Effects of surface heating on instabilities in a supersonic nozzle boundary layer

Abstract

The effects of surface heating were explored in a two-dimensional Mach 3 DeLaval nozzle to identify transition mechanisms related to heat transfer. Earlier work demonstrated that when surface heating was applied to the nozzle throat, the boundary layer at the nozzle exit could be changed from laminar to turbulent with low levels of surface heating. The present experiments show that the effect of surface heating is to reduce low-frequency disturbance growth and amplitude in the first 70% of the nozzle length. Suppression of this low-frequency activity causes turbulent bursting to be moved downstream, thereby increasing the extent of laminar flow to nearly the entire nozzle length. A mechanism for transition delay could not be found with linear stability theory. Computations predict that surface heating has only a mild stabilizing effect on Gortler vortices and first mode Tollmien Schlichting waves show negligible amplification within the nozzle. Calculations of the mean-flow also showed that neither natural cooling nor surface roughness could be responsible for the observed transition delay.