Bulletin of the American Physical Society
74th Annual Meeting of the APS Division of Fluid Dynamics
Volume 66, Number 17
Sunday–Tuesday, November 21–23, 2021; Phoenix Convention Center, Phoenix, Arizona
Session M24: Vortex Dynamics and Vortex Flows: Wakes |
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Chair: Jesse Ault, Brown Room: North 224 B |
Monday, November 22, 2021 1:10PM - 1:23PM |
M24.00001: Machine Learning Classification of Vortex Wakes from Oscillating Foils Bernardo Luiz Rocha Ribeiro, Jennifer A Franck A machine learning classification model is developed for wake patterns behind oscillating foils whose kinematics are tuned for energy harvesting from the oncoming freestream flow. The role of wake structure is particularly important for arrays of oscillating foils, since the coherent structures can cause constructive and/or destructive interference with downstream foils. This work explores 35 different oscillating foil kinematics within energy harvesting mode, with the goal of grouping and parameterizing the wake based on the input kinematic variables. An approach combining a convolutional neural network (CNN) with long short-term memory (LSTM) units is utilized to automatically classify the wakes into three groups based on foil's relative angle of attack. After revising group boundaries based on model performance, an average accuracy of 90% is achieved, demonstrating that foil relative angle of attack can be used to discern distinct wake patterns and hence provide insight for optimizing foil array configurations for energy harvesting. |
Monday, November 22, 2021 1:23PM - 1:36PM |
M24.00002: Flow past a cylinder in the Brinkman limit Sichen Liang, Guillaume Durey, Mrudhula Baskaran, Jesse T Ault We consider the quasi-2D flow past a cylinder in the limit of highly confined flow in the depth direction, often referred to as the Brinkman limit. We use microfluidics experiments and numerical simulations to consider the transition from the Brinkman limit to the infinite 2D limit. In particular, we focus on the formation of recirculation zones behind the cylinder as functions of Reynolds number. Microfluidics experiments are performed using high-speed photography and MATLAB’s PIVlab along with particle tracking to analyze both the transient and steady-state results. Numerical simulations are performed using OpenFOAM to vary the thickness of the geometry from the Brinkman limit to the 2D limit, and streamline visualizations are used to visualize the flows. The wake formation and unsteady flow behaviors in the Brinkman limit are found to vary significantly from the traditional results for the 2D case, and the characteristic von Karman vortex shedding behavior is significantly altered in the confined geometry. |
Monday, November 22, 2021 1:36PM - 1:49PM |
M24.00003: An experimental study of falling spheres in a confined density-stratified fluid Faezeh Masoomi, Pranav Mohan, Sayantan Bhattacharya, Javad Eshraghi, Pavlos P Vlachos Falling objects in a density stratified fluid have essential applications in the natural environment. For example, the vertical motion of zooplankton, and marine snow, the falling biological debris from the ocean's surface, is affected by ocean stratification. In addition, the stratification of the atmosphere can modify the settling dynamics of dust and aerosol, which directly impacts climate change. Studies have shown that density stratification can substantially alter the flow and wake structures around settling spherical particles, which results in a significant increase in the drag coefficient [1,2]. The dynamics of falling spheres are characterized by a natural reverse jet and internal waves. In this study, we investigate the transient behavior of falling spheres with moderate Reynolds number (~65 and up) in a confined linearly stratified fluid, using 3-D particle tracking velocimetry (PTV). The wake structure is compared to the homogenous counterpart. Moreover, the effect of confinement will be assessed by comparing the current results with those reported in the literature with no confinements. |
Monday, November 22, 2021 1:49PM - 2:02PM |
M24.00004: Active tail flexion in concert with passive hydrodynamic forces improves swimming speed and efficiency Haotian Hang, Sina Heydari, John H Costello, Eva Kanso Swimming fish have a universal rule in bending their body during swimming; it occurs at about one-third from the tail of fish with a maximum bending angle of about 30o. However, the hydrodynamic mechanisms that shaped this convergent design and its potential benefit to fish in terms of swimming speed and efficiency are not well understood. It is also unclear to what extent this bending is active or passively follows the interaction of a flexible posterior with the fluid. Here, we analyze the swimming performance of a self-propelled two-link plate with active and passive posterior end in the context of the vortex sheet method. Passive bending is more efficient but slower, but active bending can enhance both speed and efficiency. Importantly, we find that the phase difference between the posterior and anterior of the body is an important kinematic factor that influences performance. Active antiphase flexion, consistent with the passive flexion phase, can simultaneously enhance speed and efficiency in a region within the design space that overlaps with biological observations. Our results shows that fish that actively bend their bodies could exploits passive hydrodynamics can at once improve speed and efficiency. |
Monday, November 22, 2021 2:02PM - 2:15PM |
M24.00005: Formation and scaling of primary and secondary vortices Diego Francescangeli Vortex formation is a limiting process. |
Monday, November 22, 2021 2:15PM - 2:28PM |
M24.00006: Modelling vortex ring growth in the wake of a translating cone Guillaume De Guyon-Crozier, Karen Mulleners Vortex rings have the ability to transport fluid over long distances. They are usually produced by ejecting a volume of fluid through a circular orifice or nozzle. When the volume and velocity of the ejected fluid are known, the vortex' circulation, impulse, and energy can be estimated by the slug flow model. Vortex rings also form in the wake of accelerating axisymmetric bodies. In this configuration, the volume and velocity of the fluid that is injected into the vortex is not known a priori. Here, we present two models to predict the growth of the vortex behind disks or cones. The first model uses conformal mapping and assumes that all vorticity generated ends up in the vortex. The vortex circulation is determined by imposing the Kutta condition at the tip of the disk. The position of the vortex is integrated from an approximation of its velocity, given by Fraenkel. The model predicts well the maximum circulation of the vortex, but does not predict the tail shedding observed experimentally. A second model is based on an axisymmetric version of the discrete vortex method. The shear layer formed at the tip of the cone is discretised by point vortices, which roll-up into a coherent vortex ring. The model accurately captures the temporal evolution of the circulation and the non-dimensional energy. It also predicts the occurrence of tail shedding and the total amount of vorticity lost in the wake. The portion of the lost vorticity due to tail shedding is sensitive to the choice of the numerical parameters controlling the stability of the shear layer. |
Monday, November 22, 2021 2:28PM - 2:41PM |
M24.00007: Optimisation of a bio-inspired jet propulsor Mrudhula Baskaran, Fabio Zuliani, Alexander Gehrke, Jamie Paik, Karen Mulleners Bio-inspired underwater vehicles imitate the kinematics of biological organisms for locomotion. One mechanism exploited by marine organisms is pulsatile jet propulsion, the periodic ejection of vortex rings for thrust generation. Here, we present the design of a jellyfish-inspired device that produces vortices by compressing a bulb and ejecting fluid. The size of the orifice through which vortices are ejected can be varied in time to mimic medusae that alter their velar diameter to generate optimal vortex rings. Vortices are studied through time-resolved velocity field measurements and thrust measurements using strain gages. A multi-objective optimization is implemented to determine the time-varying compression and orifice-diameter profiles that are most energy-efficient for steady-state cruising and rapid escaping conditions. The effect of these kinematics on the vortex formation process, vortex ring circulation, impulse, and non-dimensional energy is studied. We also expand upon the specific role of secondary vortices in thrust generation. These findings enable us to optimally harness vortex rings for the energy-efficient locomotion of bio-inspired vehicles. |
Monday, November 22, 2021 2:41PM - 2:54PM |
M24.00008: Thrust generation by the coordinated suction and blowing of a cylinder array Dohyun Kim, Minhyeong Lee, Daegyoum Kim Some animals, such as cephalopods, periodically expand and contract their highly flexible bodies to intake and eject surrounding fluid for propulsion. Motivated by such flexible bodies effective in suction and blowing of the fluid, we simplify the continuous deformation of the flexible body by segmenting it into multiple moving bodies in order to examine how the coordinated oscillating motions of the segmented bodies induce suction and blowing and generate propulsive force. In this study, several rigid circular cylinders are positioned in a circle with equal distance between adjacent cylinders. If the cylinders oscillate in a radial direction with the same amplitude and frequency, suction and blowing flows occur uniformly at every exit. However, slight variation in the phases of the cylinders can induce directional flow, generating net thrust on the entire structure. Kinematic parameters for the coordinated motion of the cylinder array, oscillation amplitude, frequency, and phase difference between the cylinders, are numerically investigated to find the optimal condition for the maximum thrust. |
Monday, November 22, 2021 2:54PM - 3:07PM |
M24.00009: A comparative analysis of clapping propulsion in stationary and freely moving conditions Suyog V Mahulkar, Jaywant H Arakeri In aquatic habitats, pulse-jet type of propulsion can be observed, for example, in jellyfish and squid. In the present study, pulse jet propulsion is studied with a system consisting of two thin flat plates that clap together. We consider two cases: the body is free to move and the other in which the body is fixed. |
Monday, November 22, 2021 3:07PM - 3:20PM |
M24.00010: On wake mode transition of a foil with combined heaving and pitching motion Suyash Verma, Arman Hemmati The drag to thrust transition and its correspondence with changes in wake topology is investigated numerically for an oscillating foil with combined heaving and pitching motion. Quantitative and qualitative evaluations at a range of reduced frequency (0.16<f*<0.64), phase difference (0o<Π<315o) and Reynolds number (1000<Re<12000) reveal no direct correspondence between changes in the propulsive performance and wake modes. For f*<0.32, several wake configurations ranging from 2P+2S, wBvK and 2P are observed at increasing Π, although the propulsive performance was still observed to be drag dominated. For f*>0.24, different spatial configuration of 2P mode is observed for 90o<Π<225o, which are characterized by either vortex pairs aligned with the wake centerline (2PH) or dipoles (2PD). However, no drag-to-thrust transition coincides with these changes in wake topology. Further investigation into the mean flow corresponding to 0.24<f*<0.4 revealed bifurcation of the thrust producing jet with increasing Π from 0o to 90o. Dynamic interaction of coherent structures in the wake provides a qualitative reasoning for this bifurcation. This serves as a possible route to drag production in biological swimming and hence confines the range of Π that yields an optimum propulsive efficiency. |
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