An experimental investigation of density gradient fluctuations in high-speed jets using optical deflectometry

An optical deflectometry system is used to provide both qualitative and quantitative flowfield analyses in the jet shear layers exhausted from beveled and circular nozzles. Experiments are conducted for pure air jets as well as helium/air mixture jets (to simulate the properties of heated jets) at subsonic and supersonic Mach numbers. Applying this nonintrasive technique to obtain measurements detailing the level of correlation, spectral content, and convection velocity in jet flows is especially important to better understand the physical mechanisms associated with the turbulence large-scale structures that develop in the jet plume. Schlieren images and deflectometry measurements show good agreement with data from the literature for the pure air jet cases. However, helium/air mixture jets display much lower levels of correlation and little evidence of large-scale structures in the measured spectra. Convection velocities with helium addition are also significantly reduced. These results are being addressed with ongoing experiments. Finally, density gradient fluctuation signals acquired across the jet shear layer for supersonic, cold cases exhibit skewness variations that show some consistency with the vortex-street pattern suggested by Lau and Fisher for subsonic jets.