Compressible turbulence density gradient spectra using schlieren imaging

Scramjet engines and astrophysical flows require the understanding of turbulent Mach number (M_t) effects on turbulence. M_t values as low as 0.1 begin to exhibit compressible behaviors. Simulations show that the slope of the spectra steepens with increasing M_t. Since compressibility is changes in density, measurements of density gradients are sensitive to subtle aspects of compressible turbulence. In a pressurized vessel with a fan-generated jet, the speed of sound is adjusted using different gases including sulfur-hexafluoride (SF₆), to increase M_t up to 0.15 while holding the Taylor-Reynolds number constant at values up to 1000. This way, the role of the Reynolds number and Mach number can be distinguished independently. Schlieren imaging is implemented to visualize the density gradients, which are quantifiable with calibration techniques. With a high-speed camera, the density gradients are recorded, both spatially and temporally, to obtain the spectra at various conditions, and observe the behavior of the slope.