Uriel Goldberg

Interfacing Statistical Turbulence Closures with Large-Eddy Simulation

Progress toward a general purpose hybrid Reynolds-averaged Navier-Stokes (RANS)/large-eddy simulation (LETS) framework is described, in which large-scale, statistically represented turbulence kinetic energy is converted automatically into resolved-scale velocity fluctuations wherever the local mesh resolution is sufficient to support them. Existing hybrid RANS/LES approaches alter the nature of the local partial differential equations according to the local mesh resolution, but they do not alter the nature of the data on which these equations operate. The implications of this are discussed. Subsequently, a simple mechanism is introduced to transfer statistical kinetic energy into resolved-scale fluctuations in a manner that preserves a given set of space/time correlations and set of second moments. This process, which can appropriately be termed Large-Eddy STimulation (LEST), generates the large-scale eddies needed to form the unsteady boundary conditions at RANS interfaces to LES regions, into which turbulence energy can be deposited either through mean convection or through turbulent transport

A Wall-Distance-Free k-ε Model With Enhanced Near-Wall Treatment

We evaluate a wall-distance-free low-Re κ-e turbulence closure model which Incorporates an extra source term in the e transport equation designed to increase the level of e in nonequilibrium flow regions, thus reducing the kinetic energy and length scale magnitudes to improve prediction of adverse pressure gradient flows, including those involving separated flows regions. Two such cases are used here to test the model: one in the low speed flow regime, the other a supersonic one. Comparisons with experimental data and with an earlier version of the κ-e model, as well as with a variant of the κ-ω model (both also wall-distance-free) reveal that the proposed model enables improved prediction of adverse pressure gradient flows relative to more traditional κ-e models

Convergence acceleration for unified-grid formulation using preconditioned implicit relaxation

Improved convergence rates for a unified grid framework are achieved by combining several convergence acceleration strategies, which include local implicit time-stepping, low-speed preconditioning, and relaxation methods. It is demonstrated that good convergence can be achieved on various grid types and topologies, all speed regimes, and for both inviscid and viscous flows.

Sub-grid turbulence modeling for unsteady flow with acoustic resonance

This paper proposes a novel combination of Reynoldsaveraged Navier-Stokes (RANS) and large-eddy simulation (LES) sub-grid models, which combines the best features of time-averaged and spatially-filtered models, yielding the superior near-wall stress predictions of (algebraic or full-transport) Reynolds-stress models with the ability to override any quasi-steady grid-converged RAN’S model solution in regions of sufficiently high grid density. The proposed hybrid formulation is well suited to the coupled simulation of all flow scales in resonating high-Reynolds number flows and contains no additional empirical constants beyond those appearing in the original RANS and LES sub-grid models.

A POINTWISE VERSION OF BALDWIN-BARTH TURBULENCE MODEL

SUMMARY The Baldwin-Barth υRTone-equation turbulence model is modified by replacing the y+-dependent near-wall formulation with equivalent functions based on the ratio of the large eddy and the Kolmogorov time scales. The result is a υRT model valid in near-wall regions which, nevertheless, does not require knowledge of local distance to walls, rendering it ideal for use within unstructured computational grid frameworks as well as with traditional structured grids. The new models predictive capability is shown through a number of flow cases, including both wall-bounded and free shear flows.

Toward a Pointwise Turbulence Model for Wall-Bounded and Free Shear Flows

A modified version of the Baldwin-Barth two-equation turbulence model is proposed, in which the near-wall function is based on the ratio of the large eddy and the Kolmogorov time scales. This results in a model applicable to both wall-bounded and free shear flows which, nevertheless, does not require explicit knowledge of local distance to walls, rendering it useful within both structured and unstructured computational frameworks for flow predictions involving complex geometries. The new models predictive capability is demonstrated through a number of flow cases.

Validation Of CFD++ Code Capability For Supersonic Combustor Flowfields

Numerical simulations of several turbulent supersonic flows related to scramjet combustors are carried out using a new unified-grid computational methodology. Five problems are considered: a 2-D ramp unit problem; a reattaching turbulent shear layer, the 3-D University of Virginia two-hole supersonic transverse Air-Air injector; and the NASA P2 and P8 supersonic inlets. The numerical simulations are conducted using the Reynoldsaveraged Navier-Stokes equations along with oneequation and three-equation pointwise turbulence models. Both turbulence models enable accurate prediction of the flowfields and numerical results compare favorably with experimental data in all cases.

Variable Turbulent Schmidt and Prandtl Number Modeling

Abstract: This paper describes variable turbulent Schmidt and Prandtl number formulations, based on Rodi’s (1980) algebraic Reynolds stress model adapted to velocity-scalar correlations. Since the approach is algebraic in nature, it does not introduce transport equations beyond the ones already existing to compute the flow, rendering it a viable engineering tool. The method is scrutinized against several test cases, including a 3D Scramjet combustor problem. Results are encouraging and lend confidence in the proposed approach.

The CFD++ Computational Fluid Dynamics Software Suite

Computational Fluid Dynamics CFD is no longer the domain of just specialists. It is also being used by engineers and scientists in many disciplines who are interested in CFD as a tool to investigate other things and not as just an end in itself. The realization of this fact drives developers to produce user-friendly CFD products that automate most of the problem set up and solution process. The CFD++ software suite is a unified-grid, unified-solution, unified-computing CFD simulation capability that was designed from the outset to be effective from the users perspective. The details are explained in this paper.

The k-ε-Rt Turbulence Closure

Abstract A turbulence closure, based on transport equations for the turbulence kinetic energy, k, its dissipation rate, ε and the undamped eddy viscosity, R, is presented. The model, which is free of topography-dependent parameters, combines a k-ε closure with the Rt model so that no inflow turbulence decay occurs in external flows, an attribute often sought by aeronautical engineers using CFD for flow computations. The model is shown to revert to the k-ε closure in near-wall flow regions. Two aerodynamic flow cases are presented, comparing the original k-ε closure to the current 3-equation model.