Flow noise control of a rod-airfoil configuration using "natural rod-base blowing": Numerical experiments

When the vortices shedding from an upstream rod impinge upon the surface of a downstream airfoil, the resultant vortex-body interaction noise can be significant. The use of "natural rod-base blowing" to reduce this interaction noise is investigated numerically using a 3D hybrid computational aeroacoustics approach. The natural blowing is generated through an internal slot that interconnects the stagnation and base region of the rod. Numerical simulation is performed for a straight blowing at blowing rates (BRs) between 6.4% and 16.3% and an oblique blowing with slot-incidence angles (theta) between 0 degrees and 15 degrees, respectively. Far-field noise evaluation demonstrates that the natural rod-base blowing under tested BRs can reduce significantly the noise emission, whereas loses its efficiencies gradually with the increase of angle theta. The most effective case is the straight blowing (theta = 0 degrees) at BR >= 13.6% where the far-field tonal noise associated with the steady-periodic von Karman vortex shedding is annihilated. The changes in transient flow structures indicate that the natural blowing gradually attenuates the von Karman vortex shedding when BR increases, resulting in a mitigation of vortex-body interaction on the airfoil leading surface and hence a reduction of the unsteady lift. At BR=13.6%, as the angle theta reaches 15 degrees from below, a new von Karman vortex street reforms in the flow field, leading to another tonal noise. Linear stability analysis based on a local concept of absolute/convective instability suggests that the absolutely unstable region in the near-wake of the rod elongates first and then shrinks with the increase of BR, while the local absolute growth rate decreases monotonously. (C) 2020 Elsevier Masson SAS. All rights reserved.