Giuliano De Stefano

Sharp cutoff versus smooth filtering in large eddy simulation

The large eddy simulation (LES) equations of turbulent flows are formally derived by applying a low-pass filter to the Navier–Stokes equations. As a result the subgrid-scale (SGS) stress tensor strongly depends on the assumed filter shape, which causes a SGS model to be filter dependent. In particular, depending on the choice of the filter the corresponding SGS model should satisfy very different requirements in terms of large scale dynamics and kinetic energy budget. This paper is an attempt to systematically study the effect of the filter shape on the subgrid scale model and its subsequent effect on LES. For the sake of simplicity, we consider numerical simulation of a one-dimensional homogeneous flow, governed by the viscous Burgers equation. Large eddy simulations of the solution of the Burgers problem are performed using subgrid scale models obtained by filtering data from direct numerical simulations. Diagnostics include temporal evolution of energy and dissipation as well as energy spectra. It is d...

Localized dynamic kinetic-energy-based models for stochastic coherent adaptive large eddy simulation

Stochastic coherent adaptive large eddy simulation (SCALES) is an extension of the large eddy simulation approach in which a wavelet filter-based dynamic grid adaptation strategy is employed to solve for the most “energetic” coherent structures in a turbulent field while modeling the effect of the less energetic background flow. In order to take full advantage of the ability of the method in simulating complex flows, the use of localized subgrid-scale models is required. In this paper, new local dynamic one-equation subgrid-scale models based on both eddy-viscosity and non-eddy-viscosity assumptions are proposed for SCALES. The models involve the definition of an additional field variable that represents the kinetic energy associated with the unresolved motions. This way, the energy transfer between resolved and residual flow structures is explicitly taken into account by the modeling procedure without an equilibrium assumption, as in the classical Smagorinsky approach. The wavelet-filtered incompressible...

Wavelet-based adaptive large-eddy simulation with explicit filtering

Stochastic coherent adaptive large-eddy simulation is a novel approach to the numerical simulation of turbulence, where the coherent energetic eddies are solved for, while modeling the influence of the less energetic coherent/incoherent background flow. The formal separation between resolved and unresolved field is obtained by wavelet threshold filtering that is inherent to the adaptive wavelet collocation numerical method. A new explicit wavelet filtering strategy is introduced and tested, by considering two different filtering levels: the physical level, which controls the turbulence model, and the numerical level that is responsible for the accuracy of numerical simulations. The theoretical basis for wavelet-based adaptive large-eddy simulation with explicit filtering and consistent dynamic modeling is given. Numerical experiments are presented for unsteady homogeneous turbulence, demonstrating the existence of grid-independent solutions.

Lagrangian dynamic SGS model for stochastic coherent adaptive large eddy simulation

Stochastic coherent adaptive large eddy simulation (SCALES) is an extension of large eddy simulation that uses a wavelet filter-based dynamic grid adaptation strategy to solve for the most energetic coherent structures in a turbulent flow field, while modeling the effect of the less energetic ones. A localized dynamic subgrid scale model is needed to fully exploit the ability of the method to track coherent structures. In this paper, new local Lagrangian models based on a modified Germano dynamic procedure, redefined in terms of wavelet thresholding filters, are proposed. These models extend the original path-line formulation of Meneveau et al. [J. Fluid Mech. 319 (1996)] in two ways: as Lagrangian path-line diffusive and Lagrangian path-tube averaging procedures. The proposed models are tested for freely decaying homogeneous turbulence with initial Re λ = 72. It is shown that the SCALES results, obtained with less than 0.4% of the total non-adaptive nodes required for a DNS with the same wavelet solver, closely match reference DNS data. In contrast to classical LES, this agreement holds not only for large scale global statistical quantities, but also for energy and, more importantly, enstrophy spectra up to the dissipative wavenumber range.

Spatially Variable Thresholding for Stochastic Coherent Adaptive LES

The properties of wavelet transform, viz. the ability to identify and efficiently represent temporal/spatial coherent flow structures, self-adaptiveness, and de-noising, have made them attractive candidates for constructing multi-resolution variable fidelity schemes for simulations of turbulence (Schneider and Vasilyev, 2010). Stochastic Coherent Adaptive Large Eddy Simulation (SCALES) (Goldstein and Vasilyev, 2004) is the most recent wavelet-based methodology for numerical simulations of turbulent flows that resolves energy containing turbulent motions using wavelet multi-resolution decomposition and self-adaptivity. In this technique, the extraction of the most energetic structures is achieved using wavelet thresholding filter with a priori prescribed threshold level.

Stochastic Coherent Adaptive LES of Forced Isotropic Turbulence

The stochastic coherent adaptive large eddy simulation (SCALES) method [3] exploits a wavelet thresholding filter- ased dynamic grid adaptation strategy to solve for the energetic “coherent” eddies in a turbulent flow field. The effect of the residual less energetic flow structures is modeled by supplying the simulation with a suitable subgrid-scale (SGS) model. The SCALES approach was successfully applied to the simulation of decaying homogeneous turbulence (e.g., see [2, 10]). Here, the wavelet-based approach is applied to statistically steady turbulence by considering linearly forced homogeneous turbulence at moderate Reynolds-number.

Towards Lagrangian dynamic SGS model for SCALES of isotropic turbulence

1 Dipartimento di Ingegneria Aerospaziale e Meccanica, Seconda Universita di Napoli, 81031 Aversa, Italy (giuliano.destefano@unina2.it) 2 Department of Mechanical Engineering, University of Colorado at Boulder, 427 UCB, Boulder CO, USA (Daniel.E.Goldstein@colorado.edu, Oleg.Vasilyev@colorado.edu) 3 Department of Mathematics & Statistics, McMaster University, Hamilton, ON, Canada L8S 4K1 (kevlahan@mcmaster.ca)

A CFD Study of Wind Loads on High Aspect Ratio Ground-Mounted Solar Panels

Computational fluid dynamics is used to study the wind loads on a high aspect ratio ground-mounted solar panel. Reynolds-averaged Navier-Stokes simulations are performed using a commercial finite volume-based code with two different numerical approaches. First, the entire panel is directly simulated in a three-dimensional domain. Then, a small portion of the panel is considered, by imposing periodic boundary conditions in the spanwise homogeneous direction. The comparison shows a good match between the results obtained with the two different models, in terms of pressure coefficient and aerodynamic loads. The main consequence is a considerable reduction of the computational costs when using the reduced model.

Stochastic coherent adaptive LES with time-dependent thresholding

With the recent development of wavelet-based techniques for computational fluid dynamics, adaptive numerical simulations of turbulent flows have become feasible (Schneider and Vasilyev, 2010). Adaptive wavelet methods are based on wavelet threshold filtering that makes it possible to separate coherent energetic eddies, which are numerically simulated, from residual background flow structures that are filtered out. By varying the filter thresholding level different approaches with different fidelity are obtained: from the highly accurate wavelet-based direct numerical simulation (WDNS) that does not involve any model to the stochastic coherent adaptive large eddy simulation (SCALES) that needs a closure modeling procedure, i.e. (De Stefano et al., 2008).

A new development of the dynamic procedure for the integral-based implicit filtering in large-eddy simulation

This paper is focused on the role of integral-based Finite Volume (FV) discretizations in Large Eddy Simulation of turbulence. The integral-based form implicitly induces the top-hat filtering on the balanced variable. This leads us to rewrite also a different decomposition of the fluxes. As a consequence, the development of a new Germano identity can be achieved having some advantages over the classical differential-based form. However, the dynamic procedure requires an explicit test-filtering on a computational grid that, to be optimal, requires an evaluation of the shape of the numerical filter induced by the FV-based discretization. Therefore, the goal of this paper is the theoretical study of the effective filter shape induced by some 3D Finite Volume reconstructions. The induced shape and width are analyzed by means of a modified wavenumber-like analysis that is applied in the 3D Fourier space. Some schemes are considered and the differences in terms of either velocity or flux interpolations on either staggered or non-staggered grids are derived and analyzed.