Session A05: Animal Flight: Flying Insects I

The effect of bio-inspired butterfly wing tip scales on the growth of a leading edge vortex

Previous studies with live Monarch butterflies have shown that removal of the scales can have a large effect on flight efficiency. One potential mechanism is if the scales found on the wing tips can affect tip-vortex growth. This study investigates the effect of butterfly wing tip scales on leading edge vortex (LEV) formation and growth. It is hypothesized that the addition of wing tip scales will decrease the vortex growth as the scales impede the motion of the flow around the tip of the wing. This would result in less energy loss to the tip vortices occurring during butterfly flight. A tow tank experiment was used to replicate the butterfly wing tip scales' effect through the movement of two plates in mineral oil. The first model is a smooth-edge flat plate while the other has 3D printed scales covering the leading edge which mimic the long, thin scales found on the wing tips of the Monarch butterfly. Digital Particle Image Velocimetry (DPIV) was then used to track the development of LEVs. Results will be presented.   

Unsteady aerodynamics and odorant transport in an upwind surging flight of drosophila

For foraging and mating purposes, insects rely on their olfactory system to detect odor stimuli and track down odor sources using their high-efficient flapping wing mechanism. However, we have little understanding of how insects utilize the local filament-like odor plume structures to perceive the surrounding dynamic environments. In this study, a fully coupled three-way numerical solver is developed, which solves the 3D Navier-Stokes equations coupled with equations of motion for the passive flapping wings and the odorant advection-diffusion equation. High-fidelity numerical simulations of a model fruit fly in surging upwind flight is performed, superimposing isotropic turbulent fluctuations to the uniform inflow. A parametric study is conducted to investigate unsteady aerodynamics and odorant transport over a range of key parameters, including turbulent intensity, reduced frequency, Reynolds number, and Schmidt number. Our simulation results will provide new insights into the mechanism of how fruit flies perceive odor landscape and inspire the future design of odor-guided micro aerial vehicles for surveillance and detection missions.   

Lateral flight instability in fruit flies is determined by wing-wing interaction and wing elevation kinematics

Understanding the passive dynamics of flying insects is important for evaluating the constraints under which the insect flight control system evolved and for developing biomimetic robots. Previous studies using computational fluid dynamics (CFD) simulations on several insects found that they are passively unstable due to a non-oscillatory diverging mode, with positive coupling between roll and sideways motion. These studies assumed simplified and sinusoidal wing kinematics, with zero elevation angle and negligible wing-wing interaction. Here, we revisit these assumptions, by performing CFD-based linear stability analysis of a fruit fly with accurate experimental wing kinematics and wing-wing interactions. We find that passive lateral dynamics are unstable, but with an oscillating-diverging mode that stems from a negative coupling between roll and sideways motion: e.g., when the fly slides to the right, it rolls to the left. Due to low damping, this seemingly restoring effect results in an oscillating instability. This coupling stems from wing elevation that induces a drag-based restoring torque, and from wing-wing interaction, that reduces lift-based diverging torque. These results highlight the importance of real wing kinematics and wing-wing interactions in such analyses.   

Dimensional Analysis of Mountain Pine Beetle Flight

The Mountain Pine Beetle (MPB) is one of the most destructive pests in the pine forests of Western North America and has proven difficult to control. As the MPB disperses through flight, the study of MPB flight endurance and range is critical to estimating and controlling their spread. As the behavioral characteristics of the MPB limit free-flight measurements, flight mills (captive-flight apparatus that limit an insect to flight circular fixed path) are commonly used instead. We characterize the flight performance of the MPB on a flight mill using recent dimensional analysis of unsteady airfoil flows. The results propose specific, quantitative improvements for estimates of MPB dispersion.

Moreover, while it has been established in the literature that peak cruising performance for flapping wings, across many different species, lies within an optimal range of Strouhal numbers of 0.2 < St < 0.4, all observations of tethered MPBs in this work have been with Strouhal numbers greater than 2, which is more characteristic of rapid accelerations or escape maneuvers than efficient cruising. This indicates that the MPB consumes an extraordinary amount of energy on a flight mill, meaning that existing flight-mill derived flight range and endurance estimates may be significant underestimates.   

Data-driven olfactory search in a turbulent flow

Tracking the source of an odor which is advected by a turbulent flow is an important behavior for many flying insects and other animals. This olfactory search problem is rendered especially difficult by the intermittency intrinsic to turbulence, and it requires complex search strategies which properly leverage infrequent odor detections. In this work, we perform direct numerical simulations (DNS) of tracer particles emitted from a points source in a turbulent flow with a mean wind. We study the concentration statistics of the tracer data and use the data to extract model-based policies for search. We compare the empirical performance of near optimal policies (in the sense of partially observable Markov decision processes) to that of several heuristics.

    

Honeybee flight in windy conditions

The study of bee flight and their collective behavior has been the focus of many studies that have offered us insight into how bees are able to fly and how they communicate with one another. However not much is known about honeybee fight dynamics in turbulent and windy conditions and how they manage to maneuver in such environments. Here we use 3 GoPro cameras to track honeybees in three-dimensions (3D) during their flight to and from the hive. We create different turbulent conditions using fans and a mobile active grid, similar to one used in high Reynolds number wind tunnel experiments. We find that under the conditions investigated, honeybees seem to exhibit similar flight dynamics which are not dependent on the characteristics of the different flows they are exposed to. In flight, honeybees accelerate slowly and decelerate rapidly. While this behavior is observed in both calm and windy conditions, it is increasingly dominant in windy conditions where short straight trajectories are broken up by turns and increased maneuvering.

    

Aerodynamic characteristics of a hovering moquito wing

We numerically investigate the aerodynamic characteristics of a hovering mosquito wing. To understand the role of the deviation motion in its flapping kinematics, we perform a separate simulation without deviation motion and compare the results with those with deviation motion. The deviation motion significantly changes the angle of attack, which greatly influences the instantaneous force and power generation. In particular, the wing with deviation motion produces a large drag force at the beginning of each half-stroke by increasing the angle of attack. Furthermore, the wing uses the drag force to enhance the vertical force, and maintains positive vertical force during a whole flapping period. The present result indicates that the mosquito wing generates a larger vertical force more efficiently by including the deviation motion.   

A Numerical Study on Three-Dimensional Flapping Dragonfly Wings with Optimized Input Kinematics for hovering and forward flight

Dragonflies are capable of hover, glide, forward flight, quick turning, etc., with substantial flight times. Previous studies found that dragonfly wings require distinctive input kinematics for different flight modes. For instance, in hovering/forward flight, the wings flap out-of-phase, whereas during quick maneuvers, the wings flap in-phase. In this study, we performed numerical simulation using a dynamic mesh to analyze the wake structure over a pair of tandem dragonfly wings and further optimized their aerodynamic performance with novel flapping kinematics. The wing geometries are based on structural studies. The input kinematics are selected as the mix of pure sinusoidal and periodic Eldredge functions.  The design parameters are taken similar to those of Berman and Wang (2007) and Geherke et al. (2019). Our study found that both wings' combined vertical lift coefficient is 1.56 times more than their individual lift coefficients when summed up. Moreover, the hindwing experiences most of the enhancement in lift resulting from the vortex synergy interaction. It was found the downwash-upwash wake interactions are insignificant in the in-phase flapping, which agrees with the results of Lua et al. (2018). The detailed flow physics will be discussed in the presentation.   

Demystifying the Clap & Fling Mechanism: Insights from the Force Partitioning Method (FPM)

Flying insects employ an intricate flapping motion of their wings to sustain and control flight. Several physical mechanisms have been advanced over the years to account for their large lift coefficients. Using video recordings of hovering Encarsia Formosa (tiny wasps), Weis-Fogh (J. Exp. Biol,1973) was the first to observe the use of the clap & fling (CF) mechanism, where the interaction of the two wings generates additional lift. In CF kinematics, the wings clap together at the end of the upstroke before swiftly flinging apart as the downstroke begins. This synchronized interaction of the wings along with their coupled rotation induces leading edge vortices at the wings first during the clap phase and then again while the wings pronate.  Here we apply the force partitioning method to dissect the lift generation mechanism in wings undergoing CF. The FPM, which is derived using first principles, provides a mathematical partitioning of total pressure force on an immersed body into components due to vortical regions, added-mass effects, and viscous momentum diffusion. FPM is applied to CF kinematics over a range of Reynolds numbers and the results examined to provide new insights into this unique flight mechanism.