Session T10: Biofluids: Flying General

Golden eagles exploit turbulence intermittency

We examine the statistics of the accelerations of six golden eagles in natural flight and find that extreme vertical accelerations are consistent with the selective amplification of small-scale turbulent updrafts. The finding appears to be the first observation of wildlife exploiting turbulent fluctuations, and extends what we know about how wildlife uses relatively stationary flows to stay aloft, such as thermal updrafts and wind shear, by showing how fully unsteady flows, or gusts, can also be beneficial. The evidence in favor of our interpretation includes a probability distribution of vertical acceleration differences that exhibits long tails consistent with turbulence intermittency and inconsistent with gust suppression, or with the mitigation of extreme events. Furthermore, the breadth of the tails, measured by the acceleration difference flatness, increases toward short timescales, thereby breaking self-similarity in the same way as does turbulence intermittency. Finally, the acceleration difference flatness blows up toward large values on timescales shorter than a few seconds in a way predicted by a simple nonlinear model of the eagles' response to turbulence. The model breaks up-down symmetry in favor of upward gusts, and is consistent with the eagles' interest in staying aloft while minimizing energy expenditure.   

Feather-inspired Flow Control Devices: 2D vs 3D Comparison

Birds are superior in terms of agility and maneuverability compared to small-scale UAVs, partly because birds’ wings are equipped with multiple flow control devices. One of such flow control devices is a group of feathers known as the coverts. The coverts are passive high-lift aeroelastic flow control devices used by birds to mitigate stall and control flow separation. Prior studies by the authors have investigated the performance characteristics and physics of torsionally hinged covert-inspired flaps mounted on the suction side of airfoils in 2D settings across different Reynolds numbers. In this work, we present wind tunnel experiments using time-resolved PIV flow measurements and force data at Reynolds number O(10⁵) to investigate the effect of covert-inspired flaps on a 3D wing section. We will examine the fluid-structure interaction mechanisms for the 3D system and compare them to our prior findings for the 2D system. Results will show the effects of the spanwise flow, due to the wingtip vortices, on the aerodynamic performance of the wing-flap system and quantify how the deployment dynamics of the torsionally-hinged covert-inspired flaps change between the 2D and 3D cases.   

Flow physics of bioinspired passive multi-flap wings with variable stiffness

Covert feathers on the suction side of a bird's wing are observed to flap passively during high-angle-of-attack maneuvers. This observation inspired the incorporation of bioinspired flaps to the suction side of aerial vehicles to improve flight performance at high angles of attack. Prior experimental and computational research by the author and others investigated the effect of varying the number and chord-wise position for multi-flap systems and the variation of the flap's hinge torsional stiffness in single-flap systems. However, the impact of torsional stiffness on multi-flap systems has not been studied. In this work, we use time-resolved force and flowfield measurements to systematically study the impact of torsional stiffness in multi-flap wings on the aerodynamic performance and the flow physics of an airfoil at Reynolds number Re=200,000. In addition, we quantify the deflection dynamics of each flap and study the interplay between their motion and the coherent unsteady flow structures in the flow.   

Flow Physics of Bioinspired Wingtips for Drag Reduction

Flight mission demands for small-scale uncrewed aerial vehicles (UAVs) are increasing and require high levels of aerodynamic efficiency and maneuverability. However, current UAVs are considered point designs, meaning they are designed for only a single mission or specific flight conditions. In contrast, birds like Harris’s hawks are capable of both agile and efficient flight under various conditions, partly due to their wing morphology. For example, Harris’s hawks have moderate aspect ratio wings that enhance maneuverability but are often associated with reduced aerodynamic efficiency due to increased induced drag. However, their wing tip slots have been shown to spread vorticity both horizontally and vertically, a mechanism hypothesized to reduce induced drag. In this study, we examine the design of a biologically relevant wing with various Harris hawk-inspired wingtip configurations to study the effect of wing tip slots on aerodynamic performance.  Experiments are conducted in the 4’x4’ wind tunnel facility at Princeton University at Re = 2 x 10⁵. Time-resolved forces and flow field measurements will be used to understand how wingtips improve aerodynamic performance by altering the vortex structures in the flow. Such an understanding can enable a new generation of UAVs capable of a broad range of missions that require both agility and efficiency.     

Existence and stability of equilibria of free-falling plates

Steady gliding and diving motions in animals such as birds may be loosely modeled by considering flat plates falling freely through fluid. We investigate equilibrium solutions of a quasi-steady two-dimensional nonlinear model of thin rectangular plates subject to gravitational and fluidic forces at intermediate Reynolds numbers. We begin by proving the existence and uniqueness of such equilibrium states for a given set of fixed dimensionless geometric parameters. We then examine a broad range of these equilibria through linear stability analysis, and present phase diagrams showing a highly complex structure of stable and unstable regions including multiple Hopf bifurcation boundaries. We then verify these findings via the full nonlinear model. We identify a new flight mode in addition to those already studied in previous literature, and demonstrate its existence experimentally. Finally, we propose a necessary but insufficient condition for divergent stability based on the sign of the derivative of the aerodynamic center of pressure, as well as several other factors that substantially contribute to the stability. These results are highly generalized to flat plates with arbitrary geometry and density, as well as to fluids of arbitrary densities. We also highlight some connections to real examples of steady animal flight.   

The role of wing shape in the aerodynamic performance of tiny insects

The bristled wings of numerous species of tiny insects such as thrips and fairyflies show remarkable diversity in shape, ranging from short, teardrop-shaped to long, slender profiles. While wing area is well known to directly impact aerodynamic force generation, the effects of varying the ratio of leading edge to trailing edge wing area (LE/TE), as well as the ratio of wing span squared to wing area (AR), on the flapping flight of tiny insects are unknown. To examine the effect of wing shape, we fabricated 12 dynamically scaled wing models. Including three bristled and three solid models with AR of 3,5,7, four bristled models with LE/TE of 2, 1, 0.5, 0.25, and a thrips and fairyfly idealized replica model. These models were tested at Reynolds number = 10 on a 3D flapping robotic model to collect time varying lift and drag forces. We found that varying both LE/TE and AR had little effect on the cycle averaged lift and drag forces. Using 2D phase-locked particle image velocimetry on the LE/TE 2 and 0.25 wings, we found the strength of the leading-edge vortex was higher than that of the trailing-edge vortex during the downstroke for both wings. Flow field data will be presented on all the wings to further examine the aerodynamic effects of wing shape.   

Parametric Modelling of Vortex-Vortex Interactions in 2-DOF Flapping Wings using Empirical Methods

In our previous work, we performed numerical simulation using a dynamic mesh to analyze the wake structure over a pair of tandem dragonfly wings and further optimized their aerodynamic performance (energy-wise) with novel flapping kinematics. The input kinematics are selected as the mix of pure sinusoidal and periodic Eldredge functions. Our study found that both wings' combined vertical lift coefficient is 1.56 times more than their individual lift coefficients when summed up. Moreover, the hindwing experiences most of the enhancement in lift resulting from the vortex synergy interaction. We found the downwash-upwash wake interactions are insignificant in in-phase flapping, which agrees with previous studies.In our current work, we empirically model the Vortex-vortex interaction forces in a bid to improve the aerodynamic model of flapping wings. The results from our previous work suggested that the force enhancement experienced as a result of the vortex interactions can be modelled using a parameter k which is a function of the phase spacing (vertical alignment) and wing spacing (horizontal alignment), and the effect of k over a time period can be controlled using a piece-wise sigmoid function. A comparative study was performed over three different phase spacings of 0, 90 and 180 degrees, and four wing spacings of 0.1, 0.25, 0.5, and 1 c. It was found that though phase spacing and wing spacing are intertwined variables which inversely cancel each other, the magnitude of variation in each of the variables is considerably different and has significant ramifications on propulsive efficiency. It was found that the case with 90 degrees phase spacing and 0.1c wing spacing provided us with the best results for hovering and forward flight conditions with minor tradeoffs on power requirements and propulsive efficiency. Furthermore, 3D simulation was conducted on the 0 degrees phase spacing and 0.5c wing spacing (second closest contender case to optimal kinematics) to understand the effect of tip vortices on the hindwing. It was observed that the tip vortices from the forewing destructively interfere with the outer midspan vortices on the hindwing which are critical to generating lift. Hence, suggesting that the forewings must be longer in length to avoid its tip vortices interacting with the midspan vortices of the hindwings.   

Numerical Investigation of Flow Dynamics in Gliding Snake-Like Models: Vortex Structures and Aerodynamic Performance

This work employs a sharp interface immersed-boundary-based incompressible Navier-Stokes flow solver to study the flow dynamics of a snake-like model exhibiting periodic horizontal undulation during steady gliding. Detailed vortex dynamics analysis examines vortex structures near the snake body, revealing the formation of oblique-shaped leading-edge vortices (LEV) and trailing-edge vortices (TEV) tubes due to the span-wise velocity. During the undulating period, a secondary LEV is generated at the anterior body, resulting in a 30% increase in lift generation over the body. The angle of attack (AOA) study demonstrates that the undulating body produces the highest average lift at AOA=45°, with a delayed-stall phenomenon compared to the 2D flying snake cross-section study. Moreover, LEV strength and lift enhancement are observed as the Reynolds number (Re) increases. Additionally, undulating frequency (f) changes influence LEV formation and its strength, significantly impacting the snake's aerodynamic performance. This study brings new knowledge on the role of undulatory motion in flying snake gliding, deepens our understanding of lift generation mechanisms, and offers potential guidance for future designs of aerial gliding aircraft.