Session ZC03: Biological and Bioinspired Flight

Spinning in the rain: Maple samaras recover autorotation after drop impact

Samaras are known for their elegant and robust autorotation, a resilience that persists in the adverse conditions imposed by high-speed raindrops. Like flying insects, descending samaras are not unlikely to be struck by raindrops in an intense storm. In this study, we detail the impact dynamics for impact regions across the samara body and drop-shedding mechanisms that samaras exhibit to return to autorotation. The impact force in different impact locations can pitch the samara up to 60 degrees and, in some cases, induce spanwise roll. Raindrops may shatter or remain intact upon impact, pushing the undamaged samara downward 5-25 wingspan lengths before autorotation is recovered. Drops that strike near the wing tip elicit the greatest recovery distance, while impacts onto the nutlet mass are the least disruptive to the samara and most likely to cause the drop to induce splashing. Our results indicate that samaras are robust to raindrop impacts and, given sufficient height above the ground at impact, always recover autorotation. In so doing, a combination of water repellency, aerodynamic drag on the drop, and centrifugal forces ensure the entire drop is shed from the spinning seed.   

Influence of Mass Distribution on Samara Descent Motions.

Samaras are fruits with heavier seeds and lighter flat wings, which influence their dispersal approaches and play a key role in plant reproduction. This study presents a framework to alter mass distribution on a plate to present the diverse patterns of samara descent. We found that changing the mass distribution results in five distinct flight modes. Statistical analysis suggests that periodic descent patterns like autorotation and spiral tumbling are prevalent in nature, likely due to their stable flight performance. Further simulations reveal that vortices and wake interactions are crucial for lift production and maintaining flight modes. These results demonstrate how strategic mass distribution in samaras enhances their spread and provide new ideas for designing biomimetic flying devices.   

Inertial coupling in the flight mechanics of hummingbirds during an escape maneuver

When responding to a perceived threat, hummingbirds perform an impressive escape maneuver from hovering to backing off and then flying away. In the process, their body goes through simultaneous pitch, yaw, and roll rotations. The rotational dynamics of the body therefore involves inertial coupling from these multi-axis rotations. In this study, we analyzed high-speed videos and performed high-fidelity computational fluid dynamics modeling of the maneuver. We found that while a bird's pitch-up was occurring, inertial coupling between yaw and roll helped slow down and terminate the pitch, thus serving as a passive control mechanism for the maneuver. Furthermore, an inertial coupling between pitch-up and roll can help accelerate yaw before the yaw-roll coupling. Compared with aircraft and other animals that primarily rely on aerodynamic forces for flight control, hummingbirds seem to utilize inertial coupling as a novel mechanism to achieve superb aerial agility.   

A numerical study of the aerodynamic performance of damaged dragonfly wings in forward flight.

Dragonfly wings often suffer damage from predators or by wear over their lifespan. The damage is more likely to start from the wing tip and trailing edge, but damage patterns vary, resulting in diverse aerodynamic effects. We generate a series of damaged dragonfly wings according to the probability maps reported by Rajabi et al. [10.1242/bio.027078], and conduct computational fluid dynamics (CFD) simulations to compare the influence of wing area loss on aerodynamic performance in forward flight, as well as the flow field for analyzing the mechanisms of force changes. The simulations are made using our open-source code WABBIT, developed by Engels et al. [10.4208/cicp.OA-2020-0246]. We find that a large area lost on the forewing can lead to a decrease of horizontal force in the ipsilateral hindwing when the wings approach, due to reduced forewing-hindwing interaction. Meanwhile, the forces on the forewing keep the same as intact wing when the ipsilateral hindwing is damaged tremendously.   

Topolgy optimization of wings with serrated trailing edges using Design-by-Morphing and Bayesian optimization

Aerodynamic efficiency and noise suppression are two seemingly conflicting objectives that aerodynamicists, particularly in the wind energy sector, need to strike a balance between. Serrations on the trailing edges of wings, perhaps inspired by natural predators such as owls, is an active area of research for aerodynamicists. However, the inhibitive cost of high-fidelity simulations to optimize the designs of such wings using conventional topology optimization methods, coupled with designer-biased topological spaces, forces aerodynamicists to work within narrow design spaces where radical improvements are unlikely. In our current work, we optimize such wings with serrated trailing edges using a two-pronged approach: (1) Design-by-Morphing (DbM), a data-driven design strategy for creating a continuous and constraint-free design search space by morphing homeomorphic shapes, that can produce radical out-of-sample designs, something which is unique from conventional design strategies; and (2) a Mixed-variable, Multi-Objective Bayesian Optimization (MixMOBO) algorithm, that can optimize expensive, black-box problems with a few functions calls. DbM-MixMOBO produces non-intuitive groundbreaking designs which exhibit considerable improvements.   

Unsteady lift of flapping membrane wings: Connecting theory to experiments

The aerodynamic performance of flapping membrane wings has been studied both theoretically and experimentally to understand the remarkable maneuverability of bats and other flying mammals. These wings passively deform during flight due to the coupling between the membrane profile and the surrounding flow. The complex physics of this problem often necessitates simplifying assumptions, such as using a 2D airfoil, inviscid flow, and small membrane deformations, to achieve an analytical solution. However, these assumptions are typically not sustained in realistic experimental studies. In this work, we aim to connect the unsteady aerodynamic theory of membrane wings with experimental results. We demonstrate how a simplified theoretical model can predict the unsteady lift of a flapping membrane wing based solely on measurements of membrane deformation. To that end, we extend the unsteady aerodynamic theory of flexible membrane airfoils to include effects of the unsteady freestream velocity encountered by the wing and compare the theoretical predictions with experimental measurements of a robotic flapping wing in hover. Our results show good agreement between the theoretical and measured values of the unsteady lift due to membrane oscillations obtained for moderate to high pitch angles, in which the flow remains mostly attached.   

Quieter UAV Rotor Blades Inspired by Insect Wings: Insights from Computational Models

The application of unmanned aerial vehicles (UAVs) is surging across several industries, paralleled by growing demand for these UAVs. However, the noise emitted by UAVs remains a significant impediment to their widespread use even though in areas such as product delivery, they can be more environmentally friendly than traditional delivery methods. Nature has often been a source of inspiration for devices that are efficient and eco-friendly. In the current study, we revisit the previous work by Seo et. al. (Bioinspir. Biomim 2021) on aeroacoustics of mosquitoes and fruit fly wings to assess if we can leverage some insights from these biological models to develop strategies and designs for reducing drone rotor noise. Aerodynamic simulations are performed by our in-house flow solver ViCar3D and the aeroacoustic sound is predicted by the Ffowcs Williams–Hawkings equation. Sound directivity, power, and characteristics of those rotor designs are compared and discussed.   

Sound Generation Mechanisms in the Flapping Flight of Insects

Aero-acoustics of insect flight has received less attention than aerodynamics whch is typically modeled with an incompressible assumption. The fundamental harmonic of the generated sound corresponds to the flapping wing frequency, but the sound generation mechanisms for the higher order harmonics are not understood. The flapping wings induce complicated vortex structures which related to the sound production. The leading edge, wing tip and trailing vortices associated with the wing dynamics in both hovering and forward flight relate to fluid structure interactions from the wing deformation. Better understanding of the the fluid structure interactions in terms of the sound production as well as lift and drag is needed. The flight sound generation of adult Coconut Rhinoceros Beetle (Oryctes rhinoceros) and the Oriental Flower Beetle (Protaetia orientalis) was studied at an invasive species center at the University of Hawaii Manoa.The individual beetles were measured in terms of size, wing span and weight. A four element microphone array with a synchronized high speed camera (X-Stream, Inc.) at 1000-5000 fps measured the tethered flight sounds. The wing motion was digitized from high speed video for computation with the unsteady, compressible flow solver (CAESIM, Adaptive Research, Inc.). The models of the wing motion were obtained using mesh deformation methods for a moving body. The flapping was modeled with a prescribed bending along with a coupled rotation and translation from a hinge position..