Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session L28: Turbulence: LES - ModelingCFD Turbulence
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Chair: James Brasseur, University of Colorado Boulder Room: 207 |
Monday, November 20, 2017 4:05PM - 4:18PM |
L28.00001: Return to Isotropy of a Filtered Turbulent Field Clark Pederson, Robert Moser Large-eddy simulations are often applied to anisotropic turbulence, including shear flows, expansions, and contractions. While much focus has been directed towards proper estimation of subfilter dissipation, this is an incomplete picture. Improving LES models requires understanding the time-evolution of anisotropic turbulence and the anisotropic interactions between resolved and unresolved scales. Here, we examine the behavior of anisotropic turbulence as it returns to isotropy. Homogeneous, isotropic turbulent fields were transformed into anisotropic fields by applying rotation, shear, and axisymmetric contractions. From this initial condition, direct numerical simulations of the return to isotropy were performed. The time-evolution of various measures of anisotropy are analyzed. Finally, the implications of these results for LES models are examined. [Preview Abstract] |
Monday, November 20, 2017 4:18PM - 4:31PM |
L28.00002: Filter and Grid Resolution in DG-LES Ling Miao, Shervin Sammak, Cyrus K. Madnia, Peyman Givi The discontinuous Galerkin (DG) methodology has proven very effective for large eddy simulation (LES) of turbulent flows. Two important parameters in DG-LES are the grid resolution ($h$) and the filter size ($\Delta$). In most previous work, the filter size is usually set to be proportional to the grid spacing. In this work, the DG method is combined with a subgrid scale (SGS) closure which is equivalent to that of the filtered density function (FDF). The resulting hybrid scheme is particularly attractive because a larger portion of the resolved energy is captured as the order of spectral approximation increases. Different cases for LES of a three-dimensional temporally developing mixing layer are appraised and a systematic parametric study is conducted to investigate the effects of grid resolution, the filter width size, and the order of spectral discretization. Comparative assessments are also made via the use of high resolution direct numerical simulation (DNS) data. [Preview Abstract] |
Monday, November 20, 2017 4:31PM - 4:44PM |
L28.00003: Implicit LES using Adaptive Filtering G. Sun, J.A. Domaradzki In ILES numerical dissipation prevents buildup of small scale energy in a manner similar to the explicit SGS models. If spectral methods are used the numerical dissipation is negligible but it can be introduced by applying a low-pass filter in the physical space (Domaradzki et al., 2002), resulting in an effective ILES. We provide a comprehensive analysis of the numerical dissipation produced by different filters in a turbulent channel flow simulated using a non-dissipative pseudo-spectral solver. The amount of numerical dissipation imparted by filtering can be easily manipulated by changing how often a filter is applied. We show that when the additional numerical dissipation is close to the SGS dissipation of an explicit LES the overall accuracy of ILES is also comparable, indicating that periodic filtering can replace explicit models. Following the work of Tantikul and Domaradzki (2010) a new method is proposed, which does not require any prior knowledge of a flow, to determine the filtering period adaptively. Once an optimal filtering period is found, the accuracy of ILES is significantly improved at low implementation complexity and computational cost. The method is general, performing well for different Reynolds numbers, filter shapes, and grid resolutions. [Preview Abstract] |
Monday, November 20, 2017 4:44PM - 4:57PM |
L28.00004: On the ``optimal" spatial distribution and directional anisotropy of the filter-width and grid-resolution in large eddy simulation Siavash Toosi, Johan Larsson The accuracy of an LES depends directly on the accuracy of the resolved part of the turbulence. The continuing increase in computational power enables the application of LES to increasingly complex flow problems for which the LES community lacks the experience of knowing what the ``optimal" or even an ``acceptable" grid (or equivalently filter-width distribution) is. The goal of this work is to introduce a systematic approach to finding the ``optimal" grid/filter-width distribution and their ``optimal" anisotropy. The method is tested first on the turbulent channel flow, mainly to see if it is able to predict the right anisotropy of the filter/grid, and then on the more complicated case of flow over a backward-facing step, to test its ability to predict the right distribution and anisotropy of the filter/grid simultaneously, hence leading to a converged solution. [Preview Abstract] |
Monday, November 20, 2017 4:57PM - 5:10PM |
L28.00005: The Role of Law-of-the-Wall and Roughness Scale in the Surface Stress Model for LES of the Rough-wall Boundary Layer. James Brasseur, Paulo Paes, Marcelo Chamecki Large-eddy simulation (LES) of the high Reynolds number rough-wall boundary layer requires both a subfilter-scale model for the unresolved inertial term and a ``surface stress model'' (SSM) for space-time local surface momentum flux. Standard SSMs assume proportionality between the local surface shear stress vector and the local resolved-scale velocity vector at the first grid level. Because the proportionality coefficient incorporates a surface roughness scale $z_{0}$ within a functional form taken from law-of-the-wall (LOTW), it is commonly stated that LOTW is ``assumed,'' and therefore ``forced'' on the LES. We show that this is not the case; the LOTW form is the ``drag law'' used to relate friction velocity to mean resolved velocity at the first grid level consistent with $z_{0}$ as the height where mean velocity vanishes. Whereas standard SSMs do not force LOTW on the prediction, we show that parameterized roughness does not match ``true'' $z_{0}$ when LOTW is not predicted, or does not exist. By extrapolating mean velocity, we show a serious mismatch between true $z_{0}$ and parameterized $z_{0}$ in the presence of a spurious ``overshoot'' in normalized mean velocity gradient. We shall discuss the source of the problem and its potential resolution. [Preview Abstract] |
Monday, November 20, 2017 5:10PM - 5:23PM |
L28.00006: Multiscale correlations in highly resolved Large Eddy Simulations Luca Biferale, Michele Buzzicotti, Moritz Linkmann Understanding multiscale turbulent statistics is one of the key challenges for many modern applied and fundamental problems in fluid dynamics. One of the main obstacles is the existence of anomalously strong non Gaussian fluctuations, which become more and more important with increasing Reynolds number. In order to assess the performance of LES models in reproducing these extreme events with reasonable accuracy, it is helpful to further understand the statistical properties of the coupling between the resolved and the subgrid scales. We present analytical and numerical results focussing on the multiscale correlations between the subgrid stress and the resolved velocity field obtained both from LES and filtered DNS data. Furthermore, a comparison is carried out between LES and DNS results concerning the scaling behaviour of higher-order structure functions using both Smagorinsky or self-similar Fourier sub-grid models. [Preview Abstract] |
Monday, November 20, 2017 5:23PM - 5:36PM |
L28.00007: Entropic Lattice Boltzmann: an implicit Large-Eddy Simulation? Guillaume Tauzin, Luca Biferale, Mauro Sbragaglia, Abhineet Gupta, Federico Toschi, Matthias Ehrhardt, Andreas Bartel We study the modeling of turbulence implied by the unconditionally stable Entropic Lattice Boltzmann Method (ELBM). We first focus on 2D homogeneous turbulence, for which we conduct numerical simulations for a wide range of relaxation times $\tau$. For these simulations, we analyze the effective viscosity obtained by numerically differentiating the kinetic energy and enstrophy balance equations averaged over sub-domains of the computational grid. We aim at understanding the behavior of the implied sub-grid scale model and verify a formulation previously derived using Chapman-Enskog expansion. These ELBM benchmark simulations are thus useful to understand the range of validity of ELBM as a turbulence model. Finally, we will discuss an extension of the previously obtained results to the 3D case. [Preview Abstract] |
Monday, November 20, 2017 5:36PM - 5:49PM |
L28.00008: Subgrid-scale models for large-eddy simulation of rotating turbulent channel flows Maurits H. Silvis, Hyunji Jane Bae, F. Xavier Trias, Mahdi Abkar, Parviz Moin, Roel Verstappen We aim to design subgrid-scale models for large-eddy simulation of rotating turbulent flows. Rotating turbulent flows form a challenging test case for large-eddy simulation due to the presence of the Coriolis force. The Coriolis force conserves the total kinetic energy while transporting it from small to large scales of motion, leading to the formation of large-scale anisotropic flow structures. The Coriolis force may also cause partial flow laminarization and the occurrence of turbulent bursts. Many subgrid-scale models for large-eddy simulation are, however, primarily designed to parametrize the dissipative nature of turbulent flows, ignoring the specific characteristics of transport processes. We, therefore, propose a new subgrid-scale model that, in addition to the usual dissipative eddy viscosity term, contains a nondissipative nonlinear model term designed to capture transport processes, such as those due to rotation. We show that the addition of this nonlinear model term leads to improved predictions of the energy spectra of rotating homogeneous isotropic turbulence as well as of the Reynolds stress anisotropy in spanwise-rotating plane-channel flows. [Preview Abstract] |
Monday, November 20, 2017 5:49PM - 6:02PM |
L28.00009: Predicting viscous-range velocity gradient dynamics in large-eddy simulations of turbulence Perry Johnson, Charles Meneveau The details of small-scale turbulence are not directly accessible in large-eddy simulations (LES), posing a modeling challenge because many important micro-physical processes depend strongly on the dynamics of turbulence in the viscous range. Here, we introduce a method for coupling existing stochastic models for the Lagrangian evolution of the velocity gradient tensor with LES to simulate unresolved dynamics. The proposed approach is implemented in LES of turbulent channel flow and detailed comparisons with DNS are carried out. An application to modeling the fate of deformable, small (sub-Kolmogorov) droplets at negligible Stokes number and low volume fraction with one-way coupling is carried out. These results illustrate the ability of the proposed model to predict the influence of small scale turbulence on droplet micro-physics in the context of LES. [Preview Abstract] |
Monday, November 20, 2017 6:02PM - 6:15PM |
L28.00010: Determining flow and simulation dependent properties of turbulent flows based on an analysis of the Lyapunov exponent. Jeffrey Labahn, Gabriel Nastac, Luca Magri, Matthias Ihme The Lyapunov exponent represents the rate of separation of a chaotic solution and can produce insight on flow and simulation dependent properties of turbulent flows. In the current study, an analysis of the Lyapunov exponent is used to investigate its ability to determine the dynamic content and predictability of large-eddy simulation. By comparing inert and reacting flows, it is shown that combustion increases the predictability of the turbulent simulation as a result of the dilatation and increased viscosity by heat release. The predictability time is found to scale with the integral time scale both in the reacting and inert jet flows. It is demonstrated that the global Lyapunov exponent can be utilized as a metric to determine the spatial extent of the computational domain required to capture the dynamic nature of the flow. In addition, an analysis of the local Lyapunov exponent demonstrates that this metric can also determine flow-dependent properties, such as regions of high turbulence and regions that are sensitive to small perturbations within the flow field. [Preview Abstract] |
Monday, November 20, 2017 6:15PM - 6:28PM |
L28.00011: The coupling of high-speed high resolution experimental data and LES through data assimilation techniques S. Harris, J.W. Labahn, J.H. Frank, M. Ihme Data assimilation techniques can be integrated with time-resolved numerical simulations to improve predictions of transient phenomena. In this study, optimal interpolation and nudging are employed for assimilating high-speed high-resolution measurements obtained for an inert jet into high-fidelity large-eddy simulations. This experimental data set was chosen as it provides both high spacial and temporal resolution for the three-component velocity field in the shear layer of the jet. Our first objective is to investigate the impact that data assimilation has on the resulting flow field for this inert jet. This is accomplished by determining the region influenced by the data assimilation and corresponding effect on the instantaneous flow structures. The second objective is to determine optimal weightings for two data assimilation techniques. The third objective is to investigate how the frequency at which the data is assimilated affects the overall predictions. [Preview Abstract] |
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