Evaluation of sub grid scale and local wall models in Large-eddy simulations of separated flow (original) (raw)
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A New Wall Model for Large Eddy Simulation of Separated Flows
Fluids, 2019
The aim of this work is to propose a new wall model for separated flows which is combined with large eddy simulation (LES) of the flow field in the whole domain. The model is designed to give reasonably good results for engineering applications where the grid resolution is generally coarse. Since in practical applications a geometry can share body fitted and immersed boundaries, two different methodologies are introduced, one for body fitted grids, and one designed for immersed boundaries. The starting point of the models is the well known equilibrium stress model. The model for body fitted grid uses the dynamic evaluation of the von Kármán constant κ of Cabot and Moin (Flow, Turbulence and Combustion, 2000, 63, pp. 269-291) in a new fashion to modify the computation of shear velocity which is needed for evaluation of the wall shear stress and the near-wall velocity gradients based on the law of the wall to obtain strain rate tensors. The wall layer model for immersed boundaries is an extension of the work of Roman et al. (Physics of Fluids, 2009, 21, p. 101701) and uses a criteria based on the sign of the pressure gradient, instead of one based on the friction velocity at the projection point, to construct the velocity under an adverse pressure gradient and where the near-wall computational node is in the log region, in order to capture flow separation. The performance of the models is tested over two well-studied geometries, the isolated two-dimensional hill and the periodic two-dimensional hill, respectively. Sensitivity analysis of the models is also performed. Overall, the models are able to predict the first and second order statistics in a reasonable way, including the position and extension of the downward separation region.
International Journal of Heat and Fluid Flow, 2003
Large Eddy Simulations are presented for the ow in a periodic channel segment, which is laterally constricted by hill-shaped obstructions on one wall, having a height of 33% of the unconstricted channel. The Reynolds number, based on channel height, is 21560. Massive separation thus arises on the hills' leeward sides, the length of which is about 50% of that of the periodic segment. After reattachment, the ow is allowed to recover over about 30% of the segment length before being strongly accelerated over the windward side of the next hill. The principal challenge of this ow arises from the separation on the curved hill surface and the fact that the reattachment point, and hence the whole ow, are highly sensitive to the separation process.
An approach to wall modeling in large-eddy simulations
Physics of Fluids, 2000
Channel flow with friction Reynolds number Re as high as 80 000 is treated by large-eddy simulation at a moderate cost, using the subgrid-scale model designed for detached-eddy simulations. It includes wall modeling, and was not adjusted for this flow. The grid count scales with the logarithm of the Reynolds number. Three independent codes are in fair agreement with each other. Reynolds-number variations and grid refinement cause trades between viscous, modeled, and resolved shear stresses. The skin-friction coefficient is too low, on the order of 15%. The velocity profiles contain a ''modeled'' logarithmic layer near the wall and some suggest a ''resolved'' logarithmic layer farther up, but the two layers have a mismatch of several units in U ϩ .
Wall-Layer Modelling of massive separation in Large Eddy Simulation of coastal flows
2015
The subject of modelling flow near wall is still open in turbulent wall bounded flows, since there is no wall layer model which works perfectly. Most of the present models work well in attached flows, specially for very simple geometries like plane channel flows. Weakness of the models appears in complex geometries, and many of them do not capture flow separation accurately in detached flows, specially when the slope of wall changes gradually. In many engineering applications, we deal with complex geometries. A possible way to simulate flows influenced by complex geometry using a structured grid, is to consider the geometry as immersed boundary for the simulation. Current wall layer models for the immersed boundaries are more complex and less accurate than the body-fitted cases (cases without immersed boundaries). In this project the accuracy of wall layer model in high Reynolds number flows is targeted, using LES for attached flows as well as detached flows (flows with separation)....
Validation of a novel very large eddy simulation method for simulation of turbulent separated flow
International Journal for Numerical Methods in Fluids, 2013
The paper describes the validation of a newly developed very LES (VLES) method for the simulation of turbulent separated flow. The new VLES method is a unified simulation approach that can change seamlessly from Reynolds-averaged Navier-Stokes to DNS depending on the numerical resolution. Four complex test cases are selected to validate the performance of the new method, that is, the flow past a square cylinder at Re D 3000 confined in a channel (with a blockage ratio of 20%), the turbulent flow over a circular cylinder at Re D 3900 as well as Re D 140, 000, and a turbulent backward-facing step flow with a thick incoming boundary layer at Re D 40, 000. The simulation results are compared with available experimental, LES, and detached eddy simulation-type results. The new VLES model performs well overall, and the predictions are satisfactory compared with previous experimental and numerical results. It is observed that the new VLES method is quite efficient for the turbulent flow simulations; that is, good predictions can be obtained using a quite coarse mesh compared with the previous LES method. Discussions of the implementation of the present VLES modeling are also conducted on the basis of the simulations of turbulent channel flow up to high Reynolds number of Re D 4000. The efficiency of the present VLES modeling is also observed in the channel flow simulation. From a practical point of view, this new method has considerable potential for more complex turbulent flow simulations at relative high Reynolds numbers.
A new two-scale model for large eddy simulation of wall-bounded flows
Progress in Aerospace Sciences, 2010
A new hybrid approach to model high Reynolds number wall-bounded turbulent flows is developed based on coupling a two-level simulation (TLS) approach Menon, 2006 [1], 2007 [2] in the inner region with conventional large eddy simulation (LES) away from the wall. This new approach is significantly different from previous near-wall approaches for LES. In this hybrid TLS-LES approach, a very fine smallscale (SS) mesh is embedded inside the coarse LES mesh. The SS equations capture fine-scale temporal and spatial variations in all three Cartesian directions for all three velocity components near the wall. The TLS-LES equations are derived using a new scale separation operator that allows a smooth transition between the two regions, with the equations in the transition region obtained by blending the TLS large-scale and LES equations. New terms in the hybrid region are identified. The TLS-LES approach is used to study the near-wall features in canonical turbulent channel flows for a range of Reynolds number using relatively coarse large-scale (LS) grids. Results show that the TLS-LES approach is able to capture the effect of both the LS and SS features in the wall region consistently for the range of simulated Reynolds number.
A New Version of Detached-eddy Simulation, Resistant to Ambiguous Grid Densities
Theoretical and Computational Fluid Dynamics, 2006
Detached-eddy simulation (DES) is well understood in thin boundary layers, with the turbulence model in its Reynolds-averaged Navier–Stokes (RANS) mode and flattened grid cells, and in regions of massive separation, with the turbulence model in its large-eddy simulation (LES) mode and grid cells close to isotropic. However its initial formulation, denoted DES97 from here on, can exhibit an incorrect behavior in thick boundary layers and shallow separation regions. This behavior begins when the grid spacing parallel to the wall Δ∥ becomes less than the boundary-layer thickness δ, either through grid refinement or boundary-layer thickening. The grid spacing is then fine enough for the DES length scale to follow the LES branch (and therefore lower the eddy viscosity below the RANS level), but resolved Reynolds stresses deriving from velocity fluctuations (“LES content”) have not replaced the modeled Reynolds stresses. LES content may be lacking because the resolution is not fine enough to fully support it, and/or because of delays in its generation by instabilities. The depleted stresses reduce the skin friction, which can lead to premature separation. For some research studies in small domains, Δ∥ is made much smaller than δ, and LES content is generated intentionally. However for natural DES applications in useful domains, it is preferable to over-ride the DES limiter and maintain RANS behavior in boundary layers, independent of Δ∥ relative to δ. For this purpose, a new version of the technique – referred to as DDES, for Delayed DES – is presented which is based on a simple modification to DES97, similar to one proposed by Menter and Kuntz for the shear–stress transport (SST) model, but applicable to other models. Tests in boundary layers, on a single and a multi-element airfoil, a cylinder, and a backward-facing step demonstrate that RANS function is indeed maintained in thick boundary layers, without preventing LES function after massive separation. The new formulation better fulfills the intent of DES. Two other issues are discussed: the use of DES as a wall model in LES of attached flows, in which the known log-layer mismatch is not resolved by DDES; and a correction that is helpful at low cell Reynolds numbers.
Journal of Fluid Mechanics, 2005
High-resolution large-eddy simulation is used to investigate the mean and turbulence properties of a separated flow in a channel constricted by periodically distributed hillshaped protrusions on one wall that obstruct the channel by 33% of its height and are arranged 9 hill heights apart. The geometry is a modification of an experimental configuration, the adaptation providing an extended region of post-reattachment recovery and allowing high-quality simulations to be performed at acceptable computing costs. The Reynolds number, based on the hill height and the bulk velocity above the crest is 10 595. The simulated domain is streamwise as well as spanwise periodic, extending from one hill crest to the next in the streamwise direction and over 4.5 hill heights in the spanwise direction. This arrangement minimizes uncertainties associated with boundary conditions and makes the flow an especially attractive generic test case for validating turbulence closures for statistically two-dimensional separation. The emphasis of the study is on elucidating the turbulence mechanisms associated with separation, recirculation reattachment, acceleration and wall proximity. Hence, careful attention has been paid to resolution, and a body-fitted, low-aspect-ratio, nearly orthogonal numerical grid of close to 5 million nodes has been used. Unusually, the results of two entirely independent simulations with different codes for identical flow and numerical conditions are compared and shown to agree closely. Results are included for mean velocity, Reynolds stresses, anisotropy measures, spectra and budgets for the Reynolds stresses. Moreover, an analysis of structural characteristics is undertaken on the basis of instantaneous realizations, and links to features observed in the statistical results are identified and interpreted. Among a number of interesting features, a distinct 'splatting' of eddies on the windward hill side following reattachment is observed, which generates strong spanwise fluctuations that are reflected, statistically, by the spanwise normal stress near the wall exceeding that of the streamwise stress by a substantial margin, despite the absence of spanwise strain.
Computers & Fluids, 2012
We report on the isogeometric residual-based variational multiscale (VMS) large eddy simulation of a fully developed turbulent flow over a wavy wall. To assess the predictive capability of the VMS modeling framework, we compare its predictions against the results from direct numerical simulation (DNS) and large eddy simulation (LES) and, when available, against experimental measurements. We use C 1 quadratic B-spline basis functions to represent the smooth geometry of the sinusoidal lower wall and the solution variables. The Reynolds numbers of the flows considered are 6760 and 30,000 based on the bulk velocity and average channel height. The ratio of amplitude to wavelength (a/k) of the sinusoidal wavy surface is set to 0.05. The computational domain is 2k  1.05k  k in the streamwise, wall-normal and spanwise directions, respectively. For the Re = 6760 case, mean averaged quantities, including velocity and pressure profiles, and the separation/reattachment points in the recirculation region, are compared with DNS and experimental data. The turbulent kinetic energy and Reynolds stress are in good agreement with benchmark data. Coherent structures over the wavy wall are observed in isosurfaces of the Q-criterion and show similar features to those previously reported in the literature. Comparable accuracy to DNS solutions is obtained with at least one order of magnitude fewer degrees of freedom. For the Re = 30,000 case, good agreement was obtained for mean wall shear stress and velocity profiles compared with available LES results reported in the literature.