Multi-point Focused Laser Differential Interferometry for Noise Measurements in High-Speed Tunnels

Abstract

Acoustic radiation and entropic disturbances were investigated with a 3-point Focused Laser Differential Interferometer (FLDI) in the free-stream flow of a Mach-6 shock tunnel at varying unit Reynolds numbers (3.23-14.4x10^6 /m) and total enthalpies (0.57-0.89 MJ/kg). With the FLDI beams split along the Mach angle, simultaneous measurements were made of disturbances propagating along streamlines and Mach lines. Streamwise convection velocities measured with the FLDI were found to agree well with theoretical free-stream velocities. Shear layers originating from the boundary layer of the free-jet nozzle contaminate the FLDI's core flow signal due to their presence within the optical path, and an analysis of the extent of shear layer influence is presented. The broadband density-based turbulence intensity is compared to that from the core flow, and in both cases, a decrease in fluctuations with Reynolds number is found. The relative contributions of the acoustic and entropic disturbances to the overall noise environment is also examined through a study of the correlated signal pairs, and a slight trend of increasing proportion of acoustic noise with Reynolds number is observed. Finally, the intermittency of the various tunnel conditions is investigated; the significantly elevated intermittency at the lowest Reynolds number condition suggests that the nozzle boundary layer may be transitional at this condition.