Bulletin of the American Physical Society
61st Annual Meeting of the APS Division of Fluid Dynamics
Volume 53, Number 15
Sunday–Tuesday, November 23–25, 2008; San Antonio, Texas
Session AB: Turbulence Simulations I |
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Chair: Adonios Karpetis, Texas A&M University Room: 001B |
Sunday, November 23, 2008 8:00AM - 8:13AM |
AB.00001: Enhancement of artificial bulk viscosity method for predicting sound and aero-optics in transonic regimes Parviz Moin, Ali Mani, Johan Larsen Alternative formulations of Cook and Cabot shock capturing model (Journal of Computational Physics, 2005) is investigated using large-eddy simulation (LES) of transonic flow over a cylinder at Re=10,000 and M=0.85. The original model uses an artificial bulk viscosity to adaptively thicken shocks over a few grid cells, and uses high order Laplacian of strain rate tensor magnitude to trigger the bulk viscosity coefficient. This model is employed in a 6th order staggered LES code on a structured mesh and found to cause unnecessary dissipation of dilatation in flow regimes other than shock regions. In particular, artificial viscosity magnitude was found to be an order of magnitude higher than physical viscosity in sound propagating regimes. Using derivatives of dilatation field as model's coefficient, instead of strain rate magnitude, is shown to significantly remove the unnecessary dissipation from the leading portion of the cylinder. Furthermore, employing a shock sensor switch to turn-off the model in the far-field is found to enhance prediction of flow generated noise. The impact of artificial dissipation on aero-optics will also be discussed. [Preview Abstract] |
Sunday, November 23, 2008 8:13AM - 8:26AM |
AB.00002: Grid Independent Large-Eddy Simulation using Explicit Filtering Sanjeeb Bose, Parviz Moin, Donghyun You The governing equations for large-eddy simulation are derived from the application of a low-pass filter to the Navier-Stokes equations. It is often assumed that discrete operations performed on a particular grid act as an implicit filter causing results to be sensitive to the mesh resolution. Alternatively, explicit filtering separates the filtering operation from the underlying mesh distribution, thereby eliminating grid sensitivities. We investigate the use of explicit filtering in large-eddy simulation in order to obtain numerical solutions that are grid independent and are not influenced by numerical errors. The extension and implementation of high-order, commuting filters (Vasilyev \emph{et. al}, \emph{J. Comp. Phys.}, 1998) in the context of wall-bounded flows will be discussed. The convergence of simulations using a fixed filter width with varying mesh resolutions to a \emph{true} LES solution will be analyzed, with particular attention to the performance of the chosen subgrid scale model. Results from planar channel flow simulations using LES with explicit filtering at $Re_{\tau} = 180$ and $395$ will be presented. [Preview Abstract] |
Sunday, November 23, 2008 8:26AM - 8:39AM |
AB.00003: Minimization of the Germano--identity error in the dynamic Smagorinsky model Krishnan Mahesh, Noma Park We revisit the Germano--identity error in the dynamic modeling procedure in the sense that the current modeling procedure to obtain the dynamic coefficient may not truly minimize the error in the mean and global sense. A ``corrector step'' to the conventional dynamic Smagorinsky model is proposed to obtain a corrected eddy viscosity which further reduces the error. The change in resolved velocity due to the coefficient variation as well as nonlocal nature of the filter and flow unsteadiness is accounted for by a simplified suboptimal control formalism without resorting to the adjoint equations. The cost function chosen is the Germano--identity error integrated over the entire computational volume and pathline. In order to determine corrected eddy viscosity, the Fr\'echet derivative of the cost function is directly evaluated by a finite--differencing formula in an efficient manner. The proposed model is applied to isotropic turbulence and turbulent channel flow at various Reynolds numbers and resolutions to obtain noticeable reduction in the Germano--identity error and significantly improved flow statistics. [Preview Abstract] |
Sunday, November 23, 2008 8:39AM - 8:52AM |
AB.00004: Large eddy simulation of an urban-type boundary layer Sriharsha Kandala, Dietmar Rempfer, Paul Fischer, Candace Wark Large eddy simulations based on the scale-dependent Lagrangian dynamic model (Meneveau, Phys. Fluids (17), 2005) allow for scale dependence. This is particularly relevant when the filter scale approaches the upper limits of the inertial range, which is typically the case when modeling urban boundary layers. Scale-dependent Lagrangian dynamic models are also known to exhibit favorable dissipation characteristics. In this talk we present the results of a numerical simulation of an urban-type boundary layer described in the talk by Monnier, Neiswander, Wark {\&} Rempfer at the 2007 APS/DFD meeting. The domain consists of 4 rows of 3 cuboids placed in a wind tunnel. The inlet velocity conditions are obtained from hot-wire measurements upstream of the blocks. The flow is simulated using the dynamic Smagorinsky model available with the commercial software FLUENT and the scale-dependent Lagrangian dynamic model available with spectral element code nek5000. These results are compared with the PIV data obtained from the experiments. [Preview Abstract] |
Sunday, November 23, 2008 8:52AM - 9:05AM |
AB.00005: A dynamic wall model constrained by external Reynolds stress Noma Park, Aman Verma, Krishnan Mahesh We propose a new approach of incorporating RANS constraints into SGS models, and discuss the corresponding dynamic wall model. Unlike conventional approaches, given Reynolds stresses are not imposed as the solution, but are used as constraints on the mean SGS stress so that the given Reynolds stress closely matches the computed stress only in the mean sense. Also, since LES in general outperforms RANS even at coarse resolution except very near the wall, RANS constraints are limited to the points where the LES solution is expected to be erroneous. We use the Germano--identity error as an indicator of LES quality so that the RANS constraints are activated only where the Germano--identity error exceeds a certain bound. The proposed model is applied to LES of turbulent channel flow at various Reynolds numbers and grid resolutions to obtain significant improvement over the dynamic Smagorinsky model, especially at coarse resolutions. [Preview Abstract] |
Sunday, November 23, 2008 9:05AM - 9:18AM |
AB.00006: Theoretically Based Optimal LES Nicholas Malaya, Amitabh Bhattacharya, Robert Moser A major obstacle to the practical application of optimal LES modeling has been the use of DNS statistical data for the multi-point velocity correlations that are needed in the formulation. Here we show that for high Reynolds number turbulence for which small-scale isotropy is valid, the Kolmogorov inertial range theory, the quasi-normal approximation and a model form for the three-point third-order velocity correlation are sufficient to define the optimal LES model. Only two flow-dependent constants remain, the velocity variance $u^2$ and the dissipation $\epsilon$. These constants are determined via a dynamic procedure in which the statistics of the unfiltered turbulence are reconstructed from the LES statistics. These theoretical optimal models are applied to a finite-volume LES formulation of isotropic turbulence, and the resulting LES perform very well. In these finite-volume LES, an optimal model for the convective momentum flux through finite volume cell boundaries replaces the normal finite volume schemes. The model can thus be considered to be a finite volume LES operator. The spectral properties of the LES operators have been compared to those of standard finite volume schemes, and there are several surprising differences, which have important implications for the formulation of LES models. [Preview Abstract] |
Sunday, November 23, 2008 9:18AM - 9:31AM |
AB.00007: Large-Eddy Simulations of turbulent hydrodynamic and magnetohydrodynamic channel flows Axelle Vire, Dmitry Krasnov, Bernard Knaepen, Thomas Boeck We perform Large-Eddy Simulations of incompressible hydrodynamic and magnetohydrodynamic channel flows at low magnetic Reynolds numbers (i.e. in the framework of the quasi-static approximation where the Lorentz force is treated as an explicit contribution to the momentum balance). The computations are performed using a pseudospectral and a second-order collocated finite volume method. Two eddy-viscosity type models are compared for different mesh resolutions: the dynamic Smagorinsky (DSM) and the Wall-Adapting Local Eddy-viscosity (WALE) model. We examine in detail the contributions to the kinetic energy budget of each term appearing in the Navier-Stokes equations. In particular, the results show that the subgrid-scale dissipation measured in the finite volume simulations is systematically much lower than in the spectral ones. [Preview Abstract] |
Sunday, November 23, 2008 9:31AM - 9:44AM |
AB.00008: On discretization errors and subgrid scale model implementations in Large Eddy Simulations Bernard Knaepen, Axelle Vir\'e We analyze the impact of discretization errors on the performance of the Smagorinsky model in Large Eddy Simulations (LES). To avoid difficulties related to solid boundaries, we focus on decaying homogeneous turbulence. It is shown that two numerical implementations of the model in the same finite volume code lead to significantly different results in terms of kinetic energy decay, time evolutions of the viscous dissipation and kinetic energy spectra. In comparison with spectral LES results, excellent predictions are however obtained with a novel formulation of the model derived from the discrete Navier-Stokes equations. We also highlight the effect of discretization errors on the measurement of physical quantities that involve scales close to the grid resolution. [Preview Abstract] |
Sunday, November 23, 2008 9:44AM - 9:57AM |
AB.00009: Rapid Spin-Up of MHD Turbulence Wouter Bos, Salah Neffaa, Kai Schneider Direct numerical simulations of two-dimensional decaying MHD turbulence in bounded domains show the rapid generation of angular momentum in non-axisymmetric geometries. It is found that magnetic fluctuations enhance this mechanism. The subsequent generation of a magnetic angular momentum or angular field is due to the relaxation of the flow towards an aligned state. For axi-symmetric geometries the generation of angular momentum is absent, nevertheless a weak angular field can be observed. The derived evolution equations for both angular momentum and angular field yield possible explanations for the observed behaviour. [Preview Abstract] |
Sunday, November 23, 2008 9:57AM - 10:10AM |
AB.00010: Magnetic Structures Produced by the Small-Scale Dynamo Louise Wilkin, Carlo Barenghi, Anvar Shukurov Small-scale dynamo action has been obtained for a flow previously used to model fluid turbulence, where the sensitivity of the magnetic field parameters to the kinetic energy spectrum can be explored. We apply quantitative morphology diagnostics, based on the Minkowski functionals, to magnetic fields produced by the kinematic small-scale dynamo to show that magnetic structures are predominantly filamentary rather than sheetlike. Our results suggest that the thickness, width, and length of the structures scale differently with magnetic Reynolds number as $R_m^{-2/(1-s)}$ and $R_m^{-0.55}$ for the former two, whereas the latter is independent of $R_m$, with $s$ the slope of the energy spectrum. [Preview Abstract] |
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