Session R01: Biofluids: Flying Birds

Energy consumption for wing actuation of hummingbirds during the escape maneuver

In this study, we consider the rapid escape maneuver of hummingbirds who were initially hovering but then startled by the frontal approach of a looming object. To escape from the perceived threat, the hummingbirds substantially changed their wingbeat frequency, wing trajectory, and other kinematic parameters. Using wing kinematics reconstructed from high-speed videos and computational fluid dynamics modeling, we calculated the time-dependent aerodynamic torques and power output separately for wing stroke, pitch, and deviation, to understand the requirements for wing actuation in this process. The results show that the overall power output of the maneuver is more than twice as hovering.  Wing pitching requires positive power input and is thus active.  For wing stroking, downstroke is much more powerful than upstroke. However, wing deviation during upstroke requires significantly more power than downstroke for the wings to change their trajectories and move caudally, which as a result, redirects the aerodynamic force vector to produce the needed escape acceleration.   

Hummingbirds use active wing pitching during the escape maneuver

Previous studies suggested that wing pitching, i.e., the wing rotation around its long axis, of insects and hummingbirds is primarily driven by an inertial effect associated with stroke reversal of the wings and is thus passive.  Here we used 3D computational fluid dynamics to model hummingbird wings during a rapid escape maneuver.  The model was based on the body and wing kinematics reconstructed from high-speed videos, and both inertial and aerodynamic effects of the wings were incorporated to resolve time-dependent forces and torques acting on the wings.  The results show that although the inertial effect drove the wing flipping at stroke reversal, i.e., similar to hovering, significant power input was required to pitch up the wings during downstroke to enhance aerodynamic force production; furthermore, the net power input could be positive for wing pitching in a complete wingbeat cycle.  Therefore, our study suggests that hummingbirds utilize an active mechanism during the maneuver to drive wing pitching and achieve high maneuverability.    

Experimental investigation of wing geometrie for a better understanding of bird flight in ground effect.

For more than a century, ground effect has been known for reducing flying cost. The commonly accepted explanation relies on two principles: the increase in lift by the ram effect and the decrease in lift-induced drag by downwash disruption. Large birds like pelicans and albatrosses are often seen gliding near the water surface. Although it is often claimed that they are taking advantage of ground effect, their wing arched geometry keeps most of the lower-wing surface at more than one chord-length from the water. Hence, other phenomena may be at play since birds are not that close to the ground as required with traditional ground effect theories.

To investigate potential new phenomena, three wing geometries are studied: rectangular flat wings, with and without end-plates, and a bird-inspired anhedral wing.

Wings are first placed in a wind tunnel. The ground is simulated with a fixed plate beneath the wings. The wing lift and drag coefficients are measured. Tip vortices and ground interaction are characterized with laser visualization and PIV. Since the plate imposes an inaccurate boundary condition, other tests are underway in a tow tank facility. The wings are towed above the water surface, which enables real flight boundary conditions.   

Passive pitch induced by a spreading pigeon tail

Aspect ratio is among the most influential parameters that impact the lift and drag generation of finite-span wings, in particular their dependency in the angle of attack. During landing, birds may use their tail in a rather complex way by rapidly changing both its angle of attack (pitch motion) and spread angle (spread-fold movement), which is a proxy for aspect ratio. Generally, the pitch motion is assumed to be primarily active, we reevaluate this by estimating the passive aerodynamic effects induced by the tail's spread-fold movement. Using a biohybrid robotic tail with real feathers, we present here the lift and drag characteristics of a pigeon tail for different spread angles. Corroborating the energetic landscape for these lift-drag characteristics, we predict whether further spreading the tail passively induces an upwards or downwards pitch as a function of spread angle and flow velocity. Our results suggest a new passive control pathway for landing in birds and their robotic counterparts.   

Surfing birds: flight interactions with a structured vortex wake

Understanding the coupling between avian flight behavior and unsteady flow structures is crucial for advancements in biological and engineering research.  We present results from a wind tunnel  study of the European Starlings' response to a mechanically generated wake.  We conduct synchronized measurements of the bird position  - measured using machine-vision tracking - and the wake velocity field - measured using planar Particle Image Velocimetry (PIV) aligned in the streamwise plane.   Measurements are conducted behind the birds flying both in a clean freestream, and in a vortex dominated flow, generated by a two-dimensional wing positioned upstream and pitching at the bird flapping frequency, (11 Hz).  The animal flight results are interpreted with the assistance of an engineering model system in which the aerodynamic forces and torques on a NACA 0012 airfoil are measured as it surfs in the identical mechanically-generated vortex wake. The experiments reveal a variation in lift induced by the wake, at a frequency locked into the wake shedding frequency.  The strength of this force variation varied linearly with the Strouhal number based on the flapping frequency and freestream velocity).   

Energy-positive soaring using transient turbulent fluctuations

Soaring birds gain energy from stable ascending currents or shear. However, it remains unclear whether energy loss due to drag can be overcome by extracting work from transient turbulent fluctuations. We designed numerical simulations of gliders navigating in a kinematic model that captures the spatio-temporal correlations of atmospheric turbulence. Energy extraction is enabled by an adaptive algorithm based on Monte Carlo tree search that dynamically filters acquired information about the flow to plan future paths. We show that net energy gain is feasible under realistic constraints. Glider paths reflect patterns of foraging, where exploration of the flow is interspersed with bouts of energy extraction through localized spirals.   

Experimental Characterization of the Flow around a Bat Wing

Understanding the flow in the boundary layer around a bat’s wing is essential for elucidating the relationship between its aerial agility and wing kinematics. The flow field near the surface of a dissected bat wing is studied in a wind tunnel using stereo-PIV measurement. Imaging of the particles near the surface is challenging due to reflections from the wing features, such as bones (digits) and hairs. To resolve this problem, the tracer particles, aerosols of Polyethylene glycol generated by a Laskin Nozzle, are mixed with fluorescent Rhodamine 6G dye, and the images are recorded through a high-pass filter that removes most of the green light reflected from the surface but transmits the yellow- orange fluorescence emitted from the tracers. Settling of the aerosols on the surface, particularly the hair, over time eventually causes fluorescence from the surface, degrading the image quality, but also allows detection of the hair motion. Data are acquired at flow speeds ranging from 4 to 10 m/s, and incidence angles of 0^o to 20^o, in a series of streamwise-wall normal planes covering the entire wingspan. The PIV results highlight the impact of wing curvature on the flow structure and the effect of digits in creating region of low momentum and flow separation.   

Effect of transverse gusts on a Red-tailed Hawk

The inability of smaller-scale uncrewed aerial vehicles (UAV) to navigate gust-laden environments significantly impedes their performance envelope. In this study, the response of a red-tailed hawk to upwards transverse gusts is investigated as a bio-inspired approach to improve UAV gust-resilience. An indoor flight arena instrumented with high-speed cameras was used to quantify the 3D motion of the hawk flying through gusts of two different magnitudes - low and high gust ratios. Although the hawk maintained its flapping motion while flying through the gust, it encountered the gust at different points in the flapping cycle depending on the experimental run and gust ratio. In response to the gust, the hawk executed a continuous downward pitching motion of the wing, decreasing the wing pitch angle to between -20^○ and -5^○. Estimation of coefficient of lift (C_L) across the wing illustrated that the C_L increased at a low rate, to a maximum of around 2 to 2.5 for low gust ratio conditions. At high gust ratio conditions, the C_L initially increased rapidly and then increased at a low rate to a value around 4 to 5. A flight dynamics model accounting only for the wing mechanics was able to better predict the flight path of the bird at low gust ratio conditions when compared to high gust ratio conditions. This points to additional factors (for example, tail pitch and roll modulation), which were potentially used by the bird, that need to be included in the flight dynamics model to improve predictions.