A numerical evaluation of viscous effects on vortex induced noise

The effect of viscosity on the noise generated by interactions of simple axisymmetric vorticity structures is numerically investigated. A low Mach number is assumed, and hence the feedback of the acoustic field on the flow can be neglected. With this assumption, Lighthill's acoustic analogy enables one to determine the farfield pressure perturbation due to vorticity motion. For a swirl-free incompressible flow, numerical simulations are performed with a finite difference scheme in terms of the vorticity and the stream function. The acoustics of collision and pairing of vortex rings is investigated for growing Reynolds numbers. Results show that the quadrupole term converges to the inviscid one, while the acoustic term due to the energy dissipation vanishes, according to the theoretical prediction of Obermeier