The curved bow shock in hypersonic flow over a blunt body generates a shear layer with smoothly distributed vorticity. The vorticity magnitude is 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 non-linear 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 shear layer. Significant differences exist between plane and axisymmetric flow. The highest growth rate is observed for disturbances with wavelength equal to the nose radius. For sufficiently high density ratio the shock layer loses all order, so that the classical theoretical results on flows near the Newtonian limit (large density ratio) that assume smooth flow become suspect. A physical experiment confirms the qualitative features of the computed flows.
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