Vortex filamentation and fragmentation phenomena in flapping motion and effect of aspect ratio and frequency on global strain, rotation, and enstrophy

Abstract

In the present study, flow field around rigid flat plate wings executing main flapping motion has been studied using phaselocked two-dimensional particle image velocimetry measurements. Experiments have been conducted in water as a fluid medium for an asymmetric upper-lower stroke single degree of freedom main flapping motion. Two different aspect ratio (1.5 and 1.0) rectangular wings at 1.5 and 2.0 Hz flapping frequency in hovering flight mode (advance ratio, J ¼ 0), zero wing pitch angle, and chord-based Reynolds number of the order of 10 4 have been studied. Velocity field and vorticity field with k 2 criterion information have been obtained for the complete stroke in great detail to reveal the minute aspects of flow dynamics. The flow features during the downstroke and upstroke have been observed to be consistent for all four cases investigated. The predominant characteristic of the flow during downstroke and upstroke has been referred to as vortex filamentation and fragmentation phenomena. Quantities such as circulation, rate of strain, rate of rotation, and enstrophy have been studied to identity the effect of minor change in aspect ratio and flapping frequency. It is found that for higher aspect ratio wing hyperbolic behavior is predominant except for end of downstroke and beginning of upstroke where elliptic behavior is observed. For lower aspect ratio, wing elliptic behavior is predominant except for end of upstroke and beginning of downstroke where hyperbolic behavior is seen. The hyperbolic behavior became stronger at higher frequency. From enstrophy distribution it is evident that higher frequencies play a more dominant role than aspect ratio in determining the budget.

Velocity Field around a Rigid Flapping Wing with a Winglet in Quiescent Water

“…Goli et al described the generic flow field behavior around a rigid flapping wing without a winglet [17][18][19]. The present study compared the velocity fields of wings with and without a winglet.…”

“…PIV measurements were made to capture the velocity field in the mid-chord plane of the flapping wing. Details regarding the flapping mechanism, water tank, fabrication materials, PIV measurements, and experimental procedure are presented in the papers of Goli et al [17][18][19]. An overview of the experimental system is presented in the following.…”

Evolution of Vortex Structures Generated by a Rigid Flapping Wing with a Winglet in Quiescent Water

“…To the best of the authors knowledge, the concept of winglet in mechanical flapping motion has been introduced by Goli et al [9], while the flow phenomena around the rigid flapping wing without winglet as vortex filamentation and fragmentation has been proposed in Goli et al [17,22]. This study extends this understanding by introducing a related phenomenon, which is termed "modified vortex filamentation and fragmentation."…”

“…Currently, a 1-DOF flapping motion (Fig. 1(a)) with a four-bar mechanism configuration is deployed, and the dimensions can be found in Goli et al [11,17]. The wings, composed of 1.5 mm thick Perspex ® acrylic, are manufactured in the form of a flat plate.…”

“…Additionally, this mechanism proves advantageous as it requires minimal linkages and cranks, simplifying its construction and reducing the total weight of the flying vehicle. The optimization of weight is consistently a crucial consideration in practical flying vehicles [17][18]. Some of the recent studies providing future research prospects are discussed as follows.…”