Session R15: Biofluids: Flying and Swimming

Flow structures in the wake of heaving and pitching foils

A 10-bar mechanism drives a cambering hydrofoil in an oscillatory heaving and pitching motion that replicates the flapping motion of a dolphin tail. The mechanism sits on a force-balance with six strain gages that together measure the forces and moments experienced by the fin during an oscillation. Planar Laser-Induced Fluorescence is used to image the flow structures created downstream of the cambering fin for a range of Reynolds and Strouhal numbers. The images are taken in the mid-plane, parallel to the bottom of the water tunnel. These results are compared to a rigid foil at matching conditions to investigate the role of camber changes during the flapping cycle.   

Force measurement in heaving and pitching foils

This study analyzes the efficiency of a cambering hydrofoil built to simulate the movement of flukes on cetaceans. The mechanism is a 10 bar assembly that allows a hydrofoil to move in a cambered pitching and heaving motion similar to that of a dolphin. The mechanism sits on a force-balance with six strain gages that together measure the three forces and three moments experienced by the fin during a cycle of motion. These gages are attached to a traveling mechanism that rest on a water flume. To analyze the efficiency of the hydrofoil we took measurements at various Reynolds and Strouhal numbers. These measurements were done twice were compared to the thrust produced by a rigid (non-cambered) hydrofoil at the same conditions.   

3D Synthetic Aperture PIV of a Freely Swimming Fish

Fish owe much of their locomotive success to complex body geometries and wake interactions that cannot be fully characterized by planar experimental techniques including 2D PIV. Volumetric methods are valuable to illustrate and quantify these features, thus providing new insights for bioinspired design. In particular, synthetic aperture particle image velocimetry (SAPIV) uses light field imaging algorithms to reconstruct a 3D particle field which can then be analyzed using a 3D cross-correlation. Previous studies have shown that this technique is able to resolve all three velocity components on the same order length scale and to see around partial occlusions, such as a caudal fin, through the use of multiple cameras. To harness these capabilities for biomimetic use, SAPIV is used to depict the three-dimensional velocity field and vortical structures surrounding a freely swimming Giant danio (Devario aequipinnatus) during straight swims and turning maneuvers.   

Hydrodynamic performance of multiple bodies swimming in an in-line configuration

Experiments are reported on a pair of airfoils that are harmonically pitched about their leading edges and arranged in an in-line configuration to determine the hydrodynamic effect of drafting behind a neighbor in unsteady bio-inspired propulsion. The thrust production, power consumption, and propulsive efficiency is independently measured for the leading and trailing airfoils at a Reynolds number of 2000 for a range of streamwise airfoil spacings, Strouhal numbers, and oscillation phase differential between the airfoils. To assess the wake interactions between the panels that lead to propulsive performances observed, digital particle image velocimetry (DPIV) is used. These results are compared to an airfoil swimming in an isolated configuration to identify the parameters that lead to a benefit (or detriment) when swimming in-line with a neighbor.   

Dynamics of a heaving flexible foil in a uniform flow

Most aerial and aquatic animals produce thrust using flapping flexible appendages. The performances of such propulsion systems are strongly related to the appendages dynamics, in particular to the amplitude of the trailing edge motion and to the vortical patterns produced. A better understanding of this mode of propulsion requires to investigate the dynamics of the flexible appendages, as a response to harmonic forcing. In this context, experiments are performed with flexible foils immersed in the uniform flow of a water channel. A harmonic heaving motion, that is transverse to the foil, is then imposed to its leading edge. The response of the foil likely results from the resonance between the forcing and the natural modes of vibration. Experimental results are compared with a two-dimensional model assuming a zero-thickness flexible sheet of infinite span immersed in a potential flow.   

Experimental investigation of 2D flexible plunging hydrofoil

It is believed that both birds and insects benefit from their wing flexibility during the flapping flight. One of the possible benefits is higher lift force generation capability than that of rigid wing models. Both experimental and computational work has discovered that the leading edge vortex (LEV) plays an important role in this advantage of high lift force generating efficiency. In the present work, flow physics related to high lift-generating flexible wings are investigated experimentally. Both flexible and rigid hydrofoils (NACA0012) were actively plunged in glycerol-water solution with various amplitude, frequency and Reynolds number combinations. Phase-locked Particle Image Velocimetry (PIV) measurements were conducted to investigate the generation and evolution of the LEVs. Lift and drag forces during plunging were also measured to uncover the relationship between the force response and the surrounding flow field development. The overall results were also compared between flexible and rigid hydrofoils to provide qualitative data for validation of computational work.   

Performance of a wing with nonuniform deformability in hovering flight

The deformability of insect wings are related to the embedded skeleton (venation). In this study, the aerodynamic performance of wings with nonuniform flexibility is computationally investigated. By using a two-dimensional rendition, the underlying veins are modeled as springs, and the membrane is modeled as a flexible plate. The focus is on the effects of the detailed distribution of vein flexibility upon the performance of such a wing in the generation of lift force. Specifically, we are interested to find the importance of leading edge strengthening. Towards this end, the aerodynamic performances of three wings, a rigid wing, a flexible wing with identical veins, and a flexible wing with strengthened leading edge, are studied and compared against each other. It is shown that the flexible wing with leading edge strengthening is capable of producing significantly higher lift force. This is found to be related to the cambering effect at the leading edge, which enhances the leading edge vortices. In addition, in contrast to the other two wings, which show sensitivity to kinematic parameters, the wing with strengthened leading edge perform well over a wide range of parameters.   

Rotational inertial effects on flexible wing

To understand rotational inertial effects on aerodynamic force, the lattice Boltzmann flexible particle method (LBFPM) is employed to simulate interaction between fluid flows and flapping motion of a chord-wise flexible wing in a 3D space at two levels of pitching or rotational rates corresponding to two rotational Reynolds numbers of Rer=356 and 107 while the translation Reynolds number is kept at the same level of Re=136. At each rotational Reynolds number, flexibility and mass ratio of wing to fluid are systematically varied at different levels and lift, drag, deformation and power efficiency are computed and compared. It is found that the lift force and power efficiency increase non-linearly up to maximum values as chord-wise flexibility increases, then fall down as flexibility continuously increases for the larger rotational Reynolds number of Rer=356. As the mass ratio increases the inertial force and the lift force increase while the input power increases. The flexibility should be optimized by the lift force and the power efficiency. The simulation results indicate that rotational inertia is an important factor for flexibility to enhance lift and power efficiency. However, the case with a lower rotational Reynolds number of Rer=107 does not have this behavior. It is also found that the deflection angle and the sweeping distance in the vertical direction are much larger for trailing edge than leading edge.   

Sectional lift coefficient of a rotating wing at low Reynolds number

We investigate the characteristics of sectional lift force on a rotating wing at low Reynolds number using three-dimensional numerical simulation. Three different types of flat plate wings (fruit-fly, rectangular and triangular wings) are considered but keeping their aspect ratio (wing span/wing chord) the same at 3.74. The wings rotate at a constant angular velocity and the angle of attack is fixed during rotation ($5^\circ \sim 45^\circ$). The Reynolds number is 136 based on the wing chord length and the translational velocity at the wing tip, corresponding to that of the flapping fruit-fly wing in hovering flight. An immersed boundary method in a non-inertial reference frame (Kim and Choi, JCP, 2006) is used to simulate the flow. During the first rotation, the sectional lift coefficient decreases from the wing root to the wing tip for all cases. After several rotations, however, the sectional lift coefficient becomes nearly constant except near the wing root and tip at low angles of attack ($\leq 15^\circ$), but maintains a similar behavior to that of first rotation at high angle of attack ($\sim 45^\circ$). Finally, the wing shape does not significantly change the spanwise distribution of sectional lift coefficient.   

Aerodynamic effect of alula in avian flight

Alula is a small structure located at the joint between handwing and armwing of birds and has been suggested to function as a leading-edge slot. In this study, we investigated the functional aspect of alula in bird flight with experimental conditions that reflect the flow characteristics used by birds in their actual flight using magpies as the model species. The presence of alula enabled the bird to perform steeper descending flights with greater lateral angle changes. Force measurements showed that alula presence increased the lift when the angle of attack was high (higher than 20-45 deg), which resulted in the stall delay by 5 deg. The wake width was significantly thinner when alula was present, suggesting that boundary layer separation is delayed when alula is used. This result was corroborated by PIV; accelerated streamwise velocity over the wing surface was recovered faster and separation point was pushed downstream when alula was present. To conclude, the lift enhancement and stall delay by alula are closely related to the downstream movement of separation point and faster recovery of accelerated flow over the wing surface, which endows greater flight maneuverability to the birds.