Shock tunnel noise measurement with resonantly enhanced focused schlieren deflectometry

The character of the boundary layer noise and ambient tunnel noise are of interest in the experimental study of laminar to turbulent transition. The instability mechanism in hypersonic flow over slender bodies is the acoustic mode. A number of investigations of flow over a slender cone in high-enthalpy facilities have been performed; however, measurements of the boundary layer noise and ambient tunnel noise have not been made. In cold hypersonic facilities the frequency range of the acoustic mode typically lies below 500 kHz; in high-enthalpy facilities, 5-20 MJ/kg, the most strongly amplified acoustic mode frequency is approximately 1-3 MHz. These high frequencies are well beyond the reach of the piezo-electric pressure transducers typically used in cold hypersonic facilities. A logical approach is to investigate the use of optical methods. Measurements of the boundary layer noise and ambient tunnel noise on a five degree half angle cone in the Caltech T5 hypervelocity shock tunnel are made with a single point focused schlieren system and a resonantly enhanced focused schlieren system.