BULK VISCOSITY AS A RELAXATION PARAMETER - FACT OR FICTION

The interpretation of bulk viscosity as a relaxation parameter for des cribing nonequilibrium polyatomic gases is challenged both qualitative ly and quantitatively. Its presumed proportionality to a relaxation ti me is an artificial concept restricted to Very rapid equilibration of translational and internal modes compared with some characteristic tim e of the system. Application of a generalized methodology to small-sig nal sound waves of intermediate and high frequencies demonstrates this artificiality and shows that bulk viscosity can at best be used as a nonphysical adjustable parameter to force fit a single aspect of relax ation. Sound absorption can be force fitted in this way, but not the d ispersion of the sound speed. Accordingly, recent applications of bulk viscosity in solving such problems as heat transfer in hypersonic bou ndary layers may be seriously deficient. (C) 1996 American Institute o f Physics.