Uniform flow injection into a turbulent boundary layer for trailing edge noise reduction

Abstract

The hydrodynamic effects of inclined uniform continuous blowing on a turbulent boundary layer are investigated experimentally. A laminar flow is introduced into the boundary layer through a fence on a flat plate at a distance of 3.38δ0 upstream of the trailing edge. The effects of this open-loop technique of flow control are examined at different angles of injection and at different blowing rates. Surface pressure fluctuations acquired from flush-mounted microphones are used to estimate the trailing edge noise. Injection angles of 70° and 90° in combination with strong blowing rates enable a noise reduction of up to 15 dB at mid and high frequencies, f > 300 Hz. Similar aeroacoustic performances are obtained at a blowing angle of 50° but at lower blowing rates. At low frequencies, a penalty is expected, with the trailing edge noise increasing for all the injection angles and blowing rates under analysis. Mean velocity profiles from hot-wire anemometry reveal that high injection angles and strong blowing rates induce a flow separation, which is expected to deteriorate the aerodynamic performances. When applying a uniform blowing at 50°, however, no flow separation occurs. From an aeroacoustic and aerodynamic point of view, uniform blowing applied at 50° and at intermediate blowing rates is found to be the most promising setting.