Aerodynamic Interaction Between an Over-the-Wing Propeller and the Wing Boundary-Layer in Adverse Pressure Gradients

This experimental study focuses on the aerodynamic interaction effects that occur between an over-the-wing (OTW) propeller and a wing boundary-layer. An OTW propeller is positioned above the hinge-line of a wing featuring a plain flap. The measurements are carried out with and without axial pressure gradients by deflecting the flap and by extending the wing in streamwise direction to simulate a flat-plate configuration, respectively. Wing pressure taps and phase-free particle-image-velocimetry (PIV) are used to quantify the time-averaged interaction effects, while embedded microphones and phase-locked PIV are used to analyze unsteady interaction effects. Results show that the propeller generates an adverse pressure gradient on the wing surface which increases linearly with thrust and decreases as the blade tipclearance is increased. The pressure gradient is partially caused by the slipstream contraction, which creates a streamwise velocity deficit near the wall immediately behind the propeller disk. Moreover, the rotation of the propeller blades generates pressure fluctuations on the surface, the amplitude of which exceeds both the pressure fluctuations produced by the tip-vortices and the time-averaged pressure effect of the slipstream. Consequently, the propeller triggers flow separation when the flap is deflected. A parametric study of different propeller locations indicates that increasing the tip-clearance is not an effective way to mitigate flow separation. However, displacing the propeller half a radius upstream induces a Coanda effect which allows the flow to remain attached.