Arnold Song

Aeromechanics of Membrane Wings with Implications for Animal Flight

5. The lift and drag coefficients were measured for wings of varying aspect ratio, compliancy, and prestrain values. In addition, the static and dynamic deformations of compliant membrane wings were measured using stereo photogrammetry. A theoretical model for membrane camber due to aerodynamic loading is presented, indicating that the appropriate nondimensional parameter describing the problem is a Weber number that compares the aerodynamic load to the membrane elasticity. Excellent agreement between the theory and experiments is found. Measurements of aerodynamic performance show that, in comparison with rigid wings, compliant wings have a higher lift slope, maximum lift coefficients, and a delayed stall to higher angles of attack. In addition, they exhibit a strong hysteresis botharoundazeroangleofattackaswellasaroundthestallangle.Unsteadymembranemotionswerealsomeasured, anditisobservedthatthe membranevibrateswithaspatialstructure thatisclosely relatedto thefreeeigenmodesof themembraneundertensionandthattheStrouhalnumberatwhichthemembranevibratesriseswiththefreestream velocity, coinciding with increasing multiples of the natural frequency of the membrane.

Wing structure and the aerodynamic basis of flight in bats

Powered, flapping flight has evolved at least four times in the Animal Kingdom: in insects, birds, pterosaurs, and bats. Although some aspects of flight mechanics are probably common to all of these lineages, each of the four represents a unique solution to the challenges of maneuverable flapping flight at animal length scales. Flight is less well documented and understood for bats than birds and insects, and may provide novel inspiration for vehicle design. In particular, bat wings are made of quite flexible bones supporting very compliant and anisotropic wing membranes, and possess many more independently controllable joints than those of other animals. We show that the mechanical characteristics of wing skin play an important role in determining aerodynamic characteristics of the wing, and that motions at the many hand joints are integrated to produce complex and functionally versatile dynamic wing conformations.

Dynamics of a Compliant Membrane as Related to Mammalian Flight

Bats and other mammalian gliders exhibit extraordinary ∞ight capabilities some of which are attributable to their unique wing structure which is quite distinct from birds and insects. In mammalian ∞ight, the wing is composed of a thin compliant membrane of skin which has the ability to adapt to ∞ow conditions. In an efiort to characterize the distinctive aerodynamics of compliant membrane wings, results from wind tunnel tests of the static deformation and unsteady vibrations of a low aspect ratio wing are presented. The unsteady de∞ection of a latex membrane model was measured via high-speed, stereo photogrammetry. The tested wing has a half-span aspect ratio of 0.69 and experiments were performed with two pretensioned conflgurations: (1) 0 % prestrain and (2) 5 % prestrain. Testing was performed at angles of attack ranging from 4 i 34 ‐ and Rec = 60;000i140;000. The static normalized camber versus angle of attack, or camber slope, was found to decrease with increasing Reynolds number. In addition, high order vibrational modes were observed to occur in a range of angles of attack (fi = 16 i 28 ‐ ) above a critical value of the Reynolds number (Rec > 80;000). Fourier analysis of these membrane vibrations show that the dominant frequencies fall along distinct bands of slightly decreasing reduced frequency.

Aerodynamic Behavior of Compliant Membranes as Related to Bat Flight

We present computations of membrane airfoil behavior subject to aerodynamic loading and compare them with in vivo measurements of membrane wings of bats during flight. The computational method assumes an inviscid potential flow (with net circulation determined by a Kutta condition), is computed using XFOIL and iteratively coupled with a finite element model describing the membrane behavior. We find that a simple model assuming uniform loading is largely confirmed, particularly for very compliant membranes in which the pressure loading is focused at the center of the airfoil. Stiffer wings transition to the more traditional pressure distribution predicted by thin airfoil theory for rigid wings. Comparisons with sail theories are also made, illustrating the effect of compliance. Additionally, the in vivo measurements of membrane deformation during bat flight are acquired from detailed kinematics recorded from Cynopterus brachyotis, flying in a wind tunnel. We demonstrate that the expansion of the wing area during the downstroke of the flight cycle exhibits area increases of up to 100% during the downstroke. In addition, comparisons with the computational theory show good qualitative agreement.

Cyber-physical Energy Harvesting through Flow-Induced Oscillations of a Rectangular Plate

The dynamics of a flat plate undergoing flow-induced oscillations in a uniform airflow have been investigated experimentally using a cyber-physical system, which integrates a real-time feedback control algorithm to simulate the desired structural dynamics. The feedback control scheme utilizes the angular position and velocity measurements as inputs to the virtual system to numerically simulate the dynamics of an elastically mounted flat plate on a virtual spring-damper system. A series of experiments were conducted using two different sized rectangular plates (AR=3.5 and 5.3), and over a range of free-stream velocities (10 m/s ∼ 25 m/s), and simulated stiffness (0<k∗ v<1.5) and damping (0.01<bv<0.56), where the non-dimensionalization is based on inertial fluid characteristics. The Reynolds number, based on the chord length of the plates, were in the order of O(10) to O(10). In this paper, we demonstrate our cyber-physical system’s robust capability to simulate the effects of varying torsional spring stiffness and linear damping, confirm that inertial scalings of the system stiffness and damping are appropriate, and quantify the system’s modest abilities to harvest energy from a steady airflow.

Exploration of bat wing morphology through a strip method and visualization

We present a visual exploration tool that facilitates biologists navigating through complex bat wing geometry by combining a novel modeling method and an interactive visualization approach. Our work contributes to the following: a new method to quantify the dynamic kinematics during flight, a new curve fitting method that measures camber, and a new tool for time-varying data visualization for biological knowledge discovery.