Numerical investigation of laminar-turbulent transition in a cone boundary layer at Mach 6

Laminar-Turbulent transition in a hypersonic cone boundary layer is investigated using Direct Numerical Simulations (DNS). The flow parameters used in the simulations are based on the experimental conditions of the Boeing/AFOSR Mach 6 quiet-flow Ludwieg Tube at Purdue University. The main objective of the present research is to determine which nonlinear mechanisms may be dominant in a broad band "natural" disturbance environment and then to perform controlled transition simulations of these mechanisms. Towards this end, a "natural" transition scenario was modeled and investigated by generating wave packet disturbances. These wave packet simulations provided strong evidence for a possible presence of fundamental and subharmonic resonance mechanisms in the nonlinear transition regime. However, the fundamental resonance was much stronger than the subharmonic resonance. To gain more insight into the nonlinear development we performed controlled transition simulations of these mechanisms. We found that the strength of fundamental resonance is strongly influenced by the wave angle of the secondary oblique wave pair. For a conical geometry this issue is even more complicated than for a flat-plate as the wave angle of a disturbance wave changes in the downstream direction. Therefore, first we carried out a parameter study to find the most strongly resonating oblique wave pair. Then a set of highly resolved controlled fundamental (K-type) breakdown simulations was performed using the most strongly resonating oblique wave pair as secondary waves. These simulations demonstrated that fundamental breakdown may indeed be a viable path to complete breakdown to turbulence in hypersonic cone boundary layers.