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 KO: Turbulence: Simulations IV |
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Chair: Heinz Pitsch, Stanford University Room: Salt Palace Convention Center 251 C |
Tuesday, November 20, 2007 8:00AM - 8:13AM |
KO.00001: Coherent Vortex Simulation (CVS) of compressible turbulent mixing layers using adaptive multiresolution methods Kai Schneider, Olivier Roussel, Marie Farge Coherent Vortex Simulation is based on the wavelet decomposition of the flow into coherent and incoherent components. An adaptive multiresolution method using second order finite volumes with explicit time discretization, a 2-4 Mac Cormack scheme, allows an efficient computation of the coherent flow on a dynamically adapted grid. Neglecting the influence of the incoherent background models turbulent dissipation. We present CVS computation of three dimensional compressible time developing mixing layer. We show the speed up in CPU time with respect to DNS and the obtained memory reduction thanks to dynamical octree data structures. The impact of different filtering strategies is discussed and it is found that isotropic wavelet thresholding of the Favre averaged gradient of the momentum yields the most effective results. [Preview Abstract] |
Tuesday, November 20, 2007 8:13AM - 8:26AM |
KO.00002: Intermittency and Anomalous Scaling in Synthetic MMLM Turbulence Carlos Rosales, Charles Meneveau A simple method, the Multiscale Minimal Lagrangian Map (MMLM) approach to generate synthetic turbulent vector fields was introduced (Rosales and Meneveau, Phys. Fluids, 2006). It was shown that the synthesized fields reproduce many statistical and geometric properties observed in real, isotropic, turbulence. We investigate if this procedure, which applies a minimal Lagrangian map to deform an initial Gaussian field, can produce also anomalous scaling in the inertial range. It is found that the advection Lagrangian Map time-scale is crucial in determining anomalous scaling properties. Using a short (CFL-like) time-scale, non-Gaussian statistics and realistic geometric features are reproduced at each scale, but anomalous exponents are not observed (i.e. we observe nearly K41 scaling). Conversely,if the appropriate K-41 inertial-range turnover time-scale is used, realistic anomalous scaling exponents are produced. Remarkably, the intermittency and multifractal nature of the energy dissipation is also found to be quite realistic. The properties of the pressure field derived from the MMLM velocity field are studied. The results shed new light on the minimal dynamical requirements for the generation of anomalous scaling and intermittency in turbulent flow. [Preview Abstract] |
Tuesday, November 20, 2007 8:26AM - 8:39AM |
KO.00003: Regularization techniques for backward--in--time evolutionary PDE problems Jonathan Gustafsson, Bartosz Protas Backward--in--time evolutionary PDE problems have applications in the recently--proposed retrograde data assimilation. We consider the terminal value problem for the Kuramoto--Sivashinsky equation (KSE) in a 1D periodic domain as our model system. The KSE, proposed as a model for interfacial and combustion phenomena, is also often adopted as a toy model for hydrodynamic turbulence because of its multiscale and chaotic dynamics. Backward--in--time problems are typical examples of ill-posed problem, where disturbances are amplified exponentially during the backward march. Regularization is required to solve such problems efficiently and we consider approaches in which the original ill--posed problem is approximated with a less ill--posed problem obtained by adding a regularization term to the original equation. While such techniques are relatively well--understood for linear problems, they less understood in the present nonlinear setting. We consider regularization terms with fixed magnitudes and also explore a novel approach in which these magnitudes are adapted dynamically using simple concepts from the Control Theory. [Preview Abstract] |
Tuesday, November 20, 2007 8:39AM - 8:52AM |
KO.00004: Length scales in multi-resolution (hybrid) turbulence simulations Sunil Lakshmipathy, Sharath S. Girimaji In direct numerical simulations (DNS) of turbulence, the smallest length scale in the flow is of the order of the Kolmogorov length scale $\eta $, which is determined from molecular viscosity and dissipation. The grid resolution should be of the order of $\eta $. In large eddy simulation (LES), the filter width determines the smallest scale of motion in the simulated field. But what is the smallest scale in hybrid or multi-resolution turbulence computation schemes? In many of these schemes, the filter is implicit, rather than explicit and the filter width is not known. This renders grid resolution studies very difficult, if not impossible in hybrid methods. For such schemes, we propose that the computational Kolmogorov scale which is determined using eddy viscosity and dissipation is the smallest scale of motion. We study the length-scale distribution in severally multi-resolution Partially-averaged Navier-Stokes (PANS) calculations. It is found that the smallest scale is indeed of the order of computational Kolmogorov scale and the length-scale distribution is strikingly similar to that in DNS computations. This finding paves the way for efficient and optimal utility of grid in multi-scale resolution computations. (\textit{This work was funded by Sandia Laboratories, Albuquerque, NM}) [Preview Abstract] |
Tuesday, November 20, 2007 8:52AM - 9:05AM |
KO.00005: Resolution Effects and Small-Scale Intermittency in Direct Numerical Simulations of Turbulence Diego A. Donzis, P.K. Yeung, K.R. Sreenivasan The Reynolds number attainable in direct numerical simulation (DNS) on a domain of given size depends on both the number of grid points and the degree to which the smallest scales are resolved, which for homogeneous turbulence is usually represented by the parameter $k_{max}\eta$, where $k_{max}$ is the highest wavenumber resolved and $\eta$ is Kolmogorov scale. Recent theoretical developments have pointed to the need for highly resolved simulations, especially for high-order statistics describing small-scale intermittency. We have carried out a systematic comparison of simulations up to $2048^3$ in size and $k_{max}\eta$ varied from about 1.5 to 11 with the Taylor-scale Reynolds number fixed at around 140 and 240. The results suggest that moments of dissipation and enstrophy fluctuations are accurate up to order 4 when $k_{max}\eta\ge 3$, even if an analytic range for velocity structure functions at the corresponding order predicted by recent theory is not attained. An analysis of departures from analytic behavior gives guidance for the smallest scales that need to be resolved, as a function of the desired statistics and the Reynolds number. However normalized statistics such as the ratio of different moments, and statistics in the inertial range, are less sensitive. [Preview Abstract] |
Tuesday, November 20, 2007 9:05AM - 9:18AM |
KO.00006: Resolution and Reynolds number effects on LES of turbulent round jet mixing O.S. Sun, L.K. Su In LES, the level of resolution is determined by the filter width, which implicitly defines the cut-off length scale. Smaller-scale motions are unresolved and represented by subgrid scale (SGS) models. In most cases, the SGS model and numerical grid are coupled, precluding truly grid-independent solutions. Understanding the effects of grid and resolution on simulation results is important for determining the utility and limitations of LES. Most LES resolution studies focus primarily on numerical aspects. Here, we explicitly study the effects of resolution on LES of passive scalar mixing in a spatially-developing, round, turbulent jet. With the same initial and boundary conditions, the number of grid points ranges from $80\times 32\times 16$ ($N_{r} \times N_{\theta} \times N_{\phi}$) to $176 \times 64 \times 32$. Additionally, the simulation is performed for a range of Reynolds numbers between $Re_D=5,000$ and $Re_D=10,000$. The subgrid stress term in the momentum equation is modeled with the dynamic Smagorinsky model, while the subgrid flux term in the scalar transport equation is closed using various models, including the dynamic eddy diffusivity and dynamic mixed models. Mean and higher-order statistics, including RMS, PDFs and spectra, for the resulting velocity and scalar fields are presented, allowing detailed comparisons of the subgrid scalar flux models. [Preview Abstract] |
Tuesday, November 20, 2007 9:18AM - 9:31AM |
KO.00007: High Order Conservative Finite Difference Scheme for Variable Density Low Mach Number Turbulent Flows Olivier Desjardins, Guillaume Blanquart, Guillaume Balarac, Heinz Pitsch The high order conservative finite difference scheme of Morinishi {\it et al.} (2004) is extended to simulate variable density flows in complex geometries with cylindrical or cartesian non-uniform meshes. The formulation discretely conserves mass, momentum, and kinetic energy in a periodic domain. In the presence of walls, boundary conditions that ensure primary conservation have been derived, while secondary conservation is shown to remain satisfactory. In the case of cylindrical coordinates, it is desirable to increase the order of accuracy of the convective term in the radial direction, where most gradients are often found. A straightforward centerline treatment is employed, leading to good accuracy as well as satisfactory robustness. A similar strategy is introduced to increase the order of accuracy of the viscous terms. This numerical methodology is used to simulate a series of canonical turbulent flows ranging from isotropic turbulence to a variable density round jet. It is shown that fourth order spatial accuracy improves significantly the quality of the results over second order in most of the cases. The additional gain in accuracy provided by higher than fourth order methods does not justify their increased cost. [Preview Abstract] |
Tuesday, November 20, 2007 9:31AM - 9:44AM |
KO.00008: Assessment of effective numerical bandwidth in simulations of compressible turbulence Johan Larsson, Ankit Bhagatwala, Santhosh Shankar, Sanjiva Lele, Parviz Moin The accuracy of different numerical methods suitable for compressible turbulence calculations including shockwaves is assessed. Methods studied include WENO, compact differencing/filtering, hyperviscosity regularization, and central differencing on split convective forms. Two test cases are used: the essentially incompressible Taylor-Green vortex and isotropic decaying turbulence with shocklets. Results on coarse grids are compared to finer grid reference solutions, and particular emphasis is placed on the effective bandwidth of the methods, i.e. the range of well resolved wavenumbers. The effect of the different forms of regularization on the effective bandwidth is quantified and discussed. [Preview Abstract] |
Tuesday, November 20, 2007 9:44AM - 9:57AM |
KO.00009: Large eddy simulations of in-cylinder turbulent flows. Araz Banaeizadeh, Asghar Afshari, Harold Schock, Farhad Jaberi A high-order numerical model is developed and tested for large eddy simulation (LES) of turbulent flows in internal combustion (IC) engines. In this model, the filtered compressible Navier-Stokes equations in curvilinear coordinate systems are solved via a generalized high-order multi-block compact differencing scheme. The LES model has been applied to three flow configurations: (1) a fixed poppet valve in a sudden expansion, (2) a simple piston-cylinder assembly with a stationary open valve and harmonically moving flat piston, (3) a laboratory single-cylinder engine with three moving intake and exhaust valves. The first flow configuration is considered for studying the flow around the valves in IC engines. The second flow configuration is closer to that in IC engines but is based on a single stationary intake/exhaust valve and relatively simple geometry. It is considered in this work for better understating of the effects of moving piston on the large-scale unsteady vortical fluid motions in the cylinder and for further validation of our LES model. The third flow configuration includes all the complexities involve in a realistic single-cylinder IC engine. The predicted flow statistics by LES show good comparison with the available experimental data. [Preview Abstract] |
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