One major drawback of the eddy viscosity subgrid‐scale stress models used in large‐eddy simulations is their inability to represent correctly with a single universal constant different turbulent fields in rotating or sheared flows, near solid walls, or in transitional regimes. In the present work a new eddy viscosity model is presented which alleviates many of these drawbacks. The model coefficient is computed dynamically as the calculation progresses rather than input a priori. The model is based on an algebraic identity between the subgrid‐scale stresses at two different filtered levels and the resolved turbulent stresses. The subgrid‐scale stresses obtained using the proposed model vanish in laminar flow and at a solid boundary, and have the correct asymptotic behavior in the near‐wall region of a turbulent boundary layer. The results of large‐eddy simulations of transitional and turbulent channel flow that use the proposed model are in good agreement with the direct simulation data.

A dynamic subgrid‐scale eddy viscosity model

We present an overview of the lattice Boltzmann method (LBM), a parallel and efficient algorithm for simulating single-phase and multiphase fluid flows and for incorporating additional physical complexities. The LBM is especially useful for modeling complicated boundary conditions and multiphase interfaces. Recent extensions of this method are described, including simulations of fluid turbulence, suspension flows, and reaction diffusion systems.

Lattice boltzmann method for fluid flows

Considerable confusion surrounds the longstanding question of what constitutes a vortex, especially in a turbulent flow. This question, frequently misunderstood as academic, has recently acquired particular significance since coherent structures (CS) in turbulent flows are now commonly regarded as vortices. An objective definition of a vortex should permit the use of vortex dynamics concepts to educe CS, to explain formation and evolutionary dynamics of CS, to explore the role of CS in turbulence phenomena, and to develop viable turbulence models and control strategies for turbulence phenomena. We propose a definition of a vortex in an incompressible flow in terms of the eigenvalues of the symmetric tensor ${\bm {\cal S}}^2 + {\bm \Omega}^2$ are respectively the symmetric and antisymmetric parts of the velocity gradient tensor ${\bm \Delta}{\bm u}$. This definition captures the pressure minimum in a plane perpendicular to the vortex axis at high Reynolds numbers, and also accurately defines vortex cores at low Reynolds numbers, unlike a pressure-minimum criterion. We compare our definition with prior schemes/definitions using exact and numerical solutions of the Euler and Navier–Stokes equations for a variety of laminar and turbulent flows. In contrast to definitions based on the positive second invariant of ${\bm \Delta}{\bm u}$ or the complex eigenvalues of ${\bm \Delta}{\bm u}$, our definition accurately identifies the vortex core in flows where the vortex geometry is intuitively clear.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0022112095000462

On the identification of a vortex

http://home.eps.hw.ac.uk/~ab226/tmp/implicit/lele_92.pdf

Compact finite difference schemes with spectral-like resolution

/pdf/turbulence-and-the-dynamics-of-coherent-structures-i-1o6o6m60fm.pdf

Turbulence and the dynamics of coherent structures. I. Coherent structures

We conducted WMLES to examine the performance of a simple and widely used ODE-based equilibrium wall model in a spatially-developing 3D TBL inside a bent square duct (Schwarz and Bradshaw 1994) and 3D separated flows behind a skewed bump (Ching et al. 2018a,b; Ching and Eaton 2019). From the square duct simulation, the mean velocity profiles and crossflow angles in the outer region were predicted with high accuracy for all the considered mesh resolutions. Some disagreement was observed in the crossflow angles in the bend region where the non-equilibrium effect is most significant. Also, the simulation for the wall-mounted skewed bump showed that this simple ODE-based equilibrium wall model along with an adequate grid resolution around the 3D separation point resulted in reasonable predictions of 3D separating and reattaching flows, including mean velocity distributions, separation bubbles, and vortex structures in the bump wake.

Wall-modeled large-eddy simulation of non-equilibrium turbulent boundary layers

This study addresses the dynamics of kinetic-energy backscatter in the context of large-eddy simulations (LES) of turbulent chemically-reacting compressible flows. As a case study, a-priori analyses of direct numerical simulations (DNS) of reacting and inert supersonic, time-developing, hydrogen-air turbulent mixing layers with complex chemistry and multi-component diffusion are conducted herein to examine the effects of compressibility and combustion on subgrid-scale (SGS) backscatter of kinetic energy. General formulations of SGS backscatter with dilatation are provided, including an LESbased energy-transfer diagram that illustrates the conversion dynamics. Lastly, influences of SGS backscatter on the Boussinesq eddy viscosity are analyzed. 2. Background The energy-cascade hypothesis predicts that turbulent kinetic energy is generated at the largest scales in a flow and then transferred to progressively smaller and smaller scales until it is dissipated by molecular viscosity. This energy cascade typically holds in a statistically-averaged sense, but it does not always describe the local behavior of a turbulent flow. The turbulent dissipation associated with the smallest, viscous scales, is actually the difference between two energy fluxes, namely, the forwardscatter, corresponding to the classical energy cascade, and the backscatter, a reversal of this process in which energy is transferred from the small scales back to the large scales (Lesieur & �

/pdf/backscatter-of-turbulent-kinetic-energy-in-chemically-390dierafn.pdf

Backscatter of turbulent kinetic energy in chemically-reacting compressible flows

Large-Eddy Simulation of Reacting Turbulent Flows in

: This report contains the 2001 Annual Progress Reports of the postdoctoral fellows and visiting scholars of the Center for Turbulence Research. In 2001 CTR sponsored 15 resident Postdoctoral Fellows, 7 Research Associates and 3 Senior Research Fellows, hosted 7 visiting scholars and many shorter-term visitors, and supported 6 doctoral students. Most of the doctoral students engaged in turbulence research at CTR are supported by the U.S Office of Naval Research or the Air Force Office of Scientific Research. CTR is closely associated with the Stanford multidepartmental Center for Integrated Turbulence Simulations (CITS), funded by the Department of Energy's Accelerated Strategic Computing Initiative (ASCI). The aim of the CITS program is to compute the complete flow through an aircraft gas turbine engine. The combustion chamber is the most critical region: it is typically of very complicated geometrical shape, and the fuel is introduced as a spray of droplets which must disperse and vaporize before burning. The first paper in this volume describes large-eddy simulation of the air flow in a real aircraft gas turbine combustor, and there are several papers relevant to the crucial problem of spray combustion.

Annual Research Briefs - 2001 (Center for Turbulence Research)

/pdf/evaluation-of-drag-forcing-models-for-vertical-axis-wind-2aei54s3iq.pdf

Parviz Moin

Papers

Wall-modeled large-eddy simulation of non-equilibrium turbulent boundary layers

Backscatter of turbulent kinetic energy in chemically-reacting compressible flows

Large-Eddy Simulation of Reacting Turbulent Flows in

Annual Research Briefs - 2001 (Center for Turbulence Research)

Evaluation of drag forcing models for vertical axis wind turbine farms