COMPARISON OF NONEQUILIBRIUM SOLUTION ALGORITHMS APPLIED TO CHEMICALLY STIFF HYPERSONIC FLOWS

Three solution algorithms, explicit under relaxation, point implicit, and lower-upper symmetric Gauss-Seidel, are used to compute nonequilib rium flow around the Apollo 4 return capsule at the 62-km altitude poi nt in its descent trajectory. By varying the Mach number, the efficien cy and robustness of the solution algorithms were tested for different levels of chemical stiffness. The performance of the solution algorit hms degraded as the Mach number and stiffness of the flow increased. A t Mach 15 and 30, the lower-upper symmetric Gauss-Seidel method produc es an eight order of magnitude drop in the energy residual in one-thir d to one-half the Cray C-90 computer time as compared to the point imp licit and explicit under-relaxation methods. The explicit under-relaxa tion algorithm experienced convergence difficulties at Mach 30 and abo ve. At Mach 40 the performance of the lower-upper symmetric Gauss-Seid el algorithm deteriorates to the point that it is out performed by the point implicit method. The effects of the viscous terms are investiga ted. Grid dependency questions are explored.