The radiating pressure field of a turbulent cylindrical cavity flow.

The unsteady flow past a cylindrical cavity at M = 0.235 is investigated by Detached Eddy Simulation. Two different length to depth ratios L/D = 2.5 and L/D = 0.713 are studied to evaluate their influence on the flow. The time-resolved numerical method predicts a flow characterized by a large-scale periodic instability with a characteristic period T that changes with the depth. In the L/D = 2.5 cavity, T = 0.7745L/u∞ and the peak pressure fluctuation in the near-field p′ = 15 Pa. The L/D = 0.713 cavity is characterized by larger pressure fluctuations with a maximum of p′ = 40 Pa and a longer fundamental instability period T = 1.853L/u∞. The directivity of the radiating near-field pressure fluctuation differs between the two configurations. These have a similar upstream main directivity peak but the L/D = 0.713 cavity displays also a secondary downstream peak and it is about 15dB louder. In the spanwise plane, the acoustic wave from the L/D = 2.5 cavity is asymmetric, whereas it is symmetric in the L/D = 0.713 cavity. This characterization offers some scope for formulating airframe noise reduction strategies that are configuration specific to shallow and deep cylindrical cavities.