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

Alexander Wagner,Syed R C Ali,Adriano Cerminara,Neil D Sandham,René Petervari,Erich Schülein,Klaus Hannemann

doi:10.1017/jfm.2018.132

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. A good agreement was found between normalized Pitot pressure fluctuations converted into normalized static pressure fluctuations and the wedge probe readings. Quantitative results of the tunnel noise are provided in frequency ranges relevant for hypersonic boundary-layer transition. Complementary numerical simulations of the leading-edge receptivity to fast and slow acoustic waves were performed for the applied 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. An inclination angle was found to increase the response on the wave-facing surface of the probe and decrease the response on the opposite surface for fast acoustic waves. A frequency-dependent response was found for slow acoustic waves. 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.

Highlights

Free-stream disturbances are known to play an important role in the boundary-layer transition process
All transducers discussed in the present section provided repeatable results, allowing a determination of the spectral distribution of the surface pressure up to at least 300 kHz depending on the facility and the test condition
The slender probe design avoids the complex subsonic flow field typically found behind blunt probes and simplifies the transfer function of free-stream disturbances entering the boundary layer across the oblique shock

Summary

Introduction

Free-stream disturbances are known to play an important role in the boundary-layer transition process. An understanding of the transition process in supersonic and hypersonic boundary layers is in turn crucial for the design of vehicles operating at such speeds. Free-stream disturbances, such as vorticity, sound, entropy inhomogeneity and microscale and macroscale particulates, enter the boundary layer as unsteady fluctuations of the basic state. This process is called receptivity (Morkovin 1969). It establishes the initial conditions of the disturbance amplitude, frequency and phase leading to the breakdown of laminar flow (Saric, Reed & Kerschen 2002; Fedorov 2003). Extensive reviews of the effect of tunnel noise on high speed boundary-layer transition were conducted by Schneider (2001, 2004)

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