Dynamic density modes of high-speed cavity flow from low sampling rate Schlieren*

Open cavity flows are associated with strong tonal and broadband velocity and pressure field fluctuations. To study the dynamics of high-speed cavity flows, time-resolved measurements are generally required. In our past study, we developed a method called 'spectral analysis modal method (SAMM)' (DOI: 10.1007/s00348-020-03057-8) to extract the dynamically coherent structures using synchronized non-time-resolved (NTR) PIV and time-resolved (TR) pressure measurements. The NTR method yields the uncovers the SPOD modes from TR PIV. In the current study to investigate the compressibility effects of cavity flows at Mach 0.6, NTR Schlieren was similarly synchronized with TR surface pressure measurements in the cavity. The Schlieren images were converted into the vertical density gradient field using a knife edge cut-off calibration and the Gladstone-Dale relationship. The density field is then computed by integrating in the vertical direction from the top of the domain. Applying SAMM to snapshots of the density fields and pressure data from the three Kulite sensors, the density modes at the first four Rossiter frequencies are obtained. These modes show both downstream traveling waves and upstream propagating pressure waves identifying the feedback mechanism of the classic cavity flow.