An Evanescent Wave-Based Method for Boundary Layer Diagnostic in Compressible Flows

Compressible flows play a critical role in both industrial and defense applications, yet many aspects of their boundary layer behavior—particularly involving shock interactions—remain poorly understood due to limited measurement capabilities. Current diagnostic methods often depend on specialized materials, like pressure-sensitive paint, or intrusive techniques, such as hair-based sensors, to probe near-wall flow properties. This work introduces a novel, non-intrusive diagnostic technique that overcomes these limitations, enabling accurate measurements of near-wall properties in compressible and reacting flows. The method leverages an evanescent wave, whose phase encodes the footprint of near-wall density fluctuations and related quantities. Preliminary experiments conducted in a calibration chamber revealed density variations on par with those across a normal shock wave. These variations significantly influenced the reflected laser's phase and polarization. The approach is material-independent, robust across various flow conditions, and suitable for both wind tunnel and in-flight testing—offering a promising new avenue for diagnosing high-speed boundary layers.