Session LT: Biolocomotion VIII: Flapping and Flying III

Effect of low-amplitude vibrations on impulsively-started wings

The development and shedding of leading edge vortices (LEVs) over wings is crucial to lift generation in the flapping flight of birds and insects. Many studies have investigated the flow field empirically by means of wing models that approximate or reproduce the wing kinematics. Wing models are often made of stiff materials (e.g. aluminum, steel) or are intentionally flexible to examine aeroelastic properties. However, even stiff wings will vibrate under forces induced by accelerations, which may modify the flow field and the LEV shedding frequency. This study investigates the effects of start-up vibrations of impulsively started flat plates of different materials (Re = 60,000) at a post-stall angle of attack. Wing vibration was recorded with high-speed imaging and the flow field was analyzed with particle image velocimetry. Results do not eliminate the possibility of lock-on between the wing's natural frequency and the LEV shedding frequency.   

Investigation of vortex saturation for a simultaneously rotating and pitching wing

We investigate vortex saturation for a flat plate wing that simultaneously rotates and pitches. A vortex is saturated when it attains maximum circulation, which may occur at a non-dimensional time called the formation number. The experiment is done in a water tank and flow visualization is used to obtain the three-dimensional flow structure. We examine plate aspect ratios ranging from 2 to 4 with varying wing geometries and a Reynolds number of approximately 5000, similar to that of a hummingbird. We use digital particle image velocimetry (DPIV) to obtain the flow velocity, vorticity and circulation. The effects of spanwise (root-to-tip) flow and the tip vortex on the overall vortex structure are examined.   

Lift, Drag and Flow-field Measurements around a Single-degree-of-freedom Toy Ornithopter

The aerodynamics of a flight-worthy toy ornithopter under laminar inflow conditions are studied using a combination of load cell, flow visualization, high speed camera and PIV experiments. All the experiments were performed in the large wind tunnel facility at New Mexico State University, with the exception of a free flight test of the model. Measurements from a six-axis load cell were used to capture the variation of the lift and drag forces at various angles of attack, flapping frequencies and free-speed velocities. Smoke visualization is used to clearly demonstrate that the momentum flux in the downward direction during downstroke exceeds the upward momentum flux during upstroke due to the flexion of the wing and its angle of attack. This net surplus creates the lift in such ornithopter designs despite the stroke symmetry. PIV measurements are then performed at suitable locations to identify flow structures around the wing at various spanwise locations. A control volume analysis is performed to compare the momentum deficit in the wake to the load cell measurements.   

Three-Dimensional Wing Kinematics and Aerodynamic Characteristics of a Beetle in Free Flight

Detailed three dimensional wing kinematics and aerodynamic characteristics are experimentally presented for the free flight of a beetle, Allomyrina dichotoma, which has a pair of elytra (fore wings) and hind wings. The kinematic parameters of the wing motion, such as the wing tip trajectory, angle of attack, torsion angle, and camber deformation, are obtained from a 3D reconstruction technique that involves the use of two or three synchronized high-speed cameras to digitize various points marked on the wings. Our data show outstanding characteristics of wing deformation and flexibility in the free flight of the beetle. To find out the mechanism of aerodynamic force, the leading edge vortex (LEV) and trailing edge vortex (TEV) on both elytron and hind wing were observed by using smoke wire visualization and digital particle image velocimetry (DPIV) technique. Qualitative smoke lines in the region of the most intent vortex shedding demonstrate clearly the interaction between elytron and hind wing in hovering, forward, and climbing flight conditions. In addition, flow fields near regions of the elytron and the hind wing are quantitatively analyzed in order to visualize the LEV and calculate the circulation and lift coefficient by means of a DPIV experiment.   

Wing Flexion and Aerodynamics Performance of Insect Free Flights

Wing flexion in flapping flight is a hallmark of insect flight. It is widely thought that wing flexibility and wing deformation would potentially provide new aerodynamic mechanisms of aerodynamic force productions over completely rigid wings. However, there are lack of literatures on studying fluid dynamics of freely flying insects due to the presence of complex shaped moving boundaries in the flow domain. In this work, a computational study of freely flying insects is being conducted. High resolution, high speed videos of freely flying dragonflies and damselflies is obtained and used as a basis for developing high fidelity geometrical models of the dragonfly body and wings. 3D surface reconstruction technologies are used to obtain wing topologies and kinematics. The wing motions are highly complex and a number of different strategies including singular vector decomposition of the wing kinematics are used to examine the various kinematical features and their impact on the wing performance. Simulations are carried out to examine the aerodynamic performance of all four wings and understand the wake structures of such wings.   

Two- and three-dimensional numerical simulations of clap-fling-sweep of hovering insects

The Lighthill--Weis-Fogh clap-fling-sweep mechanism is a movement used by some insects to improve their flight performance. As first suggested by Lighthill (JFM, 1973), this mechanism allows large circulations around the wings to be established immediately as they start to move. Initially, the wings are clapped. Then they fling open like a book, and a non-zero circulation is established around each of them. Thus one wing can be considered as the starting vortex for the other. Then they sweep apart, carrying these bound vortices and generating lift. Since the insect wings have relatively low aspect ratio and rotate, three-dimensional effects are important, such as spanwise flow and stabilization of the leading edge vortices (Maxworthy, JFM 2007). To explore these effects, we perform direct numerical simulations of flapping wings, using a pseudo-spectral method with volume penalization. Comparing two- and three-dimensional simulations for the same setup clarifies the role of the three-dimensionality of the wake. Our results show that the two-dimensional approximation describes very well the flow during fling, when the wings are near, but three-dimensional effects become crucial when the wings move far apart.   

A computational study of the clap-and-fling aerodynamic mechanism

Clap-and-fling is a particular wing kinematic pattern utilized by some insects and birds to produce enhanced aerodynamic forces. It consists of two very distinct phases: i) the leading edges of the two wings are brought together near the upper limit of the upstroke and subsequently the wings are rotated around their leading edges, ``clapping'' like a closing book; ii) at the onset of the downstroke, and while they are still close, the two wings rotate around their trailing edges ``flinging'' apart. Prior theoretical and experimental work suggested that clap-and-fling is responsible for production of unusually high lift coefficients. However, due to limitations of the theoretical models and experimental techniques, detailed quantitative results are yet to be reported. In the present work we provide a concrete description of the underlying physics by means of high-fidelity three-dimensional simulations based on the Navier- Stokes equations for incompressible flow. Our results verify the lift enhancement trends observed in the experiments and indentify the particular flow patterns correlated with such increases.   

Gliding flight in snakes: non-equilibrium trajectory dynamics and kinematics

For animal gliders that live in trees, a glide trajectory begins in free fall and, given sufficient space, transitions to equilibrium gliding with no net forces on the body. However, the dynamics of non-equilibrium gliding are not well understood. Of any terrestrial animal glider, snakes may exhibit the most complicated glide patterns resulting from their highly active undulatory behavior. Our aim was to determine the characteristics of snake gliding during the transition to equilibrium. We launched ``flying'' snakes (\textit{Chrysopelea} \textit{paradisi}) from a 15 m tower and recorded the mid-to-end portion of trajectories with four videocameras to reconstruct the snake's 3D body position. Additionally, we developed a simple analytical model of gliding assuming only steady-state forces of lift, drag and weight acting on the body and used it to explore effects of wing loading, lift-to-drag ratio, and initial velocity on trajectory dynamics. Despite the vertical space provided to transition to steady-state gliding, snakes did not exhibit equilibrium gliding and in fact displayed a net positive acceleration in the vertical axis.   

Flying Snake Flight Performance: Role of Cross-sectional Shape and Orientation of Tandem Body Segments

The ``flying'' snake (\textit{Chrysopelea} \textit{paradisi}) possesses one of the most dynamically complex gliding flight patterns found in nature. Unlike other airborne animals that possess wings or flaps, this species lacks appendages to aid in controlling its flight path and producing lift. While gliding, the snake undergoes a high-amplitude undulatory motion, during which it expands its ribs to double its body width so that its cross section mimics the shape of a thick airfoil with camber and leading and trailing edges. The goal of this study was to determine the aerodynamic forces produced by the snake and investigate the underlying fluid dynamics that give the snake its unique gliding and maneuvering capabilities. Two-dimensional force measurements and CFD simulations were performed on an anatomical model of the snake's cross section to determine the steady and unsteady lift and drag coefficients, as well as the vortex shedding characteristics. These results show that the lift and drag produced by the model is dependent on Reynolds number, angle of attack, and the orientation of upstream and downstream body segments. Several tandem model configurations produced significant increases in lift and decreases in drag.   

Numerical investigation of wake structures of slow-flying bats

Recently, some unique features of wake structure in bat flight have been revealed by experiments. It is found that the flow structure of bat flight is more complex than that of bird. A conceptual wake model of bat flight has been ``rebuilt'' using 2D DPIV images, but there is some risk of missing the details regarding dynamics of 3D vortex structures. Detailed flow information is still needed to understand the unsteady flow in bat flying. In this work, we perform 3D simulation of bat flying at the Reynolds number of 1000 (based on upstream flow and mean chord length) using the immersed boundary method. The geometry and wing-beat kinematics of bat are taken from the work of Watts et al (2001). The topology and evolution of the wake structures are described. The variation of topology in wake structures with the flapping Strouhal number is investigated. Moreover, the link between the generation of high lift and leading edge vortex is also studied.