Stability and transition in boundary layers on reentry vehicle nosetips

The stability characteristics of the boundary layer on the nose tip of a reentry vehicle have been investigated for a wide range of conditions. Results based upon classical parallel-flow stability theory indicate that boundary layers on smooth-walled nose tips are stable by a wide margin at realistic Reynolds numbers. The addition of nonparallel effects, including the axisymmetric vortex stretching that is encountered as the boundary layer is swept over the nose tip, moves the neutral stability curve to lower Reynolds numbers, but by only negligible amounts, indicating that the parallel-flow analysis is more than adequate for the present problem. The stability results for rough-surface nosetips, which are based on a phenomenological model for the effects of roughness on the mean flow profiles, yield completely different conclusions. The presence of roughness can produce large, strongly unstable regions on the nosetip. In particular, the interaction between roughness and other parameters is especially important. The results indicate that in the presence of roughness, wall cooling is strongly destabilizing, whereas the effects of the pressure gradient are very weak. Both of these predictions are completely different from smooth-wall stability results but are in agreement with numerous experimental transition results. The calculations also indicate that surface mass addition is destabilizing in the presence of smooth walls but by much smaller amounts than indicated in the experiments of Demetriades, while it has very small effects in the presence of wall roughness.