Evaluation of free stream acoustic noise in a shock tunnel using focused laser differential interferometry

Free stream disturbances of acoustic nature are known to be amplified in the boundary layer on the test model and cause transition to occur much earlier than in flight, even though the exact governing mechanisms are still not fully understood. In order to enable the best use of experimental observations conducted in conventional shock tubes or shock tunnels and support the interpretation of data obtained in different facilities, it is therefore important to quantify the free stream environment, in terms of inherent acoustic disturbances. In the present work, free stream disturbance measurements in a shock tunnel are presented using the technique of Focused Laser Differential Interferometry (FLDI). The experiments are conducted in the High Enthalpy Shock Tunnel Göttingen (HEG) of the German Aerospace Center (DLR) under free stream unit Reynolds numbers of approximately 1.5e6, 2.2e6, and 2.4e6 1/m, and total enthalpies of 11.9, 9.8, and 3.5 MJ/kg, respectively. Free stream fluctuations in terms of FLDI phase shift spectra are shown. In this data, the lower and upper frequency bounds of the FLDI instrument are identified. Convection velocities are calculated by cross-correlating the time signals from the streamwise FLDI probes, which are separated by a known distance. Using the obtained convection velocities, the frequency bounds are converted into the smaller and larger detected disturbance wavelengths. RMS values of density and pressure fluctuations are calculated from the post-processed FLDI data within the identified frequency bounds. The RMS of pressure fluctuation are compared with measurements performed in a previous work using pressure transducers in a wedge probe in HEG, for the free stream unit Reynolds number 2.4e6 1/m case. The spectra of the pressure fluctuations for all cases are shown, and the effect of Reynolds number and total enthalpy are analyzed. The ease of implementation of FLDI and its clean view of the disturbance spectrum make it a promising candidate technique for the standardization of free stream disturbance measurements, aiming at improving the interpretation and comparison of boundary layer data across different facilities.