| The oscillatory flow within weapons bay cavities of military aircraft produces broadband noise superimposed by intense, low frequency tones, particularly during transonic flight. Passive control devices such as leading edge spoilers, while effective during subsonic flight, display diminished effectiveness with increasing Mach number. Alternatively, introduction of leading edge perturbations to the cavity shear layer at frequencies on the order of 10 times the dominant tonal frequency have shown promise, resulting in attenuations of up to 29 dB in wind tunnel studies [1].;This thesis provides a new perspective on the physics of "high-frequency" flow control by examining the couplings amongst the spatial scales of the flow using higher-order spectral analysis and performing low-dimensional modal analysis. In particular, the methods of bispectral analysis and Dynamic Mode Decomposition were selected for their capability to identify underlying processes in non-linear physical systems. The present study calculates acoustic power spectra at selected axial stations along the cavity walls, and the cross-bispectrum associated with unsteady axial velocity calculations along the cavity shear layer to identify quadratic phase couplings between spectral components. Subsequently, spatial structures associated with power spectral components are extracted by performing Dynamic Mode Decomposition (DMD) [9, 10] upon a discrete snapshot set sampled from the flow at equispaced time intervals.;In the present study, numerical simulations of uncontrolled and controlled M=1.19 flow traversing an L/D = 5.0, W/D = 0.5 rectangular cavity are performed. Five controlled simulations are made which approximate the presence of a leading edge, resonance tube bank issuing normal to the freestream. For a pulsing frequency of 5 kHz, blowing ratios of beta=1.2,0.6,0.3 are investigated to isolate the effect of frequency on acoustic power spectra. For a blowing ratio beta=1.2, pulsing frequencies of 1 kHz and 5 kHz are made to distinguish between "low-frequency" and "high-frequency" effects. Finally, a steady blowing simulation is made for a blowing ratio beta=1.2. Detached Eddy Simulations for the aforementioned cases are performed using FDL3DI [55], a three-dimensional, unsteady, compressible Navier-Stokes solver, pioneered at the Air Force Research Laboratory. |
