Session BM: Bio-Fluids: Flight II

Hydrodynamic aspects of thrust generation in gymnotiform swimming

The primary propulsor in gymnotiform swimmers is a fin running along most of the ventral midline of the fish. The fish propagates traveling waves along this ribbon fin to generate thrust. This unique mode of thrust generation gives these weakly electric fish great maneuverability cluttered spaces. To understand the mechanical basis of gymnotiform propulsion, we investigated the hydrodynamics of a model ribbon-fin of an adult black ghost knifefish using high-resolution numerical experiments. We found that the principal mechanism of thrust generation is a central jet imparting momentum to the fluid with associated vortex rings near the free edge of the fin. The high-fidelity simulations also reveal secondary vortex rings potentially useful in rapid sideways maneuvers. We obtained the scaling of thrust with respect to the traveling wave kinematic parameters. Using a fin-plate model for a fish, we also discuss improvements to Lighthill's inviscid theory for gymnotiform and balistiform modes in terms of thrust magnitude, viscous drag on the body, and momentum enhancement.   

A strong-coupling approach to simulate flexible flapping wing

An immersed boundary technique with strong-coupling flow-structure-interaction (FSI) is used to study the flapping and twisting of a two-dimensional flexible wing. Using the method by Zhao et al. (J. Comput. Phys., 2008), a single set of equations of motion on a fixed Eulerian mesh is solved for both fluid and solid. The solid characteristics is essentially presented as an extra elastic-stress term, which is distributed from an overlapping Lagrangian mesh for tracking the solid deformation and computing the stress. In this study, the moving trajectory is controlled by two means: 1) bodyforce term defined by traditional direct-forcing method to prescribe certain control points (e.g. pin or shake the leading edge); 2) external bodyforce term with certain frequency to push/pitch the wing. The rest of the wing kinematics and corresponding flow field is computed through FSI. Results for wings at different pitching frequencies are shown for the flow at Re=400.   

Coordinated ejection of spores allows cup fungi to control surrounding air flows

The forcibly launched spores of ascomycete fungi must eject through a boundary layer of nearly still air in order to be dispersed by vigorous air flows beyond the boundary layer. Spores are microscopic in size and experience very high fluid drag that causes \emph{single} spore to decelerate very quickly in flight. Experiments and asymptotic models show that coordination of the ejection of hundreds, thousands or tens of thousands of spores creates a favorable wind that carries spores across the boundary layer, and around any intervening obstacles.   

Summoning the wind: enhanced dispersal by synchronized discharge of fungal spores

Cup fungi use coordinated ejection to minimize air resistance upon spores in flight and increase the distance traveled by their forcibly ejected spores. Direct Numerical Simulations show how the near simultaneous release of spores mobilizes the air around the spores to create a favorable wind enhancing spore dispersal. Varying the size of the originating fruiting body and the time scale separating successive spore ejections illuminates the biomechanical challenges that these fungi have overcome on the evolutionary path to co-operative behavior.   

Optimal flexibility in flapping appendages

When oscillated in a fluid, appendages such as insect wings and fish fins can produce large thrust forces while undergoing considerable bending. We attempt to understand the role of flexibility by formulating a simple optimization problem. Can we determine the flexibility which produces maximum thrust, or a given thrust at maximum efficiency? We present first a general model for how flexible surfaces produce vorticity and bend passively in a fluid. The model combines a nonlinear ODE for elastic bodies with a singular integral equation for a potential flow with velocity discontinuities. We solve the linearized model and find a series of local thrust optima with power-law dependences on rigidity and driving frequency. These optima are resonant peaks, damped by fluid inertia, and can be predicted with a scaling analysis. We discuss extensions to large-amplitude motions, and motions of actual fish fins.   

Optimization of Unsteady Fluid-Body Interactions via Machine Learning

Optimization of the interactions between a moving body and its surrounding fluid can be extremely complicated; even optimization on simple models can be tremendously computationally expensive. In this work we demonstrate that using a state-of-art machine learning algorithm we are able to efficiently optimize a flapping strokeform for energy efficiency entirely on a laboratory experimental system (i.e., without the use of any simulation). The learning is performed in real-time on a vertically heaving wing that is free to rotate about its center in the horizontal plane as a model of forward flapping flight (Re$\sim$30,000). The learning algorithm must contend with the stochasticity and long-term correlations inherent in its being run online and on an experimental system. Despite these difficulties, we demonstrate its success at learning using several wing forms, where it is able to optimize a strokeform in approximately 1,000 flaps (less than twenty minutes).   

A Computational Study of Wing-Wing Interactions in Hovering Insect Wings

It has been discovered that flying insects may enhance their lift production through ``clap-and-fling'' during dorsal stroke reversal. In this study, an immersed-boundary-based DNS solver is used to quantitatively study the aerodynamic benefit and associated vortex dynamics of wing-wing interactions for modeled fruitfly (Drosophila) wings in hovering motion. Simulations show that about 12.8{\%} vertical force augmentation is produced over the entire stroke cycle compared to a single flapping wing with the same motion. Delayed trailing-edge vortex formation during the fling stage is also observed as reported by idealized 2-D studies in literature. Effects of different stoke trajectories on the aerodynamic performance are also investigated in this study. These simulations will allow us to investigate the ``clap-and-fling'' mechanism and help with the improvement of current analytical models on force predictions in insect flights.   

Vortex wakes of a flapping foil in a flowing soap film

We present an experimental study of an oscillating, symmetric foil in a vertically flowing soap film. By varying frequency and amplitude of the oscillation we explore and visualize a variety of wake structures, including von K\'arm\'an wake, reverse von K\'arm\'an wake, 2P wake, and 2P+2S wake. We characterize the transition from the von K\'arm\'an wake (drag) to the reverse von K\'arm\'an wake (thrust) and discuss the results in relation to fish swimming. We visualize the time evolution of the vortex shedding in detail, identify the origins of the vortices comprising the wake, and propose a simple model to account for the transition from von K\'arm\'an like wakes to more exotic wake structures.   

Coupled flutter of parallel flags

We address experimentally and theoretically the flutter instability of parallel flags in uniform airflow. Identical flags are cut from Mylar sheets and clamped into parallel streamlined masts. When placed in a low-turbulence wind tunnel, the flags flutter at a well-defined frequency if the flow velocity is above a critical value. This instability results from the competition between the destabilizing pressure forces and the stabilizing bending stiffness of the flags. Depending on the number of flags, the distance between them, their size and the flow velocity, different coupled modes can be observed with a high-speed camera. The observed modes and their frequencies are compared with a theoretical linear model assuming a potential flow and small flag deflections.   

Simplified hovering flight analyzed by a theory of force decomposition

It has been of great interest to learn how an insect flight gains lift from its aerodynamics. In this study, we examine a simplified model of hovering motion for fruit fly from the perspective of force decomposition. The force components from the decomposition include one from the vorticity within the flow, one from the surface vorticity, and two contributions credited to the motion of the insect wing. The phase difference in the models gives three types of motion: symmetric, advanced and delayed rotations. It is shown that the symmetric rotation has the maximum vorticity lift, but the optimal average lift is attained for an advanced rotation, which, compared to the symmetric rotation, increases the force contribution due to the unsteady surface motion at the expense of sacrificing contribution from the vorticity. By identifying the variations of the vorticity lift with flow characteristics, we may further explore the detailed mechanisms associated with the unsteady aerodynamics at different phases of hovering motion. Moreover, it is shown that the insect wing shares the same mechanism of gaining lift when in the phase of driving with a fuller speed, but exhibits different mechanisms at turning from one phase of motion to another.