Bulletin of the American Physical Society
60th Annual Meeting of the Divison of Fluid Dynamics
Volume 52, Number 12
Sunday–Tuesday, November 18–20, 2007; Salt Lake City, Utah
Session AO: Turbulence: Simulations I |
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Chair: Farhad Jaberi, Michigan State University Room: Salt Palace Convention Center 251 C |
Sunday, November 18, 2007 8:30AM - 8:43AM |
AO.00001: On the evaluation of the sub-filter scalar variance and dissipation rate in large eddy simulation Guillaume Balarac, Heinz Pitsch, Venkatramanan Raman In large-eddy simulations, the energy containing scales of the turbulence are resolved and the small scales have to be modeled. This is very important for flows with combustion, where the heat release typically correlates well with the rate of molecular mixing on the smallest scales. The mixture fraction describing mixing between fuel and oxidizer plays a central role in turbulent non-premixed combustion modeling. In particular, the sub-filter mixture fraction variance and the mixture fraction dissipation rate describe molecular mixing. Models for these quantities have been proposed in the past, but the performance of these models is often not of satisfactory accuracy given their importance for predicting the heat release. In the present work, a model based on a Taylor series expansion is proposed following the approach of Clark et al. [J. Fluid Mech., 1979]. The model is tested in an a priori study, and effects of expansion order and filter kernel are assessed. The results are discussed based on the notion of ``irreducible error'' recently introduced by Moreau et al. [Phys. Fluids, 2006]. The model is compared with the dynamic model and the results are analyzed to understand the validity of assumptions made in the dynamic procedure. Further numerical issues related to LES using implicit filtering are discussed. [Preview Abstract] |
Sunday, November 18, 2007 8:43AM - 8:56AM |
AO.00002: Generalized filtered boundary formulation for wall-bounded LES Henry Chang, Amitabh Bhattacharya, Robert Moser In previous work, a new treatment of wall boundary conditions for LES was developed, in which the wall was filtered along with the turbulence. In this filtered wall formulation, the computational domain is expanded beyond the wall, and a homogeneous or nearly homogeneous filter is applied in the expanded domain. In this case, the filtered Navier-Stokes equations include an extra term involving the wall tractions that arises from the filtering of the boundary. The wall tractions are modeled by requiring that no (or minimum) kinetic energy leaks out of the fluid domain. In the previous work, a global (Fourier) filter was employed, and the resulting LES simulations were in excellent agreement with filtered DNS data. In the current effort, we generalize this approach to local filters such as finite volume discretizations. This is important because finite volumes are a more generally applicable and more commonly used context for LES. Several generalizations are possible, the two most promising appear to be recasting in terms of a weak imposition of the boundary conditions, and recasting as an optimal estimation model. These approaches are evaluated in the context of an optimal LES formulation of the volumetric LES model, and compared to filtered DNS data. [Preview Abstract] |
Sunday, November 18, 2007 8:56AM - 9:09AM |
AO.00003: Characterization of Implicit LES Methods Andrew Aspden The broad range of time and length scales present in high Reynolds number turbulent flows is often prohibitively expensive for direct numerical simulations (DNS) to capture completely. Large eddy simulation (LES) attempts to circumvent this issue by filtering out the small scale motions in the flow, replacing their effects with a subgrid model. High-order finite-volume schemes can accurately capture the inviscid cascade of kinetic energy, and the inherent truncation error acts as an implicit subgrid model, forming a natural form of LES. However, the absence of a physical viscosity prohibits conventional characterization of these methods, specifically how kinetic energy is dissipated at the grid scale and how to define a relevant Reynolds number. Kolmogorov's 1941 papers achieve this characterization for real-world viscous fluids in terms of a universal equilibrium range determined uniquely by the rate of energy dissipation and physical viscosity. Analogously, this paper proposes than an ILES method can be characterized by a universal equilibrium range determined uniquely by the energy dissipation rate and computational cell width. Implicit LES simulations of maintained homogeneous isotropic turbulence are presented to support this proposal and highlight similarities and differences with real-world viscous fluids. Direct comparison with data from high resolution DNS calculations provides a basis for deriving an effective viscosity and an effective Kolmogorov length scale. [Preview Abstract] |
Sunday, November 18, 2007 9:09AM - 9:22AM |
AO.00004: A Finite--Volume Method for LES of Compressible Flows on Unstructured Grids Krishnan Mahesh, Noma Park A non--dissipative, cell--centered, finite--volume method for large--eddy simulation of compressible flows on unstructured grids is proposed. Approaches to flux reconstruction, shock-capturing, and subgrid scale modeling are discussed. The flux reconstruction seeks good modified wave-number behavior along with robustness. Least--square error minimization along with truncated Taylor--series expansion up to quadratic terms, is used for both convective and viscous terms. Singular value decomposition is used to solve problems caused by `bad' cells. Shock--capturing is performed by a corrector step which uses a characteristic--based filter. A local sensor is used to localize numerical dissipation. The dynamic Smagorinsky model is used for the subgrid terms.The grid--to--test filter width ratio is treated as being variable to achieve insensitivity of the dynamic constant to the shape and width of test filter. Results from various numerical tests ranging from isotropic turbulence to shock/turbulence interaction will be presented. [Preview Abstract] |
Sunday, November 18, 2007 9:22AM - 9:35AM |
AO.00005: A One-Equation Subgrid-Scale Estimation Model for Large-Eddy Simulation Noma Park, Krishnan Mahesh We discuss a SGS model which attempts to not only model the subgrid stress, but also determine the subgrid velocity. The model has no adjustable coefficents, does not require the use of finer grids, and does not require defiltering. The subgrid velocity is related to the resolved non--linear terms, and dissipation obtained from the dynamic Smagorinsky model, and subgrid kinetic energy obtained from its transport equation are imposed as constraints. The proposed model is validated through \emph{a priori} and \emph{a posteriori} tests on Burger's equation, decaying isotropic turbulence, and turbulent channel flow. [Preview Abstract] |
Sunday, November 18, 2007 9:35AM - 9:48AM |
AO.00006: LES of the turbulent wake of the Ahmed car model by Spectral Vanishing Viscosity approach Matthieu Minguez, Eric Serre, Richard Pasquetti One studies by Large Eddy Simulation the flow around a reference car model, the Ahmed body, for a Reynolds number of 768000 and a slant angle of 25$^{\circ}$, as defined in the ERCOFTAC benchmark. The developing tri- dimensionnal flow is fully turbulent, strongly unsteady and with large separation zones. The numerical model is based on a highly accurate method (spectral Chebyshev-Fourier approximation of the solution), including a penalization method to incorporate the obstacle and a domain decomposition technique in the streamwise direction. This LES is based on the Spectral Vanishing Viscosity stabilization method, allowing a damping of the highest frequencies of the solution. A particular attention is given to the treatment of the near wall region in to capture the crucial turbulence level of the slant mixing layer. The topology of the flow is captured with a partial separation zone on the slant and two strong contra- rotating trailing vortices coming off the slant edges. The mean velocity fields as the turbulence levels show a very good agreement with the experiments of reference. [Preview Abstract] |
Sunday, November 18, 2007 9:48AM - 10:01AM |
AO.00007: Large eddy simulation using a multidomain spectral method Kaustav Sengupta, Gustaaf Jacobs, Farzad Mashayek Spectral/hp element methods are now well established for direct simulation of turbulent flows in moderately complex configurations. However there have been limited attempts to extend them to the field of large-eddy simulation (LES). In this work we report the development of a LES procedure for compressible turbulent flows using a Chebyshev multidomain spectral method. The method uses a fully staggered grid on hexahedral sub-domains. Staggering of solution and fluxes leads to a fully conservative algorithm and imposes no smoothness requirement on grids across the sub-domain interfaces, thereby allowing use of unstructured hexahedrons. Geometric flexibility through unstructured domain decomposition and higher-order approximation within each sub-domain makes the method ideal for application to complex flow simulation in practical geometries. For LES, an extension of the dynamic model of Germano applicable to compressible flows is employed. The test filtering associated with the model, is accomplished through a sub-domain based Lagrange-interpolant projection technique. Results from decaying isotropic turbulence and channel flow simulations are presented. [Preview Abstract] |
Sunday, November 18, 2007 10:01AM - 10:14AM |
AO.00008: Applocation of DNS/LES spectral element methods in turbulent flow simulations Xu Zhang, Dan Stanescu, Jonathan W. Naughton This paper demonstrate applications of spectral element and large eddy simulation (LES) technique in turbulent flow simulations. The high-order discontinuous Galerkin(DG) spectral element method is applied to three-dimensional Navier- Stokes equations for spatial discretization. A local spectral discretization in terms of Legendre polynomials is used on each element of the hexahedra mesh, which allows for high-accurate simulations of turbulent flows. Discontinuities across the interfaces of the elements are resolved using a Riemann solver. An isoparametric representation of the geometry is implemented, with boundaries of the domain discretized to the same order of accuracy as the solution, and explicit low-storage Runge-Kutta methods are used for time integration. Large eddy simulation has proven to be a valuable technique for the calculation of turbulent flows. An element based filtering technique is used in conjunction with the standard Smagorinsky eddy viscosity model to estimate the effect of sub-grid scales stresses in this paper. The developed nonlinear model are also be added in the code. Simulations of compressible turbulent mixing layer and back-facing step are performed to evaluate the robust method. The results based on both DNS and large eddy simulations are presented in this paper. [Preview Abstract] |
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