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Linear stability of the three-dimensional compressible attachment-line boundary-layer flow on a 60° -sweep infinite-span cylinder in a Mach 3.5 free-stream is investigated by the two-dimensional (2D) eigenvalue approach. Results of this study indicate that the supersonic attachment-line boundary layer is subjected to the oblique Tollmien-Schlichting instability, and the nonparallel effect is highly destabilizing. The critical Reynolds number based upon momentum thickness is found to be about 125, in contrast with the value of 235 for incompressible flow. The frequencies of unstable travelling modes are on the order of 100KHz for the free-stream Reynolds numbers considered in the study.

Correlations for transition from laminar to turbulent flow on 45° and 60° swept cylinders based on data obtained in the NASA Langley Mach 3.5 Pilot Quiet Tunnel are presented. Variations of free-stream noise from high levels comparable to those in conventional wind tunnels to more than an order of magnitude lower had no effect on transition. However, when boundary-layer trips were attached to the leading edges, transition occurred at lower Reynolds numbers depending on both the trip height and the wind tunnel noise level. Compressible linear stability calculations have been performed for the boundary layer on an infinite swept cylinder. The boundary layer on the attachment line has a generalized inflection point similar to that present in a flat-plate boundary layer. The results show that Tollmien-Schlichting waves are amplified in the attachment line boundary layer and that oblique waves have the highest growth rates. Wall cooling tends to be stabilizing.