Letteratura scientifica selezionata sul tema "Eddy flux. Jets Turbulence"
Cita una fonte nei formati APA, MLA, Chicago, Harvard e in molti altri stili
Consulta la lista di attuali articoli, libri, tesi, atti di convegni e altre fonti scientifiche attinenti al tema "Eddy flux. Jets Turbulence".
Accanto a ogni fonte nell'elenco di riferimenti c'è un pulsante "Aggiungi alla bibliografia". Premilo e genereremo automaticamente la citazione bibliografica dell'opera scelta nello stile citazionale di cui hai bisogno: APA, MLA, Harvard, Chicago, Vancouver ecc.
Puoi anche scaricare il testo completo della pubblicazione scientifica nel formato .pdf e leggere online l'abstract (il sommario) dell'opera se è presente nei metadati.
Articoli di riviste sul tema "Eddy flux. Jets Turbulence"
1
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.
2
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.
3
Schneider, Wilhelm. "Decay of momentum flux in submerged jets". Journal of Fluid Mechanics 154 (maggio 1985): 91–110. http://dx.doi.org/10.1017/s0022112085001434.
Testo completoAbstract (sommario):
Slender laminar and turbulent, plane and axisymmetric jets emerging from orifices in plane or conical walls are studied at large distances from the orifices. The entrainment of momentum coupled with the entrainment of volume into a jet is determined, and its effect on the flow field is analysed by combining inner and outer expansions with a multiple scaling approach.In turbulent (plane or axisymmetric) jets, the axial velocity decreases more rapidly than predicted by classical boundary-layer solutions, and the momentum flux vanishes as the distance from the orifice tends to infinity. The analysis unveils a source of discrepancies in previous experimental data on turbulent jets.In a laminar plane jet, the momentum flux changes but little. In a laminar axisymmetric jet, the momentum flux changes slowly, yet considerably. When a critical distance from the orifice is approached, the momentum flux in the jet becomes very small, the jet diameter very large, and a toroidal viscous eddy is predicted. The structure of the flow field is briefly discussed.
4
Kobayashi, Hiromichi, Hiroki Shionoya e Yoshihiro Okuno. "Turbulent duct flows in a liquid metal magnetohydrodynamic power generator". Journal of Fluid Mechanics 713 (17 ottobre 2012): 243–70. http://dx.doi.org/10.1017/jfm.2012.456.
Testo completoAbstract (sommario):
AbstractWe numerically assess the influence of non-uniform magnetic flux density and connected load resistance on turbulent duct flows in a liquid metal magnetohydrodynamic (MHD) electrical power generator. When increasing the magnetic flux density (or Hartmann number), an M-shaped velocity profile develops in the plane perpendicular to the magnetic field; the maximum velocity in the sidewall layer of the M-shaped profile increases to maintain the flow rate. Under the conditions of a relaminarized flow, the turbulence structures align along the magnetic field and flow repeatedly like a von Kármán vortex sheet. At higher Hartmann numbers, the wall-shear stress in the sidewall layer increases and the sidewall jets transit to turbulence. The sidewall jets in the MHD turbulent duct flows have profiles similar to the non-MHD wall jets, i.e. a mean velocity profile with outer scaling, Reynolds shear stress with the opposite sign in a sidewall jet, and two maxima for the turbulent intensities in a sidewall jet. The Lorentz force suppresses the vortices of the secondary mean flow near the Hartmann layer for low Hartmann numbers, whereas the secondary vortices remain near the Hartmann layer for high Hartmann numbers. An optimal load resistance (or load factor) to obtain a maximum electrical efficiency exists, because the strong Lorentz force for a low load factor and unextracted eddy currents for a high load factor reduce efficiency. When the value of the load factor is changed, the profiles of mean velocity and r.m.s. for the optimal load factor produce almost the same profiles as the high load factor near the open-circuit condition.
5
Kamenkovich, Igor, Pavel Berloff e Joseph Pedlosky. "Role of Eddy Forcing in the Dynamics of Multiple Zonal Jets in a Model of the North Atlantic". Journal of Physical Oceanography 39, n. 6 (1 giugno 2009): 1361–79. http://dx.doi.org/10.1175/2008jpo4096.1.
Testo completoAbstract (sommario):
Abstract Multiple zonal jets are observed in satellite data–based estimates of oceanic velocities, float measurements, and high-resolution numerical simulations of the ocean circulation. This study makes a step toward understanding the dynamics of these jets in the real ocean by analyzing the vertical structure and dynamical balances within multiple zonal jets simulated in an eddy-resolving primitive equation model of the North Atlantic. In particular, the authors focus on the role of eddy flux convergences (“eddy forcing”) in supporting the buoyancy and relative/potential vorticity (PV) anomalies associated with the jets. The results suggest a central role of baroclinic eddies in the barotropic and baroclinic dynamics of the jets, and significant differences in the effects of eddy forcing between the subtropical and subpolar gyres. Additionally, diabatic potential vorticity sources and sinks, associated with vertical diffusion, are shown to play an important role in supporting the potential vorticity anomalies. The resulting potential vorticity profile does not resemble a “PV staircase”—a distinct meridional structure observed in some idealized studies of geostrophic turbulence.
6
Bakas, Nikolaos A., e Petros J. Ioannou. "On the Mechanism Underlying the Spontaneous Emergence of Barotropic Zonal Jets". Journal of the Atmospheric Sciences 70, n. 7 (1 luglio 2013): 2251–71. http://dx.doi.org/10.1175/jas-d-12-0102.1.
Testo completoAbstract (sommario):
Abstract Zonal jets are commonly observed to spontaneously emerge in a β-plane channel from a background of turbulence that is sustained in a statistical steady state by homogeneous stochastic excitation and dissipation of vorticity. The mechanism for jet formation is examined in this work within the statistical wave–mean flow interaction framework of stochastic structural stability theory (SSST) that makes predictions for the emergence of zonal jets in β-plane turbulence. Using the coupled dynamical SSST system that governs the joint evolution of the second-order statistics and the mean flow, the structural stability of the spatially homogeneous statistical equilibrium with no mean zonal jets is studied. It is shown that close to the structural stability boundary, the eddy–mean flow dynamics can be split into two competing processes. The first, which is shearing of the eddies by the local shear described by Orr dynamics in a β plane, is shown in the limit of infinitesimal shear to lead to the formation of jets. The second, which is momentum flux divergence resulting from lateral wave propagation on the nonuniform local mean vorticity gradient, is shown to oppose jet formation. The upgradient momentum fluxes due to shearing of the eddies are shown to act exactly as negative viscosity for an anisotropic forcing and as negative hyperviscosity for isotropic forcing. The downgradient fluxes due to wave flux divergence are shown to act hyperdiffusively.
7
Farrell, Brian F., e Petros J. Ioannou. "Sensitivity of Perturbation Variance and Fluxes in Turbulent Jets to Changes in the Mean Jet". Journal of the Atmospheric Sciences 61, n. 21 (1 novembre 2004): 2644–52. http://dx.doi.org/10.1175/jas3256.1.
Testo completoAbstract (sommario):
Abstract Synoptic-scale eddy variance and fluxes of heat and momentum in midlatitude jets are sensitive to small changes in mean jet velocity, dissipation, and static stability. In this work the change in the jet producing the greatest increase in variance or flux is determined. Remarkably, a single jet structure change completely characterizes the sensitivity of a chosen quadratic statistical quantity to modification of the mean jet in the sense that an arbitrary change in the jet influences a chosen statistical quantity in proportion to the projection of the change on this single optimal structure. The method used extends previous work in which storm track statistics were obtained using a stochastic model of jet turbulence.
8
Thompson, Andrew F., e William R. Young. "Two-Layer Baroclinic Eddy Heat Fluxes: Zonal Flows and Energy Balance". Journal of the Atmospheric Sciences 64, n. 9 (1 settembre 2007): 3214–31. http://dx.doi.org/10.1175/jas4000.1.
Testo completoAbstract (sommario):
Abstract The eddy heat flux generated by statistically equilibrated baroclinic turbulence supported on a uniform, horizontal temperature gradient is examined using a two-layer β-plane quasigeostrophic model. The dependence of the eddy diffusivity of temperature, Dτ, on external parameters such as β, bottom friction κ, the deformation radius λ, and the velocity jump 2U, is provided by numerical simulations at 110 different points in the parameter space β* = βλ2/U and κ* = κλ/U. There is a special “pivot” value of β*, βpiv* ≈ 11/16, at which Dτ depends weakly on κ*. But otherwise Dτ has a complicated dependence on both β* and κ*, highlighted by the fact that reducing κ* leads to increases (decreases) in Dτ if β is less than (greater than) βpiv*. Existing heat-flux parameterizations, based on Kolmogorov cascade theories, predict that Dτ is nonzero and independent of κ* in the limit κ* → 0. Simulations show indications of this regime provided that κ* ≤ 0.04 and 0.25 ≤ β* ≤ 0.5. All important length scales in this problem, namely the mixing length, the scale of the energy containing eddies, the Rhines scale, and the spacing of the zonal jets, converge to a common value as bottom friction is reduced. The mixing length and jet spacing do not decouple in the parameter regime considered here, as predicted by cascade theories. The convergence of these length scales is due to the formation of jet-scale eddies that align along the eastward jets. The baroclinic component of these eddies helps force the zonal mean flow, which occurs through nonzero Reynolds stress correlations in the upper layer, as opposed to the barotropic mode. This behavior suggests that the dynamics of the inverse barotropic cascade are insufficient to fully describe baroclinic turbulence.
9
Marke, Tobias, Susanne Crewell, Vera Schemann, Jan H. Schween e Minttu Tuononen. "Long-Term Observations and High-Resolution Modeling of Midlatitude Nocturnal Boundary Layer Processes Connected to Low-Level Jets". Journal of Applied Meteorology and Climatology 57, n. 5 (maggio 2018): 1155–70. http://dx.doi.org/10.1175/jamc-d-17-0341.1.
Testo completoAbstract (sommario):
AbstractLow-level-jet (LLJ) periods are investigated by exploiting a long-term record of ground-based remote sensing Doppler wind lidar measurements supported by tower observations and surface flux measurements at the Jülich Observatory for Cloud Evolution (JOYCE), a midlatitude site in western Germany. LLJs were found 13% of the time during continuous observations over more than 4 yr. The climatological behavior of the LLJs shows a prevailing nighttime appearance of the jets, with a median height of 375 m and a median wind speed of 8.8 m s−1 at the jet nose. Significant turbulence below the jet nose only occurs for high bulk wind shear, which is an important parameter for describing the turbulent characteristics of the jets. The numerous LLJs (16% of all jets) in the range of wind-turbine rotor heights below 200 m demonstrate the importance of LLJs and the associated intermittent turbulence for wind-energy applications. Also, a decrease in surface fluxes and an accumulation of carbon dioxide are observed if LLJs are present. A comprehensive analysis of an LLJ case shows the influence of the surrounding topography, dominated by an open pit mine and a 200-m-high hill, on the wind observed at JOYCE. High-resolution large-eddy simulations that complement the observations show that the spatial distribution of the wind field exhibits variations connected with the orographic flow depending on the wind direction, causing high variability in the long-term measurements of the vertical velocity.
10
Bachman, Scott D. "A Geometric Perspective on the Modulation of Potential Energy Release by a Lateral Potential Vorticity Gradient". Fluids 5, n. 3 (28 agosto 2020): 142. http://dx.doi.org/10.3390/fluids5030142.
Testo completoAbstract (sommario):
The release of available potential energy by growing baroclinic instability requires the slope of the eddy fluxes to be shallower than that of mean density surfaces, where the amount of energy released depends on both the flux angle and the distance of fluid parcel excursions against the background density gradient. The presence of a lateral potential vorticity (PV) gradient is known to affect the growth rate and energy release by baroclinic instability, but often makes the mathematics of formal linear stability analysis intractable. Here the effects of a lateral PV gradient on baroclinic growth are examined by considering its effects on the slope of the eddy fluxes. It is shown that the PV gradient systematically shifts the unstable modes toward higher wavenumbers and creates a cutoff to the instability at large scales, both of which steepen the eddy flux angle and limit the amount of energy released. This effect may contribute to the severe inhibition of baroclinic turbulence in systems dominated by barotropic jets, making them less likely to transition to turbulence-dominated flow regimes.
Tesi sul tema "Eddy flux. Jets Turbulence"
1
Ecker, Tobias. "Turbulence Statistics and Eddy Convection in Heated Supersonic Jets". Diss., Virginia Tech, 2015. http://hdl.handle.net/10919/51687.
Testo completoAbstract (sommario):
Supersonic hot jet noise causes significant hearing impairment to the military workforce and results in substantial cost for medical care and treatment. Detailed insight into the turbulence structure of high-speed jets is central to understanding and controlling jet noise. For this purpose a new instrument based on the Doppler global velocimetry technique has been developed. This instrument is capable of measuring three-component velocity vectors over ex-tended periods of time at mean data-rates of 100 kHz. As a demonstration of the applicability of the time-resolved Doppler global velocimetry (TR-DGV) measurement technique, statistics of three-component velocity measurements, full Reynolds stress tensors and spectra along the stream-wise direction in a cold, supersonic jet at exit Mach number Mj = 1.4 (design Mach number Md = 1.65) are presented. In pursuance of extending the instrument to planar op- eration, a rapid response photomultiplier tube, 64-channel camera is developed. Integrating field programmable gate array-based data acquisition with two-stage amplifiers enables high-speed flow velocimetry at up to 10 MHz. Incor- porating this camera technology into the TR-DGV instrument, an investigation of the perfectly expanded supersonic jet at two total temperature ratios (TTR = 1.6 and TTR = 2.0) was conducted. Fourth-order correlations which have direct impact on the intensity of the acoustic far-field noise as well as convective velocities on the lip line at several stream-wise locations were obtained. Comprehensive maps of the convective velocity and the acoustic Mach number were determined. The spatial and frequency scaling of the eddy convective velocities within the developing shear layer were also investigated. It was found that differences in the radial diffusion of the mean velocity field and the integral eddy convective velocity creates regions of locally high convective Mach numbers after the potential core. This, according to acoustic analogies, leads to high noise radiation efficiency. The spectral scaling of the eddy convec- tive velocity indicates intermittent presence of large-scale turbulence structures, which, coupled with the emergence of Mach wave radiation, may be one of the main driving factors of noise emission observed in heated supersonic jets.Ph. D.
2
Wille, Matthias Kurt Wilhelm. "Large eddy simulation of jets in cross flows". Thesis, Imperial College London, 1997. http://hdl.handle.net/10044/1/8322.
Testo completo
3
Stuber, Marcie Alberta. "Investigation of Noise Sources in Three-Stream Jets using Turbulence Characteristics". Thesis, Virginia Tech, 2017. http://hdl.handle.net/10919/76727.
Testo completoAbstract (sommario):
Key areas of noise sources are investigated through comparison of eddy convection velocity and turbulence measurements in three-stream nozzles. A Time-Resolved Doppler Global Velocimetry (TR-DGV) Instrument was applied to the Nozzle Acoustic Test Rig (NATR) at NASA's Aero-Acoustic Propulsion Lab (AAPL) to measure convection velocity. Particle image velocimetry (PIV) measurements provided mean velocity and turbulence intensity. Eddy convection velocity results were obtained from the TR-DGV data for three-stream nozzle configurations using a cross-correlation approach. The three-stream cases included an axisymmetric and an asymmetric nozzle configuration. Results of the VT TR-DGV convection velocity were compared to NASA PIV mean and turbulence intensity data. For the axisymmetric case, areas of high convection velocity and turbulence intensity were found to be from 4 to 6 diameters downstream. Comparison of convection velocity between the axisymmetric and offset case show this same region as the greatest reduction in convection velocity due to the offset. These findings suggest this region along the centerline near the end of the potential core is an important area for noise generation with jets and contribute to the noise reductions seen from three stream offset nozzles. An analysis of a one-dimensional wavepacket model was completed to provide understanding of the effect of the various convection velocities seen in the flow. Comparison of a wavepacket with a convection velocity of 0.6Uj to a wavepacket with a convection velocity of 0.8Uj showed that an increase in convection velocity shifts the wavenumber spectrum to higher wavenumbers as expected. It was also observed that for the higher convection velocity wavepacket, higher frequencies are more acoustically efficient, while mid frequencies are the most efficient radiators in the lower convection velocity case. Using mean velocity, turbulence intensity, and convection velocity areas of likely to generate noise are identified and possible fundamental mechanisms responsible for the noise generation are discussed.Master of Science
4
Duncan, Michael Ross. "Structure and contribution of extreme events in airbourne carbon dioxide and water vapour flux traces". Thesis, McGill University, 1990. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=59277.
Testo completoAbstract (sommario):
Conditional sampling techniques were used to analyze airbourne carbon dioxide and water vapour flux traces recorded during the FIFE experiment. Two analysis methods based on quadrant analysis were used to isolate and examine extreme contributions to estimates of the mean flux. The first method was a graphical analysis based on 'hyperbolic holes'. This method was used to attain the result that 80% of the flux-fraction is carried by 20% of the time-fraction. The second method, based on quadrant analysis, permitted the distinction of physical structures which are thought to represent the signatures of turbulent flux structures such as eddies or thermals. Overall results indicate that mean flux estimates over the FIFE site are dominated by a very few intermittent extreme events.
5
Akan, Cigdem. "Surface Mass Transfer in Large Eddy Simulation (LES) of Langmuir Turbulence". Scholar Commons, 2012. http://scholarcommons.usf.edu/etd/3944.
Testo completoAbstract (sommario):
Over the past century the study of gas exchange rates between the atmosphere and the ocean has received increased attention because of concern about the fate of greenhouse gases such as CO2 released into the atmosphere. Of interest is the oceanic uptake of CO2 in shallow water coastal regions as biological productivity in these regions is on average about three times larger than in the open ocean. It is well-known that in the absence of breaking surface waves, the water side turbulence controls gas transfer of sparingly soluble gases such as CO2 from the air to the water. The dependence of gas transfer on wind-driven shear turbulence and convection turbulence generated by surface cooling has been investigated previously by others. However, the effect of Langmuir turbulence generated by wave-current interaction has not been investigated before. More specifically, Langmuir turbulence is generated by the interaction of the wind-driven shear current with the Stokes drift velocity induced by surface gravity waves.
In this dissertation, large-eddy simulations (LES) of wind-driven shallow water flows with Langmuir turbulence have been conducted and scalar transport and surface scalar transfer dynamics analyzed. The scalar represents the concentration of a dissolved gas such as CO2 in the water. In flows with Langmuir turbulence, the largest scales of the turbulence consist of full-depth Langmuir circulation (LC), parallel downwind-elongated, counter-rotating vortices acting as a secondary structure to the mean flow.
LES guided by the full-depth LC field measurements of Gargett & Wells (2007) shows that Langmuir turbulence plays a major role in determining scalar transport throughout the entire water column and scalar transfer at the surface. Langmuir turbulence affects scalar
transport and its surface transfer through 1. the full-depth homogenizing action of the large scale LC and 2. the near-surface vertical turbulence intensity induced by the Stokes drift velocity shear. Two key parameters controlling the extent of these two mechanisms are the dominant wavelength (λ) of the surface waves generating the turbulence and the turbulent Langmuir number, Lat , which is inversely proportional to wave forcing relative to wind forcing.
Furthermore, LES representative of the field measurements of Gargett et al. (2004) shows that Langmuir turbulence increases transfer velocity (a measure of mass transfer efficiency across the air-water interface) dramatically with respect to shear-dominated turbulence.
Finally, direct resolution of the surface mass transfer boundary layer allows for the LES to serve as a testing ground for bulk parameterizations of transfer velocity. Several wellestablished
parameterizations are tested and a new parameterization based on Stokes drift velocity shear is proposed leading to encouraging results.
6
Rasam, Amin. "Explicit algebraic subgrid-scale stress and passive scalar flux modeling in large eddy simulation". Licentiate thesis, KTH, Linné Flow Center, FLOW, 2011. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-34453.
Testo completoAbstract (sommario):
The present thesis deals with a number of challenges in the field of large eddy simulation (LES). These include the performance of subgrid-scale (SGS) models at fairly high Reynolds numbers and coarse resolutions, passive scalar and stochastic modeling in LES. The fully-developed turbulent channel flow is used as the test case for these investigations. The advantage of this particular test case is that highly accurate pseudo-spectral methods can be used for the discretization of the governing equations. In the absence of discretization errors, a better understanding of the subgrid-scale model performance can be achieved. Moreover, the turbulent channel flow is a challenging test case for LES, since it shares some of the common important features of all wall-bounded turbulent flows. Most commonly used eddy-viscosity-type models are suitable for moderately to highly-resolved LES cases, where the unresolved scales are approximately isotropic. However, this makes simulations of high Reynolds number wall-bounded flows computationally expensive. In contrast, explicit algebraic (EA) model takes into account the anisotropy of SGS motions and performs well in predicting the flow statistics in coarse-grid LES cases. Therefore, LES of high Reynolds number wall-bounded flows can be performed at much lower number of grid points in comparison with other models. A demonstration of the resolution requirements for the EA model in comparison with the dynamic Smagorinsky and its high-pass filtered version for a fairly high Reynolds number is given in this thesis. One of the shortcomings of the commonly used eddy diffusivity model arises from its assumption of alignment of the SGS scalar flux vector with the resolved scalar gradients. However, better SGS scalar flux models that overcome this issue are very few. Using the same methodology that led to the EA SGS stress model, a new explicit algebraic SGS scalar flux model is developed, which allows the SGS scalar fluxes to be partially independent of the resolved scalar gradient. The model predictions are verified and found to improve the scalar statistics in comparison with the eddy diffusivity model. The intermittent nature of energy transfer between the large and small scales of turbulence is often not fully taken into account in the formulation of SGS models both for velocity and scalar. Using the Langevin stochastic differential equation, the EA models are extended to incorporate random variations in their predictions which lead to a reasonable amount of backscatter of energy from the SGS to the resolved scales. The stochastic EA models improve the predictions of the SGS dissipation by decreasing its length scale and improving the shape of its probability density function.QC 20110615
7
Marstorp, Linus. "Modelling of subgrid-scale stress and passive scalar flux in large eddy simulations of wall bounded turbulent flows". Doctoral thesis, KTH, Mekanik, 2008. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-4809.
Testo completoAbstract (sommario):
The aim of the thesis is to develop and validate subgrid-scale models that are relevant for large eddy simulations of complex flows including scalar mixing. A stochastic Smagorinsky model with adjustable variance and time scale is developed by adding a stochastic component to the Smagorinsky constant. The stochastic model is shown to provide for backscatter of both kinetic energy and scalar variance without causing numerical instabilities. In addition, new models for the subgrid-scale stress and passive scalar flux are derived from modelled subgrid scale transport equations. These models properly account for the anisotropy of the subgrid scales and have potentials wall bounded flows. The proposed models are validated in wall bounded flows with and without rotation and show potential or significantly improve predictions for such cases.QC 20100826
8
Schmitt, Thomas. "Simulation des Grandes Echelles de la combustion turbulente à pression supercritique". Thesis, Toulouse, INPT, 2009. http://www.theses.fr/2009INPT032H.
Testo completoAbstract (sommario):
Dans les chambres de combustion des moteurs fusées cryotechniques, la pression excède la pression critique des réactifs. Les interactions moléculaires ne sont plus négligeables et le comportement du fluide n’est plus celui d’un gaz parfait. Le but de cette thèse est de développer un outil de Simulation des Grandes Echelles (SGE) pour étudier la combustion et la dynamique dans des géométries réalistes de moteur fusées. L’utilisation de l’équation d’état de Peng-Robinson, associée à une formulation thermodynamique généralisée, et des coefficients de transports appropriés permettent au code de SGE AVBP du CERFACS de simuler des systèmes réactifs à pression supercritique. Les changements thermodynamiques au sein d’AVBP nécessitent également l’adaptation des conditions limites et des schémas numériques. L’outil est validé sur une configuration mono-espèce à pression supercritique, puis sur un cas représentatif d’un injecteur coaxial de moteur-fusée. Les résultats obtenus sont en bon accord avec l’expérience et offrent des perspectives encourageantes pour des études futures, telles que des configurations multi-injecteurs ou l’analyse des instabilités de combustion haute fréquenceIn cryogenic engines combustion chambers, pressure exceeds the propellants critical pressure. Molecular interactions are generally no longer negligible and fluid behavior deviates from that of a perfect gas. The objective of this thesis is to develop a Large-Eddy Simulation (LES) tool to study combustion and dynamics in realistic geometries of rocket engines. The use of the Peng-Robinson equation of state, in conjunction with a generalized treatment of thermodynamics and appropriate transport coefficients, allows the CERFACS’ LES code AVBP to handle reactive systems at supercritical pressure. Change of the thermodynamics in AVBP necessarily leads to an adaptation of boundary conditions treatment and numerical schemes. The tool is validated on a mono-species configuration at supercritical pressure, and a reactive single coaxial injector, representative of a rocket injector. Results are in good agreement with experiments and provide encouraging perspectives for future studies, such as multi-injector configurations and high-frequency combustion instabilities
9
Dupland, Laure. "Modélisation de la turbulence thermique : modèles algébriques pour la prévision des flux de chaleur turbulents". Toulouse, ENSAE, 2005. http://www.theses.fr/2005ESAE0023.
Testo completoAbstract (sommario):
Cette thèse traite des modèles thermiques algébriques explicites EAHFM pour la prévision des flux de chaleur turbulents. Moyennant une condition d’équilibre local de la turbulence, l’équation de transport de ces derniers se simplifie en une relation algébrique, s'affranchissant de l'hypothèse de nombre de Prandtl turbulent constant. Le flux de chaleur résultant est alors désaligné du gradient de température moyenne, palliant ainsi les défauts des modèles à diffusivité turbulente. L'expression du flux de chaleur turbulent dépendant des quatre échelles de la turbulence dynamique et thermique (k, ε, k[indice ϑ] et ε[indice ϑ]), la résolution de leur équation de transport est requise. Toutefois, en supposant constant le rapport r des temps caractéristiques de la turbulence, on s’exempte de la résolution des deux équations de transport thermiques. Des contraintes sur les constantes du modèle ont été développées de manière à satisfaire certains comportements physiques de base : écoulements homogènes et couche limite soumise ou non à un gradient de pression adverse. Un jeu de constantes a pu être obtenu dans chacune des deux approches (hypothèse sur r constant ou non). Un modèle de paroi a été développé de sorte que les composantes du flux de chaleur s’amortissent correctement au voisinage d’une paroi. Le modèle ainsi obtenu a été dans un premier temps appliqué aux écoulements de similitude, puis sa version simplifiée en association avec le modèle à deux équations k-kL en formulation EARSM a été implantée dans le code Navier-Stokes elsA de l'ONERA pour être validée sur les écoulements de plaque plane chauffée, de jet débouchant et de jet impactant une paroi chauffée.
10
Youssef, Jean. "Étude expérimentale d'un jet plan turbulent se développant dans un flux uniforme en co-courant". Phd thesis, Chasseneuil-du-Poitou, Ecole nationale supérieure de mécanique et d'aérotechnique, 2012. http://tel.archives-ouvertes.fr/tel-00784840.
Testo completoAbstract (sommario):
Cette étude expérimentale porte sur la mesure et l'analyse du comportement d'un rideau d'air plan, vertical descendant, lorsqu'il se développe dans un flux uniforme en co-courant. Le rapport r entre la vitesse du cocourant et la vitesse du jet évolue dans la gamme [0; 0, 3] qui comprend le cas du jet plan classique sans co-courant (r = 0). La motivation de l'étude est, pour le laboratoire d'accueil de la thèse (Irstea de Rennes, équipe Aéraulique et Contrôle des Atmosphères Turbulentes), l'apport de connaissances précises sur les rideaux d'air séparateurs d'ambiance, en propreté et en température, pour la maîtrise des ambiances locales dans l'industrie agroalimentaire et plus largement dans le domaine de la sécurité sanitaire des aliments. Cette étude porte principalement sur le cas isotherme, sans différence de température entre le jet et le co-courant, les cas anisothermes étant seulement abordés en investigation réduite dans le dernier chapitre (chap. IV). L'analyse de la turbulence est au centre de cette étude. Elle est menée à partir des différentes grandeurs caractéristiques, dont les profils de tensions de Reynolds et les échelles turbulentes caractéristiques. Elle sous-tend également l'analyse des évolutions des grandeurs moyennes, en particulier l'expansion du profil de vitesse moyenne. Le principal moyen d'investigation expérimentale est l'anémométrie par fils chauds croisés à température constante (CTA). La Vélocimétrie par Images de Particules (PIV) est utilisée dans le chapitre IV comme moyen insensible à la température, pour les études de cas anisothermes. Le rideau plan en co-courant a été mis en oeuvre dans une soufflerie verticale spécifique dont la veine d'essai a une hauteur utile de deux mètres (chap. II). L'étude s'appuie sur une analyse bibliographique (chap. I) centrée sur les équations de la turbulence appliquées aux jets plans. L'analyse du comportement dans le cas isotherme (chap. III) s'intéresse principalement à l'influence du rapport de vitesse r et du nombre de Reynolds. Un raisonnement mené sur le choix des variables d'adimensionnement pour décrire le comportement du jet amène à proposer un adimensionnement global, à la fois pour l'évolution de la vitesse moyenne sur l'axe, pour l'expansion de l'épaisseur du jet et pour l'évolution des fluctuations rms de vitesse. On obtient ainsi un modèle de comportement généralisable aux différentes valeurs de r pour les jets en co-courant, avec au passage une méthode intéressante pour évaluer par ce biais des caractéristiques du cas limite du jet plan sans co-courant. Les données complètes et précises obtenues par fils croisés permettent, en fin de chapitre III, de mener une description et analyse des échelles caractéristiques de la turbulence. Il apparaît que les échelles intégrales sont cohérentes avec le modèle proposé pour l'expansion du jet et que les échelles de Kolmogorov s'en déduisent ensuite par un recours à un rapport universel, fonction du nombre de Reynolds local.
Libri sul tema "Eddy flux. Jets Turbulence"
1
Hartmann, Jörg. Radiation and eddy flux experiment, 1991 (REFLEX 1). Bremerhaven: Alfred-Wegener-Institut für Polar-und Meeresforschung, 1992.
Cerca il testo completo
2
Kottmeier, Christoph. Radiation and eddy flux experiment, 1991 (REFLEX II). Bremerhaven: Alfred-Wegener-Institut für Polar-und Meeresforschung, 1994.
Cerca il testo completo
3
Foken, Th. Turbulenter Energieaustausch zwischen Atmosphäre und Unterlage: Methoden, messtechnische Realisierung sowie ihre Grenzen und Anwendungsmöglichkeiten. Offenbach am Main: Selbstverlag des Deutschen Wetterdienstes, 1990.
Cerca il testo completo
4
Bui, Trong T. A parallel, finite-volume algorithm for large-eddy simulation of turbulent flows. Edwards, Calif: National Aeronautics and Space Administration, Dryden Flight Research Center, 1999.
Cerca il testo completo
7
Rocky Mountain Forest and Range Experiment Station (Fort Collins, Colo.), a cura di. Eddy diffusivities for sensible heat, ozone and momentum from eddy correlation and gradient measurements. Fort Collins, Colo: U.S. Dept. of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station, 1993.
Cerca il testo completo
8
Jörg, Hartmann, a cura di. Radiation and eddy flux experiment 1995 (REFLEX III). Bremerhaven: Alfred-Wegener-Institut für Polar- und Meeresforschung, 1997.
Cerca il testo completo
9
V, Wilson Robert, e Langley Research Center, a cura di. Streamwise vorticity generation in laminar and turbulent jets. Hampton, Va: National Aeronautics and Space Administration, Langley Research Center, 1999.
Cerca il testo completo
10
Center, NASA Glenn Research, a cura di. The numerical analysis of a turbulent compressible jet. [Cleveland, Ohio]: National Aeronautics and Space Administration, Glenn Research Center, 2001.
Cerca il testo completoCapitoli di libri sul tema "Eddy flux. Jets Turbulence"
1
Patel, Sanjay, e Dimitris Drikakis. "Flux Limiting Schemes for Implicit Large Eddy Simulation of Synthetic Jets". In Computational Fluid Dynamics 2006, 439–44. Berlin, Heidelberg: Springer Berlin Heidelberg, 2009. http://dx.doi.org/10.1007/978-3-540-92779-2_68.
Testo completo
2
Gand, F., J. Verrière e S. Deck. "Effect of Upstream Turbulence on Single and Dual-Stream Jets. Assessment of Zonal Detached Eddy Simulation (ZDES)". In Progress in Hybrid RANS-LES Modelling, 79–89. Cham: Springer International Publishing, 2018. http://dx.doi.org/10.1007/978-3-319-70031-1_6.
Testo completo
3
Clayton, D. J., e W. P. Jones. "Large Eddy Simulation of Impinging Jets in a Confined Flow". In Engineering Turbulence Modelling and Experiments 6, 247–56. Elsevier, 2005. http://dx.doi.org/10.1016/b978-008044544-1/50023-6.
Testo completo
4
Beaubert, F., e S. Viazzo. "LARGE EDDY SIMULATIONS OF PLANE TURBULENT IMPINGING JETS AT MODERATE REYNOLDS NUMBERS". In Engineering Turbulence Modelling and Experiments 5, 277–86. Elsevier, 2002. http://dx.doi.org/10.1016/b978-008044114-6/50026-0.
Testo completo
5
Tsubokura, M., T. Kobayashi e N. Taniguchi. "NUMERICAL STUDY ON THE DIFFERENCE OF THE EDDY STRUCTURES BETWEEN PLANE AND ROUND IMPINGING JETS". In Engineering Turbulence Modelling and Experiments 5, 267–76. Elsevier, 2002. http://dx.doi.org/10.1016/b978-008044114-6/50025-9.
Testo completo
6
Ali Mergheni, Mohamed, Mohamed Mahdi Belhajbrahim, Toufik Boushaki e Jean-Charles Sautet. "A New Combustion Method in a Burner with Three Separate Jets". In Numerical and Experimental Studies on Combustion Engines and Vehicles. IntechOpen, 2020. http://dx.doi.org/10.5772/intechopen.90571.
Testo completoAbstract (sommario):
Oxy-flames from burners with separated jets present attractive perspectives because the separation of reactants generates a better thermal efficiency and reduction of pollutant emissions. The principal idea is to confine the fuel jet by oxygen jets to favor the mixing in order to improve the flame stability. This chapter concerns the effect of equivalence ratio on characteristics of a non-premixed oxy-methane flame from a burner with separated jets. The burner of 25 kW power is composed with three aligned jets, one central methane jet surrounded by two oxygen jets. The numerical simulation is carried out using Reynolds Average Navier-Stokes (RANS) technique with k-ε as a turbulence closure model. The eddy dissipation model is applied to take into account the turbulence-reaction interactions. The study is performed with different global equivalence ratios (0.7, 0.8 and 1). The validation of the numerical tools is done by comparison with experimental data of the stoichiometric regime (Ф = 1). The two lean regimes of Ф = 0.7 and 0.8 are investigated only by calculations. The velocity fields with different equivalence ratio are presented. It yields to increase of longitudinal and transverse velocity, promotes the fluctuation in interaction zone between fuel and oxygen also a better mixing quality and a decrease of the size of the recirculation zone.
7
Manescau, Brady, Khaled Chetehouna, Ilyas Sellami, Rachid Nait-Said e Fatiha Zidani. "BLEVE Fireball Effects in a Gas Industry: A Numerical Modeling Applied to the Case of an Algeria Gas Industry". In Fire Safety and Management Awareness. IntechOpen, 2020. http://dx.doi.org/10.5772/intechopen.92990.
Testo completoAbstract (sommario):
This chapter presents the numerical modeling of the BLEVE (Boiling Liquid Expanding Vapor Explosion) thermal effects. The goal is to highlight the possibility to use numerical data in order to estimate the potential damage that would be caused by the BLEVE, based on quantitative risk analysis (QRA). The numerical modeling is carried out using the computational fluid dynamics (CFD) code Fire Dynamics Simulator (FDS) version 6. The BLEVE is defined as a fireball, and in this work, its source is modeled as a vertical release of hot fuel in a short time. Moreover, the fireball dynamics is based on a single-step combustion using an eddy dissipation concept (EDC) model coupled with the default large eddy simulation (LES) turbulence model. Fireball characteristics (diameter, height, heat flux and lifetime) issued from a large-scale experiment are used to demonstrate the ability of FDS to simulate the various steps of the BLEVE phenomenon from ignition up to total burnout. A comparison between BAM (Bundesanstalt für Materialforschung und –prüfung, Allemagne) experiment data and predictions highlights the ability of FDS to model BLEVE effects. From this, a numerical study of the thermal effects of BLEVE in the largest gas field in Algeria was carried out.
8
Kaimal, J. C., e J. J. Finnigan. "Spectra and Cospectra Over Flat Uniform Terrain". In Atmospheric Boundary Layer Flows. Oxford University Press, 1994. http://dx.doi.org/10.1093/oso/9780195062397.003.0005.
Testo completoAbstract (sommario):
Turbulent flows like those in the atmospheric boundary layer can be thought of as a superposition of eddies—coherent patterns of velocity, vorticity, and pressure— spread over a wide range of sizes. These eddies interact continuously with the mean flow, from which they derive their energy, and also with each other. The large “energy-containing” eddies, which contain most of the kinetic energy and are responsible for most of the transport in the turbulence, arise through instabilities in the background flow. The random forcing that provokes these instabilities is provided by the existing turbulence. This is the process represented in the production terms of the turbulent kinetic energy equation (1.59) in Chapter 1. The energy-containing eddies themselves are also subject to instabilities, which in their case are provoked by other eddies. This imposes upon them a finite lifetime before they too break up into yet smaller eddies. This process is repeated at all scales until the eddies become sufficiently small that viscosity can affect them directly and convert their kinetic energy to internal energy (heat). The action of viscosity is captured in the dissipation term of the turbulent kinetic energy equation. The second-moment budget equations presented in Chapter 1, of which (1.59) is one example, describe the summed behavior of all the eddies in the turbulent flow. To understand the conversion of mean kinetic energy into turbulent kinetic energy in the large eddies, the handing down of this energy to eddies of smaller and smaller scale in an “eddy cascade” process, and its ultimate conversion to heat by viscosity, we must isolate the different scales of turbulent motion and separately observe their behavior. Taking Fourier spectra and cospectra of the turbulence offers a convenient way of doing this. The spectral representation associates with each scale of motion the amount of kinetic energy, variance, or eddy flux it contributes to the whole and provides a new and invaluable perspective on boundary layer structure. The spectrum of boundary layer fluctuations covers a range of more than five decades: millimeters to kilometers in spatial scales and fractions of a second to hours in temporal scales.
Atti di convegni sul tema "Eddy flux. Jets Turbulence"
1
Kim, Sung-Eun, Xuelei Zhu e Stefano Orsino. "Large Eddy Simulation of Confined Swirling Coaxial Jets". In ASME 2005 Fluids Engineering Division Summer Meeting. ASMEDC, 2005. http://dx.doi.org/10.1115/fedsm2005-77085.
Testo completoAbstract (sommario):
Large eddy simulations (LES) have been carried out for confined swirling coaxial jets discharged into a suddenly expanded pipe, which was studied experimentally by Roback and Johnson [1, 2]. The computations were made using a parallelized finite-volume-based Navier-Stokes solver that is second-order accurate in time and space, and permits use of unstructured meshes. The computational domain starts from an inlet placed upstream of the swirl generator, which makes the inlet boundary condition easy to specify. Subgrid-scale turbulent stresses were modeled using a dynamic Smagorinsky model applicable to complex three-dimensional flows without any statistically homogeneous directions. Subgrid-scale turbulent scalar flux is modeled using a constant Schmidt number in conjunction with the dynamically computed subgrid-scale turbulent viscosity. The LES predictions were found to closely reproduce the salient features of the flow and the species concentration downstream of the expansion. One of the conclusions was that a good resolution of the mean flow and turbulence in the upstream region is crucial in accurately predicting the mixing downstream of the expansion.
2
Li, Xueying, Yanmin Qin, Jing Ren e Hongde Jiang. "Algebraic Anisotropic Turbulence Modeling of Compound Angled Film Cooling Validated by PIV and PSP Measurements". In ASME Turbo Expo 2013: Turbine Technical Conference and Exposition. American Society of Mechanical Engineers, 2013. http://dx.doi.org/10.1115/gt2013-94662.
Testo completoAbstract (sommario):
The complex structures in the flow field of gas turbine film cooling lead to the anisotropic property of the turbulent eddy viscosity and scalar diffusivity. An algebraic anisotropic turbulence model is developed while aiming at a more accurate modeling of the Reynolds stress and turbulent scalar flux. In this study the algebraic anisotropic model is validated by a series of in-house experiments for cylindrical film cooling with compound angle injection of 0, 45, and 90 deg. Adiabatic film cooling effectiveness and flow field are measured using PSP and PIV techniques on film cooling test rig in Tsinghua University. Detailed analyses of computational simulations are performed. The algebraic anisotropic model gives a good prediction of the secondary vortices associated with the jet and the trajectory of the jet, therefore improves the prediction of the scalar field. On one hand, the anisotropic eddy viscosity improves the modeling of Reynolds stress and the predictive flow field. On the other hand, the anisotropic turbulent scalar-flux model includes the role of anisotropic eddy viscosity in modeling of scalar flux and directly improves the turbulent scalar flux prediction.
3
Stratton, Zachary T., e Tom I. P. Shih. "Effects of Velocity Ratio on Trends in Film-Cooling Adiabatic Effectiveness and Turbulence". In ASME Turbo Expo 2019: Turbomachinery Technical Conference and Exposition. American Society of Mechanical Engineers, 2019. http://dx.doi.org/10.1115/gt2019-90943.
Testo completoAbstract (sommario):
Abstract For film-cooling of a flat plate with cooling jets issuing from round holes, turbulent mixing has been shown to scale with the velocity ratio (VR). In this paper, large-eddy simulations (LES) were performed to investigate the effect of varying blowing ratio (BR = 0.5 – 1.3), density ratio (DR = 1.1 – 2.1), and momentum-flux ratio (MR = 0.2–0.8) on adiabatic effectiveness and turbulence, while keeping the VR fixed at 0.46 and 0.63. Simulations based on Reynolds-Averaged Navier-Stokes (RANS) equations with the realizable k-ϵ and shear-stress transport k-ω models were also performed. The LES results show that separation and spreading of the film-cooling jet increase as BR, DR, and MR increase while VR remains constant. For a given VR, the LES predicts an absolute difference between the minimum adiabatic effectiveness of the lowest and highest MRs to be 2 to 5 times greater than those predicted by RANS. This is because RANS with either model cannot respond appropriately to changes in MR. However, RANS can correctly predict that adiabatic effectiveness decreases as VR increases. The LES results show the turbulent kinetic energy and Reynolds stresses near the film-cooling hole to change considerably with MRs at a constant VR, while turbulent heat flux changes negligibly. This suggests that while improved turbulence models for heat flux can improve RANS prediction of spreading, capturing trends, however, requires improved modeling of the Reynolds stresses.
4
Gherman, Bogdan, Robert-Zoltan Szasz e Laszlo Fuchs. "LES of Swirling Flows in Gas Turbine Combustion Chambers". In ASME Turbo Expo 2004: Power for Land, Sea, and Air. ASMEDC, 2004. http://dx.doi.org/10.1115/gt2004-53711.
Testo completoAbstract (sommario):
The flow and mixing in a swirl-stabilized gas-turbine burner is studied by Large Eddy Simulations (LES). Each swirler has a different mass flux and swirl angle. The interaction between neighbouring jets is studied, co-rotating and counter rotating jets are considered. Another issue of importance is related to the jet inlet conditions (e.g. axial distribution and levels of turbulence). In addition to the flow field (using LES) we present results related to fuel/air mixing under different conditions. We show that the LES results can resolve several issues related to the burner that cannot be accounted for by the standard RANS computations.
5
Foroutan, Hosein, e Savas Yavuzkurt. "Simulation of the Near-Field Region of Film Cooling Jets Using RANS and Hybrid URANS/LES Models". In ASME Turbo Expo 2014: Turbine Technical Conference and Exposition. American Society of Mechanical Engineers, 2014. http://dx.doi.org/10.1115/gt2014-25959.
Testo completoAbstract (sommario):
This paper investigates the flow field and heat transfer in the near-field region of film cooling jets through numerical simulations using RANS and hybrid URANS/LES models. Detailed simulations of flow and thermal fields of a single row of film cooling cylindrical holes with 30° inline injection on a flat plate are obtained for low (M = 0.5) and high (M = 1.5) blowing ratios under high free stream turbulence (10%). The realizable k-ε model is used within the RANS framework and a realizable k-ε-based detached eddy simulation (DES) is used as a hybrid URANS/LES model. Both models are used together with the two-layer zonal model for near-wall simulations. Steady and time-averaged unsteady film cooling effectiveness obtained using these models in ANSYS-FLUENT are compared with available experimental data. It is shown that hybrid URANS/LES models (DES in the present paper) predict more mixing both in the wall-normal and spanwise directions compared to RANS models, while unsteady asymmetric vortical structures of the flow can also be captured. The turbulent heat flux components predicted by the DES model are higher than those obtained by the RANS simulations, resulting in enhanced turbulent heat transfer between the jet and mainstream, and consequently better predictions of the effectiveness. Furthermore, the unsteady physics of jet and crossflow interactions and the jet lift-off under high free stream turbulence is studied using the present DES results.
6
Li, Xueying, Jing Ren e Hongde Jiang. "Film Cooling Modeling of Turbine Blades Using Algebraic Anisotropic Turbulence Models". In ASME Turbo Expo 2014: Turbine Technical Conference and Exposition. American Society of Mechanical Engineers, 2014. http://dx.doi.org/10.1115/gt2014-25191.
Testo completoAbstract (sommario):
The complex structures in the flow field of gas turbine film cooling increase the anisotropy of turbulence making it difficult to accurately compute turbulent eddy viscosity and scalar diffusivity. An algebraic anisotropic turbulence model is developed while aiming at a more accurate modeling of the Reynolds stress and turbulent scalar flux. In this study the algebraic anisotropic model is validated by two in-house experiments. One is a leading edge with showerhead film cooling and the other is a vane with full coverage film cooling. Adiabatic film cooling effectiveness under different blowing ratios, density ratios and film cooling arrangements were measured using PSP technique. Four different turbulence models are tested and detailed analyses of computational simulations are performed. Among all the turbulence models investigated, the algebraic anisotropic model shows better agreement with the experimental data qualitatively and quantitatively. The algebraic anisotropic model gives a good prediction of the vortex strength and turbulence mixing of the jet, therefore improves the prediction of the scalar field.
7
Bhaskaran, Rathakrishnan, e Gustavo Ledezma. "Near Wall Resolution Requirements for High-Order FR/CPR Method for Wall-Resolved Large Eddy Simulations". In ASME Turbo Expo 2020: Turbomachinery Technical Conference and Exposition. American Society of Mechanical Engineers, 2020. http://dx.doi.org/10.1115/gt2020-14432.
Testo completoAbstract (sommario):
Abstract This study aims to establish near wall resolution requirements for wall-resolved Large Eddy Simulations (LES) using the Flux Reconstruction / Correction Procedure via Reconstruction (FR/CPR) method. The FR/CPR method is relatively new and its numerical capabilities for LES are not well established. A high-order unstructured LES solver (GENESIS) based on the FR/CPR approach is used to study two canonical near wall turbulent flow problems. The first problem concerns spatial development of a turbulent flat plate boundary layer. The grid resolution requirement for various polynomial orders is established and the skin-friction and near wall turbulence is compared to theory and Direct Numerical Simulation (DNS) results. The second problem studied is the two-dimensional wall film case of Kacker and Whitelaw (1968, 1969). This is a thermal mixing problem consisting of a two-dimensional jet for various mass flow ratios and plate thicknesses. This study focuses on one of the cases from this data set, corresponding to a thick plate. Well resolved LES simulations show an excellent agreement with measured adiabatic film effectiveness. The effect of polynomial order and grid resolution is investigated and near wall resolution requirements are established.
8
Kamin, Manu, e Prashant Khare. "Liquid Jet in Crossflow: Effect of Momentum Flux Ratio on Spray and Vaporization Characteristics". In ASME Turbo Expo 2019: Turbomachinery Technical Conference and Exposition. American Society of Mechanical Engineers, 2019. http://dx.doi.org/10.1115/gt2019-91972.
Testo completoAbstract (sommario):
Abstract A comprehensive study is conducted to identify the effects of momentum flux ratio on the spray and vaporization characteristics of liquid jet injected in air crossflow at elevated temperatures, a configuration relevant to high-speed propulsion systems, such as ramjets and afterburners. The physical setup consists of a straight chamber with a triangular bluff body down-stream of the liquid injection location. The numerical simulations are based on an Eulerian-Lagrangian framework, where the gas phase flow behaviors such as recirculation zones, turbulence statistics, mixing of vaporized liquid and gas streams are resolved by solving the complete set of three-dimensional conservation equations of mass, momentum, energy and species, and the liquid phase is treated using the blob approach and tracked in a Lagrangian coordinate system. Turbulence closure is achieved using Large Eddy Simulation (LES) technique. Primary breakup of the liquid jet is simulated using the K-H wave breakup model, and the Taylor Analogy Breakup (TAB) model is used for secondary breakup. Two-way coupling between the liquid and gas phases is implemented in the LES framework to systematically model the exchange of mass, momentum and energy between the two phases. The formulation is validated against experimental measurements of liquid jet penetration and sauter mean diameter for a Weber number of 68 and momentum flux ratio of 9 at two temperatures, 298K and 573K. Results show excellent agreement with measurements for both cases. Next, simulations are conducted for a range of momentum flux ratios from 10–140 to identify the detailed gas and spray fields for vaporizing flow cases. This study helps to estimate the penetration of the liquid jet, droplet distribution, and then, location of the core of evaporated liquid in the gas-phase are quantitatively identified.
9
Herrmann, Marcus, Marco Arienti e Marios Soteriou. "The Impact of Density Ratio on the Primary Atomization of a Turbulent Liquid Jet in Crossflow". In ASME Turbo Expo 2010: Power for Land, Sea, and Air. ASMEDC, 2010. http://dx.doi.org/10.1115/gt2010-23016.
Testo completoAbstract (sommario):
Atomizing liquids by injecting them into crossflows is a common approach in gas turbines and augmentors. Much of our current understanding of the processes resulting in atomization of the jets, the resulting jet penetration and spray drop size distribution have been obtained by performing laboratory experiments at ambient conditions. Yet, operating conditions under which jets in crossflows atomize can be far different from ambient. Hence, several dimensionless groups have been identified that are believed to determine jet penetration and resulting drop size distribution. These are usually the jet and crossflow Weber and Reynolds numbers and the momentum flux ratio. In this paper we aim to answer the question of whether an additional dimensionless group, the liquid to gas density ratio must be matched. To answer this question, we perform detailed simulations of the primary atomization region using the Refined Level Set Grid (RLSG) method to track the motion of the liquid/gas phase interface. We employ a balanced force, interface projected curvature method to ensure high accuracy of the surface tension forces, use a multi-scale approach to transfer broken off, small scale nearly spherical drops into a Lagrangian point particle description allowing for full two-way coupling and continued secondary atomization, and employ a dynamic Smagorinsky large eddy simulation (LES) approach in the single phase regions of the flow to describe turbulence. We present simulation results for a turbulent liquid jet (q = 6.6, We = 330, Re = 14,000) injected into a gaseous crossflow (Re = 740,000) analyzed under ambient conditions (density ratio 816) experimentally by Brown and McDonnel (2006). We compare simulation results obtained using a liquid to gas density ratio of 10 to those obtained using a density ratio of 100, a value typical for gas turbine combustors. The results show that the increase in density ratio results in a notice-able increase in liquid core penetration with reduced bending in the crossflow and spreading in the transverse directions. The post-primary atomization spray, however, penetrates further in both the jet and transverse direction. Results further show that penetration correlations for the windward side trajectory commonly reported in the literature strongly depend on the value of threshold probability used to identify the leading edge. Correlations based on the penetration of the jet’s liquid core center of mass, on the other hand, can provide a less ambiguous measure of jet penetration. Finally, the increase in density ratio results in a decrease in wavelength of the most dominant feature associated with a traveling wave along the jet as determined by proper orthogonal decomposition.
10
Galeazzo, Flavio Cesar Cunha, Georg Donnert, Peter Habisreuther, Nikolaos Zarzalis, Richard J. Valdes e Werner Krebs. "Measurement and Simulation of Turbulent Mixing in a Jet in Crossflow". In ASME Turbo Expo 2010: Power for Land, Sea, and Air. ASMEDC, 2010. http://dx.doi.org/10.1115/gt2010-22709.
Testo completoAbstract (sommario):
Computational Fluid Dynamics (CFD) has an important role in current research. While Large Eddy Simulations (LES) are now common practice in academia, Reynolds-averaged Navier-Stokes (RANS) simulations are still very common in industry. Using RANS allows faster simulations, however the choice of the turbulence model has a bigger impact on the results. An important influence of the turbulence modeling is the description of turbulent mixing. Experience has shown that often inaccurate simulations of combustion processes originate from an inadequate description of the mixing field. A simple turbulent flow and mixing configuration of major theoretical and practical importance is the jet in crossflow (JIC). Due to its good fuel-air mixing capability over a small distance JIC is favored by gas turbine manufacturers. As the design of the mixing process is the key to creating stable low NOx combustion systems, reliable predictive tools and detailed understanding of this basic system are still demanded. Therefore the current study has re-investigated the JIC configuration under engine relevant conditions both experimentally and numerically using the most sophisticated tools available today. The combination of planar Particle Image Velocimetry (PIV) and Laser Induced Fluorescence (LIF) was used to measure the turbulent velocity and concentration fields as well as to determine the correlations of the Reynolds stress tensor ui′uj′ and the Reynolds flux vector ui′c′. Boundary conditions were determined using Laser Doppler Velocimetry. The comparisons between the measurements and simulation using RANS and LES showed that the mean velocity field was well described using the SST turbulence model. However, the Reynolds stresses and more so the Reynolds fluxes deviate substantially from the measured data. The systematic variation of the turbulent Schmidt number reveals the limited influence of this parameter indicating that the basic modeling is amiss. The results of the LES simulation using the standard Smagorinsky model were found to provide much better agreement with experiments also in the description of turbulent mixing.