Insect-inspired, tailless, hover-capable flapping-wing robots: Recent progress, challenges, and future directions

At rest, beetles fold and tuck their hindwings under the elytra. For flight, the hindwings are deployed through a series of unfolding configurations that are passively driven by flapping forces. The folds lock into place as the wing fully unfolds and thereafter operates as a flat membrane to generate the aerodynamic forces. We show that in the rhinoceros beetle (Allomyrina dichotoma), these origami-like folds serve a crucial shock-absorbing function during in-flight wing collisions. When the wing collides with an object, it collapses along the folds and springs back in place within a single stroke. Collisions are thus dampened, helping the beetle to promptly recover the flight. We implemented this mechanism on a beetle-inspired wing on a flapping-wing robot, thereby enabling it to fly safely after collisions.

https://science.sciencemag.org/content/sci/370/6521/1214.full.pdf

Mechanisms of collision recovery in flying beetles and flapping-wing robots.

This paper reports a fundamental investigation on the aerodynamics of two-dimensional flapping wings in tandem configuration in forward flight. Of particular interest are the effects of phase angle (φ) and center-to-center distance (L) between the front wing and the rear wing on the aerodynamic force generation at a Reynolds number of 5000. Both experimental and numerical methods were employed. A force sensor was used to measure the time-history aerodynamic forces experienced by the two wings and digital particle image velocimetry was utilized to obtain the corresponding flow structures. Both the front wing and the rear wing executed the same simple harmonic motions with φ ranging from −180° to 180° and four values of L, i.e., 1.5c, 2c, 3c, and 4c (c is the wing chord length). Results show that at fixed L = 2c, tandem wings perform better than the sum of two single wings that flap independently in terms of thrust for phase angle approximately from −90° to 90°. The maximum thrust on the rear wing occurs du...

Aerodynamics of two-dimensional flapping wings in tandem configuration

Development of a novel non-contact piezoelectric wind energy harvester excited by vortex-induced vibration

Energy harvesting for jet engine monitoring

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.

http://iopscience.iop.org/article/10.1088/1748-3190/11/3/036005/pdf

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

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.

https://iopscience.iop.org/article/10.1088/1748-3190/11/5/056013/pdf

Aspect ratio effects on revolving wings with Rossby number consideration

A silicon chip integrated microelectromechanical (MEMS) wind energy harvester, based on the vortex-induced vibration (VIV) concept, has been designed, fabricated, and tested as a proof-of-concept demonstration. The harvester comprises of a cylindrical oscillator attached to a piezoelectric MEMS device. Wind tunnel experiments are conducted to measure the power output of the energy harvester. Additionally, the energy harvester is placed within a formation of up to 25 cylinders to test whether the vortex interactions of multiple cylinders in formation can enhance the power output. Experiments show power output in the nanowatt range, and the energy harvester within a formation of cylinders yield noticeably higher power output compared to the energy harvester in isolation. A more detailed investigation conducted using computational fluid dynamics simulations indicates that vortices shed from upstream cylinders introduce large periodic transverse velocity component on the incoming flow encountered by the downstream cylinders, hence increasing VIV response. For the first time, the use of formation effect to enhance the wind energy harvesting at microscale has been demonstrated. This proof-of-concept demonstrates a potential means of powering small off-grid sensors in a cost-effective manner due to the easy integration of the energy harvester and sensor on the same silicon chip.

/pdf/vortex-induced-vibration-wind-energy-harvesting-by-1r27xkxhkj.pdf

Vortex-induced vibration wind energy harvesting by piezoelectric MEMS device in formation.

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

Ground effect on the aerodynamics of three-dimensional hovering wings.

Flapping of an insect wing can be broadly separated into sweeping, elevating, and rotational motions. The sweeping motion generates forward velocity, and the rotational motion imposes an appropriate angle of attack; both are vital to lift generation. However, the purpose of elevating motion in insect flight remains unclear. In this paper, the aim is to better understand the effects of elevating motion to lift generation and vortex structure development when rigid wings are subjected to three-dimensional simple harmonic motion and hovering hawkmoth flapping motion. Both experimental and numerical techniques are used, and results show that, among the different types of simple harmonic motions considered here, only figure-of-eight motions at a relatively low midstroke angle of attack (25 deg) outperform flapping motions without elevating motion. In this case, lift is enhanced by approximately 11% with insignificant cost to hovering efficiency. The lift enhancement could be attributed to rapid growth of the l...

Y. J. Lee

Papers

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

Aspect ratio effects on revolving wings with Rossby number consideration

Vortex-induced vibration wind energy harvesting by piezoelectric MEMS device in formation.

Ground effect on the aerodynamics of three-dimensional hovering wings.

Aerodynamic effects of elevating motion on hovering rigid hawkmothlike wings