Study On Fluid-resonant Acoustic Oscillation Induced By Mean Flow In Deep Cavity

The phenomenon that self-sustained oscillation is induced by mean flow passing the opening of the cavity is named mean flow induced cavity oscillation. The cavity geometry and flow condition affects oscillation phenomenon and inherent mechanism. Of these, with excellent monofrequency feature and remarkable acoustic energy-flux density, fluid-resonant oscillation has broad application prospects in mean flow energy harvesting. Despite much study on understanding and modeling of this phenomenon, the mechanism of the fluid-resonant oscillation is still unclear; the prediction of the pulsation amplitude is still problematic for practical cases. In order to explore the working mechanism, the theoretical, numerical and experimental study on fluid-resonant oscillation is carried.In this thesis, the research progresses of the mean flow induced oscillation are introduced; parameters of the geometry and fluid dynamics which affect the cavity flow were enumerated. The research shows that the oscillation mechanisms are dependent on these parameters and influencing ways. It seems necessary to draw some classifications.Vortex-sound theory, Green’s Function and other theories are applied to explain the phenomenon. The mathematic equations of longitudinal mode and acoustic power in low Mach number is obtained.Compressible Large-Eddy Simulation (LES) was applied to simulate the fluid-resonant acoustic oscillation induced by the mean flow in a cross-junction deep cavity, in which Favre fitter is employed. Simulations were performed with three grids, two subgrid-scale models, with central difference method in space and implicit second order scheme in time. Numerical simulation shows an accurate prediction in the variation of the frequency and the pressure amplitude as a function of the mean flow velocity. The pressure amplitude damps as finer mesh, and the dynamic Smagorinsky model and the wall-adapted local eddy-viscosity (WALE) model are similar in the flow prediction. On this basis, the relationship between the motion of large-scale vortex structure in the opening of the resonator tube and the standing wave acoustic field distribution inside the resonator tube is analyzed, and the mechanism of the fluid-resonant oscillation induced by the mean flow is revealed. Rate of vorticity shedding at the upstream edge of the resonator tube is significantly influenced by the acoustic field. On the other hand, vortex structures are convected along the resonator opening at a constant phase speed and transfer energy into the acoustic field. The analyses show that resonant oscillations occur in the specific region of the Strouhal number. A three-dimensional model of mean flow induced oscillation in the cross-junction configuration is applied. The results predict the development of resonant oscillation in low Mach number.A test rig of mean-flow-induced cavity oscillation was developed and preliminary experiments with the cross junction configuration were conducted. In experiments, the features of acoustic field at different mean flow velocities were shown and the reliability of numeric model was verified.