Space-time aspects of a three-dimensional multi-modulated open cavity flow

Side-band frequencies in an incompressible flow past a rectangular cavity are characterized through their space-time coherent structures. A parametric study over a range of dimensionless cavity length L/θ0 has been carried out in the incompressible regime. It yields the general evolution of self-sustained oscillations, for which primary characteristics match results in the literature. The modulating frequencies associated with side-band frequencies are usually imputed either to the two-dimensional (vortex-edge) interaction at the impingement or to three-dimensional dynamics induced by centrifugal instabilities in the inner-flow. However, secondary order features sometimes depart from commonly accepted scheme. In addition to the salient features of the flow, our observations bring to light another modulation, which may be related to the main recirculation inside the cavity. That modulation even becomes predominant for peculiar configurations. The present work focuses on such a configuration with a cavity length/depth ratio L/D = 1.5 and dimensionless cavity length L/θ0 = 76. Based on time-resolved velocity measurements, the extensive analysis is concerned with the non-linear interactions within the flow. Using laser Doppler velocimetry and time-resolved particle image velocimetry in two planes, this multi-modulated regime is so addressed through both local and global aspects. Time-resolved velocity fields provide space-time coherent data that are analysed using transfer functions, space-time diagrams, and space-extended time-Fourier decomposition.