Aeromechanics of membrane and rigid wings in and out of ground-effect at moderate Reynolds numbers

Abstract

Wind tunnel experiments are conducted using membrane wings and rigid flat-plates in ground-effect at a moderate Reynolds number of Re = 56,000 with ground clearances from 1% to 200% chord length measured from their trailing-edge. A six-axis load-cell captures time-resolved forces and moment while time-resolved stereo digital image correlation (DIC) measurements are performed to capture membrane motions. The lift and drag coefficients of the rigid wing in ground-effect follow well-established trends while the membrane wing appears to exhibit improved coefficients and efficiency (compared to the rigid wing) when in ground-effect. Proper orthogonal decomposition (POD) is applied to study the spatiotemporal structure of membrane vibrations. With increasing angles-ofattack and/or decreasing heights above ground, mode shapes of membrane deformation are dominated by large-scale fluctuations that have a smaller number of local extrema along the chord. Ground-effect induces modifications to the membrane deformation, which appear to be similar to the modifications induced by increasing angles-of-attack in free-flight. At high angles-of-attack in free-flight or at moderate angles in ground-effect, two POD modes of membrane fluctuations are found to be sufficient to capture 90% of all membrane deformations. Under these conditions, a membrane deformation with maximum camber near the trailing edge of the membrane wing is found to correlate with high lift, low drag and a nose down pitching moment. The extrema in membrane deformations and lift and drag forces occur simultaneously, while there is a time-lag between the deformation and the pitching moment.

On the fluid-structure interaction of flexible membrane wings for MAVs in and out of ground-effect

“…The experimental setup (Fig. 2) is similar to those used in our previous studies on time-synchronised load and membrane measurements (Bleischwitz et al, 2015a(Bleischwitz et al, , 2016a and adds further time-synchronised flow measurements. Key elements of the setup consist of a robotic sting arm for pitch and altitude control of the wings and a rolling road system for ground-effect.…”

“…The ability of flexible membrane wings to respond and interact with growing vortical flow structures opens the question how membrane wings perform in the vicinity of ground-effect. Recent work at the University of Southampton acquired time-synchronised load and membrane deformation measurements and focused on the aerodynamic performance and load-to-membrane interaction in and out of ground-effect (Bleischwitz et al, 2015a(Bleischwitz et al, , 2016a. Fig.…”

“…Current study Rigid (Bleischwitz,2016) Membrane (Bleischwitz,2016) (b) Aerodynamic efficiency Fig. 1 Background information extracted from the initial study (Bleischwitz et al, 2016a): (a) Static lift capability (b) and aerodynamic efficiency for rigid flat-plate and membrane wings going from free-flight (h/c = 2) into ground-effect conditions (h/c ≤ 1), measured at three angles-of-attack (α R = α Rigid , α M = α Membrane .). Points of interest in the current paper are marked in blue.…”

“…However, membrane wings come with superiority in separated (stall related) flow conditions, showing further lift (and drag) enhancement to rigid, similar cambered plates due to their ability to couple membrane dynamics with leading-edge vortex-shedding for further energy entrainment close to the wing surface. The vortex-shedding related oscillation behaviour of the membrane was suggested to be either triggered by high angles-of-attack in free-flight conditions or by ground-effect conditions (Bleischwitz et al, 2015a(Bleischwitz et al, , 2016a. However, these first studies on membrane wings in ground-effect focused on pure loads and membrane interaction, but lacked measurements of the flow development and coupling characteristics between the flow and the membrane.…”

Hydrodynamics of a self-propelled flexible fin in perturbed flows

“…Motivated by many animals swimming near the ground, Sung Goon PARK* and Hyung Jin SUNG* *Department of Mechanical Engineering, KAIST 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Korea E-mail: hjsung@kaist.ac.kr Park and Sung, Mechanical Engineering Reviews, Vol.5, No.1 (2018) [DOI: 10.1299/mer. many researchers have performed experimental (Blevins and Lauder 2013;Quinn et al 2014aQuinn et al , 2014bFernandez-prats et al 2015;Bleishwitz et al 2016) and numerical (Dai et al 2016;Ryu et al 2016;Tang et al 2016;Park et al 2017) studies on the ground effect, and observed the hydrodynamic advantages such as increments of cruising speed, thrust and propulsive efficiency depending on the body motion of a swimmer. Fish can benefit from swimming in the wake of an inanimate structure as well as swimming near the ground.…”