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

Abstract

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.

Numerical Investigation of Propeller–Flap Interaction in Inclined Over-the-Wing Distributed-Propulsion Systems

“…This corresponds to approximately 50% of the boundary-layer thickness that would be obtained without propeller or flap deflection at the baseline propeller location, as determined in Ref. [24]. The definition of the boundary-layer thickness, 99 , is based on the total pressure that is associated with 99% free-stream velocity.…”

“…For validation purposes, the wing and propeller geometry of the experimental study described in Ref. [24] is used. The wing has a chord of = 1.042 m and features a flat upper surface, on which a pressure gradient can be generated by deflecting a plain flap of 20% chord.…”

“…Details of the geometry can be found in Ref. [24]. The propeller is positioned above the flap hinge at / = 0.79.…”

“…However, both numerical [23] and experimental [24] studies have shown that, in OTW systems, the interaction between the propeller and the wing boundary-layer can lead to flow separation over the flap in high-lift conditions. This detrimental effect cannot be accurately captured with steady RANS simulations [23] and is particularly important to consider at low advance ratios and high-thrust settings [24], i.e., in take-off conditions. Moreover, it is unclear how well unsteady simulations capture this effect, and whether the flow separation is effectively mitigated when the propeller is inclined with the flap.…”

Aerodynamic Performance and Interaction Effects of Circular and Square Ducted Propellers

“…In order to improve the convergence of the simulations, the corners of the square duct were slightly rounded, so that the minimum corner radius (at the airfoils' thickest point) would be 1% of the propeller radius, R p . The minimum gap between the propeller tips and duct surface (or tip clearance) was maintained at 0.3% of the propeller radius, leading to a duct chord of approximately 0.217 m. The propeller geometry was based on an existing design, denoted "XPROP", that has been characterized previously in both experimental and numerical studies [15][16][17][18]. The six-bladed wind-tunnel model has a radius of R p = 0.2032 m. Details regarding the propeller geometry, such as chord and twist distributions, can be found in Ref.…”