Passive hypersonic boundary layer transition control using an ultrasonically absorptive coating with random microstructure - Part 1: Ultrasonic absorption properties of carbon-carbon

Hypersonic boundary layer transition control offers the potential to reduced surface heat flux rates and in consequence the reduction of the thermal protection system mass. Preceding studies have shown that ultrasonically absorptive carbon fiber reinforced carbon ceramic (C/C) with random microstructure can be applied to damp the second mode growth in hypersonic boundary layers and thus to delay boundary transition. To better understand the potential of C/C materials for use as passive transition control a test rig was set up to determine the acoustic absorption characteristics in absence of the external boundary layer flow. Various flat plate samples covered with porous carbon-carbon layers of different thickness were investigated in the range of 200 kHz to 400 kHz at ambient pressures between 5x103 Pa and 1x105 Pa. A set of piezo ceramic sound transducers were used as transmitter and receiver to generate and reveive the acoustic waves. All material properties necessary to predict the acoustic characteristics by means of acoustic porous absorbers theory are introduced. The predicted theoretical results are compared to the results obtained in the experiment