Turbulent transport models for scramjet flowfields

Turbulence modeling approaches were examined from the standpoint of their capability to predict the complex flowfield features observed in scramjet combustions. Thus, for example, the accuracy of each turbulence model, with respect to the prediction of recirculating flows, was examined. It was observed that for large diameter ratio axisymmetric sudden expansion flows, a choice of turbulence model was not critical because of the domination of their flowfields by pressure forces. For low diameter ratio axisymmetric sudden expansions and planar backward-facing steps flows, where turbulent shear stresses are of greater significance, the algebraic Reynolds stress approach, modified to increase its sensitivity to streamline curvature, was found to provide the best results. Results of the study also showed that strongly swirling flows provide a stringent test of turbulence model assumptions. Thus, although flows with very high swirl are not of great practical interest, they are useful for turbulence model development. Finally, it was also noted that numerical flowfields solution techniques have a strong interrelation with turbulence models, particularly with the turbulent transport models which involve source-dominated transport equations

Assessment of turbulent models for scramjet flowfields

The behavior of several turbulence models applied to the prediction of scramjet combustor flows is described. These models include the basic two equation model, the multiple dissipation length scale variant of the two equation model, and the algebraic stress model (ASM). Predictions were made of planar backward facing step flows and axisymmetric sudden expansion flows using each of these approaches. The formulation of each of these models are discussed, and the application of the different approaches to supersonic flows is described. A modified version of the ASM is found to provide the best prediction of the planar backward facing step flow in the region near the recirculation zone, while the basic ASM provides the best results downstream of the recirculation. Aspects of the interaction of numerica modeling and turbulences modeling as they affect the assessment of turbulence models are discussed

Development of code evaluation criteria for assessing predictive capability and performance

Computational Fluid Dynamics (CFD), because of its unique ability to predict complex three-dimensional flows, is being applied with increasing frequency in the aerospace industry. Currently, no consistent code validation procedure is applied within the industry. Such a procedure is needed to increase confidence in CFD and reduce risk in the use of these codes as a design and analysis tool. This final contract report defines classifications for three levels of code validation, directly relating the use of CFD codes to the engineering design cycle. Evaluation criteria by which codes are measured and classified are recommended and discussed. Criteria for selecting experimental data against which CFD results can be compared are outlined. A four phase CFD code validation procedure is described in detail. Finally, the code validation procedure is demonstrated through application of the REACT CFD code to a series of cases culminating in a code to data comparison on the Space Shuttle Main Engine High Pressure Fuel Turbopump Impeller

Comparative Study of Advanced Turbulence Models for Turbomachinery

A computational study has been undertaken to study the performance of advanced phenomenological turbulence models coded in a modular form to describe incompressible turbulent flow behavior in two dimensional/axisymmetric and three dimensional complex geometry. The models include a variety of two equation models (single and multi-scale k-epsilon models with different near wall treatments) and second moment algebraic and full Reynolds stress closure models. These models were systematically assessed to evaluate their performance in complex flows with rotation, curvature and separation. The models are coded as self contained modules that can be interfaced with a number of flow solvers. These modules are stand alone satellite programs that come with their own formulation, finite-volume discretization scheme, solver and boundary condition implementation. They will take as input (from any generic Navier-Stokes solver) the velocity field, grid (structured H-type grid) and computational domain specification (boundary conditions), and will deliver, depending on the model used, turbulent viscosity, or the components of the Reynolds stress tensor. There are separate 2D/axisymmetric and/or 3D decks for each module considered. The modules are tested using Rocketdyn's proprietary code REACT. The code utilizes an efficient solution procedure to solve Navier-Stokes equations in a non-orthogonal body-fitted coordinate system. The differential equations are discretized over a finite-volume grid using a non-staggered variable arrangement and an efficient solution procedure based on the SIMPLE algorithm for the velocity-pressure coupling is used. The modules developed have been interfaced and tested using finite-volume, pressure-correction CFD solvers which are widely used in the CFD community. Other solvers can also be used to test these modules since they are independently structured with their own discretization scheme and solver methodology. Many of these modules have been independently tested by Professor C.P. Chen and his group at the University of Alabama at Huntsville (UAH) by interfacing them with own flow solver (MAST)

Advanced turbulence models for turbomachinery

Development and assessment of the single-time-scale k-epsilon turbulence model with different near-wall treatments and the multi-scale turbulence model for rotating flows are presented. These turbulence models are coded as self contained module decks that can be interfaced with a number of CFD main flow solvers. For each model, a stand-alone module deck with its own formulation, discretization scheme, solver and boundary condition implementations is presented. These satellite decks will take as input (from a main flow solver) the velocity field, grid, boundary condition specifications and will deliver turbulent quantities as output. These modules were tested as separate entities and, although many logical and programming problems were overcome, only wider use and further testing can render the modules sufficiently 'fool proof'

Overview of turbulence model development and applications at Rocketdyne

This viewgraph presentation discusses turbulence modeling requirements, development philosophy, and approach; two major areas of concentration (high speed and low speed turbulence modeling); high speed turbulence modeling; compressibility effects; turbulence models adapted to USA code; M = 9.2 flat plate flow; Mach 7.05 flow over axisymmetric flare; Mach 8.6 flow over cold wall edge; low speed turbulence modeling; turbulence models being assessed; turbulence model deck structure and integration with Navier-Stokes solver; nonlinear algebraic-stress model; rotation modified k-epsilon model; and Reynolds stress model

Les Using A Spectral Element Method And Eddy-Viscosity Type Subgrid Models

Introduction Computational modeling techniques, primarily computational fluid dynamics (CFD), together with select ground and flight testing, provide the best potential to be the engineering tools of choice in the new Air Force and NASA advanced propulsion programs. Currently one of the biggest hindrances to the more extensive use of computational tools in engineering is the lack of reliable physical process models (e.g. turbulence, transition, chemistry). Turbulence is the pacing item and has the most bearing on the fidelity of the calculations. The current work horse turbulence models used in engineering are of the 1- and 2- equation variety designed for Reynolds Averaged Navier-Stokes equations. The performance of these models appear to be application dependent and range from fair to poor for complex geometries. Higher order phenomenological models such as algebraic stress and full Reynolds stress models are yet to be demonstrated conclusively on realistic 3D problems. Th

Spectral Element Analysis of Sound hopagation in a Mu~er

Abstract:~is paper is concerned with a Imt-squares specti element analysis of sound propagation in an expansion chamber muffler with and without a mean flow. The atgorithm is based on the least-squmes finite element methodology with spectral collocation discretization in space and three-time-level discretization in time to solve the Iintized acoustic field equations, Effects of the mean flow on the acoustic wave propagation in the muffler were tien into consideration. TRODUC~ON An accurate determination of acoustic wave propagation in a lined expansion duct is viti for noise connol and reduction in an engine exhaust system. The noise originating from the internal-combustion engine chamber flows with the burning gas through the exaust pipe and muffler and discharges into the ambient environment. The process of noise propagation in an engine piping system is quite complicated. Moreover, the pipe lining is often treated to dissipate acoustic energy for noise reduction, making the modeling of the acoustic waves in the engine exhaust system even more involved. Analytical solution of sound propagation in such a system generally is not possible; and hence most computations of acoustic modes in lined ducts are analyzed numerically by finite difference [e.g. This synopsis presents a least-squares spectral element method for sound wave propagation in a lined expansion chamber muffler. The method solves the first-order acoustic field equations derived from the full Navier-Stokes equations in a finite number of elements, which represent the muffler. Within each element we first approximate the solution to the acoustic field equations by a series of unknown coefficients with known basis functions, form the residual of the approximation, and then minimize the integral of the squares of the residual with respect to the unknown coefficients. The resultant system equations are written in a matrix form and discretized by the spectral element method for spatial derivatives and by the three-level time stepping for temporal derivatives. A similar approach was used by Chan [3] to develop an incompressible viscous flow solver to compute time accurate flows. Finally the discretized equations are solved for the unknown coefficients by the Jacobian preconditioned conjugate gradient method. Numerical results were presented for pressure contours of sound waves propagating at frequency of 1 Hz with and without flow effect, and at 1~Hz without flow effect. MA~MA~CAL MODEL~G Consider an expansion chamber muffler, as shown i