Direct numerical simulations (DNS) of passive scalar mixing in isotropic turbulence is used to study, analyze, and, subsequently, model the role of small (subgrid) scales in the mixing process. In particular, we attem...
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Direct numerical simulations (DNS) of passive scalar mixing in isotropic turbulence is used to study, analyze, and, subsequently, model the role of small (subgrid) scales in the mixing process. In particular, we attempt to model the dissipation of the large-scale (supergrid) scalar fluctuations caused by the subgrid scales by decomposing it into two parts: (i) the effect due to the interaction among the subgrid scales, E(phi)(>);and, (ii) the effect due to interaction between the supergrid and the subgrid scales, E(phi)(><). Model comparison with DNS data shows good agreement. (C) 1996 American institute of Physics.
Unstructured grid flow solvers for the compressible Navier-Stokes equations are surveyed. Significant progress are made in the areas of spatial and temporal discretization and adaptative and parallel algorithms. Newly...
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Unstructured grid flow solvers for the compressible Navier-Stokes equations are surveyed. Significant progress are made in the areas of spatial and temporal discretization and adaptative and parallel algorithms. Newly developed field equation turbulence models seem to mesh nicely with the unstructured grid framework. Based on these developments, it is safe to say that unstructured grid technology is almost on par with structured grid technology, although encumbered with additional memory andcomputational costs.
The Crow constraint on time-dependent turbulence models is generalized to effects of second order in the strain rate. For simple shear flow, this generalization requires agreement of the time-dependent normal stress d...
The Crow constraint on time-dependent turbulence models is generalized to effects of second order in the strain rate. For simple shear flow, this generalization requires agreement of the time-dependent normal stress deviators with the short-time rapid distortion limit and with long-time equilibrium data. A model consistent with these requirements is proposed based on a perturbative solution of the equations of the direct interaction approximation for shear flow. It requires a decomposition of the Reynolds stress into tensor components which satisfy coupled linear relaxation equations. This approach to time-dependent turbulence modeling based on analytical theories offers an alternative to the standard ones, in which various hypotheses are introduced to close correlations in the exact transport equation for second-order single-point moments.
Modeled dissipation rate transport equations are often derived by invoking various hypotheses to close correlations in the corresponding exact equations. D. C. Leslie [Modern Developments in the Theory of Turbulence (...
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Modeled dissipation rate transport equations are often derived by invoking various hypotheses to close correlations in the corresponding exact equations. D. C. Leslie [Modern Developments in the Theory of Turbulence (Oxford University, Oxford, 1972)] suggested that these models might be derived instead from Kraichnan's [J. Fluid Mech. 47 (1971)] wavenumber space integrals for inertial range transport Stower. This suggestion is applied to the destruction terms in the dissipation rate equations for incompressible turbulence, buoyant turbulence, rotating incompressible turbulence, and rotating buoyant turbulence. Model constants like C-epsilon 2 are expressed as integrals;convergence of these integrals implies the absence of Reynolds number dependence ii the corresponding destruction term. The dependence of C-epsilon 2 on rotation rate emerges naturally;sensitization of the modeled dissipation rats equation to rotation is not required. A buoyancy related effect which is absent in the exact transport equation for temperature variance dissipation, but which sometimes improves computational predictions;also arises naturally, The time scale in the modeled transport equation depends on whether Bolgiano or Kulmogorov inertial range scaling applies. A simple extension af these methods leads to a preliminary dissipation rate equation for rotating buoyant turbulence. (C) 1996 American institute of Physics.
An agglomeration multigrid strategy is developed and implemented for the solution of three-dimensional steady viscous flows. The method enables convergence acceleration with minimal additional memory overhead and is c...
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An agglomeration multigrid strategy is developed and implemented for the solution of three-dimensional steady viscous flows. The method enables convergence acceleration with minimal additional memory overhead and is completely automated in that it can deal with grids of arbitrary construction. The multigrid technique is validated by comparing the delivered convergence rates with those obtained by a previously developed overset-mesh multigrid approach and by demonstrating grid-independent convergence rates for aerodynamic problems on very large grids. Prospects for further increases in multigrid efficiency for high-Reynolds-number viscous flows on highly stretched meshes are discussed.
The sound radiated by isotropic turbulence is computed using inertial range scaling expressions for the relevant two-time and two-point correlations. The result depends on whether the decay of Eulerian time correlatio...
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The sound radiated by isotropic turbulence is computed using inertial range scaling expressions for the relevant two-time and two-point correlations. The result depends on whether the decay of Eulerian time correlations is dominated by large scale sweeping or by local straining: the straining hypothesis leads to an expression for total acoustic power given originally by Proudman, whereas the sweeping hypothesis leads to a more recent result due to Lilley. (C) 1996 American institute of Physics.
An implicit method for the computation of unsteady Rows on unstructured grids is presented. Following a finite difference approximation for the time derivative, the resulting nonlinear system of equations is solved at...
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An implicit method for the computation of unsteady Rows on unstructured grids is presented. Following a finite difference approximation for the time derivative, the resulting nonlinear system of equations is solved at each time step by using an agglomeration multigrid procedure. The method allows for arbitrarily large time steps and is efficient in terms of computational effort and storage. Inviscid and viscous unsteady flows are computed to validate the procedure. The issue of the mass matrix which arises with vertex-centered finite volume schemes is addressed. The present formulation allows the mass matrix to be inverted indirectly. A mesh point movement and reconnection procedure is described that allows the grids to evolve with the motion of bodies. As an example of flow over bodies in relative motion, flow over a multi-element airfoil system undergoing deployment is computed. (C) 1996 Academic Press, Inc.
An enhanced diffusion-reaction system (DRS) is proposed as a statistical model for the evolution of multiple scalars undergoing mixing and reaction in an isotropic turbulence field. The DRS model is dose enough to the...
An enhanced diffusion-reaction system (DRS) is proposed as a statistical model for the evolution of multiple scalars undergoing mixing and reaction in an isotropic turbulence field. The DRS model is dose enough to the scalar equations in a reacting how that other statistical models of turbulent mixing, which decouple the velocity field from scalar mixing and reaction (e.g. mapping closure model, assumed-pdf models), cannot ditinguish the model (DRS) equations from the exact equations. Numerical simulations of the DRS equations are performed for three scalars evolving from non-premixed initial conditions. A simple one-step reversible reaction is considered. The data from the simulations are used to (i) study the effect of chemical conversion on the evolution of scalar statistics, and (ii) evaluate other models (mapping-closure model, assumed multivariate beta-pdf model) leading to inferences about engineering approaches to turbulent combustion modeling.
Parallel direct execution simulation is an important tool for performance and scalability analysis of large message passing parallel programs executing on top of a virtual computer. However, detailed simulation of mes...
Compressible mixing layers are analyzed using a dilatational covariances model based on a pseudo-sound constitutive relation. The calculations are used to evaluate the different physical phenomena affecting compressib...
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