Force production and flow structure of the leading edge vortex on flapping wings at high and low Reynolds numbers

Abstract

The elevated aerodynamic performance of insects has been attributed in part to the generation and maintenance of a stable region of vorticity known as the leading edge vortex (LEV). One explanation for the stability of the LEV is that spiraling axial flow within the vortex core drains energy into the tip vortex, forming a leading-edge spiral vortex analogous to the flow structure generated by delta wing aircraft. However, whereas spiral flow is a conspicuous feature of flapping wings at Reynolds numbers (Re) of 5000, similar experiments at Re=100 failed to identify a comparable structure. We used a dynamically scaled robot to investigate both the forces and the flows created by a wing undergoing identical motion at Re of ~120 and ~1400. In both cases, motion at constant angular velocity and fixed angle of attack generated a stable LEV with no evidence of shedding. At Re=1400, flow visualization indicated an intense narrow region of spanwise flow within the core of the LEV, a feature conspicuously absent at Re=120. The results suggest that the transport of vorticity from the leading edge to the wake that permits prolonged vortex attachment takes different forms at different Re.

Rotational accelerations stabilize leading edge vortices on revolving fly wings

“…1C; supplementary material Movie 3), as found in prior studies (Dickinson et al, 1999;Usherwood and Ellington, 2002;Birch et al, 2004), but not on a unidirectionally translating wing (Ro=ϱ, A*=ϱ; Fig. 1B; supplementary material Movie 2).…”

“…The lift and drag forces acting on the wing were measured with a custom-built force sensor as previously described (Dickinson et al, 1999;Birch and Dickinson, 2001;Birch et al, 2004;Dickson and Dickinson, 2004). For post-processing we down-sampled the measurements at 300 Hz, which is still approximately 1400 times the flapping frequency.…”

“…Whereas a LEV is shed after a few chord lengths of travel on a translating 2D model of an insect wing (Dickinson and Götz, 1993;Dickinson, 1994;Miller and Peskin, 2004;Lentink et al, 2008), it remains stably attached on a 3D model wing that revolves about its base (Dickinson et al, 1999;Usherwood and Ellington, 2002;Birch et al, 2004). Van Den Berg and Ellington (Van Den Berg and Ellington, 1997) note that the spiral LEV generated by their mechanical model of a hawkmoth wing is remarkably similar to the spiral LEV generated by delta and swept wings (Ellington et al, 1996;Van Den Berg and Ellington, 1997).…”

“…Based on the notion that revolving insect wings also stabilize LEVs (Dickinson et al, 1999;Usherwood and Ellington, 2002;Birch et al, 2004) we then apply the Navier-Stokes equations for flapping wings using a coordinate transformation that attaches the frame of reference to the surface of the flapping wing [these equations have been derived in the accompanying paper (Lentink and Dickinson, 2009)]. This analysis shows how wing kinematics can potentially stabilize the LEV on a revolving wing.…”

Unsteady high-lift mechanisms from heaving flat plate simulations

“…Of particular interest is the delayed stall phenomena, in which a leading edge vortex (LEV) on the upper edge of the wing enhances lift throughout the downstroke, which has been shown experimentally [5,6] and computationally [7,8]. The presence of LEVs and other large flow structures indicates that traditional quasi-steady flow analysis that relies on inviscid flow theory is not capturing the correct physics.…”