Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session E20: Biological Fluid Dynamics: Locomotion Flapping |
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Chair: Sachin Shinde, Indian Institute of Technology, Kanpur Room: Georgia World Congress Center B308 |
Sunday, November 18, 2018 5:10PM - 5:23PM |
E20.00001: Flow underneath the wing of a honey bee locomoting on a water surface Chris Roh, Morteza Gharib In our previous studies, we observed honey bee’s locomotion at the air-water interface. Their ventrally wetted wings beat at high frequency (30-220 Hz), which propel them forward. Using the kinematics of body we measured average thrust generated by the wing to be O(100 μN). To understand the thrust generation by the beating wing, we constructed a mechanical model that can mimic the bee’s wing kinematics. The flow under the wing was quantitatively measured using digital particle image velocimetry. The flow field shows that the wing is pumping the fluid underneath the wing backwards, consistent with the forward thrusting by the bee. |
Sunday, November 18, 2018 5:23PM - 5:36PM |
E20.00002: Towards Self Similarity of Wake Transitions in Flapping Foils Nikolaos S. Lagopoulos, Gabriel D. Weymouth, Bharathram Ganapathisubramani The present study aims to examine the key factors responsible for wake transitions in flapping foils regardless of the imposed kinematics. Here we focus on two types of transition: Von Karman wake reversal and wake deflection, which loosely correspond to the generation of thrust and side force respectively. Using two-dimensional numerical simulations of a Boundary Data Immersion (BDIM) solver, we examine these transitions for sinusoidal heaving, pitching and coupled (pitching and heaving) motions. Other factors under consideration include different effective angles of attack as well as pivot points when present. We explore a wide range of thickness based Strouhal numbers (Sr) and non-dimensional TE amplitudes (AD) as done in previous studies. In the AD-Sr phase map, the transitions are found to be clearly dependent on the kinematics. Various possible length and time scales that can universally capture the transitions are explored. The cycle averaged swept area of the foil appears to be a suitable length scale that can capture these transitions independent of the kinematics. We link the flapping kinematics and wake formation by shedding light on the underlined physics. Key Words: Flapping Foils, BvK Wake, Deflection. |
Sunday, November 18, 2018 5:36PM - 5:49PM |
E20.00003: Hydrodynamics of a trapezoidal self-propelled flexible plate. Jaeha Ryu, Hyung Jin Sung A trapezoidal self-propelled flexible plate in a quiescent flow was simulated using an immersed boundary method. The shape ratio (S = Wt/Wl) was defined as the ratio of the trailing edge width (Wt) to the leading edge width (Wl). To reveal the hydrodynamics of the plate, the averaged cruising speed (Uc), the input power (P), and the swimming efficiency (η) were analyzed as a function of the bending rigidity (γ) and the shape ratio (S). The three-dimensional effect on the dynamics of the plate was scrutinized with the kinematics such as the peak-to-peak amplitude (At/A) and the Strouhal number (St). The elongated body theory was adopted to see the relation between the kinematics and dynamics. The maximum angle of attack (Φmax) and the mean effective length (Leff) were examined to account 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. |
Sunday, November 18, 2018 5:49PM - 6:02PM |
E20.00004: 3D Influence on Propulsive Flapping at Low Reynolds Number Andhini Novrita Zurman-Nasution, Gabriel Weymouth, Bharathram Ganapathisubramani Many of the 2D approximations used for flapping studies at low Reynolds number are questionable. The objective of this study is to characterize the 3-dimensional behaviour of flapping foils with applications to biomimetic flying and swimming robots. The 3D fluid effects of oscillating foils are studied through the comparison of their thrust force, vorticity and energy. We simulate the unsteady flow on an infinite-span flapping foil with heaving, pitching and coupled motion using the Boundary Data Immersion Methods, which is documented to accurately predict low Re stationary and dynamic foils. A range of Strouhal numbers St = AD*f*D/U (D=foil thickness, f=frequency, U=freestream velocity and AD= trailing-edge amplitude/D) are examined by increasing AD=0.0625-3.75 with constant f*D/U. We find that the flow transitions from turbulent 3D structures into uniform 2D flow at around AD=1 (St=0.3). Suitable range of St for each motion is determined where the flow remains 2D and it provides a window over which 2D simulations can represent the realistic 3D flow. Outside of it, running 3D simulation is critical as the flow field, forces, efficiencies and wake energy are all different than their 2D prediction for the same geometry and kinematics. |
Sunday, November 18, 2018 6:02PM - 6:15PM |
E20.00005: Flexibility induces ‘unsteady actuator disk’ type of action for a foil flapping in the absence of free stream Sachin Yashavant Shinde, Jaywant H Arakeri Birds, insects, fish use wing, fin flexibility to their advantage, but knowing ‘how?’ is non-trivial due to the intricately coupled fluid-flexible-surface interactions. The surface selected to understand such interactions is a two-dimensional rigid foil, to which is attached at the trailing edge a thin chordwise flexible flap, pitching in quiescent water - a case relevant to hovering. Flapping flexible foil produces a reverse Benard-Karman vortex jet. Interestingly, time mean shows an accelerating flow in the downstream, similar to an ‘idealized actuator disk’, but, with some differences. We show (being reported for the first time) that the flapping flexible foil can be thought of as an ‘unsteady actuator disk’. Ellington (1984) modeled unsteady force generation for a hovering insect with ‘pulsed actuator disk’ that applies pressure impulses to the air passing through. Using PIV data, we present mechanism of how flexibility induces the unsteady actuator disk type action: deformations of flexible flap during ‘active period’ orient the generated pressure gradients in the jet direction, thus causing the flow acceleration. In stark contrast, if rigid, the same flap would orient the pressure gradients in transverse direction, which is not useful for propulsion as well as hovering. |
Sunday, November 18, 2018 6:15PM - 6:28PM |
E20.00006: Fin morphology interplay in the trajectory optimization of single fin flapping propulsion Cecilia Huertas-Cerdeira, Nathan K Martin, Morteza Gharib Flapping propulsion has received recent attention as an alternative propulsion method for unmanned underwater vehicles. By allowing a single fin to move in all three degrees of rotation, instead of following simple pitching and heaving motions, this project aims to achieve an efficiency and maneuverability that will allow to remove any additional surfaces. In order to find the optimal 3D fin trajectory that generates a specific thrust force, side force or turning moment, a setup has been developed that can experimentally perform and evaluate the trajectories generated in the optimization steps of an evolutionary algorithm. In this study we analyze the effect that the fin morphology has on the optimal fin trajectory for different maneuvers. For that purpose, the optimal trajectories for flat plates of varying shapes and aspect ratios are obtained and analyzed, with a special emphasis on understanding and comparing the evolution of caudal fin morphologies of different fish. |
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