Detecting Low-Frequency Oscillations of Separated Shocks in Supersonic Overexpanded Nozzle Flows Using Shock Detection

Supersonic nozzles operating at off-design conditions experience detrimental flow phenomena such as low-frequency unsteadiness, which is commonly-observed in such flows and can negatively affect performance and structural integrity. In the current work, an asymmetric planar de Laval nozzle with a design Mach number of 2.59 is operated at a nozzle pressure ratio (NPR) below its design value. Schlieren images are obtained experimentally for the overexpanded flow, which is characterized by shock-induced flow separation in the nozzle diverging section, followed by a series of shocks. The separation location is unsteady and exhibits low-frequency oscillations in the streamwise direction. The Schlieren images are used to analyze the oscillations, with edge detection schemes used to locate the shock and characterize the extent and frequency behavior of the intermittent region.