Hypersonic Flow Over a Flat Plate: CFD Comparison with Experiment

Abstract

Experimental results of a hypersonic flow over a flat plate are compared with a CFD simulation. Slip boundary conditions are employed in the CFD simulation, with the accommodation coefficient being varied between 0.5 and 1.0. Detailed velocity profiles in both the x- and y-directions are compared with the experimental data, as well as slip velocity at the wall. A brief comparison is also made to similar simulation results using DSMC. I. Introduction The design of hypersonic vehicles requires accurate prediction of the gas flow around the vehicles surfaces. During it's trajectory through an atmosphere, a hypersonic vehicle will experience vastly different flow regimes due to the variation of atmospheric density with altitude. In addition, the high temperatures encountered due to the high velocities cause dissociation and ionization of the atmospheric gases. Reproduction of these varied flow conditions in ground-based laboratory facilities is both expensive and technically challenging. Hence, there is an extremely important role for computational models in the development of hypersonic vehicles. In the continuum regime flows around hypersonic vehicles can be accurately simulated using traditional Computational Fluid Dynamics (CFD) by solving either the Euler or preferably the Navier-Stokes (NS) equations. These continuum methods assume small perturbations from local thermodynamic equilibrium. In the rarefied flow regime the flow can be computed using the direct simulation Monte Carlo (DSMC) method. 1 The DSMC method does not depend on assumptions involving a small perturbation from equilibrium and hence is more accurate than CFD methods for non-equilibrium flows. Generally speaking, CFD methods for solving the NS equations are about an order of magnitude faster than the DSMC method. Note that in continuum regimes, locally a flow may behave like a rarefied flow if the local characteristic length scale is very small. Previous work by the authors comparing CFD and DSMC focused purely on numerical results, with CFD simulations being compared directly to DSMC simulations. 7-10 In particular, the walls were assumed to be fully diffusive; that is, the gas molecules colliding with the wall were assumed to accommodate fully to the wall conditions. Hence, an accommodation coefficient of unity was used for the CFD slip boundary conditions. In this paper, two-dimensional CFD solutions are compared with experimental measurements of a hypersonic flow of nitrogen over a flat plate. 4 Several different values for the accommodation coefficient are evaluated. In addition, the CFD solutions are also indirectly compared to DSMC solutions of the same flow. 11 Thus, the relative accuracy of CFD and DSMC can be evaluated against a realistic flow.