Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session G9: Swimming IIIBio Fluids: External
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Chair: Daniel Floryan, Princeton University Room: 502 |
Monday, November 20, 2017 10:35AM - 10:48AM |
G9.00001: Squid-inspired vehicle design using coupled fluid-solid analytical modeling Francesco Giorgio-Serchi, Gabriel Weymouth The need for enhanced automation in the marine and maritime fields is fostering research into robust and highly maneuverable autonomous underwater vehicles. To address these needs we develop design principles for a new generation of soft-bodied aquatic vehicles similar to octopi and squids. In particular, we consider the capability of pulsed-jetting bodies to boost thrust by actively modifying their external body-shape and in this way benefit of the contribution from added-mass variation. We present an analytical formulation of the coupled fluid-structure interaction between the elastic body and the ambient fluid. The model incorporates a number of new salient contributions to the soft-body dynamics. We highlight the role of added-mass variation effects of the external fluid in enhancing thrust and assess how the shape-changing actuation is impeded by a confinement-related unsteady inertial term and by an external shape-dependent fluid stiffness contribution. We show how the analysis of these combined terms has guided us to the design of a new prototype of a squid-inspired vehicle tuning of the natural frequency of the coupled fluid-solid system with the purpose of optimizing its actuation routine. [Preview Abstract] |
Monday, November 20, 2017 10:48AM - 11:01AM |
G9.00002: Intermittent Swimming with a Flexible Propulsor Emre Akoz, Keith Moored Aquatic animals use a variety of swimming gaits to propel themselves efficiently through the oceans. One type of gait known as intermittent or burst-and-coast swimming is used by species such as saithe, cod and trout. Recent studies have shown that this gait can save up to 60\% of a swimmer’s energy by exploiting an inviscid Garrick mechanism. These detailed studies have examined the effects of an intermittent swimming gait on rigid propulsors, yet the caudal fins of intermittent swimmers are in fact highly flexible propulsors. In this respect, to gain a comprehensive understanding of intermittent swimming, the effect of elasticity on the swimming performance and wake flow of an intermittent swimmer is investigated. To accomplish this a torsional spring structural model is strongly coupled to a fast boundary element method solver that captures the fluid-structure interaction of a two-dimensional self-propelled intermittently pitching hydrofoil. It is shown that flexibility introduces extra vortices to the coasting phase of motion that can either promote or diminish thrust production depending upon the hydrofoil parameters. An optimal intermittent flexible swimmer is shown to increase its efficiency by as much as 28\% when compared to an optimal continuous flexible swimmer. [Preview Abstract] |
Monday, November 20, 2017 11:01AM - 11:14AM |
G9.00003: Computational investigation of feedback loop as a potential source of neuromechanical wave speed discrepancy in swimming animals. Namu Patel, Neelesh A. Patankar Aquatic locomotion relies on feedback loops to generate the flexural muscle moment needed to attain the reference shape. Experimentalists have consistently reported a difference between the electromyogram (EMG) and curvature wave speeds. The EMG wave speed has been found to correlate with the cross-sectional moment wave. The correlation, however, remains unexplained. Using feedback dependent controller models, we demonstrate two scenarios -- one at higher passive elastic stiffness and another at lower passive elastic stiffness of the body. The former case becomes equivalent to the penalty type mathematical model for swimming used in prior literature and it does not reproduce neuromechanical wave speed discrepancy. The latter case at lower elastic stiffness does reproduce the wave speed discrepancy and appears to be biologically most relevant. These findings are applied to develop testable hypotheses about control mechanisms that animals might be using at during low and high Reynolds number swimming. [Preview Abstract] |
Monday, November 20, 2017 11:14AM - 11:27AM |
G9.00004: Effects of traveling waves on flow separation and turbulence Amir Mahdi Akbarzadeh, Iman Borazjani Stable leading edge vortex (LEV) is observed in many flying, hovering and also some aquatic creatures. However, the LEV stability in aquatic animal, in contrast to hovering ones, is not well understood. Here, we study the flow over an inclined plate with an undulatory motion inspired from aquatic swimmers using our immersed boundary, large-eddy simulations (LES). The angle of attack is five degrees and Reynolds number (Re) is 20,000. The undulation is a traveling wave, which has a constant amplitude of 0.01 with respect to chord length and a different wavelength and Strouhal number (St$=$fA/U, f: frequency, A: amplitude, and U: free stream velocity) for each case. Over a fixed plate the LEV becomes unstable as it reaches the trailing edge and sheds to the wake, whereas over the undulating plate with St$=$0.2 the LEV becomes stable. The visualization of time average results shows there is a favorable pressure gradient along the tangential direction in cases the LEV becomes stable, which we explain analytically by showing the correlation between the average pressure gradient, St, and wavelength. Finally, the effects of undulatory moving walls of a channel flow on the turbulent statistics is shown. [Preview Abstract] |
Monday, November 20, 2017 11:27AM - 11:40AM |
G9.00005: Scaling laws for a compliant biomimetic swimmer Florence Gibouin, Christophe Raufaste, Yann Bouret, Mederic Argentina Motivated by the seminal work of Lord Lighthill in the sixties, we study the motion of inertial aquatic swimmers that propels with undulatory gaits. In 2014, Gazzola et al. have uncovered the law linking the swimming velocity to the kinematics of the swimmer and the fluid properties. At high Reynolds numbers, the velocity appears to be equal to $0.4 A f/(2\pi)$, where $A$ and $f$ are respectively the amplitude and the frequency of the oscillating fin. We have constructed a compliant biomimetic swimmer, whose muscles have been modeled through a torque distribution thanks to a servomotor. A soft polymeric material mimics the flesh and provides the flexibility. By immersing our robot into a water tunnel, we find and characterize the operating point for which the propulsive force balances the drag. We bring the first experimental proof of the former law and probe large amplitude undulations which exhibits nonlinear effects. All data collapse perfectly onto a single master curve. We investigate the role of the fin flexibility by varying its length and its thickness and we figured out the existence of an efficient swimming regime. [Preview Abstract] |
Monday, November 20, 2017 11:40AM - 11:53AM |
G9.00006: Distributed flexibility in inertial swimmers Daniel Floryan, Clarence W. Rowley, Alexander J. Smits To achieve fast and efficient swimming, the flexibility of the propulsive surfaces is an important feature. To better understand the effects of distributed flexibility (either through inhomogeneous material properties, varying geometry, or both) we consider the coupled solid and fluid mechanics of the problem. Here, we develop a simplified model of a flexible swimmer, using Euler-Bernoulli theory to describe the solid, Theodorsen’s theory to describe the fluid, and a Blasius boundary layer to incorporate viscous effects. Our primary aims are to understand how distributed flexibility affects the thrust production and efficiency of a swimmer with imposed motion at its leading edge. In particular, we examine the modal shapes of the swimmer to gain physical insight into the observed trends. [Preview Abstract] |
Monday, November 20, 2017 11:53AM - 12:06PM |
G9.00007: Self-propulsion of a pitching foil Anil Das, Ratnesh Shukla, Raghuraman Govardhan Undulatory motions serve as a fundamental mechanism for bio-locomotion at moderate and high Reynolds numbers. An understanding of the interactions between self-propelling undulatory motions and the surrounding fluid, not only provides insight into the efficiency of bio-locomotion, but also yields valuable pointers for the design of autonomous under-water and micro-aerial vehicles. Here, we investigate a simplified model of a self-propelling pitching foil that undergoes time-periodic oscillations about its quarter chord. We consider two-dimensional configurations in which the foil is free to propel along only longitudinal and both transverse and longitudinal directions. In both the configurations, the time-averaged self-propelling velocity increases monotonically with the Reynolds number Re (based on trailing edge speed and chord as the characteristic velocity and length). The rate of increase is particularly pronounced in the low Re regime (Re \textless 400) over which the closely-spaced wake vortices dissipate within a few chord lengths. At moderate and high Re, the wake exhibits increasingly complex structure in both the configurations. For a fixed Re, the foil with a single translational degree of freedom propels at a higher speed for a higher input power requirement. Differences between the two configurations will be discussed within the context of undulatory self-propulsion observed in nature. [Preview Abstract] |
Monday, November 20, 2017 12:06PM - 12:19PM |
G9.00008: Hydrodynamics of a three-dimensional self-propelled flexible plate. Jaeha Ryu, Hyung Jin Sung A three-dimensional self-propelled flexible plate in a quiescent flow was simulated using the immersed boundary method. The clamped leading edge of the flexible plate was forced into a vertical oscillation, while free to move horizontally. To reveal the hydrodynamics of the plate, the averaged cruising speed ($\bar{U}_{C})$, the input power ($\bar{P})$, and the swimming efficiency ($\eta )$ were analyzed as a function of the bending rigidity ($\gamma )$ and the flapping frequency ($f)$. The velocity field around the plate and the exerted force on the plate were demonstrated to find out the dynamic interaction between the plate and the surrounding fluid. The kinematics of the plate, the maximum angle of attack ($\phi _{max})$, and the mean effective length ($\bar{L}_{eff})$ were examined accounting for the hydrodynamics of the self-propelled flexible plate. The vortical structures around the plate were visualized, and the influence of the tip vortex on the swimming efficiency was explored qualitatively and quantitatively. [Preview Abstract] |
Monday, November 20, 2017 12:19PM - 12:32PM |
G9.00009: Scaling Laws for the Propulsive Performance of Self-Propelled Three-Dimensional Pitching Panels Fatma Ayancik, Keith Moored Inviscid computational results are presented within a boundary element numerical framework on a self-propelled virtual body combined with a rigid three-dimensional rectangular plate undergoing pitching motions about its leading edge. New scaling laws have been developed for the thrust and power as well as self-propelled speed, efficiency, and cost of transport by incorporating three-dimensional effects for varying aspect ratios. A lifting line theory correction is applied to account for the variation of the circulatory forces due to three-dimensional effects and the alteration of the added mass forces with aspect ratio changes is also considered. The scaling laws show that when accounting for three-dimensional effects, the physics of mean thrust production follows linear theory well, while the power must be modified with nonlinear corrections. [Preview Abstract] |
Monday, November 20, 2017 12:32PM - 12:45PM |
G9.00010: The wake dynamics leading to higher efficiency versus larger thrust for heaving and pitching panels Arman Hemmati, Alexander J. Smits The flow around a heaving-pitching rectangular panel is examined using Immersed Boundary Method incorporated to Direct Numerical Simulations, over a range of Reynolds numbers and Strouhal numbers. This study is aimed at determining the effects of a combined heaving-pitching motion on propulsive efficiency and thrust generation of new underwater propulsors. Preliminary results indicate that high efficiency and large thrust cannot be achieved simultaneously using a combined heaving-pitching motion. In particular, the leading (LEV) and trailing edge (TEV) vortex dynamics, which lead to enhanced thrust generation on the one hand and better propulsive efficiency on the other, were dictated by the oscillation frequencies and the heaving-pitching phase difference. For example, a higher heaving frequency accelerates the formation and detachment of LEVs, which favors higher efficiency while decreasing thrust generation. A higher pitching frequency, however, enhances thrust generation by accelerating the TEV detachment with a negative impact on the efficiency. [Preview Abstract] |
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