Leading-edge vortices over swept-back wings with varying sweep geometries

Abstract

Micro air vehicles are used in a myriad of applications, such as transportation and surveying. Their performance can be improved through the study of wing designs and lift generation techniques including leading-edge vortices (LEVs). Observation of natural fliers, e.g. birds and bats, has shown that LEVs are a major contributor to lift during flapping flight, and the common swift ( Apus apus ) has been observed to generate LEVs during gliding flight. We hypothesize that nonlinear swept-back wings generate a vortex in the leading-edge region, which can augment the lift in a similar manner to linear swept-back wings (i.e. delta wing) during gliding flight. Particle image velocimetry experiments were performed in a water flume to compare flow over two wing geometries: one with a nonlinear sweep (swift-like wing) and one with a linear sweep (delta wing). Experiments were performed at three spanwise planes and three angles of attack at a chord-based Reynolds number of 26 000. Streamlines, vorticity, swirling strength, and Q -criterion were used to identify LEVs. The results show similar LEV characteristics for delta and swift-like wing geometries. These similarities suggest that sweep geometries other than a linear sweep (i.e. delta wing) are capable of creating LEVs during gliding flight.

Laminar separated flows over finite-aspect-ratio swept wings

“…Lambert et al. (2019) investigated the formation of LEVs on swept wings with different leading-edge geometries. Their results showed similar LEV characteristics for swept wings with either straight or curved leading edges.…”

“…Videler, Stamhuis & Povel (2004) identified LEVs in the swept wings of common swifts (Apus apus) during gliding. Lambert et al (2019) investigated the formation of LEVs on swept wings with different leading-edge geometries. Their results showed similar LEV characteristics for swept wings with either straight or curved leading edges.…”

Effects of uniform vertical inflow perturbations on the performance of flapping wings

“…The vorticity field was statistically characterized to demonstrate the existence of LEV using Q -criterion method [29,30]. A vortex was identified based on areas where the magnitude of the rotation rate tensor false([italicΩ]= 1/2(normal∇v−false(∇boldvfalse)bold-italicT)false) was greater than the magnitude of the strain rate tensor false(false[ψfalse]=1/2false(∇boldv+(normal∇v)Tfalse) which led to a positive value of Q calculated as (1/2false[∥false[Ωfalse]2∥−∥false[ψfalse]2∥false]), where the double vertical bars denote Frobenius norm.…”

Wake characteristics of a freely rotating bioinspired swept rotor blade

“…When a stable LEV is formed over the wing of common swifts, it augments lift as well as causes delayed stall [ 29 ]. These are commonly known to form over linear (delta wing) [ 30 , 31 ] and more recently over nonlinear swept-back (swift-like) wing geometries [ 32 , 33 ]. Ashwill et al .…”

“…When a stable LEV is formed over the wing of common swifts, it augments lift as well as causes delayed stall [29]. These are commonly known to form over linear (delta wing) [30,31] and more recently over nonlinear swept-back (swift-like) wing geometries [32,33]. Ashwill et al [34] developed a 27 m long swept blade (Sweep-Twist Adaptive Rotor blade) to be royalsocietypublishing.org/journal/rsos R. Soc.…”