum at the stator blade passing frequency and shaft frequency of adjacent stator multiples. With the number of blades increasing, the "hump" becomes more obvious, the characteristic peak changes periodically and reaches the minimum when the number of blades is the number of rotors. Due to the use of the stator and duct, the amplitude of the unsteady force broadband spectrum of the pump-jet is higher than propeller, but the "hump" is not as obvious as propeller. The research is helpful to clarify the unsteady force characteristics of pump-jet induced by turbulence, and provide ideas for the vibration and noise reduction of pump-jet.
45
Pramanik, Shantanu, e Manab Kumar Das. "Computational study of a turbulent wall jet flow on an oblique surface". International Journal of Numerical Methods for Heat & Fluid Flow 24, n. 2 (25 febbraio 2014): 290–324. http://dx.doi.org/10.1108/hff-01-2012-0005.
Testo completoAbstract (sommario):
Purpose – The purpose of the present study is to investigate the flow and turbulence characteristics of a turbulent wall jet flowing over a surface inclined with the horizontal and to investigate the effect of variation of the angle of inclination of the wall on the flow structure of the wall jet. Design/methodology/approach – The high Reynolds number two-equation κ− model with standard wall function is used as the turbulence model. The Reynolds number considered for the present study is 10,000. The Reynolds averaged Navier-Stokes (RANS) equations are used for predicting the turbulent flow. A staggered differencing technique employing both contravariant and Cartesian components of velocity has been applied. Results for distribution of wall static pressure and skin friction, decay of maximum streamwise velocity, streamwise variation of integral momentum and energy flux have been compared for the cases of α=0°, 5°, and 10°. Findings – Flow field has been represented in terms of streamwise and lateral velocity contours, static pressure contour, vorticity contour and streamwise velocity and static pressure profiles at different locations along the oblique offset plate. Distribution of Reynolds stresses in terms of spanwise, lateral and turbulent shear stresses, and turbulent kinetic energy and its dissipation rate have been presented to describe the turbulent characteristics. Similarity of streamwise velocity and the velocity parallel to the oblique wall has been observed in the developed region of the wall jet flow. A decaying trend is observed in the variation of total integral momentum flux in the developed region of the wall jet which becomes more evident with increase in oblique angle. Developed flow region has indicated trend of similarity in profiles of streamwise velocity as well as velocity component parallel to the oblique wall. A depression in wall static pressure has been observed near the nozzle exit when the wall is inclined and the depression increases with increase in inclination. Effect of variation of oblique angles on skin friction coefficient has indicated that it decreases with increase in oblique angle. Growth of the outer and inner shear layers and spread of the jet shows linear variation with distance along the oblique wall. Decay of maximum streamwise velocity is found to be unaffected by variation in oblique angle except in the far downstream region. The streamwise variation of spanwise integral energy shows increase in oblique angle and decreases the magnitude of energy flux through the domain. In the developed flow region, streamwise variation of centreline turbulent intensities shows increased values with increase in oblique angle, while turbulence intensities along the jet centreline in the region X<12 remain unaffected by change in oblique angles. Normalized turbulent kinetic energy distribution highlights the difference in turbulence characteristics between the wall jet and reattached offset jet flow. Near wall velocity distribution shows that the inner region of boundary layer of the developed oblique wall jet follows a logarithmic profile, but it shows some difference from the standard logarithmic curve of turbulent boundary layers which can be attributed to an increase in skin friction coefficient and a decrease in thickness of the wall attached layer. Originality/value – The study presents an in-depth investigation of the interaction between the jet and the inclined wall. It is shown that due to the Coanda effect, the jet follows the nearby wall. The findings will be useful in the study of combined flow of wall jet and offset jet and dual offset jet on oblique surfaces leading to a better design of some mechanical jet flow devices.
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Koch, Steven E., Brian D. Jamison, Chungu Lu, Tracy L. Smith, Edward I. Tollerud, Cecilia Girz, Ning Wang et al. "Turbulence and Gravity Waves within an Upper-Level Front". Journal of the Atmospheric Sciences 62, n. 11 (1 novembre 2005): 3885–908. http://dx.doi.org/10.1175/jas3574.1.
Testo completoAbstract (sommario):
Abstract High-resolution dropwindsonde and in-flight measurements collected by a research aircraft during the Severe Clear-Air Turbulence Colliding with Aircraft Traffic (SCATCAT) experiment and simulations from numerical models are analyzed for a clear-air turbulence event associated with an intense upper-level jet/frontal system. Spectral, wavelet, and structure function analyses performed with the 25-Hz in situ data are used to investigate the relationship between gravity waves and turbulence. Mesoscale dynamics are analyzed with the 20-km hydrostatic Rapid Update Cycle (RUC) model and a nested 1-km simulation with the nonhydrostatic Clark–Hall (CH) cloud-scale model. Turbulence occurred in association with a wide spectrum of upward propagating gravity waves above the jet core. Inertia–gravity waves were generated within a region of unbalanced frontogenesis in the vicinity of a complex tropopause fold. Turbulent kinetic energy fields forecast by the RUC and CH models displayed a strongly banded appearance associated with these mesoscale gravity waves (horizontal wavelengths of ∼120–216 km). Smaller-scale gravity wave packets (horizontal wavelengths of 1–20 km) within the mesoscale wave field perturbed the background wind shear and stability, promoting the development of bands of reduced Richardson number conducive to the generation of turbulence. The wavelet analysis revealed that brief episodes of high turbulent energy were closely associated with gravity wave occurrences. Structure function analysis provided evidence that turbulence was most strongly forced at a horizontal scale of 700 m. Fluctuations in ozone measured by the aircraft correlated highly with potential temperature fluctuations and the occurrence of turbulent patches at altitudes just above the jet core, but not at higher flight levels, even though the ozone fluctuations were much larger aloft. These results suggest the existence of remnant “fossil turbulence” from earlier events at higher levels, and that ozone cannot be used as a substitute for more direct measures of turbulence. The findings here do suggest that automated turbulence forecasting algorithms should include some reliable measure of gravity wave activity.
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Ströher, G. R., C. A. Martins e C. R. De Andrade. "NUMERICAL AND EXPERIMENTAL STUDY OF A FREE INCOMPRESSIBLE ISOTHERMAL TURBULENT COAXIAL JET". Revista de Engenharia Térmica 9, n. 1-2 (31 dicembre 2010): 98. http://dx.doi.org/10.5380/reterm.v9i1-2.61939.
Testo completoAbstract (sommario):
In the present study the free incompressible isothermal turbulent coaxial jet problem is numerically solved, and compared with experimental measurements for different velocity ratio between the inner and the outer streams of the jet. The radial profile of the axial mean velocity was obtained with hot anemometry at different axial positions. Governing equations (mass conservation, momentum, turbulence model) were discretized employing the finite volume method with a segregated solver. The analysis of the experimental results showed that coaxial jet flow fields did not present self-similarity up to z/D=25, and the numerical solution using the Shih’s k ε turbulence model did not match reasonably with the experimental data, with a difference of about ± 10%.
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Skotnicka-Siepsiak, Aldona. "Comparing selected parameters of a two-dimensional turbulent free jet on the basis of experimental results, digital simulations, and theoretical analyses". Technical Sciences 1, n. 20 (28 dicembre 2016): 31–48. http://dx.doi.org/10.31648/ts.2907.
Testo completoAbstract (sommario):
The presented experimental and digital examinations of a two-dimensional turbulent free jet are a first phase of in the study of the Coandă effect and its hysteresis. Additionally, basing on theoretical analyses, selected results for a turbulent jest have been also mentioned, considering theoretical assumptions for the wall layer. As the result, on the basis of experimental, digital, and analytical methods, a review of characteristic jet properties has been prepared, which includes a jet spreading ratio, its cross and longitudinal sections, and turbulence level. The jet spreading radio has been expressed as a non-linear function of the x : b relative length.
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Gerodimos, G., e R. M. C. So. "Near-Wall Modeling of Plane Turbulent Wall Jets". Journal of Fluids Engineering 119, n. 2 (1 giugno 1997): 304–13. http://dx.doi.org/10.1115/1.2819135.
Testo completoAbstract (sommario):
In most two-dimensional simple turbulent flows, the location of zero shear usually coincides with that of vanishing mean velocity gradient. However, such is not the case for plane turbulent wall jets. This could be due to the fact that the driving potential is the jet exit momentum, which gives rise to an outer region that resembles a free jet and an inner layer that is similar to a boundary layer. The interaction of a free-jet like flow with a boundary-layer type flow distinguishes the plane wall jet from other simple flows. Consequently, in the past, two-equation turbulence models are seldom able to predict the jet spread correctly. The present study investigates the appropriateness of two-equation modeling; particularly the importance of near-wall modeling and the validity of the equilibrium turbulence assumption. An improved near-wall model and three others are analyzed and their predictions are compared with recent measurements of plane wall jets. The jet spread is calculated correctly by the improved model, which is able to replicate the mixing behavior between the outer jet-like and inner wall layer and is asymptotically consistent. Good agreement with other measured quantities is also obtained. However, other near-wall models tested are also capable of reproducing the Reynolds-number effects of plane wall jets, but their predictions of the jet spread are incorrect.
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Sun, Mingbo, Yuan Liu e Zhiwei Hu. "Turbulence decay in a supersonic boundary layer subjected to a transverse sonic jet". Journal of Fluid Mechanics 867 (21 marzo 2019): 216–49. http://dx.doi.org/10.1017/jfm.2019.158.
Testo completoAbstract (sommario):
The turbulence state in a supersonic boundary layer subjected to a transverse sonic jet is studied by conducting direct numerical simulations. Turbulence statistics for two jet-to-cross-flow momentum flux ratios $(J)$ of 2.3 and 5.5 based on the previous simulation (Sun & Hu, J. Fluid Mech., vol. 850, 2018, pp. 551–583) are given and compared with a flat-plate boundary layer without a jet $(J=0.0)$. The instantaneous and time-averaged flow features around the transverse jet in the supersonic boundary layer are analysed. It is found that, in the near-wall region, turbulence is suppressed significantly with increasing $J$ in the lateral boundary layer around the jet and the turbulence decay is retained in the downstream recovery region. The local boundary-layer thickness decreases noticeably in the lateral downstream of the jet. Analysis of the cross-flow streamlines reveals a double-expansion character in the vicinity of the jet, which involves the reattachment expansion related to the flow over the jet windward separation bubble and the jet lateral expansion related to the flow around the jet barrel shock. The double expansion leads to the turbulence decay in the jet lateral boundary layer and causes a slow recovery of the outer layer in the far-field boundary layer. A preliminary experiment based on the nanoparticle laser scattering technique is conducted and confirms the existence of the turbulence decay phenomenon.