Measurements of Free stream Density Fluctuations in the T4 Stalker Tube

Abstract

Interpreting the results of boundary Layer transition studies and testing scramjets at realistic flight conditions in impulse facilities like the T4 Stalker tube requires prior knowledge of freestream noise levels. To quantify these noise levels, an experimental study was conducted using a focused laser differential interferometer (FLDI) and density fluctuations in the core flow of T4’s Mach 6b, 7, and 8b nozzles were determined. FLDI also probed inside and outside the Mach 6b nozzle core flow. When probing outside the nozzle core flow, the FLDI results show increased fluctuations for frequencies above 400 kHz. Such behaviour is not observed in the spectra measured inside the core flow, demonstrating that the current FLDI instrument has an adequate spatial resolution to attenuate far-field disturbances generated outside the nozzle core flow. T4’s noise level is quantified by determining the RMS of the density fluctuations normalised by mean density in Octave bands for frequencies ranging between 22 and 22 kHz. For this frequency range, the noise level typically ranges between 0.1% and 0.6% for the Mach 6b and 7 nozzle flows. For the Mach 8b nozzle, the noise level is between 0.2% and 1.7%. The freestream sound pressure levels (SPLs), determined from the density fluctuations using isentropic relations, are between 105 and 130 dB for all three nozzle flows. The measured noise levels indicate that T4 is suitable for conducting fundamental studies in the hypersonic flow regime and for scramjet testing.