The role of the inflow momentum thickness in subsonic cylindrical cavity noise generation

At typical landing speeds, the cylindrical cavity flow that develops past an aircraft fuel vent displays tonal convective streamwise instabilities. The higher frequency range of the noise radiated by such a cavity compared to flap noise is perceived by a ground observer as louder with respect to what its amplitude in decibel would suggest, due to the dB(A) weighting. A threedimensional time-dependent numerical model of a cylindrical cavity flow is obtained using an in-house three-dimensional compressible laminar solver. This simulation predicts the flow instability and gives a preliminary understanding of the influence of the inflow momentum thickness (µ) on the flow unsteadiness. Time-dependent cavity flow models are obtained at two different Reynolds numbers (Reµ) based on the inflow momentum thickness, Reµ = 8850 and Reµ = 10750, for two diameter to depth ratios (L/D), 0.71 and 2.5. The near-field sound pressure level (SPL), the pressure coefficient Cp, and the shear layer spanning the cavity are analyzed. The numerical experiments suggest that the deep cavity is characterized by a selfsustained instability and that the shallow cavity is characterized by a steady flow recirculation. The near-field SPL was compared with past Euler predictions to study the influence of the shear layer growth on the radiating pressure field. In the laminar predictions, it was found that the amplitude of the outgoing pressure waves is lower, due to a weaker interaction of the open cavity shear-layer with the downstream solid edge.