Hybrid prediction of the aerodynamic noise radiated by a rectangular cylinder at incidence

Abstract

The acoustic radiation by a laminar flow over a rectangular cylinder at incidence is predicted using a two-step approach. The acoustic pressure is evaluated from the compact source approximation of Curle’s analogy, where the fluctuation of the aerodynamic force is the source quantity. The latter is provided by numerical simulation of the incompressible flow, the presence of the bluff body being modelled via an immersed boundary method. The approach is validated by comparison with a direct noise computation of the aeolian tone produced by the flow over a circular cylinder at Re=150 and M=0.2. Ten values of incidence are considered, from 0° to 90° for the 2D flow, at Re=200, over the rectangular cylinder, whose aspect ratio is 4. The acoustic power is strongly enhanced in comparison with the circular cylinder (by 6–15dB) and with the case without incidence (by 30–40dB). The contribution of the drag dipole is also significantly increased. The relative fluctuations of lift and drag drive the directivity for each case. Depending on the incidence, a block rotation of ±15° is observed on the directivity diagram. This is closely linked to the wake organisation, in particular the position of the stagnation point, and the orientation of the fluctuation of the aerodynamic force, all of these features undergoing a qualitative change at an incidence angle of 40°. One of the key results is that the acoustic efficiency increases quadratically with respect to the Mach number and to (rms) fluctuations of lift and drag coefficients, and depends linearly on the Strouhal number.