Quantifying density fluctuations in compressible turbulence

Numerical simulations show compressible shock-like structures at turbulent Mach numbers as low as 0.1. Quantifying the conditions that generate these structures is key to understanding how they behave in nature and engineered settings. Our goal is to quantify the density gradient field and to visualize shocks. We examine a turbulent jet inside a pressurized vessel with the ability to adjust the speed of sound using different gases including SF₆, reaching up to Re_λ~1000 and Ma_t~0.15. We perform Schlieren imaging and use the Gladstone-Dale relation to infer the density field from the image intensities. We visualize the turbulent jet flow and identify areas associated with large changes in density. These regions are then used to report spatial density gradient distributions and time-varying density fluctuations. With this information, we determine the frequency of highly compressive structures and validate published computer simulations.