Aerodynamics and aeroacoustics of flap side-edges

Abstract

An experimental and computational investigation was carried out to determine the aerodynamics and aeroacoustics of a flap side-edge. A porous side-edge treatment was applied to the flap side-edge in an attempt to reduce airframe noise. Measurements taken as part of the experimental study were forces, on-surface pressures, particle image velocimetry, hotwire anemometry and on-surface microphones. Oil flow was performed to visualise the on-surface flow. A detached eddy simulation was performed on a geometry that consisted of a main element and a half span flap to understand the flowfield. From the experimental and computational investigation four sources of vorticity in the flowfield were identified, i.e. the main element cove, the main element trailing-edge, separation on the flap suction surface, and the flap side-edge vortical system. These sources of vorticity interacted to produce a significantly unsteady flowfield above the solid flap surface. Three potential acoustic sources on the flap were identified. The first two sources were the turbulent shear layers that rolled up to form the flap side-edge vortex, reattaching firstly on the side-edge and secondly on the suction surface of the flap. A mid-frequency broadband hump was measured by an on-surface microphone at the point of reattachment of the turbulent shear layer on the flap side-edge. The third source was a low frequency instability in the off-surface vortex due to non-linear vortical interactions upstream of the flap. This instability was measured by a hotwire in the downstream vortex and by an on-surface microphone in the main element flap cove. The application of a porous flap side-edge had two favourable effects. Firstly, it reduced the magnitude of vorticity in the turbulent shear layer and the vortex. This reduced the magnitude of the hydrodynamic instabilities induced by the flap side-edge vortex. Secondly, it displaced the vortex further away from the flap surface due to the finite mass flux allowed through the porous material. This reduced the magnitude of the disturbances that interacted with the solid flap surface by moving them further away. The effect of applying a porous flap side-edge was most noticeable in reducing the mid frequency broadband hump in the on-surface microphone measurements.

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