# Combined free-stream disturbance measurements and receptivity studies in hypersonic wind tunnels by means of a slender wedge probe and DNS

## TL;DR

This study combines experimental measurements and direct numerical simulations to analyze free-stream disturbances and receptivity in hypersonic wind tunnels, revealing the dominance of slow acoustic waves and the amplification of fast modes at the leading edge.

## Contribution

It provides a comprehensive comparison of tunnel noise across Mach 3 to 7.4 and investigates the receptivity of a slender wedge probe to different acoustic waves using combined experimental and numerical methods.

## Key findings

- Fast acoustic waves show early amplification at the leading edge.
- Receptivity to fast waves is higher than to slow waves at all Mach numbers.
- Slow acoustic waves are the dominant mode in hypersonic wind tunnel noise.

## Abstract

Combined free-stream disturbance measurements and receptivity studies in hypersonic wind tunnels were conducted by means of a slender wedge probe and direct numerical simulation. The study comprises comparative tunnel noise measurements at Mach 3, 6 and 7.4 in two Ludwieg tube facilities and a shock tunnel. Surface pressure fluctuations were measured over a wide range of frequencies and test conditions including harsh test environments not accessible to measurement techniques such as pitot probes and hot-wire anemometry. Quantitative results of the tunnel noise are provided in frequency ranges relevant for hypersonic boundary layer transition. In combination with the experimental studies, direct numerical simulations of the leading-edge receptivity to fast and slow acoustic waves were performed for the slender wedge probe at conditions corresponding to the experimental free-stream conditions. The receptivity to fast acoustic waves was found to be characterized by an early amplification of the induced fast mode. For slow acoustic waves an initial decay was found close to the leading edge. At all Mach numbers, and for all considered frequencies, the leading-edge receptivity to fast acoustic waves was found to be higher than the receptivity to slow acoustic waves. Further, the effect of inclination angles of the acoustic wave with respect to the flow direction was investigated. The combined numerical and experimental approach in the present study confirmed the previous suggestion that the slow acoustic wave is the dominant acoustic mode in noisy hypersonic wind tunnels.