Measurements of flow around a flap side-edge with porous edge treatment

Wind tunnel experiments were performed to investigate a nap side-edge nowneld. A porous side-edge treatment was applied to a half span nap in an attempt to reduce the flap side-edge noise. Measurements taken were forces, on-surface pressures, particle image velocimetry, hotwire anemometry and on-surface microphones. Oil now was performed to visualise the on-surface now. Three potential acoustic sources were identified. The first two sources were the turbulent shear layer reattaching on the side-edge and on the upper surface. 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 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 effect of the porous side-edge was to reduce the magnitude of vorticity in the turbulent shear layer and the vortex. It was most noticeable in reducing the mid frequency broadband hump. It also had the effect of displacing the vortex further away from the flap surface. This reduced the magnitude of the low frequency perturbations from the unstable vortex that interacted with the solid surface.