The influence of a surface roughness element in the form of a two-dimensional hump on the transition location in a two-dimensional subsonic flow with a free-stream Mach number up to 0.8 is evaluated. Linear stability theory, coupled with the eN transition criterion, is used in the evaluation. The mean flow over the hump is calculated by solving the interacting boundary-layer equations; the viscous–inviscid coupling is taken into consideration, and the flow is solved within the separation bubble. The effects of hump height, length, location, and shape; unit Reynolds number; free-stream Mach number; continuous suction level; location of a suction strip; continuous cooling level; and location of a heating strip on the transition location are evaluated. The N-factor criterion predictions agree well with the experimental correlation of Fage (A. Fage, Br. Aero. Res. Council Report No. 2120, 1943); in addition, the N-factor criterion is more general and powerful than experimental correlations. The theoretically predicted effects of the hump’s parameters and flow conditions on transition location are consistent and in agreement with both wind-tunnel and flight observations.

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Transition on a swept-wing leading-edge model at Mach 3.5 is investigated. Surface pressure and temperature measurements are obtained in the NASA Langley Research Center Supersonic Low-Disturbance llinnel. For one case, temperature-sensitive paint and a sublimating chemical are used to visualize surface flow features such as transition location. The experimental data are compared with 1) mean-flow results computed as solutions to the thin-layer Navier-Stokes equations and 2) W-factors obtained using the envelope e N method. The experimental and computational results compare favorably in most cases. In particular, N •= 13 correlates best with the observed transition location over a range of freestream unit Reynolds numbers and angles of attack. Computed traveling crossflow disturbances with frequencies of 40-60 kHz have the largest N factors, and the surface flow visualizations reveal smooth transition fronts with only faint evidence of stationary crossflow vortices. These results suggest that transition is probably dominated by traveling, rather than stationary, crossflow disturbances for the present model.