Search

In this study, the effects of fin stroke phase offset, flapping frequency, and freestream flow speed on thrust production are investigated for a median paired (pectoral) fin – caudal fin configuration. Fin geometries and kinematics are selected to be representative of common bio-inspired unmanned vehicles as well as the fish that inspire them, and for this study the fins are rigid to limit the variable parameter space. Experimental results demonstrate a significant correlation between freestream flow speed and thrust-optimal stroke phase offset, as well as between fin flap speed and thrust-optimal stroke phase offset. The results also highlight differences between the time-histories of thrust at high- and low-thrust generating stroke phase offsets suggesting a significant difference in wake signatures. This study provides the foundation for development of a comprehensive analytical tool for flapping fin and vehicle design.

The response of a rigid flapping wing in forward flight, at Re = 10,000, subjected to frontal gusts has been investigated. The phasing and duration of the gusts and their impact on the various unsteady mechanisms are analyzed within a single flapping cycle. The gust is characterized by a step function with integral length scale much larger than that of the physical dimension of the micro air vehicle and with time scale much smaller than the flapping time period. The instantaneous lift and thrust profiles were observed to be influenced by a combination of the effective angle of attack, wing rotation, and the leading-edge vortex structures existing in the flow at any given time, with the leading-edge vortices themselves being influenced by the duration and magnitude of the change in effective angle of attack. Frontal gusts applied during the downstroke accelerated the development of the flow, resulting in the formation and detachment of multiple leading-edge vortices on the wing surface that increased the lift and thrust, illustrating the importance of the leading-edge vortex dynamics to force production. The effect of the gust is observed to be diminished when it occurs during rapid supination of the wing. The lift and thrust profiles are found to react in a similar fashion for gusts applied during the downstroke, whereas they experienced opposite effects during the upstroke. During the upstroke, force characteristics are shown to primarily react to effective angle-of-attack changes more than to changes in flow structures.

The climbing flight flapping kinematics obtained on a fruit bat, Cynopterus brachyotis, is deconstructed using proper orthogonal decomposition to shed light on the functional kinematic modes which enables bat flight. Mode 1 and cumulative mode 1 + 2, compared against the native kinematic, is simulated in a Immersed Boundary CFD framework to observe the effect these modes have on the unsteady transient mechanisms of the flow produced by the flapping wings. From the obtained data, the bat exhibits fine control of its mechanics by actively varying wing camber, wing area, torsional rotation of the wing, forward and backward translational sweep of the wing, and wing conformation to dictate the fluid dynamics. As is common in flapping flight, the primary force generation is through the attached unsteady vortices on the wing surface. Mode 1 is seen to act as the power stroke dumping energy into the fluid. Mode 2 adds the mechanism which maneuvers the flow structures to enable forward flight.

The response of a flexible wing and a rigid wing in flapping forward flight, at Re=10,000, subjected to frontal gusts is investigated. The impact of a gust of chosen phasing and duration on the various unsteady mechanisms are analyzed within a single flapping cycle. The gust is characterized by a step function with integral length scale much larger than that of the physical dimension of the MAV and with time scale much smaller than the flapping time period. The instantaneous lift and thrust were observed to be influenced by a combination of the effective angle of attack, chosen kinematics, transient Leading Edge Vortex(LEV) structures, and the dynamic cambering in the case of the membrane wing. The effect of the gust on the lift profile is observed to have opposite effects for the rigid vs. flexible case while the thrust behaves in a similar profile. Nomenclature C = Airfoil chord length CL = Coefficient of lift CT = Coefficient of thrust h = wing thickness J = Advance ratio; U ∞/U f ; Ratio of the flight velocity to the flapping velocity Nξ = pre-stress along the chord Nζ = pre-stress along the span Nξζ = pre-stress in shear p = Pressure Re = Reynolds number; UfC/ν Re t = Inverse of the turbulent viscosity U∞ = Free stream velocity; Forward flight velocity Uf = Flapping velocity; 2 �fR w = Out of plane deformation x = Physical space coordinate

From flight data obtained on a fruit bat, Cynopterus brachyotis, a kinematic model for straight-line flapping motion is extracted and analyzed in a computational fluid dynamics (CFD) framework to gain insight into the complexity of bat flight. The intricate functional mechanics and architecture of the bat wings set it apart from other vertebrate flight. The extracted kinematic model is simulated for a range of Reynolds numbers, to observe the effect these phenomena have on the unsteady transient mechanisms of the flow produced by the flapping wings. The Strouhal number calculated from the data is high indicating that the oscillatory motion dominates the flow physics. From the obtained data, the bat exhibits fine control of its mechanics by actively varying wing camber, wing area, torsional rotation of the wing, forward and backward translational sweep of the wing, and wing conformation to dictate the fluid dynamics. As is common in flapping flight, the primary force generation is through the attached unsteady vortices on the wing surface. The bat through varying the wing camber and the wing area modulates this force output. The power requirement for the kinematics is analyzed and correlated with the aerodynamic performance.