LES and RANS Studies of Oscillating Flows over Flat Plate

The separated turbulent flow past an inclined flat plate with sharp leading and trailing edges was computed based on three different simulation approaches for a reynolds number Rec= 20000 and a high angle of attack α=18°. The simulation techniques applied were the Reynolds‐averaged Navier–Stokes (RANS) equations combined with a one‐equation Spalart–Allmaras turbulence model, the large‐eddy simulation (LES) based on an algebraic eddy‐viscosity model, and a hybrid approach known as detached‐eddy simulation (DES) applying a slightly modified Spalart–Allmaras model in the entire integration domain. DES is a non‐zonal coupling technique of RANS and LES developed in the hope of reducing the large computational resources required for LES computations of turbulent flows with practical relevance. However, the objective of the present study was not to compare the resources (CPU time, memory) required for all three techniques but to investigate and evaluate the quality of the predicted results for RANS, DES and LES. For this purpose, a test case which is favourable to the basic concept of DES and which places special emphasis on the LES part of the DES concept was chosen. This last issue was important since the modified Spalart–Allmaras model applied as a subgrid scale model in the LES part is not as well validated as other models usually applied in LES. For this purpose, an LES prediction on a very fine grid served as a reference case for the evaluation. For all three techniques, predictions with different grid resolutions were carried out and compared with each other based on important integral parameters (e.g. Strouhal number, mean drag and lift coefficients and their standard deviations), the instantaneous and time‐averaged flow structures, and higher‐order statistics. As expected, the pure RANS calculation, although applied as unsteady RANS, failed to predict the unsteady characteristics of the separated flow. In contrast, the DES approach yielded reasonably the shedding phenomenon and some integral parameters. However, analysing the results in more detail led to remarkable deviations between the DES and LES predictions also when the same grid resolution was applied. Especially the free shear layer originating from the leading edge of the plate was not well reproduced by DES, showing strong deficiencies of the model applied as a subgrid scale model. The reasons for this behaviour of the model were analysed in detail. Two basic causes were identified; the first is given by some near‐wall corrections in the finite Reynolds number version of the model which are not working properly in the LES mode of DES. The second is a modified definition of the filter width typically applied for DES, leading to strongly increased values of the eddy viscosity. A revised version of the S–A model taking both issues seriously into account was used as a subgrid scale model in the LES mode. As a direct consequence, much better agreement with the reference LES solution was found for the DES prediction on the coarse grid, eliminating the deficiencies of the original formulation. Copyright© 2003 John Wiley & Sons, Ltd.

The Discontinuous Galerkin (DG) method provides numerical solutions of the NavierStokes equations with high order of accuracy in complex geometries and allows for highly efficient parallelization algorithms. These attributes make the DG method highly attractive for large eddy simulation (LES). The main goal of this work is to investigate the feasibility of adopting an explicit filter to the numerical solution of the Navier-Stokes equations to increase the numerical stability of underresolved simulations such as LES and to use the explicit filter in dynamic subgrid scale (SGS) models for LES. The explicit filter takes advantage of DG’s framework where the solution is approximated using a polynomial basis. The higher modes of the solution correspond to a higher order polynomial basis, therefore by removing high order modes the filtered solution contains only lower frequency content. The explicit filter is successfully used here to remove the effects of aliasing in underresolved simulations of the Taylor-Green vortex case at a Reynolds number Re = 1600. The de-aliasing is achieved by evaluating a solution at a higher order polynomial (effectively increasing the number of quadrature points used for integration) and then projecting the solution down to a lower order polynomial. The SGS models investigated include the constant coefficient Smagorinsky Model (CCSM), Dynamic Smagorinsky Model (DSM), and Dynamic Heinz Model (DHM). The Taylor-Green Vortex case exhibits a laminar-turbulent transition and it is shown that the dynamic SGS models capture this transition more accurately than the CCSM when a sufficiently high polynomial order is used. The explicit test-filter operation for the dynamic models introduces a commutation error. A brief comparison of the effects of the commutative error that exists with this filter implementation is shown although further investigation is needed to determine the more appropriate order of operations.

CFD simulations of two opposing plane wall jets in a generic empty airplane cabin: Comparison of RANS and LES

A computational study of the standard large eddy simulation (LES) technique employing the dynamic Smagorinsky subgrid scale (SGS) model is compared to results using a highorder, implicit LES (ILES) technique with no SGS modeling. It has been shown in the past that ILES presents a viable substitute to the standard LES technique on canonical turbulent problems. The purpose of this paper is to evaluate the use of implicit LES with and without explicit subgrid scale models applied to problems of transitional ow over the SD7003 airfoil for various grid resolutions and Reynolds numbers. It will be shown that the addition of the SGS model does not oer a signicant advantage on coarser meshes when compared to the ILES approach. The ILES technique is shown to be an attractive alternative to SGS modeling in solution quality and computational cost and robustness for the ows examined.

The present study consists of three campaigns - (i) on the development of a dynamic subgrid scale (SGS) model for the turbulent flux of a passive scalar; (ii) on the formulation of a hybrid RANS/LES model for transitional flows; and (iii) on the large eddy simulation (LES) of passive scalar transport in transitional boundary layer over a flate plate and development toward an improved Higher Order Generalized Gradient Diffusion Hypothesis (HOGGDH) model. In the first project, a dynamic subgrid-scale (SGS) scalar-flux model, based on the exact rate of production of turbulent scalar fluxes, is proposed. The model is derived from an assumption that the pressure-scalar correlation in the equation for turbulent scalar flux is a vector that is approximately aligned with the scalar flux itself. The formulation then yields a tensor diffusivity which allows nonalignment of the SGS scalar fluxes with respect to the resolved scalar gradient. In contrast to eddy diffusivity models and to general gradient diffusion hypothesis models, for which the diffusivity tensor is symmetric, the present formulation produces an asymmetric diffusion tensor; for theoretical and experimental reasons, that tensor is known to be very asymmetric. The model contains a single coefficient, which is determined dynamically. The model is validated in fully developed turbulent channel flow and in separated and reattaching flow over a backstep. In the second project, a simple alternative to locally compute the model coefficient CDES in the ℓ2 − ω Delayed Detached Eddy Simulation (DDES) model is presented. The formula for the coefficient is derived from the structural function Bβ of Vreman. It, therefore, does not involve explicit filtering or averaging procedures. By virtue of the variable coefficient being based on Bβ, the model is expected to retain the property of relatively small dissipation in transitional and nearwall regions. This property enables the present formulation to be an excellent candidate to predict transitional flows. The formulation is validated in the canonical, fully developed turbulent channel flow and backward facing step flows, followed by simulations of the orderly, bypass and separation induced laminar-to-turbulent transition in a spatially developing boundary layer over a flat plate. In the third project, flat plate transition under freestream turbulence with heat transfer is studied using subgrid dynamic scalar-flux models. The advantage of adopting tensorial subgrid diffusivity is quantified. A set of Reynolds Averaged scalar-flux models is also evaluated using the LES data. Based upon the observations, an improved Higher Order Generalized Gradient Diffusion Hypothesis (HOGGDH) model is proposed with a modified, spatially varying model coefficient. The model coefficient is made a function of the turbulent Reynolds number RT , which enables it to detect the near wall region, thereby increasing the accuracy down to the wall. The mean temperature predictions and the scalar fluxes in the wall-normal as well as the streamwise directions by the improved version of the HOGGDH model are shown to be accurate compared to the HOGGDH model with a constant coefficient and other considered models.

Introduction. The reduction in beach width due to erosion is a significant issue that can either be mitigated or exacerbated by coastal protection structures. Modelling breaking waves near the coast and around coastal structures can be used to determine their impact on the dynamics of the coastal zone. The objective of this study is to model and analyze the dynamics of turbulent structures around a single breakwater, obtained using two turbulence modelling schemes: RANS and LES.Materials and Methods. Turbulence induced by breaking waves was investigated. The modelling was based on bathymetric measurements conducted along the Azov Sea coast and a three-dimensional wave hydrodynamics model supplemented with various turbulence calculation configurations.Results. Modelling results of wave processes generating turbulent flows in the presence of coastal protection structures using different turbulence models were obtained. Results obtained based on Reynolds-averaged Navier-Stokes (RANS) equations are compared with the results of Large Eddy Simulation (LES) approach with Smagorinsky dynamic subgrid-scale model (DSM).Discussion and Conclusions. The results showed that wave heights simulated by LES were higher than those simulated by RANS in the front and leeward regions of the coastal protection structure and were lower in its upper part. Thus, according to LES, a greater amount of wave energy was preserved after passing over the breakwater. Velocity vectors of the water medium showed the formation of a vortex when LES was used, whereas no evidence of such turbulent vortices was detected in the case of RANS, confirming the better performance of LES for turbulence modelling in the coastal zone. According to the presented results, LES is the preferred tool for generating turbulence under incoming wave conditions in engineering practices.

A computational study of the HyShot II combustor performance

This paper is devoted to the challenges arising in Reynolds-Averaged Navier-Stokes (RANS) modeling of confined coaxial jet mixing. The main drawbacks of the gradient diffusion hypothesis and of the constant turbulent Schmidt number approach for the considered flow type are elucidated. In the first part of the present work, the mean and fluctuating velocity and scalar fields obtained in RANS simulations are directly validated against experimental data and compared with large-eddy simulation (LES) results. Three different turbulence models and three different turbulent Schmidt numbers values are selected for the present RANS computations. The accuracy given by LES is superior in comparison to RANS results especially for scalar mixing. The inability of the chosen RANS approach to represent counter-gradient diffusion in the axial direction characteristic for the considered test case is discussed. In the second part of the present study, the time-averaged mean and fluctuating velocity and scalar fields from LES are used for the evaluation of turbulent viscosities, turbulent scalar diffusivities, and turbulent Schmidt numbers as they appear in the RANS equations. Specific problems of the evaluation of these parameters from LES data are addressed. The importance of accounting for the turbulent scalar fluxes in all coordinate directions for evaluation of turbulent scalar diffusivity is revealed. The values obtained from LES results are compared with those given by RANS modeling. The deviations and their consequences for RANS predictions are discussed.

An efficient recycling algorithm is developed for injecting resolved turbulent content in a boundary layer as it switches from a Reynolds Averaged Navier-Stokes (RANS) type treatment to a Large Eddy Simulation (LES) type treatment inside a generalized Detached-Eddy Simulation (DES). The motivation is to use RANS in the thinnest boundary-layer area, following the original argument in favour of DES, and LES in the thicker boundary-layer areas especially approaching separation, to improve accuracy and possibly obtain unsteady outputs. The algorithm relies on an overlap of the RANS and LES domains and, therefore, the availability of both RANS and LES solutions in the recycling region, which is about 5 boundary-layer thicknesses long. This permits a smooth transfer of the turbulent stresses from this section to the LES inflow. The continuity of the skin-friction distribution is very good, reflecting the excellent viability of the resolved turbulence. The approach is validated in a flat-plate boundary layer and an airfoil near stall, with mild pressure gradient near the interface, and then applied to the compressible flow over an idealized airliner windshield wiper. The pressure fluctuations at reattachment are 12dB more intense than under a simple boundary layer at the same speed, and the output contains all the quantities needed to calculate the transmission of sound through the glass.

Flow and heat transfer inside an idealized electronics system is simulated using large‐eddy simulation (LES)‐related approaches that have been validated for simpler canonical flows. These include: Yoshizawa LES, detached eddy simulation (DES), limited numerical scales (LNS) and other hybrid LES–RANS (Reynolds‐averaged Navier–Stokes) approaches including a new ILES (implicit LES)–RANS method. For the ILES, dissipation from the one legged temporal discretization is used to drain turbulence. The use of differential equations, including the Hamilton–Jacobi and Eikonal, to model turbulence distance functions is explored. The Hamilton–Jacobi is shown to be especially compatible with the zonal RANS–ILES approach and the Eikonal with DES. Performances of the LES‐related methods are compared with explicit algebraic stress unsteady RANS (URANS) results and also measurements. Considering the problem complexity, generally, for all methods, predicted mean velocities and turbulence intensities are in a reasonable agreement with measurements. Average errors are 15 and 25%, respectively. With the exception of the zonal ILES–RANS method, turbulence intensities are under‐predicted. For heat transfer, none of the models performs well giving circa 100% errors. Notably, the LNS performs poorly for both the flow field and heat transfer giving a highly complex RANS–LES interface with inappropriate upstream LES boundary conditions. DES is found impossible to converge. This is partly attributed to the irregular LES–RANS interface arising with the method. All the LES approaches significantly underpredict heat transfer and the URANS over‐predicts. Even the increased flow activity arising from use of the less dissipative ILES element does not prevent the significant heat transfer under‐prediction. Copyright © 2005 John Wiley & Sons, Ltd.

It is well known that the correlation between the Smagorinsky model and the subgrid scale stress is low, while the model based on the scale similarity assumption has considerably higher correlation. However, the scale similarity model by itself was found to be insufficiently dissipative. Therefore, the model is usually used together with the Smagorinsky model. Model coefficients are commonly computed using the two-parameter dynamic procedure. Nevertheless, the dynamic two-parameter mixed model still does not work well for wall bounded flows, since the model predicts a high value of the wall shear stress. In this study, we propose a modification to the two-parameter dynamic procedure for wall bounded flows, which removes that defect: the Smagorinsky parameter, CS, is computed exactly the same way as in the dynamic Smagorinsky model, then the other parameter, CL, is computed dynamically as CS is known. This ensures that the mixed model provides proper wall shear stress and mean velocity profile. Computational tests are done for turbulent channel flow where the Reynolds numbers based on the channel half-width and wall friction velocity are 395 and 1400. To remove the ambiguity regarding the accuracy of the finite difference scheme, we use high (up to 12th) order accurate fully conservative finite difference schemes in a staggered grid system.

The critical review discusses the most accurate methods for description of turbulent flows: the computationally very expensive direct numerical simulation (DNS) and slightly less accurate and slightly less expensive large eddy simulation (LES) methods. Both methods have found their way into nuclear thermal hydraulics as tools for studies of the fundamental mechanisms of turbulence and turbulent heat transfer. In the first section of this critical review, both methods are briefly introduced in parallel with the basic properties of the turbulent flows. The focus is on the DNS method, the so-called quasi-DNS approach, and the coarsest turbulence modeling approach discussed in this work, which is still on the very small-scale, wall-resolved LES. Other, coarser turbulence modeling approaches (such as wall-modeled LES, Reynolds Averaged Navier-Stokes (RANS)/LES hybrids, or RANS) are beyond the scope of the present work. Section II answers the question: “How do the DNS and LES methods work?” A short discussion of the computational requirements, numerical approaches, and computational tools is included. Section III is about the interpretation of the DNS and LES results and statistical uncertainties. Sections IV and V give some examples of the DNS and wall-resolved LES results relevant for nuclear thermal hydraulics. The last section lists the conclusions and some of the challenges that might be tackled with the most accurate techniques like DNS and LES.

In the present study, numerical simulations were conducted to investigate the behavior of a 60° inclined dense jet discharged onto horizontal (0°) and sloped (5°) bottoms in a stagnant environment. The objective was to evaluate the capability of Large Eddy Simulation (LES) in capturing both the kinematic and mixing characteristics of inclined dense jets interacting with different bottom boundaries. A Reynolds-Averaged Navier–Stokes (RANS) model was also included for comparison. The LES simulations successfully reproduced the key kinematic and mixing characteristics, including the jet trajectory, centerline peak location, impact point, and terminal rise height, and showed strong agreement with the experimental observations. LES also predicted the concentration distributions and variations along both the horizontal and sloped bottoms, whereas the RANS model tended to underestimate both geometrical and dilution properties. A Gaussian fitting function was proposed to estimate the concentration distribution under both bottom conditions. Analysis of the spreading layer indicated that the concentration profiles exhibited self-similarity. Energy spectrum analysis showed that the sloped bottom enhanced shear-induced turbulence, thereby improving the mixing efficiency. Results confirm the reliability of LES for describing jet–bed interactions and emphasize the influence of bed slope on jet dilution and mixing behavior.

This paper presents results from numerical simulations of a 3-bladed horizontal axis tidal stream turbine (TST). A two-equation transient Reynolds Averaged Navier Stokes (RANS) k-! Shear Stress Transport (SST)model was used for code validation and testing of a newly implemented sliding mesh technique for Code Saturne. Wall- and blade-resolved large-eddy simulations (LES) were then performed to study the complete geometry at various tip speed ratios (TSR). Thrust and power coefficients were compared to experimental measurements in a towing tank for a range of TSR (4, 5, 6, 7 & 8) at a fixed hub pitch angle. A strong flow meandering was observed with the wake recovery and velocity deficit showing a high sensitivity to upstream turbulence intensities. The mean thrust and power coefficients by RANS k-! SST model were found to be more sensitive to the upstream turbulence than the LES. A comparison of all models are also presented for the mean sectional blade pressures and mean wake velocity profiles. The paper also presents an overview of modelling and numerical issues relating to fine LES for such rotating geometries

Turbulent flow and loading on a tidal stream turbine by LES and RANS