Measurement of freestream noise in a hypersonic wind tunnel.

Quantitative characterization of freestream fluctuations in a hypersonic flow facility is important for proper interpretation of results from any boundary-layer flow transition experiments conducted in the facility. Direct measurement of freestream flow quantities (like pressure, temperature, velocity) using a physical probe is not possible due to the modulation introduced by the leading shock wave and the downstream flow region. A transfer function that models the complex modulation process, which is specific to a given probe geometry, is therefore needed to estimate the freestream flow condition from the physical probe output. The general practice across different experimental hypersonic flow facilities globally is to employ custom designed probes for estimation of freestream fluctuations. This hinders a direct comparison of freestream fluctuations between two different flow facilities since such a comparison is limited by the accuracy of the two different transfer functions that are used. The present work is motivated by the need to have a standard probe geometry and an associated transfer function for enabling an objective comparison of freestream fluctuations across experimental facilities. A wedge probe along with a linear transfer function used in the recent work by Wagner et al. (J Fluid Mech 842:495–531, 2018. https://doi.org/10.1017/jfm.2018.132) is proposed for consideration as a standard hypersonic freestream flow characterization method. The utility of the measurement method is demonstrated by making spectral measurements of freestream pressure fluctuations at Mach 6 in the Roddam Narasimha Hypersonic Wind Tunnel at IISc and performing a direct quantitative comparison with the results obtained by Wagner et al. in the RWG Ludwieg tube (Göttingen, Germany). [ABSTRACT FROM AUTHOR]