Direct Numerical Simulations of Nonlinear Entropy-Layer Instability Waves

Direct Numerical Simulations (DNS) were carried out for a blunted straight cone geometry at Mach 6 in order to investigate the nonlinear stages of the transition process initiated by disturbances in the entropy layer. The flow conditions of the Boeing/AFOSR Mach 6 Quiet Tunnel (BAM6QT) at Purdue University were used for the numerical investigations. In the simulations, “controlled” disturbances were introduced locally into the entropy layer by volume forcing of the energy equation. Low amplitude (linear) DNS revealed an unstable region that was not found using conventional Linear Stability Theory (LST), which only found a weak entropy mode instability for two-dimensional waves. A highly-resolved oblique breakdown DNS has shown that transition can be initiated by forcing large amplitude, oblique disturbances in the entropy layer. Streamwise “hot” streaks were observed in Stanton number contours on the surface of the cone. The streak spacing in the azimuthal direction corresponds to the azimuthal wavenumber of the steady streamwise mode that is nonlinearly generated by the forced oblique waves.