Abstract
Low-speed wind tunnel tests of a flexible wing semi-span model have been implemented in the 9times 7 ft de Havilland wind tunnel at the University of Glasgow. The main objective of this investigation is to quantify the effect of removing the traditional peniche boundary layer spacer utilised in this type of testing. Removal of the peniche results in a stand-off gap between the wind tunnel wall and the model’s symmetry plane. This offers the advantage of preventing the development of a horseshoe vortex in front of the model, at the peniche/wall juncture. The formation of the horseshoe vortex is known to influence the flow structures around the entire model and thus alters the model’s aerodynamic behaviours. To determine the influence of the stand-off gap, several gap heights have been tested for a range of angles of attack at Re=1.5times 10^6, based on the wing mean aerodynamic chord (MAC). Force platform data have been used to evaluate aerodynamic coefficients, and how they vary with stand-off heights. Stereoscopic Particle Imaging Velocimetry (sPIV) was used to examine the interaction between the tunnel boundary layer and model’s respective stand-off gap. In addition, clay and tuft surface visualisation enhanced the understanding of how local flow structures over the length of the fuselage vary with stand-off height and angle of attack. The presented results show that a stand-off gap of four-to-five times the displacement thickness of the tunnel wall boundary layer is capable of achieving a flow field around the model fuselage that is representative of what would be expected for an equivalent full-span model in free-air—this cannot be achieved with the application of a peniche.
Highlights
Semi-span testing techniques have been widely adopted as a tool to provide state-of-the-art wind tunnel research capabilities (Lynch 1992; Viehweger and Ewald 1994; Silva et al 2012; Nguyen et al 2015)
Over the linear lift region of the model (− 5 ≤ ≤ + 7), deviation of the aerodynamic coefficients is a function of the distance between the fuselage symmetry plane and the tunnel wall, and not a function of the angle of attack
The lift-curve slope decreased with the stand-off gap in a linear fashion; this trend is independent of the Reynolds number
Summary
Semi-span testing techniques have been widely adopted as a tool to provide state-of-the-art wind tunnel research capabilities (Lynch 1992; Viehweger and Ewald 1994; Silva et al 2012; Nguyen et al 2015). Yokokawa et al (2010) conducted a comprehensive experimental-numerical study focused on how the aerodynamic influence of a half span model changes with a peniche in an attempt to strategize the appropriate selection of the peniche stand-off height. While they observed no changes in surface flow patterns (using oil flow visualisations) with increasing peniche heights, strong changes in the aerodynamic coefficients and pressure distributions are noted. The objective of the present experimental investigation is to vary the stand-off gap and examine the resulting interactions with the tunnel wall boundary layer and its aerodynamic influence over the semi-span model. [Pa] Wing area (S) [m2] MAC (c) [m] Wing span (b) [m] Taper ratio ( ) Aspect ratio (AR)
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