Effect of Wall Blowing on Hypersonic Boundary-Layer Transition

An investigation of outgassing effects on boundary-layer transition was carried out. This joint computational–experimental work mimicked heat-shield pyrolysis outgassing in atmospheric reentry conditions. A slender 7 deg half-angle cone with air wall blowing through a porous section near the apex was tested in the VKI-H3 Mach 6 hypersonic blowdown noisy wind tunnel. The steady transition onset location was measured using infrared thermography, whereas gaseous-naphthalene-based planar laser-induced fluorescence imaging enabled the visualization and quantification of the spatial characteristics of the instabilities. Linear stability theory, combined with the semiempirical eN method with N=5.9, was used to predict the location of transition onset in the same configurations. Studies were performed at different freestream unit Reynolds numbers and blowing rates. Numerical and experimental results agreed within the experimental uncertainties, and they coincided with the previously reported advancement of the transition location due to wall blowing. The wave numbers of the most amplified second-mode instabilities obtained with the linear stability theory matched the observations done with planar laser-induced fluorescence, suggesting it was the growth of such perturbations that led to the transitioning of the boundary layer. The porous section was seen to destabilize the boundary layer for nonblowing configurations. Regarding the upstream advancement of transition associated with wall blowing, the numerical analysis suggested that it is a consequence of the increase in the range of unstable frequencies in the wall-blowing region. Blowing nonuniformities in the cone’s longitudinal direction were observed to have no influence on the local transition location under noisy conditions.