Positive and Negative Spanwise Flow Development on an Insect-Like Rotating Wing

Abstract

This paper presents an experimental investigation of the spanwise flow development on an impulsively started rotating rectangular wing at 45 deg angle of attack. Reynolds numbers of 500 and 15,000 were studied, which correspond to insect and flapping-wing micro-air-vehicle scales, respectively. Stereoscopic particle image velocimetry flowfield measurements were taken at various spanwise locations and sweep angles (angle of rotation from start) on a motor-driven rectangular wing immersed in a seeded water tank. At both Reynolds numbers, the development of the flow was characterized by the formation of strong positive (root-to-tip) and negative (tip-to-root) spanwise flows, which lost strength toward the tip and root, respectively. The negative spanwise flow appeared to be created by the tip vortex, which accelerated fluid from beyond the tip to the root and turned a portion of the positive spanwise flow back on itself, inducing a secondary vortex inboard of the tip vortex and rotating in the opposite sense. Peak positive and negative spanwise flows in both cases were on the order of the wingtip speed. The extent of negative spanwise flow formation on rectangular wings versus tapered wings, as observed in other studies, suggests that perhaps the formation of negative spanwise flow is dependent on wing geometry and the size of the tip vortex.

Aspect-ratio effects on rotating wings: circulation and forces

“…Note that Carr et al (2013a) showed both positive (outboard) and negative (inboard) spanwise velocity in the vicinity of the LEV for A = 2 and 4, which was also found by e.g. Lu & Shen (2008), Jardin et al (2012), Phillips & Knowles (2013) and Garmann & Visbal (2014). To gain insight into the individual contributions of the positive and negative u axial , we calculate the non-dimensional volume flow rate conditionally on its positive and negative components; this is given in figure 10(a,b) for A = 2 and 4, respectively.…”

“…Granlund et al (2013) showed that the lift and drag coefficients for wing rotation increase at a lower rate and achieve lower final values than for translation, and that C L is generally Re-independent for Re = 14-10 000. Phillips & Knowles (2013) used S-DPIV in select constant-span planes to investigate spanwise velocity for an A = 2.5 rotating rectangular wing at 45 • angle of attack. Similar to Lu & Shen (2008), they measured strong positive and negative velocities, attributing the latter to an effect of the TV velocity and commenting on a likely dependence on wing geometry.…”

Dynamic experimental rigs for investigation of insect wing aerodynamics

“…For liquids such as oils and water, seeding particles are normally solid and of similar density to that of the working fluid. Examples include hollow glass spheres of varying sizes (typically tens of microns in diameter) [72,75,89,106]; powders such as polyamide powder [124,[158][159][160] or polymethylmethacrylate powder [111,112,153]; and metallic-coated hollow plastic spheres [27,88,99,167,168]. Occasionally bubbles have been used as seeder particles; however, these can be challenging to work with [59,120,169,170].…”

“…The Cranfield University research team conducted studies to ascertain spanwise flow at Reynolds numbers of 500 and 15 000 [75,89], figure 9 d . Glass seeding particles were used for the PIV system to take quantitative flow measurements.…”

“…The PIV system allows two velocity components in the plane of the laser sheet to be measured, along both the span and chord—depending on the laser sheet orientation [75]. In a later study, stereoscopic PIV flow measurements were also considered [89]. Spanwise flow was relatively weak after an impulsive start, with strong spanwise flow developing soon after.…”