Attenuation of hypersonic second-mode boundary-layer instability with an ultrasonically absorptive silicon-carbide foam

Unsteady surface pressure measurements have been carried out on a flat-plate test article in the Air Force Research Laboratory’s Mach-6 Ludwieg Tube at a negative angle of attack using PCB pressure sensors. The acoustic waves of the second-mode boundary-layer instability mechanism were successfully measured as indicated by the fluctuating surface pressure spectra. The effectiveness of a novel porous material (Silicon-Carbide (SiC) foam) at absorbing these fluctuations was investigated by comparing porous-wall and impermeable-wall spectra. The impermeable-wall spectra indicate second-mode peaks in amplitude, which agree with companion computations, while the equivalent porous material showed no signs of the instability in the spectra therefore exhibiting a strong dampening ability. However, the porous-wall spectra demonstrate similar breakdown to turbulence to the impermeable material at higher length Reynolds numbers, which might be attributed to surface-roughness effects. Acoustic-absorption bench tests also indicated high levels of ultrasonic absorption for the SiC foam—the acoustic absorption coefficient was consistently higher than 0.90 for second-mode relevant frequencies. A band-limited amplitude integration of the experimental PCB spectra indicate lower amplitude growth in the porous material compared to the impermeable material; however, beyond a length Reynolds number of ≈ 4 · 10 6 , the band-limited amplitude of the porous material approach a similar value to the impermeable one. Linear-stability (LST) computations confirmed the experimentally observed frequencies only when supported by base-flow calculations capturing the leading-edge shock, which entails sudden thickening of the boundary layer at the tip not captured by the Blasius solution. Computed N factors for the porous material were approximately 30% of the associated impermeable values where the material inserts were located, downstream of which, they approached the same value. This study indicates that SiC foam has a strong ability to absorb acoustic waves characteristic of the hypersonic second-mode boundary-layer instability mechanism, and is therefore a candidate material for passive boundary-layer control.