The Passage toward Stall of Nonslender Delta Wings at Low Reynolds Number

Graduate Aeronautical Laboratories
California Institute of Technology

Abstract-
Delta wings with sharp leading edges exhibit a particular kind of separated flow, wherein the windward-surface boundary layer separates at the leading edges and the resulting shear layer rolls up into organized streamwise vortices over the leeward surface. These "leading edge vortices" have profound impact of the aeronautical application of delta wings, as they appreciably improve the wing lift and the high angle of attack controllability. However, the instability and eventual bursting of these vortices result in a stall-like process detrimental to performance.

Delta wings of high leading edge sweep, referred to as "slender wings", undergo a vortex burst process at high angles of the attack, with well-known effects on the kinematics of the rest of the flowfield. For nonslender wings, the situation is far less well known. Vortex burst occurs at comparatively small angles of attack, in the vicinity of the leeward boundary layer. Curiously, leading edge shear layer rollup appear to be unaffected by the onset of vortex burst. Velocity distribution in the core of the leading edge vortices is markedly different from that of slender wings. Strong core suction is absent, with particular consequences for the balance between the formation and convection of vorticity.

The role of low Reynolds number (O(10000)) in affecting the details of the flowfield is more profound than initially expected. In particular, flow over the leeward surface outboard of the leading edge vortices was found to be nearly stagnant in many cases, in stark distinction to what is generally observed for higher Reynolds numbers.

The velocity field of delta wings with 65° and 50° leading edge sweep was experimentally studied in a water tunnel. A version of stereoscopic digital particle image velocimetry was used to obtain quantitative data, while qualitative results were obtained with flow visualization by dye injection.

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