The role of carbon/carbon composites in the delay of hypersonic laminar to turbulent boundary layer transition

Kurzfassung

Ultrasonically absorptive carbon-carbon ceramic was shown for the first time to delay hypersonic laminar to turbulent boundary-layer transition. A 7° half-angle cone with a total length of 1100 mm was tested at zero angle of attack in the High Enthalpy Shock Tunnel Göttingen (HEG) of the German Aerospace Center (DLR) at Mach 7.5. One model was equipped with an in-house manufactured ultrasonically absorptive carbon-carbon ceramic insert with random microstructure covering one third of the model surface in circumferential direction. The remaining model surface consisted of polished steel and served as reference surface. The free-stream unit Reynolds number was varied over a range of Rem = 1.5x 10^6 m^-1 to Re_m = 9.8x10^6 m^-1 at stagnation enthalpies of h_0=3.2 MJ/kg and a wall temperature to total temperature ratio of Tw/T0= 0.1 . The ultrasonic absorption properties of carbon-carbon ceramic (C/C) were assessed theoretically by means of the quasi-homogeneous absorber theory and experimentally by means of direct reflection coefficient measurements at varying ambient pressure levels. For the first time broadband ultrasonic sound transducers with resonance frequencies of up to 490 kHz were applied to directly cover the frequency range of interest with respect to the second mode instabilities observed on cone geometries in HEG. C/C was found to absorb acoustic power transmitted towards the surface.