Journal articles on the topic 'Anisotropic turbulence models'
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1
Nazarov, F. Kh. "Comparing Turbulence Models for Swirling Flows." Herald of the Bauman Moscow State Technical University. Series Natural Sciences, no. 2 (95) (April 2021): 25–36. http://dx.doi.org/10.18698/1812-3368-2021-2-25-36.
Abstract:
The paper considers a turbulent fluid flow in a rotating pipe, known as the Taylor --- Couette --- Poiseuille flow. Linear RANS models are not suitable for simulating this type of problems, since the turbulence in these flows is strongly anisotropic, which means that solving these problems requires models accounting for turbulence anisotropy. Modified linear models featuring corrections for flow rotations, such as the SARC model, make it possible to obtain satisfactory solutions. A new approach to turbulence problems has appeared recently. It allowed a novel two-fluid turbulence model to be created. What makes this model different is that it can describe strongly anisotropic turbulent flows; moreover, it is easy to implement numerically while not being computationally expensive. We compared the results of solving the Taylor --- Couette --- Poiseuille flow problem using the novel two-fluid model and the SARC model. The numerical investigation results obtained from the novel two-fluid model show a better agreement with the experimental data than the results provided by the SARC model
2
CUI, G. X., C. X. XU, L. FANG, L. SHAO, and Z. S. ZHANG. "A new subgrid eddy-viscosity model for large-eddy simulation of anisotropic turbulence." Journal of Fluid Mechanics 582 (June 14, 2007): 377–97. http://dx.doi.org/10.1017/s002211200700599x.
Abstract:
A new subgrid eddy-viscosity model is proposed in this paper. Full details of the derivation of the model are given with the assumption of homogeneous turbulence. The formulation of the model is based on the dynamic equation of the structure function of resolved scale turbulence. By means of the local volume average, the effect of the anisotropy is taken into account in the generalized Kolmogorov equation, which represents the equilibrium energy transfer in the inertial subrange. Since the proposed model is formulated directly from the filtered Navier–Stokes equation, the resulting subgrid eddy viscosity has the feature that it can be adopted in various turbulent flows without any adjustments of model coefficient. The proposed model predicts the major statistical properties of rotating turbulence perfectly at fairly low-turbulence Rossby numbers whereas subgrid models, which do not consider anisotropic effects in turbulence energy transfer, cannot predict this typical anisotropic turbulence correctly. The model is also tested in plane wall turbulence, i.e. plane Couette flow and channel flow, and the major statistical properties are in better agreement with those predicted by DNS results than the predictions by the Smagorinsky, the dynamic Smagorinsky and the recent Cui–Zhou–Zhang–Shao models.
3
Barbi, G., A. Chierici, V. Giovacchini, F. Quarta, and S. Manservisi. "Numerical simulation of a low Prandtl number flow over a backward facing step with an anisotropic four-equation turbulence model." Journal of Physics: Conference Series 2177, no. 1 (April 1, 2022): 012006. http://dx.doi.org/10.1088/1742-6596/2177/1/012006.
Abstract:
Abstract In recent years the use of liquid metals has become more and more popular for heat transfer applications in many fields ranging from IV generation fast nuclear reactors to solar power plants. Due to their low Prandtl number values, the similarity between dynamical and thermal fields cannot be assumed and sophisticated heat turbulence models are required to take into account the anisotropy of the turbulent heat transfer involving liquid metals. In the present work, we solve an anisotropic four-equation turbulence model coupled with the Reynolds Averaged Navier Stokes system of equations to simulate a turbulent flow of liquid sodium over a vertical backward-facing step. We implement an explicit algebraic model for Reynolds stress tensor and turbulent heat flux that takes into account flow anisotropic behavior. We study forced and mixed convection regimes when a uniform heat flux is applied on the wall behind the step. Linear isotropic approximations for eddy viscosity and eddy thermal diffusivity underestimate the turbulent heat flux components while this anisotropic model shows a better agreement with DNS results.
4
Chang, Ning, Zelong Yuan, Yunpeng Wang, and Jianchun Wang. "The effect of filter anisotropy on the large eddy simulation of turbulence." Physics of Fluids 35, no. 3 (March 2023): 035134. http://dx.doi.org/10.1063/5.0142643.
Abstract:
We study the effect of filter anisotropy and sub-filter scale (SFS) dynamics on the accuracy of large eddy simulation (LES) of turbulence, by using several types of SFS models including the dynamic Smagorinsky model (DSM), dynamic mixed model (DMM), and the direct deconvolution model (DDM) with the anisotropic filter. The aspect ratios (AR) of the filters for LES range from 1 to 16. We show that the DDM is capable of predicting SFS stresses accurately at highly anisotropic filter. In the a priori study, the correlation coefficients of SFS stress reconstructed by the DDM are over 90%, which are much larger than those of the DSM and DMM models. The correlation coefficients decrease as the AR increases. In the a posteriori studies, the DDM outperforms DSM and DMM models in the prediction of various turbulence statistics, including the velocity spectra, and probability density functions of the vorticity, SFS energy flux, velocity increments, strain-rate tensors and SFS stress. As the anisotropy increases, the results of DSM and DMM become worse, but DDM can give satisfactory results for all the filter-anisotropy cases. These results indicate that the DDM framework is a promising tool in developing advanced SFS models in the LES of turbulence in the presence of anisotropic filter.
5
Faragó, Dávid, and Péter Bencs. "Measurement of turbulence properties." Analecta Technica Szegedinensia 14, no. 1 (June 8, 2020): 67–75. http://dx.doi.org/10.14232/analecta.2020.1.67-75.
Abstract:
The aim of the research is to investigate anisotropic turbulence intensities, id est to investigate the distribution of Reynolds stresses and energy spectra in a square cross-section channel, downstream of a semi-active jet turbulence grid generating anisotropic turbulent airflow. In addition to the semi-active jet turbulence grid, another type of turbulence grid was developed and experimentally investigated. This grid contains vertical, flexible strips of aluminum (in this case, there are no perpendicular (horizontal) grid elements), which vibrate at a frequency depending on the velocity of the main airflow. Besides the investigation of the velocity- and turbulence intensity distributions, another main objective of the research is to measure the von Kármán energy spectrum when the turbulence cannot be considered isotropic. This aspiration of ours is justified by the knowledge gap present in the literature in this specific field. Monin has carried out a theoretical study to extend and generalize the von Kármán – Howarth isotropic principal stress equation to the anisotropic regime. The proposed new experimental work aims to provide a solid experimental background for verifying and validating the physical correctness of the Monin equation, which may result in a new theoretical understanding and perception of the major issues and the nature of anisotropic turbulence. Since the anisotropic energy spectra are expected to exhibit different characteristics from the isotropic Kolmogorov spectra, these new experimental results may contribute to the development of new anisotropic and engineering turbulence models that can be used in industrial applications.
6
Cui, Linyan. "Atmosphere turbulence MTF models in moderate-to-strong anisotropic turbulence." Optik 130 (February 2017): 68–75. http://dx.doi.org/10.1016/j.ijleo.2016.11.012.
7
DEN TOONDER, J. M. J., M. A. HULSEN, G. D. C. KUIKEN, and F. T. M. NIEUWSTADT. "Drag reduction by polymer additives in a turbulent pipe flow: numerical and laboratory experiments." Journal of Fluid Mechanics 337 (April 25, 1997): 193–231. http://dx.doi.org/10.1017/s0022112097004850.
Abstract:
In order to study the roles of stress anisotropy and of elasticity in the mechanism of drag reduction by polymer additives we investigate a turbulent pipe flow of a dilute polymer solution. The investigation is carried out by means of direct numerical simulation (DNS) and laser Doppler velocimetry (LDV). In our DNS two different models are used to describe the effects of polymers on the flow. The first is a constitutive equation based on Batchelor's theory of elongated particles suspended in a Newtonian solvent which models the viscous anisotropic effects caused by the polymer orientation. The second is an extension of the first model with an elastic component, and can be interpreted as an anisotropic Maxwell model. The LDV experiments have been carried out in a recirculating pipe flow facility in which we have used a solution of water and 20 w.p.p.m. Superfloc A110. Turbulence statistics up to the fourth moment, as well as power spectra of various velocity components, have been measured. The results of the drag-reduced flow are first compared with those of a standard turbulent pipe flow of water at the same friction velocity at a Reynolds number of Reτ≈1035. Next the results of the numerical simulation and of the measurements are compared in order to elucidate the role of polymers in the phenomenon of drag reduction. For the case of the viscous anisotropic polymer model, almost all turbulence statistics and power spectra calculated agree in a qualitative sense with the measurements. The addition of elastic effects, on the other hand, has an adverse effect on the drag reduction, i.e. the viscoelastic polymer model shows less drag reduction than the anisotropic model without elasticity. Moreover, for the case of the viscoelastic model not all turbulence statistics show the right behaviour. On the basis of these results, we propose that the viscous anisotropic stresses introduced by extended polymers play a key role in the mechanism of drag reduction by polymer additives.
8
Cambon, Claude, and Julian F. Scott. "LINEAR AND NONLINEAR MODELS OF ANISOTROPIC TURBULENCE." Annual Review of Fluid Mechanics 31, no. 1 (January 1999): 1–53. http://dx.doi.org/10.1146/annurev.fluid.31.1.1.
9
Hocking, W. K., and J. Röttger. "The structure of turbulence in the middle and lower atmosphere seen by and deduced from MF, HF and VHF radar, with special emphasis on small-scale features and anisotropy." Annales Geophysicae 19, no. 8 (August 31, 2001): 933–44. http://dx.doi.org/10.5194/angeo-19-933-2001.
Abstract:
Abstract. An overview of the turbulent structures seen by MF, HF and VHF radars in the troposphere, stratosphere and mesosphere is presented, drawing on evidence from previous radar measurements, in situ studies, laboratory observations, observations at frequencies other than those under focus, and modelling studies. We are particularly interested in structures at scales less than one radar pulse length, and smaller than the beam width, and especially the degree of anisotropy of turbulence at these scales. Previous radar observations are especially important in regard to the degree of anisotropy, and we highlight the role that these studies have had in furthering our understanding in this area. The contrasts and similarities between the models of anisotropic turbulence and specular reflection are considered. The need for more intense studies of anisotropy at MF, HF and VHF is especially highlighted, since this is an area in which these radars can make important contributions to the understanding of atmospheric turbulence.Key words. Meteorology and atmospheric dynamics (turbulence) – Atmospheric composition and structure (instruments and techniques) – History of geophysics (atmospheric sciences)
10
Myong, Hyon Kook, and Toshio Kobayashi. "Prediction of Three-Dimensional Developing Turbulent Flow in a Square Duct With an Anisotropic Low-Reynolds-Number k-ε Model." Journal of Fluids Engineering 113, no. 4 (December 1, 1991): 608–15. http://dx.doi.org/10.1115/1.2926523.
Abstract:
Three-dimensional developing turbulent flow in a square duct involving turbulence-driven secondary motion is numerically predicted with an anisotropic low-Reynolds-number k-ε turbulence model. Special attention has been given to both regions close to the wall and the corner, which are known to influence the characteristics of secondary flow a great deal. Hence, the no-slip boundary condition at the wall is directly used in place of the common wall function approach. The resulting set of equations simplified only by the boundary layer assumption are first compared with previous algebraic stress models, and solved with a forward marching numerical procedure for three-dimensional shear layers. Typical predicted quantities such as mean axial and secondary velocities, friction coefficients, turbulent kinetic energy, and Reynolds shear stress are compared with available experimental data. These results indicate that the present anisotropic k-ε turbulence model performs quite well for this complex flow field.
11
Majda, A. J., and M. J. Grote. "Mathematical test models for superparametrization in anisotropic turbulence." Proceedings of the National Academy of Sciences 106, no. 14 (March 18, 2009): 5470–74. http://dx.doi.org/10.1073/pnas.0901383106.
12
Briggs, D. A., J. H. Ferziger, J. R. Koseff, and S. G. Monismith. "Entrainment in a shear-free turbulent mixing layer." Journal of Fluid Mechanics 310 (March 10, 1996): 215–41. http://dx.doi.org/10.1017/s0022112096001784.
Abstract:
Results from a direct numerical simulation of a shear-free turbulent mixing layer are presented. The mixing mechanisms associated with the turbulence are isolated. In the first set of simulations, the turbulent mixing layer decays as energy is exchanged between the layers. Energy spectra with E(k) ∼ k2 and E(k) ∼ k4 dependence at low wavenumber are used to initialize the flow to investigate the effect of initial conditions. The intermittency of the mixing layer is quantified by the skewness and kurtosis of the velocity fields: results compare well with the shearless mixing layer experiments of Veeravalli & Warhaft (1989). Eddies of size of the integral scale (k3/2/∈) penetrate the mixing layer intermittently, transporting energy and causing the layer to grow. The turbulence in the mixing layer can be characterized by eddies with relatively large vertical kinetic energy and vertical length scale. In the second set of simulations, a forced mixing layer is created by continuously supplying energy in a local region to maintain a stationary kinetic energy profile. Assuming the spatial decay of r.m.s. velocity is of the form u &∞ yn, predictions of common two-equation turbulence models yield values of n ranging from -1.25 to -2.5. An exponent of -1.35 is calculated from the forced mixing layer simulation. In comparison, oscillating grid experiments yield decay exponents between n = -1 (Hannoun et al. 1989) and n = -1.5 (Nokes 1988). Reynolds numbers of 40 and 58, based on Taylor microscale, are obtained in the decaying and forced simulations, respectively. Components of the turbulence models proposed by Mellor & Yamada (1986) and Hanjalić & Launder (1972) are analysed. Although the isotropic models underpredict the turbulence transport, more complicated anisotropic models do not represent a significant improvement. Models for the pressure-strain tensor, based on the anisotropy tensor, performed adequately.
13
Wall, Dylan, and Eric Paterson. "Anisotropic RANS Turbulence Modeling for Wakes in an Active Ocean Environment." Fluids 5, no. 4 (December 18, 2020): 248. http://dx.doi.org/10.3390/fluids5040248.
Abstract:
The problem of simulating wakes in a stratified oceanic environment with active background turbulence is considered. Anisotropic RANS turbulence models are tested against laboratory and eddy-resolving models of the problem. An important aspect of our work is to acknowledge that the environment is not quiescent; therefore, additional sources are included in the models to provide a non-zero background turbulence. The RANS models are found to reproduce some key features from the eddy-resolving and laboratory descriptions of the problem. Tests using the freestream sources show the intuitive result that background turbulence causes more rapid wake growth and decay.
14
Yushkov, E. V., R. Allahverdiyev, and D. D. Sokoloff. "Mean-Field Dynamo Model in Anisotropic Uniform Turbulent Flow with Short-Time Correlations." Galaxies 8, no. 3 (September 19, 2020): 68. http://dx.doi.org/10.3390/galaxies8030068.
Abstract:
The mean-field model is one of the basic models of the dynamo theory, which describes the magnetic field generation in a turbulent astrophysical plasma. The first mean-field equations were obtained by Steenbeck, Krause and Rädler for two-scale turbulence under isotropy and uniformity assumptions. In this article we develop the path integral approach to obtain mean-field equations for a short-correlated random velocity field in anisotropic streams. By this model we analyse effects of anisotropy and show the relation between dynamo growth and anisotropic tensors of helicity/turbulent diffusivity. Considering particular examples and comparing results with isotropic cases we demonstrate several mean-field effects: super-exponential growth at initial times, complex dependence of harmonics growth on the helicity tensor structure, when generation is possible for near-zero component or near-zero helicity trace, increase of the averaged magnetic field inclined to the initial current density that leads to effective Lorentz back-reaction and violation of force-free conditions.
15
Li, Yunxiao, Zhao Zhang, Ruyi Li, Dong Xu, Hao Zhang, Yangjian Cai, and Jun Zeng. "The Spiral Spectrum of a Laguerre–Gaussian Beam Carrying the Cross-Phase Propagating in Weak-to-Strong Atmospheric Turbulence." Photonics 11, no. 2 (February 4, 2024): 148. http://dx.doi.org/10.3390/photonics11020148.
Abstract:
In communication links, the presence of atmospheric turbulence leads to crosstalk between the orbital angular momentum (OAM) states, thereby limiting the performance of information transmission. Thus, knowledge of the effect of turbulence on the spiral spectrum (also named the OAM spectrum) is of utmost importance in the field of optical communications. However, most of the existing studies are limited to weak turbulence calculation models. In this paper, based on the extended Huygens–Fresnel integral, the analytical expression is derived for the mutual coherence function of a Laguerre–Gaussian beam carrying the cross-phase and propagating through weak-to-strong anisotropic Kolmogorov atmospheric turbulence; subsequently, the analytical expression is used to study the behavior of the spiral spectrum. The discrepancies in the spiral spectrum between weak and strong turbulence are comparatively studied. The influences of the cross-phase and the anisotropy of turbulence on the spiral spectrum are investigated through numerical examples. Our results reveal that the cross-phase determines the distribution of the spiral spectrum. The spiral spectrum can be tuned to multiple OAM modes through the adaptation of the cross-phase coefficient. Moreover, increasing the cross-phase coefficient can reduce both the discrepancies of the spiral spectrum under two computational methods and the effects of the anisotropic factors of turbulence on the spiral spectrum.
16
Mathis, S., V. Prat, L. Amard, C. Charbonnel, A. Palacios, N. Lagarde, and P. Eggenberger. "Anisotropic turbulent transport in stably stratified rotating stellar radiation zones." Astronomy & Astrophysics 620 (November 23, 2018): A22. http://dx.doi.org/10.1051/0004-6361/201629187.
Abstract:
Context. Rotation is one of the key physical mechanisms that deeply impact the evolution of stars. Helio- and asteroseismology reveal a strong extraction of angular momentum from stellar radiation zones over the whole Hertzsprung–Russell diagram. Aims. Turbulent transport in differentially rotating, stably stratified stellar radiation zones should be carefully modelled and its strength evaluated. Stratification and rotation imply that this turbulent transport is anisotropic. So far only phenomenological prescriptions have been proposed for the transport in the horizontal direction. This, however, constitutes a cornerstone in current theoretical formalisms for stellar hydrodynamics in evolution codes. We aim to improve its modelling. Methods. We derived a new theoretical prescription for the anisotropy of the turbulent transport in radiation zones using a spectral formalism for turbulence that takes simultaneously stable stratification, rotation, and a radial shear into account. Then, the horizontal turbulent transport resulting from 3D turbulent motions sustained by the instability of the radial differential rotation is derived. We implemented this framework in the stellar evolution code STAREVOL and quantified its impact on the rotational and structural evolution of solar metallicity low-mass stars from the pre-main-sequence to the red giant branch. Results. The anisotropy of the turbulent transport scales as N4τ2/(2Ω2), N and Ω being the buoyancy and rotation frequencies respectively and τ a time characterizing the source of turbulence. This leads to a horizontal turbulent transport of similar strength in average that those obtained with previously proposed prescriptions even if it can be locally larger below the convective envelope. Hence the models computed with the new formalism still build up too steep internal rotation gradients compared to helioseismic and asteroseismic constraints. As a consequence, a complementary transport mechanism such as internal gravity waves or magnetic fields is still needed to explain the observed strong transport of angular momentum along stellar evolution. Conclusions. The new prescription links for the first time the anisotropy of the turbulent transport in radiation zones to their stratification and rotation. This constitutes important theoretical progress and demonstrates how turbulent closure models should be improved to get firm conclusions on the potential importance of other processes that transport angular momentum and chemicals inside stars along their evolution.
17
Kaltenbach, H. J., T. Gerz, and U. Schumann. "Large-eddy simulation of homogeneous turbulence and diffusion in stably stratified shear flow." Journal of Fluid Mechanics 280 (December 10, 1994): 1–40. http://dx.doi.org/10.1017/s0022112094002831.
Abstract:
By means of large-eddy simulation, homogeneous turbulence is simulated for neutrally and stably stratified shear flow at gradient-Richardson numbers between zero and one. We investigate the turbulent transport of three passive species which have uniform gradients in either the vertical, downstream or cross-stream direction. The results are compared with previous measurements in the laboratory and in the stable atmospheric boundary layer, as well as with results from direct numerical simulations. The computed and measured flow properties agree with each other generally within the scatter of the measurements. At strong stratification, the Froude number becomes the relevant flow-controlling parameter. Stable stratification suppresses vertical overturning and mixing when the inverse Froude number based on a turn-over timescale exceeds a critical value of about 3. The turbulent diffusivity tensor is strongly anisotropic and asymmetric. However, only the vertical and the cross-stream diagonal components are of practical importance in shear flows. The vertical diffusion coefficient is much smaller than the cross-stream one at strong stratification. This anisotropy is stronger than predicted by second-order closure models. Turbulence fluxes in downstream and cross-stream directions follow classical mixing-length models.
18
Fraschetti, F. "Cross-field transport and pitch-angle anisotropy of solar energetic particles in MHD turbulence." ASTRA Proceedings 2 (January 15, 2016): 63–65. http://dx.doi.org/10.5194/ap-2-63-2016.
Abstract:
Abstract. Recent modelling of solar energetic particles (SEPs) propagation through the heliospheric turbulence, also discussed in this workshop, has investigated the role of the pitch-angle scattering and the perpendicular transport in spreading particles in heliolongitude, as shown by multi-spacecraft measurements (STEREO A/B, ACE, SOHO, etc.) at 1 AU in various energy ranges. In some events the first-order pitch-angle anisotropy of the particles distribution is not-negligible. We calculate the average perpendicular displacement due to the gradient/curvature drift in an inhomogeneous turbulence accounting for pitch-angle dependence for two MHD turbulence models: (a) 3-D isotropic, (b) anisotropic as conjectured by Goldreich-Sridhar. We find in both cases that the drift scales as (1 − μ2)2 with the cosine of pitch-angle μ, in contrast with previous models for transport of SEPs. This result can impact the models of propagation of SEPs through the heliosphere.
19
Barbi, Giacomo, Valentina Giovacchini, and Sandro Manservisi. "A New Anisotropic Four-Parameter Turbulence Model for Low Prandtl Number Fluids." Fluids 7, no. 1 (December 22, 2021): 6. http://dx.doi.org/10.3390/fluids7010006.
Abstract:
Due to their interesting thermal properties, liquid metals are widely studied for heat transfer applications where large heat fluxes occur. In the framework of the Reynolds-Averaged Navier–Stokes (RANS) approach, the Simple Gradient Diffusion Hypothesis (SGDH) and the Reynolds Analogy are almost universally invoked for the closure of the turbulent heat flux. Even though these assumptions can represent a reasonable compromise in a wide range of applications, they are not reliable when considering low Prandtl number fluids and/or buoyant flows. More advanced closure models for the turbulent heat flux are required to improve the accuracy of the RANS models dealing with low Prandtl number fluids. In this work, we propose an anisotropic four-parameter turbulence model. The closure of the Reynolds stress tensor and turbulent heat flux is gained through nonlinear models. Particular attention is given to the modeling of dynamical and thermal time scales. Numerical simulations of low Prandtl number fluids have been performed over the plane channel and backward-facing step configurations.
20
Slama, Myriam, Cédric Leblond, and Pierre Sagaut. "A Kriging-based elliptic extended anisotropic model for the turbulent boundary layer wall pressure spectrum." Journal of Fluid Mechanics 840 (February 6, 2018): 25–55. http://dx.doi.org/10.1017/jfm.2017.810.
Abstract:
The present study addresses the computation of the wall pressure spectrum for a turbulent boundary layer flow without pressure gradient, at high Reynolds numbers, using a new model, the Kriging-based elliptic extended anisotropic model (KEEAM). A space–time solution to the Poisson equation for the wall pressure fluctuations is used. Both the turbulence–turbulence and turbulence–mean shear interactions are taken into account. It involves the mean velocity field and space–time velocity correlations which are modelled using Reynolds stresses and velocity correlation coefficients. We propose a new model, referred to as the extended anisotropic model, to evaluate the latter in all regions of the boundary layer. This model is an extension of the simplified anisotropic model of Gavin (PhD thesis, 2002, The Pennsylvania State University, University Park, PA) which was developed for the outer part of the boundary layer. It relies on a new expression for the spatial velocity correlation function and new parameters calibrated using the direct numerical simulation results of Sillero et al. (Phys. Fluids, vol. 26, 2014, 105109). Spatial correlation coefficients are related to space–time coefficients with the elliptic model of He & Zhang (Phys. Rev. E, vol. 73, 2006, 055303). The turbulent quantities necessary for the pressure computation are obtained by Reynolds-averaged Navier–Stokes solutions with a Reynolds stress turbulence model. Then, the pressure correlations are evaluated with a self-adaptive sampling strategy based on Kriging in order to reduce the computation time. The frequency and wavenumber–frequency wall pressure spectra obtained with the KEEAM agree well with empirical models developed for turbulent boundary layer flows without pressure gradient.
21
Djeddou, Mokhtar, Amine Mehel, Georges Fokoua, Anne Tanière, and Patrick Chevrier. "On the application of statistical turbulence models to the simulation of airflow inside a car cabin." Physics of Fluids 35, no. 2 (February 2023): 025106. http://dx.doi.org/10.1063/5.0132677.
Abstract:
Computational fluid dynamics simulations of airflow inside a full-scale passenger car cabin are performed using the Reynolds averaged Navier–Stokes equations. The performance of a range of turbulence models is examined by reference to experimental results of the streamwise mean velocity and turbulence intensity profiles, obtained using the hot-wire anemometry technique at different locations inside the car cabin. The models include three linear eddy-viscosity-based variants, namely, the realizable k– ε, the renormalization group k– ε, and the shear-stress transport k– ω models. The baseline Reynolds stress model (BSL-RSM), a second-moment-closure variant, and an Explicit Algebraic Reynolds Stress Model (BSL-EARSM) are also investigated. Visualization of velocity vectors and streamlines in different longitudinal planes shows a similar airflow pattern. The flow topology is mainly characterized by jet flows developing from the dashboard air vents and extending to the back-seats compartment resulting in a large vortex structure. Additionally, a comparison between numerical and experimental results shows a relatively good agreement of the mean velocity profiles. However, all models exhibit some limitations in predicting the correct level of turbulence intensity. Moreover, the realizability of the modeled Reynolds stresses and the structure of turbulence are analyzed based on the anisotropy invariant mapping approach. All models reveal a few amounts of non-realizable solutions. The linear eddy-viscosity-based models return a prevailing isotropic turbulence state, while the BSL-RSM and the BSL-EARSM models display pronounced anisotropic turbulence states.
22
Milne, I. A., R. N. Sharma, and R. G. J. Flay. "The structure of turbulence in a rapid tidal flow." Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences 473, no. 2204 (August 2017): 20170295. http://dx.doi.org/10.1098/rspa.2017.0295.
Abstract:
The structure of turbulence in a rapid tidal flow is characterized through new observations of fundamental statistical properties at a site in the UK which has a simple geometry and sedate surface wave action. The mean flow at the Sound of Islay exceeded 2.5 m s −1 and the turbulent boundary layer occupied the majority of the water column, with an approximately logarithmic mean velocity profile identifiable close to the seabed. The anisotropic ratios, spectral scales and higher-order statistics of the turbulence generally agree well with values reported for two-dimensional open channels in the laboratory and other tidal channels, therefore providing further support for the application of universal models. The results of the study can assist in developing numerical models of turbulence in rapid tidal flows such as those proposed for tidal energy generation.
23
Khani, Sina, and Michael L. Waite. "An Anisotropic Subgrid-Scale Parameterization for Large-Eddy Simulations of Stratified Turbulence." Monthly Weather Review 148, no. 10 (October 1, 2020): 4299–311. http://dx.doi.org/10.1175/mwr-d-19-0351.1.
Abstract:
AbstractSubgrid-scale (SGS) parameterizations in atmosphere and ocean models are often defined independently in the horizontal and vertical directions because the grid spacing is not the same in these directions (anisotropic grids). In this paper, we introduce a new anisotropic SGS model in large-eddy simulations (LES) of stratified turbulence based on horizontal filtering of the equations of motion. Unlike the common horizontal SGS parameterizations in atmosphere and ocean models, the vertical derivatives of the horizontal SGS fluxes are included in our anisotropic SGS scheme, and therefore the horizontal and vertical SGS dissipation mechanisms are not disconnected in the newly developed model. Our model is tested with two vertical grid spacings and various horizontal resolutions, where the horizontal grid spacing is comparatively larger than that in the vertical. Our anisotropic LES model can successfully reproduce the results of direct numerical simulations, while the computational cost is significantly reduced in the LES. We suggest the new anisotropic SGS model as an alternative to current SGS parameterizations in atmosphere and ocean models, in which the schemes for horizontal and vertical scales are often decoupled. The new SGS scheme may improve the dissipative performance of atmosphere and ocean models without adding any backscatter or other energizing terms at small horizontal scales.
24
Маликов, Зафар Маматкулович, and Дилшод Примкулович Наврузов. "Моделирование турбулентной естественной конвекции на основе 2-жидкостного подхода." Computational Continuum Mechanics 17, no. 1 (May 12, 2024): 111–18. http://dx.doi.org/10.7242/1999-6691/2024.17.1.10.
Abstract:
The paper provides mathematical modeling of turbulent natural air convection at a heated vertical plate based on a fairly recently developed two-liquid turbulence model. The considered problem, despite its relative simplicity, contains all the main elements characteristic of the currents near the wall due to buoyancy forces. A significant disadvantage of the RANS turbulence models used to solve such problems is that for their numerical implementation it is necessary to set the laminar-to-turbulent transition point, which must be determined experimentally. Thus, all RANS models are unable to describe a laminar-to-turbulent transition zone. Therefore, the main purpose of the work is to test the ability of the two-liquid turbulence model to describe the transition zone. Well-known publications have shown that the two-liquid model has high accuracy and stability, and is also able to adequately describe anisotropic turbulence. The turbulence model used in this work is supplemented with an additional thermal force, which can be ignored in many flows with forced convection. However, in the natural convection currents, it is this force that contributes to the transition of the flow regime. To validate the model, as well as to verify the computational procedure, the numerical results obtained are compared with the results of the well-known RANS turbulence models (the one-parameter Spalart-Allmaras (SA) model and the Reynolds stress transfer (RSM) model), as well as with the available experimental data. It is shown that the two-liquid model adequately reproduces the laminar-to-turbulent transition zone, and the numerical results obtained are in good agreement with experimental data.
25
Kolesnichenko, Aleksandr Vladimirovich. "Toward a theory of spiral turbulence of a nonmagnetic astrophysical disk. Formation of large-scale vortex structures." Keldysh Institute Preprints, no. 9 (2024): 1–56. http://dx.doi.org/10.20948/prepr-2024-9.
Abstract:
A closed system of three-dimensional hydrodynamic equations of the mean motion scale is presented for modeling spiral turbulence in a rotating astrophysical disk. The diffusion equations for the averaged vortex and the integral vortex spirality transport equation are derived. A general concept of the emergence of energy-consuming mezoscale coherent vortex structures in a thermodynamically open subsystem of turbulent chaos, associated with the realization of the inverse cascade of kine-tic energy in mirror-nonsymmetric disk turbulence, is formulated. It is shown that negative viscosity in the rotating disk three-dimensional system is apparently a manifestation of cascade processes in spiral turbulence, when an inverse energy transfer from small vortices to larger ones is realized. It is also shown that a relatively long decay of turbulence in the disk is associated with the absence of reflective symmetry of the anisotropic field of turbulent velocities relative to its equatorial plane. The work is of a review character, made with the aim of improving new models of astrophysical nonmagnetic disks for which the effects of spiral turbulence play a determining role.
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Zhai, Chao. "Performance of rectangular QAM/FSO communication systems in the anisotropic non-Kolmogorov ground-to-satellite uplink." Journal of Optical Communications and Networking 14, no. 9 (August 12, 2022): 713. http://dx.doi.org/10.1364/jocn.456657.
Abstract:
Quadrature amplitude modulation (QAM) is an exciting technology to accomplish a high communication data rate without increasing the system bandwidth, which can undoubtedly greatly improve the performance of free-space optical (FSO) communication systems. With the increase of experimental and theoretical results about atmospheric turbulence, scientists have confirmed that the anisotropy and non-Kolmogorov property cannot be ignored for atmospheric turbulence. In this paper, utilizing the new anisotropic non-Kolmogorov (ANK) turbulence spectrum models for the satellite links that are applicable to both vertical and slant links, we derive the average bit error rate (BER) expression of rectangular QAM for a Gaussian-beam transmission through the weak ANK ground-to-satellite uplink under the influences of both scintillation and beam wander. Numerical results display that, when the zenith angle is less than a certain angle, the average BER of the ground-to-satellite uplink decays with the enhancement of the anisotropic factor, but while the zenith angle is greater than the certain angle, the contrary trend will occur. Our work will benefit the design optimization of QAM/FSO communication systems in ANK satellite links.
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Li, Xueying, Jing Ren, and Hongde Jiang. "Application of algebraic anisotropic turbulence models to film cooling flows." International Journal of Heat and Mass Transfer 91 (December 2015): 7–17. http://dx.doi.org/10.1016/j.ijheatmasstransfer.2015.07.098.
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Wang, C., S. P. Oh, and M. Ruszkowski. "Turbulent heating in a stratified medium." Monthly Notices of the Royal Astronomical Society 519, no. 3 (January 9, 2023): 4408–23. http://dx.doi.org/10.1093/mnras/stad003.
Abstract:
ABSTRACT There is considerable evidence for widespread subsonic turbulence in galaxy clusters, most notably from Hitomi. Turbulence is often invoked to offset radiative losses in cluster cores, both by direct dissipation and by enabling turbulent heat diffusion. However, in a stratified medium, buoyancy forces oppose radial motions, making turbulence anisotropic. This can be quantified via the Froude number Fr, which decreases inward in clusters as stratification increases. We exploit analogies with MHD turbulence to show that wave–turbulence interactions increase cascade times and reduce dissipation rates ϵ ∝ Fr. Equivalently, for a given energy injection/dissipation rate ϵ, turbulent velocities u must be higher compared to Kolmogorov scalings. High-resolution hydrodynamic simulations show excellent agreement with the ϵ ∝ Fr scaling, which sets in for Fr ≲ 0.1. We also compare previously predicted scalings for the turbulent diffusion coefficient D ∝ Fr2 and find excellent agreement, for Fr ≲ 1. However, we find a different normalization, corresponding to stronger diffusive suppression by more than an order of magnitude. Our results imply that turbulent diffusion is more heavily suppressed by stratification, over a much wider radial range, than turbulent dissipation. Thus, the latter potentially dominates. Furthermore, this shift implies significantly higher turbulent velocities required to offset cooling, compared to previous models. These results are potentially relevant to turbulent metal diffusion in the galaxy groups and clusters (which is likewise suppressed), and to planetary atmospheres.
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Syed, Abdul Haseeb, and Jakob Mann. "Simulating low-frequency wind fluctuations." Wind Energy Science 9, no. 6 (June 25, 2024): 1381–91. http://dx.doi.org/10.5194/wes-9-1381-2024.
Abstract:
Abstract. Large-scale flow structures are vital in influencing the dynamic response of floating wind turbines and wake meandering behind large offshore wind turbines. It is imperative that we develop an inflow wind turbulence model capable of replicating the large-scale and low-frequency wind fluctuations occurring in the marine atmosphere since the current turbulence models do not account well for this phenomenon. Here, we present a method to simulate low-frequency wind fluctuations. This method employs the two-dimensional (2D) spectral tensor for low-frequency, anisotropic wind fluctuations presented by Syed and Mann (2024) to generate stochastic wind fields. The simulation method generates large-scale 2D spatial wind fields for the longitudinal u and lateral v wind components, which can be converted into a frequency domain using Taylor's frozen turbulence hypothesis. The low-frequency wind turbulence is assumed to be independent of the high-frequency turbulence; thus, a broad spectral representation can be obtained just by superposing the two turbulent wind fields. The method is tested by comparing the simulated and theoretical spectra and co-coherences of the combined low- and high-frequency fluctuations. Furthermore, the low-frequency wind fluctuations can also be subjected to anisotropy. The resulting wind fields from this method can be used to analyze the impact of low-frequency wind fluctuations on wind turbine loads and dynamic response and to study the wake meandering behind large offshore wind farms.
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Zahiri, Amir-Pouyan, and Ehsan Roohi. "Assessment of anisotropic minimum-dissipation (AMD) subgrid-scale model: Gently-curved backward-facing step flow." International Journal of Modern Physics C 32, no. 05 (February 18, 2021): 2150068. http://dx.doi.org/10.1142/s0129183121500686.
Abstract:
The impetus of this study is to evaluate the performance of the anisotropic minimum-dissipation (AMD) subgrid-scale model (SGS) for flow over a gently-curved backward-facing step (BFS) at a Reynolds number of 13 700. Minimum-dissipation sub-grid models were developed as simple alternatives to the dynamic eddy-viscosity SGS models. AMD model is a static type of eddy-viscosity SGS model incorporating anisotropic SGS effects into numerical predictions through the large-eddy simulation (LES) approach. The open-source CFD package of OpenFOAM was used to implement the AMD model. Before focusing on the BFS flow, we investigated the impact of the AMD model coefficient magnitude on the numerical predictions of the decaying isotropic turbulence flow. In the next step, numerical solutions were obtained for the curved backward-facing step using the AMD model and Dynamic Smagorinsky model (DSM). The curved backward-facing step was considered here for the evaluation of the SGS model predictions due to its weak adverse pressure gradient and high sensitive flow mechanism. The rescaling/recycling method was employed as a turbulent inflow generation technique. The AMD model results were compared with the prediction of the DSM and Vreman model. Moreover, AMD model predictions were compared with the reported solutions obtained using different turbulent inflow generation methods. The assessments revealed the high capability of the AMD model to capture decaying turbulence and predict velocity profiles and resolved flow statistics turbulent parameters in the gently-curved backward step flow.
31
Im, Yong H., Kang Y. Huh, and Kwang-Yong Kim. "Analysis of Impinging and Countercurrent Stagnating Flows by Reynolds Stress Model." Journal of Fluids Engineering 124, no. 3 (August 19, 2002): 706–18. http://dx.doi.org/10.1115/1.1493815.
Abstract:
Numerical simulation is performed for stagnating turbulent flows of impinging and countercurrent jets by the Reynolds stress model (RSM). Results are compared with those of the k−ε model and available data to assess the flow characteristics and turbulence models. Three variants of the RSM tested are those of Gibson and Launder (GL), Craft and Launder (GL-CL) and Speziale, Sarkar and Gatski (SSG). As is well known, the k−ε model significantly overestimates turbulent kinetic energy near the wall. Although the RSM is superior to the k−ε model, it shows considerable difference according to how the redistributive pressure-strain term is modeled. Results of the RSM for countercurrent jets are improved with the modified coefficients for the dissipation rate, Cε1 and Cε2, suggested by Champion and Libby. Anisotropic states of the stress near the stagnation region are assessed in terms of an anisotropy invariant map (AIM).
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Brearley, Peter, Umair Ahmed, Nilanjan Chakraborty, and Markus Klein. "Scaling of Second-Order Structure Functions in Turbulent Premixed Flames in the Flamelet Combustion Regime." Fluids 5, no. 2 (June 2, 2020): 89. http://dx.doi.org/10.3390/fluids5020089.
Abstract:
The second-order velocity structure function statistics have been analysed using a DNS database of statistically planar turbulent premixed flames subjected to unburned gas forcing. The flames considered here represent combustion for moderate values of Karlovitz number from the wrinkled flamelets to the thin reaction zones regimes of turbulent premixed combustion. It has been found that the second-order structure functions exhibit the theoretical asymptotic scalings in the dissipative and (relatively short) inertial ranges. However, the constant of proportionality for the theoretical asymptotic variation for the inertial range changes from one case to another, and this value also changes with structure function orientation. The variation of the structure functions for small length scale separation remains proportional to the square of the separation distance. However, the constant of proportionality for the limiting behaviour according to the separation distance square remains significantly different from the theoretical value obtained in isotropic turbulence. The disagreement increases with increasing turbulence intensity. It has been found that turbulent velocity fluctuations within the flame brush remain anisotropic for all cases considered here and this tendency strengthens towards the trailing edge of the flame brush. It indicates that the turbulence models derived based on the assumptions of homogeneous isotropic turbulence may not be fully valid for turbulent premixed flames.
33
Beattie, James R., Christoph Federrath, and Amit Seta. "Magnetic field fluctuations in anisotropic, supersonic turbulence." Monthly Notices of the Royal Astronomical Society 498, no. 2 (August 6, 2020): 1593–608. http://dx.doi.org/10.1093/mnras/staa2257.
Abstract:
ABSTRACT The rich structure that we observe in molecular clouds is due to the interplay between strong magnetic fields and supersonic (turbulent) velocity fluctuations. The velocity fluctuations interact with the magnetic field, causing it too to fluctuate. Using numerical simulations, we explore the nature of such magnetic field fluctuations, $\delta \mathrm{{\boldsymbol {\mathit {B}}}}$, over a wide range of turbulent Mach numbers, $\operatorname{\mathcal {M}}= 2\!-\!20$ (i.e. from weak to strong compressibility), and Alfvén Mach numbers, $\operatorname{\mathcal {M}_{\text{A0}}}= 0.1\!-\!100$ (i.e. from strong to weak magnetic mean fields, B0). We derive a compressible quasi-static fluctuation model from the magnetohydrodynamical (MHD) equations and show that velocity gradients parallel to the mean magnetic field give rise to compressible modes in sub-Alfvénic flows, which prevents the flow from becoming two dimensional, as is the case in incompressible MHD turbulence. We then generalize an analytical model for the magnitude of the magnetic fluctuations to include $\operatorname{\mathcal {M}}$, and find $|\delta \mathrm{{\boldsymbol {\mathit {B}}}}| = \delta B = c_{\rm s}\sqrt{\pi \rho _0}\operatorname{\mathcal {M}}\operatorname{\mathcal {M}_{\text{A0}}}$, where cs is the sound speed and ρ0 is the mean density of gas. This new relation fits well in the strong B-field regime. We go on to study the anisotropy between the perpendicular (B⊥) and parallel (B∥) fluctuations and the mean-normalized fluctuations, which we find follow universal scaling relations, invariant of $\operatorname{\mathcal {M}}$. We provide a detailed analysis of the morphology for the δB⊥ and δB∥ probability density functions and find that eddies aligned with B0 cause parallel fluctuations that reduce B∥ in the most anisotropic simulations. We discuss broadly the implications of our fluctuation models for magnetized gases in the interstellar medium.
34
Kholboev, B. M., D. P. Navruzov, D. S. Asrakulova, N. R. Engalicheva, and A. A. Turemuratova. "Comparison of the Results for Calculation of Vortex Currents After Sudden Expansion of the Pipe with Different Diameters." International Journal of Applied Mechanics and Engineering 27, no. 2 (June 1, 2022): 115–23. http://dx.doi.org/10.2478/ijame-2022-0023.
Abstract:
Abstract In this work, a numerical study of a sharply expanding highly swirling flow is carried out using v2-f models based on the Comsol Multiphysics 5.6 software package and a two-fluid turbulence model. The results obtained are compared with known experimental data with different pipe diameters. The purpose of this work is to test the ability of models to describe anisotropic turbulence. It is shown that the two-fluid model is more suitable for studying such flows.
35
Barbi, G., A. Chierici, L. Chirco, V. Giovacchini, S. Manservisi, and L. Sirotti. "Numerical simulation of a low Prandtl number flow with a four-parameters turbulence model through an explicit algebraic definition of Reynolds stress and turbulent heat flux." Journal of Physics: Conference Series 2177, no. 1 (April 1, 2022): 012005. http://dx.doi.org/10.1088/1742-6596/2177/1/012005.
Abstract:
Abstract Computational Fluid Dynamics codes usually adopt the Reynolds analogy in order to simulate dynamic and thermal flow fields for ordinary fluids like water and air. On the other hand, in low Prandtl fluids, such as heavy liquid metals like Lead-Bismuth Eutectic (LBE), the time scales of temperature and velocity fields are rather different and therefore similarity hypothesis cannot be used. Furthermore, to properly predict a complex flow field characterized by anisotropic behavior, it is necessary to overcome eddy-viscosity models and move to more advanced turbulence models. In the present work, we propose a nonlinear method for the computation of the Reynolds stress tensor and of the turbulent heat flux. Explicit algebraic models (EAM) and new time scales have been implemented using a logarithmic four parameters turbulence model (i.e. K-Ω-K θ -Ω θ ). This new model is validated through the simulation of plane channel and cylinder flows and results are compared with DNS data.
36
van den Berg, J. P., N. E. Engelbrecht, N. Wijsen, and R. D. Strauss. "On the Turbulent Reduction of Drifts for Solar Energetic Particles." Astrophysical Journal 922, no. 2 (November 30, 2021): 200. http://dx.doi.org/10.3847/1538-4357/ac2736.
Abstract:
Abstract Particle drifts perpendicular to the background magnetic field have been proposed by some authors as an explanation for the very efficient perpendicular transport of solar energetic particles (SEPs). This process, however, competes with perpendicular diffusion caused by magnetic turbulence, which can also disrupt the drift patterns and reduce the magnitude of drift effects. The latter phenomenon is well known in cosmic-ray studies, but not yet considered in SEP models. Additionally, SEP models that do not include drifts, especially for electrons, use turbulent drift reduction as a justification of this omission, without critically evaluating or testing this assumption. This article presents the first theoretical step for a theory of drift suppression in SEP transport. This is done by deriving the turbulence-dependent drift reduction function with a pitch-angle dependence, as is applicable for anisotropic particle distributions, and by investigating to what extent drifts will be reduced in the inner heliosphere for realistic turbulence conditions and different pitch-angle dependencies of the perpendicular diffusion coefficient. The influence of the derived turbulent drift reduction factors on the transport of SEPs are tested, using a state-of-the-art SEP transport code, for several expressions of theoretically derived perpendicular diffusion coefficients. It is found, for realistic turbulence conditions in the inner heliosphere, that cross-field diffusion will have the largest influence on the perpendicular transport of SEPs, as opposed to particle drifts.
37
Chernyshov, A. A., K. V. Karelsky, and A. S. Petrosyan. "Large eddy simulations in plasma astrophysics. Weakly compressible turbulence in local interstellar medium." Proceedings of the International Astronomical Union 6, S274 (September 2010): 80–84. http://dx.doi.org/10.1017/s1743921311006612.
Abstract:
AbstractWe apply large eddy simulation technique to carry out three-dimensional numerical simulation of compressible magnetohydrodynamic turbulence in conditions relevant local interstellar medium. According to large eddy simulation method, the large-scale part of the flow is computed directly and only small-scale structures of turbulence are modeled. The small-scale motion is eliminated from the initial system of equations of motion by filtering procedures and their effect is taken into account by special closures referred to as the subgrid-scale models. Establishment of weakly compressible limit with Kolmogorov-like density fluctuations spectrum is shown in present work. We use our computations results to study dynamics of the turbulent plasma beta and anisotropic properties of the magnetoplasma fluctuations in the local interstellar medium.
38
Pinarbasi, A., and M. W. Johnson. "Detailed Stress Tensor Measurements in a Centrifugal Compressor Vaneless Diffuser." Journal of Turbomachinery 118, no. 2 (April 1, 1996): 394–99. http://dx.doi.org/10.1115/1.2836654.
Abstract:
Detailed flow measurements have been made in the vaneless diffuser of a large low-speed centrifugal compressor using hot-wire anemometry. The three time mean velocity components and full stress tensor distributions have been determined on eight measurement planes within the diffuser. High levels of Reynolds stress result in the rapid mixing out of the blade wake. Although high levels of turbulent kinetic energy are found in the passage wake, they are not associated with strong Reynolds stresses and hence the passage wake mixes out only slowly. Low-frequency meandering of the wake position is therefore likely to be responsible for the high kinetic energy levels. The anisotropic nature of the turbulence suggests that Reynolds stress turbulence models are required for CFD modeling of diffuser flows.
39
Jones, Raymond M., Albert D. Harvey, and Sumanta Acharya. "Two-Equation Turbulence Modeling for Impeller Stirred Tanks." Journal of Fluids Engineering 123, no. 3 (March 5, 2001): 640–48. http://dx.doi.org/10.1115/1.1384568.
Abstract:
In this study, the predictive performance of six different two-equation turbulence models on the flow in an unbaffled stirred tank has been investigated. These models include the low Reynolds number k-ε model of Rodi, W., and Mansour, N. N., “Low Reynolds Number k-ε Modeling With the Aid of Direct Simulation Data,” J. Fluid Mech., Vol. 250, pp. 509–529, the high and low Reynolds number k-ω models of Wilson, D. C., 1993, Turbulence Modeling for CFD, DCW Industries, La Canada, CA., the RNG k-ε model, and modified k-ω and k-ε models which incorporate a correction for streamline curvature and swirl. Model results are compared with experimental laser Doppler velocimetry (LDV) data for the turbulent velocity field in an unbaffled tank with a single paddle impeller. An overall qualitative agreement has been found between the experimental and numerical results with poor predictions observed in some parts of the tank. Discrepancies in model predictions are observed in the anisotropic regions of the flow such as near the impeller shaft and in the impeller discharge region where the model overpredicts the radial velocity component. These results are discussed and a strategy for improving two-equation models for application to impeller stirred tanks is proposed.
40
SHEN, LIAN, and DICK K. P. YUE. "Large-eddy simulation of free-surface turbulence." Journal of Fluid Mechanics 440 (August 10, 2001): 75–116. http://dx.doi.org/10.1017/s0022112001004669.
Abstract:
In this paper we investigate the large-eddy simulation (LES) of the interaction between a turbulent shear flow and a free surface at low Froude numbers. The benchmark flow field is first solved by using direct numerical simulations (DNS) of the Navier–Stokes equations at fine (1282 × 192 grid) resolution, while the LES is performed at coarse resolution. Analysis of the ensemble of 25 DNS datasets shows that the amount of energy transferred from the grid scales to the subgrid scales (SGS) reduces significantly as the free surface is approached. This is a result of energy backscatter associated with the fluid vertical motions. Conditional averaging reveals that the energy backscatter occurs at the splat regions of coherent hairpin vortex structures as they connect to the free surface. The free-surface region is highly anisotropic at all length scales while the energy backscatter is carried out by the horizontal components of the SGS stress only. The physical insights obtained here are essential to the efficacious SGS modelling of LES for free-surface turbulence. In the LES, the SGS contribution to the Dirichlet pressure free-surface boundary condition is modelled with a dynamic form of the Yoshizawa (1986) expression, while the SGS flux that appears in the kinematic boundary condition is modelled by a dynamic scale-similarity model. For the SGS stress, we first examine the existing dynamic Smagorinsky model (DSM), which is found to capture the free-surface turbulence structure only roughly. Based on the special physics of free-surface turbulence, we propose two new SGS models: a dynamic free-surface function model (DFFM) and a dynamic anisotropic selective model (DASM). The DFFM correctly represents the reduction of the Smagorinsky coefficient near the surface and is found to capture the surface layer more accurately. The DASM takes into account both the anisotropy nature of free-surface turbulence and the dependence of energy backscatter on specific coherent vorticity mechanisms, and is found to produce substantially better surface signature statistics. Finally, we show that the combination of the new DFFM and DASM with a dynamic scale-similarity model further improves the results.
41
Salunkhe, Sanchit, Oumnia El Fajri, Shanti Bhushan, David Thompson, Daphne O’Doherty, Tim O’Doherty, and Allan Mason-Jones. "Validation of Tidal Stream Turbine Wake Predictions and Analysis of Wake Recovery Mechanism." Journal of Marine Science and Engineering 7, no. 10 (October 11, 2019): 362. http://dx.doi.org/10.3390/jmse7100362.
Abstract:
This paper documents the predictive capability of rotating blade-resolved unsteady Reynolds averaged Navier-Stokes (URANS) and Improved Delayed Detached Eddy Simulation (IDDES) computations for tidal stream turbine performance and intermediate wake characteristics. Ansys/Fluent and OpenFOAM simulations are performed using mixed-cell, unstructured grids consisting of up to 11 million cells. The thrust, power and intermediate wake predictions compare reasonably well within 10% of the experimental data. For the wake predictions, OpenFOAM performs better than Ansys/Fluent, and IDDES better than URANS when the resolved turbulence is triggered. The primary limitation of the simulations is under prediction of the wake diffusion towards the turbine axis, which in return is related to the prediction of turbulence in the tip-vortex shear layer. The shear-layer involves anisotropic turbulent structures; thus, hybrid RANS/LES models, such as IDDES, are preferred over URANS. Unfortunately, IDDES fails to accurately predict the resolved turbulence in the near-wake region due to the modeled stress depletion issue.
42
Malikov, Zafar, Dilshod Navruzov, and Xikmatulla Djumayev. "Models results Comparison of different approaches to turbulence for flow past a heated flat plate." E3S Web of Conferences 264 (2021): 01008. http://dx.doi.org/10.1051/e3sconf/202126401008.
Abstract:
This paper compares the results of the well-known Spalart-Allmares (SA) model and the two-fluid model for the flow around a heated flat plate. These models represent different approaches to the problem of turbulence. The SA model is a one-parameter model and a representative of the RANS models. This model is currently the most popular and is used to solve many practical problems. The advantage of this model is that its accuracy is quite good and simple for numerical implementation. Therefore, the SA model is included in almost all the codes of the software package. The two-fluid model used in this work has been developed recently [15]. In the pioneering works, it is shown that the basis for constructing this model is the possibility of representing a turbulent flow in the form of a heterogeneous mixture of two liquids. Therefore, this model is derived from the dynamics of two liquids. In these works, it is also shown that the developed two-fluid model is able to adequately describe complex anisotropic turbulences. The fundamental difference between these two models is that the SA model uses the substance transfer equation, while the two-fluid model uses the dynamics equation. To compare the two models, we compare their numerical results with the known experimental data. It is shown that the results of both models are close to each other and are in good agreement with the experimental data.
43
Hwang, C. C., Genxing Zhu, M. Massoudi, and J. M. Ekmann. "A Comparison of the Linear and Nonlinear k–ε Turbulence Models in Combustors." Journal of Fluids Engineering 115, no. 1 (March 1, 1993): 93–102. http://dx.doi.org/10.1115/1.2910119.
Abstract:
In swirling turbulent flows, the structure of turbulence is nonhomogeneous and anisotropic and it has been observed that the assumptions leading to the formulation of the k-ε model, which is used very often in many engineering applications, are inadequate for highly swirling flows. Furthermore, even with the various modifications made to the k-ε model, it is still not capable of describing secondary flows in noncircular ducts and it cannot predict non-zero normal-Reynolds-stress differences. Recently Speziale (1987) has developed a nonlinar k-ε model, which extends the range of validity of the standard k-ε model while maintaining most of the interesting features of the k-ε model; for example, the ease of application in existing Computational Fluid Dynamics (CFD) codes. In this work, we will use the nonlinear k-ε closure to model the turbulence in combustors. The particular combustor geometries selected for this study are (i) the flow in a round pipe entering an expansion into another coaxial round pipe, and (ii) the flow in two confined co-axial swirling jets. The results show that there are no significant differences in the performance of the two models. It is speculated that the inlet conditions for k and ε may play as crucial a role in achieving predicted accuracy as turbulence modeling details. Also it is possible that weaknesses in the performance of the modeled equations for k and ε may have masked differences in the two models.
44
Herbert-Acero, José F., Oliver Probst, Carlos I. Rivera-Solorio, Krystel K. Castillo-Villar, and Santos Méndez-Díaz. "An Extended Assessment of Fluid Flow Models for the Prediction of Two-Dimensional Steady-State Airfoil Aerodynamics." Mathematical Problems in Engineering 2015 (2015): 1–31. http://dx.doi.org/10.1155/2015/854308.
Abstract:
This work presents the analysis, application, and comparison of thirteen fluid flow models in the prediction of two-dimensional airfoil aerodynamics, considering laminar and turbulent subsonic inflow conditions. Diverse sensitivity analyses of different free parameters (e.g., the domain topology and its discretization, the flow model, and the solution method together with its convergence mechanisms) revealed important effects on the simulations’ outcomes. The NACA 4412 airfoil was considered throughout the work and the computational predictions were compared with experiments conducted under a wide range of Reynolds numbers (7e5≤Re≤9e6) and angles-of-attack (-10°≤α≤20°). Improvements both in modeling accuracy and processing time were achieved by considering the RS LP-S and the Transition SST turbulence models, and by considering finite volume-based solution methods with preconditioned systems, respectively. The RS LP-S model provided the best lift force predictions due to the adequate modeling of the micro and macro anisotropic turbulence at the airfoil’s surface and at the nearby flow field, which in turn allowed the adequate prediction of stall conditions. The Transition-SST model provided the best drag force predictions due to adequate modeling of the laminar-to-turbulent flow transition and the surface shear stresses. Conclusions, recommendations, and a comprehensive research agenda are presented based on validated computational results.
45
Du, Guang Yu, Zhen Tan, Wei An, and De Сhun Ba. "Research on Numerical Simulation Method for 3D Complex Flow in Rotating Machinery." Applied Mechanics and Materials 226-228 (November 2012): 52–55. http://dx.doi.org/10.4028/www.scientific.net/amm.226-228.52.
Abstract:
A numerical simulation method with gas-structure interaction to analyze 3D complex flow in rotating machinery was presented and the effects with different aerodynamic turbulence model for gas-structure interaction was also presented. The blades are an important component in rotating machinery. Gas flow is unsteady three-dimensional turbulence motion with transient and anisotropic. Then the gas flow and the vibration of rotating blades interfere with each other, resulting in a complex coupling effect. It affects the machine efficiency directly. For discussing the effects on flow field of the coupling field, the blade model was built. And flow around the blades was simulated by gas-structure interaction with three turbulence models respectively. The turbulence models were standard κ-ε, renormalization group κ-ε and Smagorinsky LES. A feasible method was provided for flow field analysis in rotating machinery.
46
Brouwers, J. J. H. "Statistical Model of Turbulent Dispersion Recapitulated." Fluids 6, no. 5 (May 18, 2021): 190. http://dx.doi.org/10.3390/fluids6050190.
Abstract:
A comprehensive summary and update is given of Brouwers’ statistical model that was developed during the previous decade. The presented recapitulated model is valid for general inhomogeneous anisotropic velocity statistics that are typical of turbulence. It succeeds and improves the semiempirical and heuristic models developed during the previous century. The model is based on a Langevin and diffusion equation of which the derivation involves (i) the application of general principles of physics and stochastic theory; (ii) the application of the theory of turbulence at large Reynolds numbers, including the Lagrangian versions of the Kolmogorov limits; and (iii) the systematic expansion in powers of the inverse of the universal Lagrangian Kolmogorov constant C0, C0 about 6. The model is unique in the collected Langevin and diffusion models of physics and chemistry. Presented results include generally applicable expressions for turbulent diffusion coefficients that can be directly implemented in numerical codes of computational fluid mechanics used in environmental and industrial engineering praxis. This facilitates the more accurate and reliable prediction of the distribution of the mean concentration of passive or almost passive admixture such as smoke, aerosols, bacteria, and viruses in turbulent flow, which are all issues of great societal interest.
47
Sadiki, A., W. Bauer, and K. Hutter. "Thermodynamically consistent coefficient calibration in nonlinear and anisotropic closure models for turbulence." Continuum Mechanics and Thermodynamics 12, no. 2 (July 1, 2000): 131–49. http://dx.doi.org/10.1007/s001610050132.
48
Van Fossen, G. J., R. J. Simoneau, and C. Y. Ching. "Influence of Turbulence Parameters, Reynolds Number, and Body Shape on Stagnation-Region Heat Transfer." Journal of Heat Transfer 117, no. 3 (August 1, 1995): 597–603. http://dx.doi.org/10.1115/1.2822619.
Abstract:
This experiment investigated the effects of free-stream turbulence intensity, length scale, Reynolds number, and leading-edge velocity gradient on stagnation-region heat transfer. Heat transfer was measured in the stagnation region of four models with elliptical leading edges downstream of five turbulence-generating grids. Stagnation-region heat transfer augmentation increased with decreasing length scale but ann optimum scale was not found. A correlation was developed that fit heat transfer data for isotropic turbulence to within ±4 percent but did not predict data for anisotropic turbulence. Stagnation heat transfer augmentation caused by turbulence was unaffected by the velocity gradient. The data of other researchers compared well with the correlation. A method of predicting heat transfer downstream of the stagnation point was developed.
49
Leo, Annalisa De, Greg Collecutt, Mitchell Smith, Alessandro Stocchino, and Bill Syme. "ANISOTROPIC EDDY VISCOSITY A BENCHMARK CASE STUDY IN AN IDEALISED TIDAL ESTUARY." Coastal Engineering Proceedings, no. 37 (September 1, 2023): 30. http://dx.doi.org/10.9753/icce.v37.sediment.30.
Abstract:
Numerical schemes for solving the full shallow water equations require a turbulence closure model to represent the momentum diffusion caused by turbulence that the velocity fields do not explicitly capture. Typical models include Smagorinsky, depth-U*, Prandlt, and k-epsilon. All of these models predict isotropic eddy viscosity – i.e. the diffusion of momentum acts equally in all directions. However, it is known that when modelling saline transport within an estuary with a 2D scheme, mass dispersion is present in the longitudinal direction due to the depth velocity profile. In a 2D numerical scheme, should diffusion of momentum also be applied anisotropically? In this paper we present a benchmark case study between laboratory measured velocity results and those from numerical simulations using TUFLOW.
50
Azzi, A., and D. Lakehal. "Perspectives in Modeling Film Cooling of Turbine Blades by Transcending Conventional Two-Equation Turbulence Models." Journal of Turbomachinery 124, no. 3 (July 1, 2002): 472–84. http://dx.doi.org/10.1115/1.1485294.
Abstract:
The paper presents recent trends in modeling jets in crossflow with relevance to film cooling of turbine blades. The aim is to compare two classes of turbulence models with respect to their predictive performance in reproducing near-wall flow physics and heat transfer. The study focuses on anisotropic eddy-viscosity/diffusivity models and explicit algebraic stress models, up to cubic fragments of strain and vorticity tensors. The first class of models are direct numerical simulation (DNS) based two-layer approaches transcending the conventional k−ε model by means of a nonisotropic representation of the turbulent transport coefficients; this is employed in connection with a near-wall one-equation model resolving the semi-viscous sublayer. The aspects of this new strategy are based on known channel-flow and boundary layer DNS statistics. The other class of models are quadratic and cubic explicit algebraic stress formulations rigorously derived from second-moment closures. The stress-strain relations are solved in the context of a two-layer strategy resolving the near-wall region by means of a nonlinear one-equation model; the outer core flow is treated by use of the two-equation model. The models are tested for the film cooling of a flat plate by a row of streamwise injected jets. Comparison of the calculated and measured wall-temperature distributions shows that only the anisotropic eddy-viscosity/diffusivity model can correctly predict the spanwise spreading of the temperature field and reduce the strength of the secondary vortices. The wall-cooling effectiveness was found to essentially depend on these two particular flow features. The non-linear algebraic stress models were of a mixed quality in film-cooling calculations.