Investigation of the unsteady flow field inside a leading edge slat cove

Abstract

An experimental investigation of the unsteady flow field inside a slat cove was performed to identify possible sound source generation mechanisms. Tests were carried out on a swept constant chord half-model with a stored chord length of 0.45 m. Experiments were conducted in the 2.0 m x 1.4 m wind tunnel facility of the Technical University of Berlin at an angle of attack of 4°, 8°, 12° and 16° and free stream velocities ranging from 20 m/s up to 35 m/s. The slat cove flow field was investigated by Particle Image Velocimitry (PIV) while the radiated sound was recorded with a single microphone. Both measurements were conducted in a synchronized manner to allow a correlation between the flow field and the sound pressure. An inspection of instantaneous PIV images shows a free shear layer emanating from the slat cusp with discrete vortices further downstream. The shear layer impinges on the inner slat surface and some vortices get trapped inside the recirculation area. Based on a simplified form of the Lighthill-equation sound source terms are calculated. A dipole type sound source distribution is found along the shear layer with decreasing strength further down the shear layer. In addition the vortex shedding frequency of the shear layer vortices is estimated from the instantaneous PIV data. The shedding frequency has a Poisson-like probability distribution with center frequencies ranging from 4 kHz to 10 kHz. The mean shedding frequency scales linearly with the mean velocity of the shear layer. It could not clearly be verified that vortex shedding from the slat cusp is the cause for low frequency broad band noise. For a particular configuration several discrete tones can be observed. It is suspected that this tonal noise originates from the junction between the slat and the fuselage.