Shock layer instability near the Newtonian limit of hypervelocity flows

The curved bow shock in hypersonic flow over a blunt body generates a shear layer with smoothly distributed vorticity. The vorticity magnitude is approximately proportional to the density ratio across the shock, which may be very large in hypervelocity flow, making the shear layer unstable. A computational study of the instability reveals that two distinct nonlinear growth mechanisms occur in such flows: First, the vortical structures formed in the layer move supersonically with respect to the flow beneath them and form shock waves that reflect from the body and reinforce the structures. Second, the structures deform the bow shock, forming triple points from which shear layers issue that feed the main shear layer. Significant differences exist between plane and axisymmetric flow. Particularly rapid growth is observed for free-stream disturbances with the wavelength approximately equal to the nose radius. The computational study indicates that the critical normal shock density ratio for which disturbances grow to large amplitudes within a few nose radii is approximately 14. This served as a guide to the design of a physical experiment in which a spherical projectile moves at high speed through propane or carbon dioxide gas. The experiment confirms the approximate value of the critical density ratio, as well as the features of the computed flows. Comparisons of calculations of perfect gas flows over a sphere with shadowgraphs of the projectile show very good agreement. The Newtonian theory of hypersonic flow, which applies at high density ratio, makes the assumption that the flow remains smooth. The results show that high density ratio also causes this assumption to fail.