Reflection of Acoustic Disturbances from a Porous Coating of Regular Microstructure

Abstract

Laminar flow control (LFC) technologies reduce heat-transfer rates as well as the weight and complexity of thermal protection systems (TPS). Previous theoretical and experimental hypersonic wind tunnel studies showed that the laminar run could be substantially increased using an ultrasonically absorptive coating (UAC) that suppresses the second mode instability. In order to design and fabricate porous materials providing integrated UAC and TPS functions, theoretical and experimental tools are needed to perform quick and economical assessments of the UAC characteristics associated with reflection and absorption of the boundary-layer disturbances. Nature of the second mode instability suggests that basic features of the second mode interaction with a porous coating can be captured by considering the reflection of acoustic disturbances from the UAC surface without external boundary-layer flow. In this connection, the reflection of acoustic waves from a flat plate surface covered by a UAC with a regular micro-structure is studied experimentally and theoretically. An experimental setup was assembled to perform benchmark (no flow) measurements of the reflection coefficient at various ambient pressures with emphasis on low pressures relevant to high-altitude hypersonic flights. The theoretical reflection coefficients are in good agreement with the benchmark measurements conducted at an ultrasonic frequency of 180 kHz in the range of ambient pressures from 20 to 800 mbar. This new testing technique provides an economical method to evaluate the robustness of UACTPS articles prior to their testing in hypersonic wind tunnels.