Measurements of Boundary-Layer Instabilities with Near-Nose Blowing on Straight Cones at Mach 6

Hypersonic boundary-layer transition from laminar to turbulent flow is affected by many factors and remains difficult to predict. In the case of blunted cones, the physical mechanisms leading to transition are poorly understood. Ablative thermal protection systems create several effects that can influence boundary-layer transition, including: mass addition to the flow through gaseous blowing, the addition of carbon species into the boundary layer, outer mold line changes, and changes in surface roughness. This research considers the effects of ablation-representative blowing and bluntness on boundary-layer instabilities that lead to transition.

Experiments were performed in the Boeing/AFOSR Mach 6 Quiet Tunnel (BAM6QT) at Purdue University. The facility offers a low-disturbance environment that more closely represents a free-flight environment and is well-suited for the study of hypersonic boundary-layer instabilities. A gas injection system was designed, built, and integrated with the BAM6QT. A 5 deg half-angle straight cone model was designed, built, and instrumented for these experiments. The model could accommodate various nose tips. Air was injected through porous nose tips to simulate ablative outgassing. Several different sphere-cone porous nose tips of varying nose tip radii were used to represent nose tip shape change associated with ablation.

The outflow through the porous nose tips was characterized using a benchtop apparatus and hot-wire anemometry. The outflow was non-uniform along the axial dimension of the nose tips, but the characterization allowed the experimental blowing profiles to be provided as boundary conditions for computations, which were compared with the experimental results. The permeability of the porous nose tips was also measured.

In all cases, near-nose blowing moved transition upstream, but with increasing bluntness more blowing was required to similarly advance transition. The blowing did not appear to generate nascent instabilities; blowing appeared to cause transition by amplifying already-present disturbances. Transition appeared to be dominated by the second-mode instability for cases run with a nominally-sharp nose tip and a nose tip with a 1.60 mm nose tip radius. With the sharp nose tip, blowing caused the second-mode amplitude to reach a peak amplitude of 25 % of mean surface pressure.

With a nose tip radius of 3.18 mm, blowing did not generate detectable second-mode waves, but blowing nonetheless moved transition upstream. Amplification of low-frequency instabilities near 25 kHz, 45 kHz, ad 90 kHz appeared to lead to transition. The amplitude of these instabilities appeared to grow with blowing rate. The frequencies of these instabilities were found to be insensitive to free-stream Reynolds number. Spectral proper orthogonal decomposition based on high-speed schlieren, along with comparisons to computations, provide additional evidence that these instabilities were not modal (that is, first- or second-mode) instabilities. Infrared thermography revealed stream-wise streak patterns associated with blowing-induced transition that were different for the nominally-sharp geometry compared to the 3.18 mm nose tip radius geometries. On the nominally-sharp geometry, a dominant azimuthal wavenumber of 13 was measured. On the 3.18 mm nose tip radius geometry, a dominant azimuthal wavenumber of 8 was measured.