Passive Shock Control Concept for Drag Reduction in Transonic Flow

Abstract

The operating principle of a novel passive shock control concept for drag reduction on swept wings called D-Strips is introduced, and results of a first experimental proof of concept are presented. Wind-tunnel experiments are conducted at transonic airspeeds using an airfoil VC-Opt (9.2% relative thickness) with forced boundary-layer transition. Schlieren pictures confirm the operating principle. Wake measurements demonstrate that, locally, in the wake of D-Strips the total drag increases. However, more globally, at spanwise locations away from these positions the total drag is measurably reduced if a sufficiently strong shockwave is present above the wing-suction-side surface. In contrast to passive control by ventilation, outside of the wake of the D-Strips a wave and viscous drag reduction and observed. Furthermore, the maximum lift tends to be increased by D-Strips suggesting that the buffet-onset limit is delayed. An application of D-Strips to a swept wing at cruise conditions can yield net aircraft drag reduction because the drag rise is limited to the D-Strips and its wake, but the shock-weakening effect and the drag reduction are distributed in the wide-splayed characteristics upstream of the shock front. The effectiveness of D-Strips is less sensitive to the location of the shock wave than control concepts like two-dimensional bumps, which are only effective at the design conditions. Another application of D-Strips that can be considered is the regions with a distinct shock, for example, between engines or in the wing-root region or at the inboard engine. However, there is a need for further investigations in order to optimize D-Strips and to quantify their impact on aircraft performance. Recent experimental results indicate that airscoop-type configurations might perform better as D-Strips than Velcro-type strips. Because D-Strips are easy to apply on existing wings, an application of D-Strips for retrofit is possible.