Session AX: Aerodynamics I: Flapping Wings

Aerodynamics of high frequency flapping wings

We investigated the aerodynamic performance of high frequency flapping wings using a 2.5 gram robotic insect mechanism developed in our lab. The mechanism flaps up to 65Hz with a pair of man-made wing mounted with 10cm wingtip-to-wingtip span. The mean aerodynamic lift force was measured by a lever platform, and the flow velocity and vorticity were measured using a stereo DPIV system in the frontal, parasagittal, and horizontal planes. Both near field (leading edge vortex) and far field flow (induced flow) were measured with instantaneous and phase-averaged results. Systematic experiments were performed on the man-made wings, cicada and hawk moth wings due to their similar size, frequency and Reynolds number. For insect wings, we used both dry and freshly-cut wings. The aerodynamic force increase with flapping frequency and the man-made wing generates more than 4 grams of lift at 35Hz with 3 volt input. Here we present the experimental results and the major differences in their aerodynamic performances.   

Aerodynamic damping during body translation in animal flight: modeling and experimental results of flapping counter force (FCF)

Body movements of flying animals change their effective wing kinematics and influence aerodynamic forces. Our previous studies found that substantial aerodynamic damping was produced by flapping wings during body rotation through a passive mechanism we termed flapping counter-torque (FCT). Here we present the aerodynamic damping produced by flapping wings during body translations, which we termed flapping counter-forces (FCFs). Analytical models were derived and the aerodynamic effect of spanwise flow and wing-wake interaction were also explored. The FCFs are dependent on body velocities, wing beat amplitude and frequency. Aerodynamic force and PIV measurements were compared with the analytical models. The experiments were conducted on a pair of dynamically scaled robotic model wings in an oil tank. Experiments in air using a pair of high frequency flapping wing further validate the models. Complete 6-DOF flight dynamic model was derived.   

Aerodynamics of flapping wings with fluttering trailing edges

Our previous work on the aerodynamics of passive flexible flapping wings showed that there is a strong relationship between the dynamics of trailing edge and the size of the leading edge vortex, therefore aerodynamic forces. Here we investigated the aerodynamic effects of active trailing edges. The experiments were conducted on a model flapping wing in an oil tank. During static tests, the trailing edge bending angle was held constant from the angle of attack of the upper portion of the rigid wing. For dynamic cases, the trailing edge was controlled to flutter with a prescribed frequency and amplitude. Force measurements and PIV results show that trailing edge flexion/camber strongly correlates with the leading edge vortex and the aerodynamic forces. In addition, large instantaneous force variations are observed in the dynamic fluttering cases, suggesting that trailing edge can be used for force modulation in MAVs.   

Effect of gust on flow patterns around a robotic hummingbird wing

Numerous studies have demonstrated the importance of the leading edge vortex (LEV) in enhancing lift production during hovering flight for a hummingbird. Almost all of these experiments have been performed under laminar inflow conditions without the presence of transient flow phenomena (e.g. gust). And yet, real-life ornithopters in the field have to routinely tackle gust and directional changes in the wind. In this talk, preliminary results from an investigation of the flow field modulation around a hummingbird wing under well-controlled gusty conditions are presented. Using a 2-degree of freedom robotic hummingbird model wing mounted on a translation stage, conditions replicating a gust impacting a wing are created at the NMSU water channel facility. Phase-locked PIV velocity measurements were obtained around the wing in the presence of gusts varying from 5-30{\%} of the mean tangential wing velocity. These measurements, in combination with force and moment measurements from a six-axis load cell, are used to understand transient flow phenomena induced by the gust, and their effect on the net thrust and lift forces on the robot's wings over a range of Reynolds number (1400$<$Re$<$20000).   

Clap and fling in tiny insects with porous wings

In contrast to the flapping flight of insects of length scales ranging from the fruit fly to the hawk moth, the aerodynamics of flight in insects such as thrips that are 1 mm or less in length is not as well understood. Viscous effects become significant at this range of flight where Reynolds number Re $<$ 10, and lift forces drop significantly relative to drag forces. These insects have been proposed to augment lift through adaptations in the flight kinematics, wing flexibility and wing morphology. With reference to the flight kinematics, thrips and other tiny insects clap their wings at the end of each upstroke and fling them apart at the beginning of each downstroke (see Ellington, J. Exp. Biol., 1980). Furthermore, these insects have highly bristled wing surfaces as opposed to solid wings. We explore the role of bristled wings on the flapping flight of thrips using 2D numerical fluid-structure interaction simulations. The input parameters for the simulations are obtained from high-speed video recordings of actual insects. An idealized form of the `clap and fling' motion of two wings immersed in fluid is considered herein, and the bristles on the wings are modeled as a homogeneous porous layer using the immersed boundary method. The effect of having bristles on the flow field is examined and compared to that of an equivalent solid wing.   

Flexible Flapping Wings' Flow Fields and Deformations

The flow field and structural deformations of flexible Zimmerman planform wings are investigated in a simulated hovering environment. The wings are manufactured from a carbon fiber skeleton, reinforcing the wing root, leading edge, and chordwise battens at a few spanwise locations, and covered with a Capran membrane. The flow field is measured using Particle Image Velocimetry (PIV) at several spanwise locations, and wing deformations using Digital Image Correlation (DIC). The results are phase averaged, resulting in three dimensional, three component flow field data, and phase averaged wing deformation data, so that a the fluid-structure interactions can be better understood. The wing deformation data will be presented superimposed on wing deformation data. The measured vorticity will be studied and related to the generation of aerodynamic forces. Vortical structures are investigated and identified using the $Q$ criterion in order to demonstrate the structures formed around the wing propagates into the wake.   

Should planes look like birds?

The dominant aircraft configuration, often known as tube-and-wings, could have established itself either because it is optimal, or because it is simply good enough and has become entrenched in practice. By contrast, tailless designs have been tried throughout aviation's history but have rarely succeeded in displacing the tube-and-wing. Here we propose a new configuration which is not tailless, but in which the overall shape may offer a superior wing-body circulation distribution. Wind tunnel tests will be described in which the cases: wing-alone, wing+body, wing+body+tail are compared for the spanwise downwash distribution in their wake. Based on these measurements, a different configuration is proposed, where the primary function of the tail is circulation control over the wing and body. The fact that this configuration bears more than a passing resemblance to certain birds is noted.   

Interaction of Two Flapping Flags in Axial Flow

The flapping of two parallel flags in axial low turbulence flow is investigated experimentally inside a small scale wind tunnel test section. The variables of the problem are the size and flexural rigidity of the flags, and the distance that separates the two flags. The flow velocity represents the control parameter that governs the coupling and flapping mode of the flags. Two flapping modes, in-phase and out-of-phase modes, were observed in the experiment. Image processing technique was used and the time series of a given point on the flag edge was analyzed. The stability condition of the flags was obtained and compared to the recent theoretical models. The dynamics of the coupling between the two flags is also studied.   

Effects of Wing Platform on the Aerodynamic Performance of Finite-Span Flapping Wings

A numerical study is conducted to investigate the effects of wing platform on the aerodynamics performance of finite-span flapping wings. A three-dimensional high-order Navier-Stokes compressible flow solver was developed using the spectral difference method and dynamic grids. An AUSM$^{+}$-up Riemann solver was implemented to simulate the unsteady low Mach number flows over finite-span flapping wings with explicit third order Runge-Kutta time integration. The studied finite-span flapping wings, which include a rectangular flapping wing, an elliptic flapping wing and a bio-inspired flapping wing, have the same wing span, aspect ratio of the platform and the characteristics of the flapping motion (i.e., sinusoidal trajectory of the flapping wing tip, Strouhal number and reduced frequency). In the present study, the Strouhul number (\textit{Str}) of the finite-span flapping wings was selected to be well within the optimal range usually used by flying insects and birds and swimming fishes (i.e., 0.2 $<$\textit{ Str} $<$ 0.4). The effects of the wing platform on the aerodynamics performance of the finite-span flapping wings were elucidated in the terms of the evolutions and dynamic interaction between the leading edge vortices (LEV) and the wing tip vortices as well as the resultant aerodynamic forces (both lift and thrust) generated by the flapping wings.   

Dynamics of Flapping Flag in Axial Flow

We investigate experimentally the phenomenon of the flapping of a flag, placed within a low turbulent axial flow inside a small scale wind tunnel test section. Flags of different sizes and flexural rigidities were used. Image processing technique was used and the time series of a given point on the edge of the flag was analyzed. The stability condition of the flag was obtained and compared to the recent theoretical models and numerical simulations. Afterwards, the nonlinear dynamics of the flapping was investigated using nonlinear time series method. The nonlinear dynamics is depicted in phase space and the correlation dimension of the attractors is determined. On the basis of observations made in this study, some conclusions on the existing models were drawn.