GENERATION OF AN ACOUSTIC FIELD BY A FLUID FLOW OVER TWO-DIMENSIONAL CYLINDRICAL CAVITIES OF VARIOUS DEPTH

The hydrodynamic and acoustic fields generated by the flow of viscous fluid around a cross cylindrical groove cut in a solid wall are investigated. Therelevance of this study is caused by the wide use of such surface irregularities in technical devices. The problem is solved numerically under the assumptionof a small Mach number using a hybrid method that combines an independent estimate of the near hydrodynamic field with the acoustic Lighthill analogy. The vortex numerical scheme is used to simulate the viscous incompressible flow near the wall with a cylindrical groove; the sound generated by the groove in the far field is estimated based on the Ffowcs William-Hawkings equation in the frequency domain. A cylindrical groove is characterized by an angular size , which is directly proportional to its depth. The grooves with   40 , 60 and 90 are considered, which are shallow according to the accepted classification. The boundary layer developed in front of the groove is assumed to be laminar, and its thickness is much less than the depth of the wall irregularity. The flow is characterized by the Reynolds number Re  2104 based on the flow velocity and the chord of the groove. The characteristics of the near hydrodynamic field point out that the groove operates in the shear-layer mode, which is characterized by self-sustained oscillations of the flow caused by collision of large-scale vortices developed in the shear layer with the groove trailing edge. It is also obtained that the shear layer localized between the free flow and the recirculation zone is more stable for the deeper groove. When increasing the groove depth, both the self-oscillation frequency and the mean drag coefficient drop. The convective velocity of vortex structures in the shear layer does not depend on the groove angular size and is approximately 0.42 of the velocity of the undisturbed flow. The flow oscillations radiate a dipole with non-uniform lobes in the far acoustic field so that the sound intensity in the backward direction is higher than in the forward direction. As the angular size of the groove increases, the amplitude of sound vibrations in the far field grows for all directions.