Effects of surface heating on stability and transition in a supersonic nozzle boundary layer

Acoustic fluctuations that originate from transitional and turbulent boundary layers in a supersonic wind tunnel limit the capabilities of ground test facilities for boundary layer transition research and testing. The present work explores boundary layer stability and transition with and without surface heating on one contoured wall of a low-disturbance Mach 3 two-dimensional wind tunnel at Montana State University.; The throat area of the lower contoured surface was heated to a steady state temperature of 13% and 22% over the stagnation temperature at unit Reynolds numbers of 5.2 × 106/m and 6.2 × 106/m, respectively. Boundary layer measurements with a small, fast-response, pitot probe were used to characterize fluctuation magnitude, frequency content, and the rate of amplification with and without surface heating. The effect of surface heating was to reduce the amplitude of a low frequency disturbance at all streamwise positions. Suppressing this low frequency activity caused turbulent bursting to be moved downstream, thereby increasing the extent of laminar flow to nearly the entire nozzle length. Predictions with linear stability theory showed that heat has a mild stabilizing effect on Görtler vortices, and first-mode Tollmien Schlichting waves could be suppressed with a proper heating distribution by moving the neutral point downstream and reducing the subsequent amplification rates. However, the $eN$ method with linear stability theory completely failed to predict the observed transition in the nozzle boundary layer due to unsteady oscillations, even in the case without surface heating. Calculations of the mean-flow also showed that natural cooling (heat followed by cooling) and roughness arguments do not appear to explain the observed stability events.; The experiment and theory show at least two different paths to turbulence suppression by heating the surface of a supersonic nozzle. (1) The experiment demonstrates that heat suppresses a bypass mechanism triggered by receptivity events near and possibly upstream of the nozzle throat. (2) The computations show that a proper heating distribution can also be used to suppress the growth of linear instabilities in the nozzle if bypass were not present.