Mack-mode instabilities on a cooled flared cone with discrete roughness elements at Mach 6

Boundary-layer transition, and the increased heating that accompanies it, has a great effect on how hypersonic vehicles and projectiles are designed. The Mack-mode instability is the most dominant cause of boundary-layer transition at Mach numbers greater than 4 and is being studied here. Boundary-layer transition was tracked using several diagnostics. Thermocouples embedded in the model were used to track elevated heating levels from boundary-layer transition. Visualization of the Mack-mode disturbances was achieved and spectral data acquired using a focusing schlieren system, Mack-mode frequencies ranging from 281 to 315 kHz were recorded. 2-D mass-flux maps of the boundary layer were created using hot-film probes, azimuthal sweeps showed periodic mass-flux variations with a wavenumber of 90, which were attributed to Gortler vortices. Distributed roughness elements were placed at the neutral point with the same wavenumber seen from the hot-film data to force a 3-D breakdown, and experiments repeated. Linear array-focused laser differential interferometry was used to acquire simultaneous measurements at six streamwise points in the boundary layer and observe boundary-layer transition, while varying wall temperature and Reynolds number, wall normal measurements were also made. Computations were done to support the experimental data and agreed favorably. This work furthers the understanding of boundary-layer transition at hypersonic speeds and demonstrates the effectiveness of several diagnostic systems.