Effect of clap-and-fling mechanism on force generation in flapping wing micro aerial vehicles

The clap-and-fling effect, first observed in a number of insects, serves as a lift-enhancing mechanism for bio-inspired flapping wing micro aerial vehicles (MAV). In our comprehensive literature survey, we observe that the effect manifests differently in insects and contemporary MAVs; insects have active control over the angle of attack and stroke plane of the wing, whereas a number of kinematic parameters of an MAV's flexible wings are determined passively. Although there is consensus that flinging motion significantly enhances aerodynamic lift, the effect of clapping motion is not well-studied. To address this gap, we experimentally quantify the contribution of clapping motion using force measurement and particle image velocimetry. No significant enhancement in lift was observed due to clapping motion, because the momentum jet was too weak. However, the kinematics and flow conditions in our study were notably different from those in the previous studies on insect models. The wings of the MAV are flexible, and deform passively. Hence, the clapping of the trailing edges, and the appearance of a trailing edge momentum jet, was delayed and significantly suppressed. Using force measurement and CFD simulations, it was also found that the lesser the distance between the leading edges of the wings at the end of clap, the higher is the lift due to the subsequent fling.

Wing-wake interaction: comparison of 2D and 3D flapping wings in hover flight

The wing-wake interaction of flapping wings while hovering has been investigated, with the focus on the difference in wing-wake interaction between 2D and 3D flapping wings. Numerical simulations are conducted at a Reynolds number of 100, and the flapping configurations are divided into the 2D, quasi-3D and 3D categories. Variations of the aspect ratio and Rossby number allow the flapping configuration to morph gradually between categories. The wing-wake interaction mechanisms are identified and the effect of three-dimensionality on these mechanisms is discussed. Three-dimensionality affects wing-wake interaction through four primary aerodynamic mechanisms, namely, induced jet, downwash/upwash, leading-edge vortex (LEV) shedding due to vortex pairing, and the formation of a closely attached LEV. The first two mechanisms are well-established in the literature. With regard to the LEV shedding mechanism, it is revealed that the interaction between the LEV and the residue vortex from the previous stroke plays an important role in the early vortex shedding of 2D flapping wings. This effect diminishes with increasing three-dimensionality. With regard to the mechanism of the closely attached LEV, the wake encourages the formation of an LEV that is closely attached to the wing's top surface, which is beneficial to lift generation. This closely attached LEV mechanism accounts for most of the lift enhancement that arises from wake effects. Three-dimensionality alters the efficacy of the different aerodynamic mechanisms. Consequently, the dual peak lift coefficient pattern typically seen on 2D flapping wings transforms into the single peak lift coefficient pattern of the 3D flapping wing. It is also demonstrated that the mean lift enhancement due to wing-wake interaction diminishes rapidly when three-dimensionality is introduced. Results suggest that, for wings with parameters close to those of natural flyers, wing-wake interaction yields marginal lift enhancement and a small increase in energy consumption.

Ground effect on the aerodynamics of three-dimensional hovering wings

This paper reports the results of combined experimental and numerical studies on the ground effect on a pair of three-dimensional (3D) hovering wings. Parameters investigated include hovering kinematics, wing shapes, and Reynolds numbers (Re). The results are consistent with the observation by another study (Gao and Lu, 2008 Phys. Fluids, 20 087101) which shows that the cycle-averaged aerodynamic forces generated by two-dimensional (2D) wings in close proximity to the ground can be broadly categorized into three regimes with respect to the ground clearance; force enhancement, force reduction, and force recovery. However, the ground effect on a 3D wing is not as significant as that on a 2D flapping wing reported in (Lu et al 2014 Exp. Fluids, 55 1787); this could be attributed to a weaker wake capture effect on 3D wings. Also, unlike a 2D wing, the leading edge vortex (LEV) remains attached on a 3D wing regardless of ground clearance. For all the wing kinematics considered, the three above-mentioned regimes are closely correlated to a non-monotonic trend in the strength of downwash due to the restriction of root and tip vortex formation, and a positional shift of wake vortices. The root vortices in interaction with the ground induce an up-wash in-between the two wings, causing a strong 'fountain effect' (Maeda and Liu, 2013 J. Biomech. Sci. Eng., 8 344) that may increase the body lift of insects. The present study further shows that changes in wing planform have insignificant influence on the overall trend of ground effect except for a parallel shift in force magnitude, which is caused mainly by the difference in aspect ratio and leading edge pivot point. On the two Reynolds numbers investigated, the results for the low Re case of 100 do not deviate significantly from those of a higher Re = 5000 except for the difference in force magnitudes, since low Reynolds number generates lower downwash, weaker LEV, and lower rotational circulation. Additionally, lower Re leads to a weaker fountain effect.

Aspect ratio effects on revolving wings with Rossby number consideration

Numerical simulations have been conducted to investigate the effect of aspect ratio (AR) on the mean lift generation of a revolving flat rectangular wing. The purpose of the study is to address some discrepancies reported in the literature regarding the influence of AR on mean lift coefficient. Here, we consider a range of AR from 1 to 10 and Rossby number (Ro) from 0.58 to 7.57, and our results show that different degrees of coupling between AR and Ro yield different trends of a mean lift coefficient with respect to increasing AR. The choice of reference velocity for the normalisation of mean lift forces also has a significant effect on the perceived AR effect. By isolating the effect of Ro, we found that higher AR produces higher mean lift coefficient until it plateaus at a sufficiently high AR. This finding is consistent with conventional fixed wing aerodynamics. Additionally, our results show that increasing AR reduces the three-dimensional wing tip effect and is beneficial to mean lift generation while higher Ro increases leading-edge vortex instability, which is detrimental to mean lift generation. Therefore, mean lift generation on revolving wings is dictated by the competition between these two factors, which represent two fundamentally independent phenomena.

A quasi-steady aerodynamic model for flapping flight with improved adaptability

An improved quasi-steady aerodynamic model for flapping wings in hover has been developed. The purpose of this model is to yield rapid predictions of lift generation and efficiency during the design phase of flapping wing micro air vehicles. While most existing models are tailored for a specific flow condition, the present model is applicable over a wider range of Reynolds number and Rossby number. The effects of wing aspect ratio and taper ratio are also considered. The model was validated by comparing against numerical simulations and experimental measurements. Wings with different geometries undergoing distinct kinematics at varying flow conditions were tested during validation. Generally, model predictions of mean force coefficients were within 10% of numerical simulation results, while the deviations in power coefficients could be up to 15%. The deviation is partly due to the model not taking into consideration the initial shedding of the leading-edge vortex and wing-wake interaction which are difficult to account under quasi-steady assumption. The accuracy of this model is comparable to other models in literature, which had to be specifically designed or tuned to a narrow range of operation. In contrast, the present model has the advantage of being applicable over a wider range of flow conditions without prior tuning or calibration, which makes it a useful tool for preliminary performance evaluations.