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

A new approach to Reynolds averaged turbulence modeling is proposed which has a computational cost comparable to two equation models but a predictive capability approaching that of Reynolds stress transport models. This approach isolates the crucial information contained within the Reynolds stress tensor, and solves transport equations only for a set of 'reduced' variables. In this work, Direct Numerical Simulation (DNS) data is used to analyze the nature of these newly proposed turbulence quantities and the source terms which appear in their respective transport equations. The physical relevance of these quantities is discussed and some initial modeling results for turbulent channel flow are presented.

https://ntrs.nasa.gov/api/citations/19970014654/downloads/19970014654.pdf

A new approach to turbulence modeling

Scattering of plane sound waves by a compressible vortex is investigated by direct computation of the two-dimensional Navier-Stokes equations. Nonreflecting boundary conditions are utilized, and their accuracy is established by comparing results on different sized domains. Scattered waves are directly measured from the computations. The resulting amplitude and directivity pattern of the scattered waves is discussed, and compared to various theoretical predictions. For compact vortices (zero circulation), the scattered waves directly computed are in good agreement with predictions based on an acoustic analogy. Strong scattering at about + or - 30 degrees from the direction of incident wave propagation is observed. Back scattering is an order of magnitude smaller than forward scattering. For vortices with finite circulation refraction of the sound by the mean flow field outside the vortex core is found to be important in determining the amplitude and directivity of the scattered wave field.

/pdf/scattering-of-sound-waves-by-a-compressible-vortex-2pojv64olb.pdf

Scattering of sound waves by a compressible vortex

A temporally developing turbulent round mixing layer at Mach number M(j) = 1.92 has been simulated, and results compared to a similar nearly incompressible simulation at M(j) = 0.4. The supersonic mixing layer growth rate, defined with a delta99 thickness parameter, is found to be suppressed relative to its low Mach number counterpart. The Reynolds stresses in the supersonic flow are also suppressed in a manner similar to experimental results. Flow visualization, 1D energy spectra, and two point correlations indicate that a realistic turbulent flow is being simulated. At a given thickness in the development of the two mixing layers, the mean velocity, mean temperature, Reynolds stresses, Reynolds shear stress anisotropy, pressure fluctuations, budgets of Reynolds stresses, and turbulent kinetic energy are compared. It is found that even though maximum turbulent Mach number reaches 0.43 in the compressible case, dilatational effects are minimal, and the total viscous dissipation is nearly the same fraction of the production of turbulent kinetic energy for both flows. (Author)

Direct simulation of a supersonic round turbulent shear layer

In this study, a wavelet-based method for extraction of clusters of inertial particles in turbulent flows is presented that is based on decomposing Eulerian particle-number-density fields into the sum of coherent (organized) and incoherent (disorganized) components. The coherent component is associated with the clusters and is extracted by filtering the wavelet-transformed particle-number-density field based on an energy threshold. The method is applied to direct numerical simulations of homogeneous-isotropic turbulence laden with small Lagrangian particles. The analysis shows that in regimes where the preferential concentration is important, the coherent component representing the clusters can be described by just 1.6% of the total number ofwavelet coefficients, thereby illustrating the sparsity of the particle-number-density field. On the other hand, the incoherent portion is visually structureless and much less correlated than the coherent one. An application of the method, motivated by particle-laden radiative-heat-transfer simulations, is illustrated in the form of a grid-adaptation algorithm that results in nonuniform meshes with fine and coarse elements near and away from particle clusters, respectively. In regimes where preferential concentration in clusters is important, the grid adaptation leads to a significant reduction of the number of control volumes by one to two orders of magnitude.

Extraction of coherent clusters and grid adaptation in particle-laden turbulence using wavelet filters

The objective of this work is the development of a practical computational model of a carbonate fuel cell stack. Previously published carbonate fuel cell models have focused more on the fundamental mechanisms of fuel cell operation than on evaluation of practical fuel cell product designs. Efficient development of a fuel cell product requires a predictive tool that couples all the important mechanisms with the capability to evaluate the performance of many design iterations quickly. The important mechanisms typically include three dimensional fluid flow, heat and mass transfer, gas-phase and surface chemistry, electrochemistry and structural mechanics. For large-scale fuel cells with applications in the power generation industry, thermal management is of significant interest for stack performance, reliability and life. Minimizing peak cell temperatures improves cell life and minimizing stack temperature gradients reduces stack thermal stress and improves cell performance. In this paper, the fuel cell model is presented along with experimental data validating its accuracy and predictive capabilities. The tool has proven to be a valuable asset for design evaluation and optimization of fuel cell stack designs at FuelCell Energy.

Parviz Moin

Papers

A new approach to turbulence modeling

Scattering of sound waves by a compressible vortex

Direct simulation of a supersonic round turbulent shear layer

Extraction of coherent clusters and grid adaptation in particle-laden turbulence using wavelet filters

Mathematical modeling of an internal-reforming, carbonate fuel cell stack