Parviz Moin

Bio: Parviz Moin is an academic researcher from Stanford University. The author has contributed to research in topics: Turbulence & Large eddy simulation. The author has an hindex of 116, co-authored 473 publications receiving 60521 citations. Previous affiliations of Parviz Moin include Center for Turbulence Research & Ames Research Center.

Large eddy simulation of multi-phase turbulent flows in realistic combustors

Abstract: An overview is provided of the recent advances in performing LES in complex combustor geometries using a non-dissipative, massively parallel, unstructured grid solver (CDP). A new numerical algorithm that is discretely energy conserving on hybrid unstructured grids is developed. This allows simulations at high Reynolds numbers without the use of numerical dissipation. This LES methodology has been extended to include advanced turbulent combustion models and a Lagrangian particle tracking methodology for simulating turbulent multiphase flows. Results for single and multi-phase flow simulations in realistic Pratt & Whitney (P&W) combustor geometries are discussed.

Computational Study of Aero-Optical Distortion by Turbulent Wake

Abstract: Optical aberrations induced by turbulent flows are a serious concern in airborne communication and imaging systems. In these applications an optical beam is required to be transmitted through a relatively long distance, over which the quality of the beam can degrade due to variations of the index of refraction along its path. For air and many fluids, the refractive index is linearly related to the density of the fluid through the Gladstone-Dale relation (see Wolf & Zizzis 1978), and therefore density fluctuations due to flow turbulence are the root cause of optical aberrations. An airborne optical beam generally encounters two distinct turbulent flow regimes: the turbulence in the vicinity of the aperture produced by the presence of solid boundaries, and atmospheric turbulence. Aero-optics is the study of optical distortions by the near-field turbulent flows, typically involving turbulent boundary layers, mixing layers, and wakes (see Gilbert 1982). The depth of the aberrating flowfield is usually smaller than or comparable to the projecting (or imaging) aperture. When an initially planar optical wavefront passes a compressible flow, different parts of the wavefront experience different density in the medium and hence have different propagation speeds. Consequently the wavefront becomes deformed. A small initial deformation of the wavefront can lead to large errors on a distant target. The consequences of such deformations include optical beam deflection (bore-sight error) and jitter, beam spread, and loss of intensity. Wavefront distortions can also cause reductions of resolution, contrast, effective range, and sensitivity for airborne electro-optical sensors and imaging systems (Jones & Bender 2001). Research in the area of turbulent distortions of optical waves can be traced back to the 1950s and 1960s (see, for example, Chernov 1960; Tatarski 1961) when the scattering of acoustic and electromagnetic waves due to random fluctuations of refractive index were studied, mostly in the context of atmosphere propagation. Most of the early studies are based on statistical analysis with simplifying assumptions such as homogeneous and isotropic turbulence, and therefore are not directly applicable in realistic aero-optical flowfields. Sutton (1969) characterized different regimes based on optical and flow parameters for the case of homogeneous and isotropic turbulence and developed statistical models to predict far-field optical aberrations. It was in the late 1980s when aero-optics in the modern sense, i.e., the study of optical distortions due to near-aperture turbulence, came into consideration. Many experimental studies have been performed to develop high-speed wavefront measurement tools (e.g., Jumper & Fitzgerald 2001; Cheung & Jumper 2004), study the refractive index structures (e.g., Catrakis & Aguirre 2004; Dimotakis et al. 2001; Fitzgerald & Jumper 2004), develop distortion scaling laws (e.g., Gordeyev et al. 2003), and devise control techniques to suppress or modify optically important turbulence structures (e.g., Gordeyev et al. 2004; Sinha et al. 2004). Despite advances in wavefront sensor technology, significant limitations

Advances in large eddy simulation methodology for complex flows

Abstract: A review is provided of the recent advances in the derivation of the constitutive equations for large eddy simulation, subgrid scale modeling, wall modeling and applications of LES to turbulent combustion. The majority of the paper focuses on a review of two numerical methods for LES in complex geometry: the immersed boundary method and an unstructured mesh scheme. The latter scheme is applied to LES of a sector of a combustor of an operational gas turbine engine.

Computation of turbulent flows over backward-facing step

Abstract: A numerical method for computing incompressible turbulent flows is presented. The method is tested by calculating laminar recirculating flows and is applied in conjunction with a modified Kappa-epsilon model to compute the flow over a backward-facing step. In the laminar regime, the computational results are in good agreement with the experimental data. The turbulent flow study shows that the reattachment length is underpredicted by the standard Kappa-epsilon model. The addition of a term to the standard model that accounts for the effects of rotation on turbulent flow improves the results in the recirculation region and increases the computed reattachment length.

A dynamic subgrid‐scale eddy viscosity model

Abstract: 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.

Lattice boltzmann method for fluid flows

Abstract: 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.

On the identification of a vortex

Abstract: 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.