Session D6: Insect Flight II

How mosquitoes fly in the rain

Mosquitoes thrive during rainfall and high humidity. If raindrops are 50 times heavier than mosquitoes, how do mosquitoes fly in the rain? In this combined experimental and theoretical study, we measure the impact force between a falling drop and a free-flying mosquito. High-speed videography of mosquitoes and custom-built mimics reveals a mosquito's low inertia renders it impervious to falling drops. Drops do not splash on mosquitoes, but simply push past them allowing a mosquito to continue on its flight path undeterred. We rationalize the force imparted using scaling relations based on the time of rebound between a falling drop and a free body of significantly less mass.   

The Timing in the Control of Insect Flight Instability

Flapping flight of insects is intrinsically unstable. Using 3D dynamic simulation of flapping flight, we analyze the stability of periodic states associated with the limit cycles of the dynamical system. We construct a discrete time-delayed linear controller and examine the controllability condition. The controller's effectiveness depends in a subtle manner on the timing of the sensory measurement combined with the delay time in actuation.   

Uncovering the aerodynamics of the smallest insects using numerical and physical models

A vast body of research has described the complexity of flight in insects ranging from the fruit fly, \textit{Drosophila melanogaster}, to the hawk moth, \textit{Manduca sexta}. The smallest flying insects have received far less attention, although previous work has shown that flight kinematics and aerodynamics can be significantly different. In this presentation, three-dimensional direct numerical simulations are used to compute the lift and drag forces generated by flexible wings to reveal the aerodynamics of these tiny fliers. Results are validated against dynamically scaled physical models. At the lowest Reynolds numbers relevant to insect flight, the relative forces required to rotate the wings and fling them apart become substantially greater. Wing flexibility can reduce these forces and improve efficiency in some situations.   

Flow Structure on a Flapping Wing: Quasi-Steady Limit

The flapping motion of an insect wing typically involves quasi-steady motion between extremes of unsteady motion. This investigation characterizes the flow structure for the quasi-steady limit via a rotating wing in the form of a thin rectangular plate having a low aspect ratio (AR =1). Particle Image Velocimetry (PIV) is employed, in order to gain insight into the effects of centripetal and Coriolis forces. Vorticity, velocity and streamline patterns are used to describe the overall flow structure with an emphasis on the leading-edge vortex. A stable leading-edge vortex is maintained over effective angles of attack from 30$^\circ$ to 75$^\circ$ and it is observed that at each angle of attack the flow structure remains relatively same over the Reynolds number range from 3,600 to 14,500. The dimensionless circulation of the leading edge vortex is found to be proportional to the effective angle of attack. Quasi-three-dimensional construction of the flow structure is used to identify the different regimes along the span of the wing which is then complemented by patterns on cross flow planes to demonstrate the influence of root and tip swirls on the spanwise flow. The rotating wing results are also compared with the equivalent of translating wing to further illustrate the effects of the rotation.   

Computational Aerodynamics of Insects' Flapping Flight

The kinematics of the Insects' flapping flight is modeled through mathematical and computational observations with commercial software. Recently, study on the insects' flapping flight became one of the challenging research subjects in the field of aeronautics because of its potential applicability to intelligent micro-robots capable of autonomous flight and the next generation aerial-vehicles. In order to uncover its curious unsteady characteristics, many researchers have conducted experimental and computational studies on the unsteady aerodynamics of insects' flapping flight. In the present paper, the unsteady flow physics around insect wings is carried out by utilizing computer software e-AIRS. The e-AIRS (e-Science Aerospace Integrated Research System) analyzes and models the results of computational and experimental aerodynamics, along with integrated research process of these two research activities. Stroke angles and phase angles, the important two factors in producing lift of the airfoils are set as main parameters to determine aerodynamic characteristics of the insects' flapping flight. As a result, the optimal phase angle to minimize the drag and to maximize the lift are found. Various simulations indicate that using proper value of variables produce greater thrust due to an optimal angle of attack at the initial position during down stroke motion.   

Butterfly scales and their local surface drag dependence on flow orientation

An experimental study was carried out to measure surface drag over embedded cavity models based on the geometry of butterfly wing scales. Monarch (\textit{Danaus plexippus}) scales, each measuring about 0.1 mm in length, were observed using microscopy to evaluate the microgeometry. Two separate, fabricated models scaled up (300:1) the geometry for dynamically similar testing in a Couette flow oil tank facility. The drag induced over the patterned surfaces was measured using a force gauge. Flow transverse to the rows of scales resulted in a significant drag decrease ($>$ 30{\%}), with dependence on Re. This drag reduction is attributed to the formation of embedded vortices forming between the rows of scales resulting in a ``roller bearing'' effect. Flow parallel to the rows, as expected, resulted in larger drag increases, especially at lower Re. Both effects may prove beneficial to the butterfly, during flapping and gliding flight, and will be discussed based on the observed orientation of the scales on real specimens.   

The importance of being top-heavy: Intrinsic stability of flapping flight

We explore the stability of flapping flight in a model system that consists of a pyramid-shaped object that freely hovers in a vertically oscillating airflow. Such a ``bug'' not only generates sufficient aerodynamic force to keep aloft but also robustly maintains balance during free-flight. Flow visualization reveals that both weight support and intrinsic stability result from the periodic shedding of dipolar vortices. Counter-intuitively, the observed pattern of vortex shedding suggests that stability requires a high center-of-mass, which we verify by comparing the performance of top- and bottom-heavy bugs. Finally, we visit a zoo of other flapping flyers, including Mary Poppins' umbrella, a flying saucer or UFO, and Da Vinci's helicopter.   

Vortex Tilting and the Enhancement of Spanwise Flow in Flapping Flight

The leading edge vortex is key in lift generation on flapping wings. Its stability depends on the transport of the entrained vorticity into the wake via spanwise flow. This study investigates the generation and enhancement of spanwise flow based on the chordwise vorticity that results from the tilting of the leading edge vortex and trailing edge vortex. Two dynamically scaled robotic model wings, one rectangular and one insect wing shaped based on \textit{Drosophila melanogaster} (fruit fly), are submerged in a tank of mineral oil and actuated into flapping motion. The overall flow structure was visualized and measured by a Volumetric 3-component Velocimetry (V3V) system (TSI, Inc.). From the three dimensional flow measurements obtained, the chordwise vorticity resulting from the vortex tilting is shown. The distribution of the resulting spanwise flow induced by the vortex tilting is shown using isosurfaces and on a planar cross section downstream of the leading edge. It is observed that the largest spanwise flow is located in the area between the tilted leading edge vortex and the tilted trailing edge vortex, supporting our hypothesis that the vortex tilting enhances the spanwise flow. This vortex tilting mechanisms helps to explain the overall flow structure and the stability of the leading edge vortex.   

Simulation of a prescribed fruitfly flapping motion by the ALE-GFD method on a hybrid Cartesian-meshfree grid

The flapping-wing aerodynamics of insects has been a subject of immense interest for many years. The Arbitrary Lagrangian-Eulerian -- Generalized Finite Difference (ALE-GFD) computational scheme on hybrid convecting Cartesian-meshfree grid systems represents a viable alternative to existing mesh-based and immersed boundary approaches for simulating the highly complex and unsteady flows generated by the flapping wings of insects. The three-dimensional flapping-wing flows of a fruitfly (Drosophila) are simulated in the present study. As the forces generated are very sensitive to the acceleration of the wings, a smoothing process was applied on the flapping kinematics to suppress non-physical fluctuations and spikes from the force outcome. The resulting lift and drag forces are then validated with the experimental results, obtained from a parallel experimental study conducted by the research group, measured on a fruitfly-like wing profile executing the identical motion. The excellent agreement between the results demonstrates the feasibility and efficacy of the ALE-GFD numerical approach.