Showing papers on "K-epsilon turbulence model published in 2003"
TL;DR: A generalization of a class of differential length-scale equations typically used in second-order turbulence models for oceanic flows is suggested in this article, which can be recovered as special cases of this generic model, and thus can be rationally compared.
Abstract: A generalization of a class of differential length-scale equations typically used in second-order turbulence models for oceanic flows is suggested. Commonly used models, like the κ-e model and the Mellor-Yamada model, can be recovered as special cases of this generic model, and thus can be rationally compared. In addition, a method is proposed that yields a generalized framework for the calibration of the most frequently used class of differential length-scale equations. The generic model, calibrated with this method, exhibits a greater range of applicability than any of the traditional models. Stratified flows, plane mixing layers, and turbulence introduced by breaking surface waves are considered besides some classical test cases.
882 citations
TL;DR: In this paper, the authors present numerical simulations and explore scalings and anisotropy of compressible magnetohydrodynamic (MHD) turbulence, including super-Alfvenic turbulence.
Abstract: We present numerical simulations and explore scalings and anisotropy of compressible magnetohydrodynamic (MHD) turbulence Our study covers both gas-pressure-dominated (high β) and magnetic-pressure-dominated (low β) plasmas at different Mach numbers In addition, we present results for super-Alfvenic turbulence and discuss in what way it is similar to sub-Alfvenic turbulence We describe a technique of separating different magnetohydrodynamic modes (slow, fast and Alfven) and apply it to our simulations We show that, for both high- and low-β cases, Alfven and slow modes reveal a Kolmogorov k - 5 / 3 spectrum and scale-dependent Goldreich-Sridhar anisotropy, while fast modes exhibit a k - 3 / 2 spectrum and isotropy We discuss the statistics of density fluctuations arising from MHD turbulence in different regimes Our findings entail numerous astrophysical implications ranging from cosmic ray propagation to gamma ray bursts and star formation In particular, we show that the rapid decay of turbulence reported by earlier researchers is not related to compressibility and mode coupling in MHD turbulence In addition, we show that magnetic field enhancements and density enhancements are marginally correlated Addressing the density structure of partially ionized interstellar gas on astronomical-unit scales, we show that the viscosity-damped regime of MHD turbulence that we reported earlier for incompressible flows persists for compressible turbulence and therefore may provide an explanation for these mysterious structures
598 citations
TL;DR: An experimental investigation of the transition to turbulence in a pipe over approximately an order of magnitude range in the Reynolds number Re indicates that the amplitude of perturbation required to cause transition scales as O(Re-1).
Abstract: We report the results of an experimental investigation of the transition to turbulence in a pipe over approximately an order of magnitude range in the Reynolds number Re. A novel scaling law is uncovered using a systematic experimental procedure which permits contact to be made with modern theoretical thinking. The principal result we uncover is a scaling law which indicates that the amplitude of perturbation required to cause transition scales as O(Re–1).
294 citations
TL;DR: In this paper, a similarity formulation is proposed to describe the streamwise turbulence intensity across the entire smooth-wall zero-pressure-gradient turbulent boundary layer, which is an extension of the Marusic, Uddin and Perry [Phys. Fluids 9, 3718 (1997)] formulation that was restricted to the outer region of the boundary layer.
Abstract: A similarity formulation is proposed to describe the streamwise turbulence intensity across the entire smooth-wall zero-pressure-gradient turbulent boundary layer. The formulation is an extension of the Marusic, Uddin, and Perry [Phys. Fluids 9, 3718 (1997)] formulation that was restricted to the outer region of the boundary layer, including the logarithmic region. The new formulation is found to agree very well with experimental data over a large range of Reynolds numbers varying from laboratory to atmospheric flows. The formulation is founded on physical arguments based on the attached eddy hypothesis, and suggests that the boundary layer changes significantly with Reynolds number, with an outer flow influence felt all the way down to the viscous sublayer. The formulation may also be used to explain why the empirical mixed scaling of DeGraaff and Eaton [J. Fluid Mech. 422, 319 (2000)] appears to work.
244 citations
TL;DR: In this paper, the influence of an external turbulence field on the bed load sediment transport in an open channel was investigated, where the external turbulence was generated by a horizontal pipe placed halfway through the depth and a series of grids with a clearance of about one third of the depth from the bed, and extending over a finite length of the flume.
Abstract: This paper summarizes the results of an experimental study on the influence of an external turbulence field on the bed load sediment transport in an open channel. The external turbulence was generated by (1) a horizontal pipe placed halfway through the depth \ih; (2) a series of grids with a clearance of about one-third of the depth from the bed, and extending over a finite length of the flume; and (3) a series of grids with a clearance in the range (0.1–1.0)\ih from the bed, but extending over the entire length of the flume. Two kinds of experiments were conducted: plane-bed experiments and ripple-covered-bed experiments. In the former case, the flow in the presence of the turbulence generator was adjusted so that the mean bed shear stress was the same as in the case without the turbulence generator in order to single out the effect of the external turbulence on the sediment transport. In the ripple-covered-bed case, the mean and turbulence quantities of the streamwise component of the velocity were measured, and the Shields parameter, due to skin friction, was determined. The Shields parameter, together with the RMS value of the streamwise velocity fluctuations, was correlated with the sediment transport rate. The sediment transport increases markedly with increasing turbulence level.
239 citations
TL;DR: A weak wave turbulence theory is established for incompressible fluids under rapid rotation using a helicity decomposition, and the kinetic equations for energy E and helicity H are derived for three-wave coupling and it is proved that the energy cascade is to small scales.
Abstract: A weak wave turbulence theory is established for incompressible fluids under rapid rotation using a helicity decomposition, and the kinetic equations for energy E and helicity H are derived for three-wave coupling. As expected, nonlinear interactions of inertial waves lead to two-dimensional behavior of the turbulence with a transfer of energy and helicity mainly in the direction perpendicular to the rotation axis. For such a turbulence, we find, analytically, the anisotropic spectra E approximately k(-5/2)(perpendicular)k(-1/2)(parallel), H approximately k(-3/2)(perpendicular)k(-1/2)(parallel), and we prove that the energy cascade is to small scales. At lowest order, the wave theory does not describe the dynamics of two-dimensional (2D) modes which decouples from 3D waves.
236 citations
TL;DR: In this paper, it is shown that the transfer of energy to small scales produces a tremendous growth of helicity separately in the + and − helical modes at high wave numbers, diverging in the limi...
Abstract: Three-dimensional (3D) turbulence has both energy and helicity as inviscid constants of motion. In contrast to two-dimensional (2D) turbulence, where a second inviscid invariant—the enstrophy—blocks the energy cascade to small scales, in 3D there is a joint cascade of both energy and helicity simultaneously to small scales. It has long been recognized that the crucial difference between 2D and 3D is that enstrophy is a nonnegative quantity whereas the helicity can have either sign. The basic cancellation mechanism which permits a joint cascade of energy and helicity is illuminated by means of the helical decomposition of the velocity into positively and negatively polarized waves. This decomposition is employed in the present study both theoretically and also in a numerical simulation of homogeneous and isotropic 3D turbulence. It is shown that the transfer of energy to small scales produces a tremendous growth of helicity separately in the + and − helical modes at high wave numbers, diverging in the limi...
233 citations
TL;DR: It is found that for explicit filtering, increasing the reconstruction levels for the RSFS stress improves the mean velocity as well as the turbulence intensities, when compared to LES without explicit filtering.
Abstract: Turbulent channel flow simulations are performed using second- and fourth-order finite difference codes. A systematic comparison of the large-eddy simulation (LES) results for different grid resolutions, finite difference schemes, and several turbulence closure models is performed. The use of explicit filtering to reduce numerical errors is compared to results from the traditional LES approach. Filter functions that are smooth in spectral space are used, as the findings of this investigation are intended for application of LES to complex domains. Explicit filtering introduces resolved subfilter-scale (RSFS) as well as subgrid-scale (SGS) turbulence terms. The former can be theoretically reconstructed; the latter must be modelled. The dynamic Smagorinsky model, the dynamic mixed model, and the new dynamic reconstruction model are all studied. It is found that for explicit filtering, increasing the reconstruction levels for the RSFS stress improves the mean velocity as well as the turbulence intensities. When compared to LES without explicit filtering, the difference in the mean velocity profiles is not large; however the turbulence intensities are improved for the explicit filtering case.
218 citations
01 Jan 2003
TL;DR: In this article, an optimally formulated two-equation model in combination with an automatic wall treatment and a model for transition from laminar to turbulent flow was implemented in CFX-5 and applied to different test cases.
Abstract: Heat transfer is of key importance in many gas turbine components. Most of the CFD development in this area is focused on advanced turbulence model closures including second moment closure models, and so called Low-Reynolds (low-Re) number and two-layer turbulence models. However, in many cases CFD heat transfer predictions based on these standard models still show a large degree of uncertainty, which can be attributed to the use of the -equation as the turbulence scale equation and the associated limitations of the near wall treatment. The present paper demonstrates that an optimally formulated two-equation model in combination with advanced wall treatment can overcome many problems of previous models. The SST (Shear Stress Transport) model in combination with an automatic wall treatment and a model for transition from laminar to turbulent flow was implemented in CFX-5 and applied to different test cases. In all cases the SST model shows to be superior, as it gives more accurate predictions and is less sensitive to grid variations. NOMENCLATURE Symbols
216 citations
TL;DR: In this article, the authors present an overview of Turbulent Flow Physics and Equations and their application in various areas of physics, including Turbulence Modeling, Free Shear Flows, and Large Eddy Simulations.
Abstract: Preface. Acknowledgments. 1. Preliminaries. 2. Overview of Turbulent Flow Physics and Equations. 3. Experimental and Numerical Methods. 4. Properties of Bounded Turbulent Flows. 5. Properties of Turbulent Free Shear Flows. 6. Turbulent Transport. 7. Theory of Idealized Turbulent Flows. 8. Turbulence Modeling. 9. Applications of Turbulence Modeling. 10. Large Eddy Simulations. 11. Analysis of Turbulent Scalar Fields. 12. Turbulence Theory. Author Index. Subject Index.
212 citations
TL;DR: In this paper, four different turbulence models were employed to study their influence on the results of pulsatile turbulent flow in axisymmetric stenoses and it was found that the low Reynolds number k-omega turbulence model was in much better agreement with previous experimental measurements than both the low and high Reynolds number versions of the RNG (renormalization-group theory) k-epsilon turbulence model and the standard k-EPsilon model, with regard to predicting the mean flow distal to the stenosis including aspects of the vortex shedding process and the turbulent flow
Abstract: Pulsatile turbulent flow in stenotic vessels has been numerically modeled using the Reynolds-averaged Navier-Stokes equation approach. The commercially available computational fluid dynamics code (CFD), FLUENT, has been used for these studies. Two different experiments were modeled involving pulsatile flow through axisymmetric stenoses. Four different turbulence models were employed to study their influence on the results. It was found that the low Reynolds number k-omega turbulence model was in much better agreement with previous experimental measurements than both the low and high Reynolds number versions of the RNG (renormalization-group theory) k-epsilon turbulence model and the standard k-epsilon model, with regard to predicting the mean flow distal to the stenosis including aspects of the vortex shedding process and the turbulent flow field. All models predicted a wall shear stress peak at the throat of the stenosis with minimum values observed distal to the stenosis where flow separation occurred.
TL;DR: In this paper, the authors explore the turbulence near a freely deformable interface in a countercurrent air-water flow, at a shear Reynolds number Re� = 171, and compare the results for the gas side with open-channel flow data at the same shear-Reynolds number, placing emphasis upon the influence of the waves in the interfacial viscosity-affected region, and away from it in the outer core flow.
Abstract: Direct numerical simulation has been performed to explore the turbulence near a freely deformable interface in a countercurrent air–water flow, at a shear Reynolds number Re� = 171. The deformations of the interface fall in the range of capillary waves of waveslope ak =0 .01, and very small phase speed-to-friction velocity ratio, c/u� .T heresults for the gas side are compared to open-channel flow data at the same shear Reynolds number, placing emphasis upon the influence of the waves in the interfacial viscosity-affected region, and away from it in the outer core flow. Comparison shows a similarity in the distribution of the turbulence intensities near the interface, confirming that for the range of flow conditions considered, the lighter phase perceives the interface like a flexible solid surface, at least in the limit of non-breaking waves. Overall, in a time-averaged sense, the interfacial motion affects the turbulence in the near-interface region; the most pertinent effect is a general dampening of the turbulent fluctuating field which, in turn, leads to a reduction in the interfacial dissipation. Furthermore, the turbulence is found to be less anisotropic at the interface than at the wall. This is confirmed by the analysis of the pressure– rate-of-strain tensor, where the effect of interfacial motion is shown to decrease the pressure strain correlation in the direction normal to the interface and in the spanwise direction. The analysis of the turbulent kinetic energy and Reynolds stress budgets reveals that the interface deformations mainly affect the so-called boundary term involving the redistribution of energy, i.e. by the action of pressure, turbulent fluctuations and molecular viscosity, and the dissipation terms, leaving the production terms almost unchanged. The non-zero value of the turbulent kinetic energy at the interface, together with the reduced dissipation, implies that the turbulent activity persists near the interface and contributes to accelerating the turbulent transfer mechanisms. Away from the interface, the decomposition of the fluctuating velocity gradient tensor demonstrates that the fluctuating rate-of-strain and rate-of-rotation at the interface influence the flow throughout the boundary layer more vigorously. The study also reveals the streaky structure over the deformable interface to be less organized than over a rigid wall. However, the elongation of the streaks does not seem to be much affected by the interfacial motion. A simple qualitative analysis of the quasi–streamwise vortices using different eduction techniques shows that the interfacial turbulent structures do not change with a change of boundary conditions.
TL;DR: In this article, the authors show that the turbulence of the Earth's plasma sheet appears to be a turbulent of eddies, rather than a turbulence of Alfven waves or other MHD modes.
Abstract: [1] The turbulent flows and tangled magnetic fields of the Earth's plasma sheet are explored theoretically and by means of ISEE-2 plasma and magnetic-field measurements. The goal is to obtain a basic understanding of (1) the dynamics, (2) the driving, and (3) the dissipation of the turbulence. Dynamically, the turbulence of the plasma sheet appears to be a turbulence of eddies, rather than a turbulence of Alfven waves or other MHD modes. In this respect, it is similar to the two-dimensional turbulence of the solar wind. Previously published statistical arguments that the correlation length (= integral scale = eddy size) of the turbulence fluctuations in the plasma sheet is leddy ∼ 1.6 RE are confirmed by analyzing plasma sheet magnetic-field measurements during two special “sweeping” intervals that follow the passage of interplanetary shocks. For dissipation of the turbulence, two mechanisms appear to be important. The first is a cascade of energy in the turbulence to small spatial scales, where internal dissipation at non-MHD spatial scales should occur. The second mechanism is electrical coupling of the turbulent flows to the resistive ionosphere, which introduces a “quasi-viscosity” to the plasma sheet. This quasi-viscosity is complicated owing (1) to a time delay associated with Alfven-transit-time coupling (introducing a viscoelasticity to the turbulence), (2) to a scalesize dependence of the coupling (introducing a hypoviscosity), and (3) to a dependence of the coupling on the sign of the flow shear (introducing a sign-vorticity effect). For the coupling of the plasma sheet turbulence to the ionosphere, retarded-time Reynolds numbers R* are derived to describe the importance of the resulting dissipation (quasi-viscosity) and a Deborah number D is derived to describe the importance of the time lag (elasticity). The difference of the plasma sheet turbulence from homogeneous turbulence is discussed; this dissimilarity is owed to (a) dissipation at all wave numbers in the plasma sheet, (b) the presence of boundaries, and (c) the limited range of spatial scales that will allow scale-invariant dynamics. A better description of the plasma sheet turbulence would be “turbulence in a box” or a turbulent wake.
TL;DR: In this paper, a comprehensive study using LDA (Laser Doppler Anemometry), HWA (Hot-Wire Anemometers) and static pressure measurements was performed in order to investigate the flow and turbulence structure around a simplified car model.
Abstract: A comprehensive study using LDA (Laser Doppler Anemometry), HWA (Hot-Wire Anemometry) and static pressure measurements was performed in order to investigate the flow and turbulence structure around a simplified car model.The aim was to supply a detailed data set acquired under well defined boundary conditions to be used as reference data for numerical simulations in general and the validation and verification of refined turbulence models in particular. Because of the fact that the losses in the detached wake region make the major contribution to the aerodynamic drag and the prediction accuracy of the wake is a quite selective criteria for the turbulence models the study focused to the wake behind a simplified car model.
TL;DR: Nuclear magnetic resonance measurements and numerical simulations indicate the existence of sharp transition to turbulence in the B phase of superfluid 3He, which is insensitive to the fluid velocity, in striking contrast to current textbook knowledge of turbulence.
Abstract: Hydrodynamic flow in classical and quantum fluids can be either laminar or turbulent. Vorticity in turbulent flow is often modelled with vortex filaments. While this represents an idealization in classical fluids, vortices are topologically stable quantized objects in superfluids. Superfluid turbulence1 is therefore thought to be important for the understanding of turbulence more generally. The fermionic 3He superfluids are attractive systems to study because their characteristics vary widely over the experimentally accessible temperature regime. Here we report nuclear magnetic resonance measurements and numerical simulations indicating the existence of sharp transition to turbulence in the B phase of superfluid 3He. Above 0.60Tc (where Tc is the transition temperature for superfluidity) the hydrodynamics are regular, while below this temperature we see turbulent behaviour. The transition is insensitive to the fluid velocity, in striking contrast to current textbook knowledge of turbulence2. Rather, it is controlled by an intrinsic parameter of the superfluid: the mutual friction between the normal and superfluid components of the flow, which causes damping of the vortex motion.
TL;DR: In this paper, a review of the use of high-resolution methods in turbulent flow computations is presented, and theoretical arguments and recent research aiming at justifying their use as an implicit turbulence model are discussed.
01 Jan 2003
TL;DR: In this article, it is shown that using a forcing function which is directly proportional to the velocity has physical meaning and gives results which are closer to both homogeneous and non-homogeneous turbulence.
Abstract: Stationary isotropic turbulence is often studied numerically by adding a forcing term to the Navier-Stokes equation. This is usually done for the purpose of achieving higher Reynolds number and longer statistics than is possible for isotropic decaying turbulence. It is generally accepted that forcing the Navier-Stokes equation at low wave number does not influence the small scale statistics of the flow provided that there is wide separation between the largest and smallest scales. It will be shown, however, that the spectral width of the forcing has a noticeable effect on inertial range statistics. A case will be made here for using a broader form of forcing in order to compare computed isotropic stationary turbulence with (decaying) grid turbulence. It is shown that using a forcing function which is directly proportional to the velocity has physical meaning and gives results which are closer to both homogeneous and non-homogeneous turbulence. Section 1 presents a four part series of motivations for linear forcing. Section 2 puts linear forcing to a numerical test with a pseudospectral computation.
TL;DR: A scaling is proposed to collapse the departure from the critical Reynolds number for pure fluid as a function of concentration into a single master curve.
Abstract: We investigate experimentally the influence of suspended particles on the transition to turbulence. The particles are monodisperse and neutrally buoyant with the liquid. The role of the particles on the transition depends upon both the pipe to particle diameter ratios and the concentration. For large pipe-to-particle diameter ratios the transition is delayed while it is lowered for small ratios. A scaling is proposed to collapse the departure from the critical Reynolds number for pure fluid as a function of concentration into a single master curve.
TL;DR: In this article, the authors investigated the possible causes of the turbulence encounter and used very high-resolution two-and three-dimensional numerical simulations to investigate the possible cause of the turbulent encounter.
Abstract: An investigation of the generation of turbulence above deep convection is presented. This investigation is motivated by an encounter between a commercial passenger aircraft and severe turbulence above a developing thunderstorm near Dickinson, North Dakota, on 10 July 1997. Very high-resolution two- and three-dimensional numerical simulations are used to investigate the possible causes of the turbulence encounter. These simulations explicitly resolve the convection and the turbulence-causing instabilities. The configurations of the models are consistent with the meteorological conditions surrounding the event. The turbulence generated in the numerical simulations can be placed into two general categories. The first category includes turbulence that remains local to the cloud top, and the second category includes turbulence that propagates away from the convection and owes its existence to the breakdown of convectively generated gravity waves. In both the two- and three-dimensional calculations, th...
TL;DR: In this article, the authors show that the transition to turbulence is connected with the formation of a chaotic saddle in the phase space of the system and quantify a sensitive dependence on initial conditions and find in a statistical analysis that the distribution of turbulent lifetimes follows an exponential law.
Abstract: The experiments by Darbyshire and Mullin (J. Fluid Mech. 289, 83 (1995)) on the transition to turbulence in pipe flow show that there is no sharp border between initial conditions that trigger turbulence and those that do not. We here relate this behaviour to the possibility that the transition to turbulence is connected with the formation of a chaotic saddle in the phase space of the system. We quantify a sensitive dependence on initial conditions and find in a statistical analysis that in the transition region the distribution of turbulent lifetimes follows an exponential law. The characteristic mean lifetime of the distribution increases rapidly with Reynolds number and becomes inaccessibly large for Reynolds numbers exceeding about 2200. Suitable experiments to further probe this concept are proposed.
TL;DR: In this paper, the Lagrangian averaged Navier-Stokes (LANS-alpha) equations were used to simulate three-dimensional homogeneous and isotropic turbulence.
Abstract: Capabilities for turbulence calculations of the Lagrangian averaged Navier-Stokes (LANS-alpha) equations are investigated in decaying and statistically stationary three-dimensional homogeneous and isotropic turbulence. Results of the LANS-alpha computations are analyzed by comparison with direct numerical simulation (DNS) data and large eddy simulations. Two different decaying turbulence cases at moderate and high Reynolds numbers are studied. In statistically stationary turbulence two different forcing techniques are implemented to model the energetics of the energy-containing scales. The resolved flows are examined by comparison of the energy spectra of the LANS-alpha with the DNS computations. The energy transfer and the capability of the LANS-alpha equations in representing the backscatter of energy is analyzed by comparison with the DNS data. Furthermore, the correlation between the vorticity and the eigenvectors of the rate of the resolved strain tensor is studied. We find that the LANS-alpha equations capture the gross features of the flow, while the wave activity below the scale alpha is filtered by a nonlinear redistribution of energy.
TL;DR: In this paper, a three-dimensional CFD model was used to compute the formation of the meandering pattern in an initially straight alluvial channel, based on the finite volume method using an unstructured grid with dominantly hexahedral cells.
Abstract: A three-dimensional CFD model was used to compute the formation of the meandering pattern in an initially straight alluvial channel. The numerical model was based on the finite volume method using an unstructured grid with dominantly hexahedral cells. The k-e model was used to predict turbulence and the SIMPLE method was used to compute the pressure. The sediment transport was computed as bed load in addition to solving the convection-diffusion equation for suspended sediment transport. The bed changes were calculated and the grid was altered during the computation as channel erosion and deposition caused wetting and drying. The model was tested by comparing with results from physical model studies carried out at Colorado State Univ., Fort Collins, Colo. The results showed successfully the replication of many of the meander characteristics, including secondary currents, cross-sectional profiles, meander planform, meander wavelength, downstream meander migration, and chute formation.
TL;DR: In this paper, the authors applied the CIRA RANS flow solver by employing a large set of turbulence models, to typical aerodynamic applications for which certified experimental data are available in literature.
TL;DR: In this paper, a Kolmogorov's relation allows for an analytical solution to be calculated within the portion of a dense and homogeneous canopy where the mixing length does not vary.
Abstract: The k - e turbulence model is a standard of computational software packages for engineering, yet its application to canopy turbulence has not received comparable attention This is probably due to the additional source (and/or sink) terms, whose parameterization remained uncertain This model must include source terms for both turbulent kinetic energy (k) and the viscous dissipation rate (e), to account for vegetation wake turbulence budget In this note, we show how Kolmogorov's relation allows for an analytical solution to be calculated within the portion of a dense and homogeneous canopy where the mixing length does not vary By substitution within model equations, this solution allows for a set of constraints on source term model coefficients to be derivedThose constraints should meet both Reynolds averaged Navier–Stokes equationsand large-eddy simulation sub-grid scale turbulence modelling requirementsAlthough originating from within a limited portion of the canopy, the predictedcoefficients values must be valid elsewhere in order to make the model capable of predicting the whole canopy-layer flow with a single set of constants
TL;DR: In this article, the authors compared four low-Reynolds-number (LRN) turbulence models and found that the renormalization group (RNG) and Menter κ-ω models amplify the flow instabilities after tubular constrictions and hence fail to capture the laminar flow behavior at low Reynolds numbers.
Abstract: In numerous internal flow systems the velocity field can undergo all flow regimes, that is, from laminar, via transitional, to fully turbulent. Considering two test conduits with local constrictions, four turbulence models, with an emphasis on low-Reynolds-number (LRN) turbulence models, were compared and evaluated. The objective was to identify a readily available LRN turbulence model with which incompressible laminar-to-turbulent velocity and pressure fields in complex three-dimensional conduits can be directly computed. The comparison study revealed that the renormalization group (RNG) κ-e and Menter κ-ω models amplify the flow instabilities after tubular constrictions and hence fail to capture the laminar flow behavior at low Reynolds numbers
TL;DR: In this article, power-law spectra of velocity were established for dilute polymer solutions when polymers are strongly stretched by the flow, and such spectra were not associated with a flux of a conserved quantity, in two cases.
Abstract: Turbulence in dilute polymer solutions when polymers are strongly stretched by the flow is investigated. We establish power-law spectra of velocity, that are not associated with a flux of a conserved quantity, in two cases. First, such spectrum is formed in the elastic waves range of high Reynolds number turbulence of polymer solutions above the coil–stretch transition. Second, such spectrum is characteristic of the elastic turbulence, where chaotic flow is excited due to elastic instabilities at small Reynolds numbers.
TL;DR: A review of existing and new considerations for the assessment of wind loads on low-rise structures from wind tunnel simulation experiments is presented in this paper, where the aerodynamics of surface-mounted prisms are given.
TL;DR: In this paper, four different two-equation turbulence models for geophysical flows are compared: the k − ϵ model, two new versions of the k - ω model, and the Mellor-Yamada model.
TL;DR: Mahesh et al. as discussed by the authors used the interaction of homogeneous isotropic turbulence with a normal shock to suggest improvements in the k-e model applied to shock/turbulence interaction.
Abstract: The RANS (Reynolds averaged Navier–Stokes) equations can yield significant error when applied to practical flows involving shock waves. We use the interaction of homogeneous isotropic turbulence with a normal shock to suggest improvements in the k–e model applied to shock/turbulence interaction. Mahesh et al. [J. Fluid Mech. 334, 353 (1997)] and Lee et al. [J. Fluid Mech. 340, 22 (1997)] present direct numerical simulation (DNS) and linear analysis of the flow of isotropic turbulence through a normal shock, where it is found that mean compression, shock unsteadiness, pressure-velocity correlation, and up-stream entropy fluctuations play an important role in the interaction. Current RANS models based on the eddy viscosity assumption yield very high amplification of the turbulent kinetic energy, k, across the shock. Suppressing the eddy viscosity in a shock improves the model predictions, but is inadequate to match theoretical results at high Mach numbers. We modify the k equation to include a term due to s...
TL;DR: In this article, a non-linear k-model is adopted as a turbulence model, which can take into account the anisotropy of turbulence with less CPU time and computer memory than RSM or LES.
Abstract: SUMMARY The incompressibleow around blubodies (a square cylinder and a cube) is investigated numerically using turbulence models. A non-linear k - � model, which can take into account the anisotropy of turbulence with less CPU time and computer memory than RSM or LES, is adopted as a turbulence model. In tuning of the model, the model coecients of the non-linear terms are adjusted through the examination of previous experimental studies in simple shearows. For the tuning of the coecient in the eddy viscosity (= C� ), the realizability constraints are derived in three types of basic 2Dow patterns, namely, a simple shearow, �ow around a saddle and a focal point. Cis then determined as a function of the strain and rotation parameters to satisfy the realizability. The turbulence model is �rst applied to a 2Dow around a square cylinder and the model performance for unsteadyows is examined focussing on the period and the amplitude of theow oscillation induced by Karman vortex shedding. The applicability of the model to 3Dows is examined through the computation of theow around a surface-mounted cubic obstacle. The numerical results show that the present model performs satisfactorily to reproduce complex turbulentows around blubodies. Copyright ? 2003 John Wiley & Sons, Ltd.