Mechanisms of trailing edge noise reduction by means of micro slots or porosity

Kurzfassung

Trailing edge noise reduction by means of replacing the rear 11 percent of the chord c of an airfoil with porous material was investigated experimentally and numerically. As simple porosity streamwise oriented micro slots were studied, periodic along the span. The vorticity of any pertubation vortex in the boundary layer induces flow around the edge. For the solid edge the perturbation flow strictly points away from the edge, representing sound while for the slotted edge, the flow is sucked through the slot back into the vortex system, i.e. does not represent sound. Measurements show noise reduction for slots and various porous materials in quantitative agreement with the computations. Apparently the mechanism is not driven, but only modified by viscosity.