Bulletin of the American Physical Society
77th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 24–26, 2024; Salt Lake City, Utah
Session C31: Interact: The Dynamics of Vortices |
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Chair: Philip Marcus, University of California, Berkeley Room: 255 C |
Sunday, November 24, 2024 10:50AM - 11:20AM |
C31.00001: INTERACT FLASH TALKS: The Dynamics of Vortices Each Interact Flash Talk will last around 1 minute, followed by around 30 seconds of transition time. |
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C31.00002: The Transition of a Stable Vortex from Hollow to Non-Hollow to Unstable - such as Jupiter's Great Red Spot Philip S Marcus, Aidi Zhang, Sungkyu Kim A hollow 3D vortex is one in which the absolute vertical vorticity has a local minimum at its center. Hollow vortices have never been seen in the lab, but hollow anticyclones exist in stratified, rotating astrophysical flows such as numerically-computed zombie vortices in protoplanetary disks and Jupiter’s Great Red Spot. The physics of vortex hollowness are not fully understood, but recent numerical simulations provide clues, and it may be that only anticyclones, rather than cyclones, can be hollow and that the hollowness is due to the fact that the fluid near the mid-plane of an anticyclone is always less stratified than its surrounding fluid. Simulations show that stable hollow vortices transition to stable unhollow vortices if the vortex becomes too vertically thin or too thick. If the resulting non-hollow vortex continues to have its thinness or thickness increased, the vortex becomes convectively unstable. In addition, if the ambient flow around a stable hollow vortex becomes sufficiently destratified, the vortex becomes un-hollow, and if it is further destratified, the vortex becomes unstable. A model based on hydrostatic equilibrium and geostrophic balance explains these observations and may allow us to find a prescription for creating stable hollow vortices in the lab. The model also explains many of the recent observations of Jupiter's Great Red Spot by the Hubble Space Telescope, James Webb Space Telescope, and Juno satellite. |
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C31.00003: ABSTRACT WITHDRAWN
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C31.00004: When a particle meets a vortex ring... Guilherme Siqueira de Aquino, Sylvain Viroulet, Julie Albagnac Particle-laden flows are ubiquitous in a wide range of natural phenomena and industrial applications (e.g. volcanoes, fluidized beds, etc.), which justifies a large number of studies on the subject. Nevertheless, despite considerable progress in modelling these flows, the control or even prediction of particle transport and dispersion remains an open question in many situations. To quantify the fundamental physical mechanisms of interaction, this study focuses on the interaction of a single inertial particle and a model flow, namely a vortex ring. Vortex rings can be generated whenever an impulsive motion occurs and, consequently, are one of the most widespread vortical structures. Such structure constitutes the main part of flow dynamics in many geophysical and industrial contexts. The approach chosen here was first to map the interactions as a function of the non-dimensional parameters of the problem, using 3D visualization and PIV. Next, numerical simulations (DNS, Basilisk) were performed to reproduce the experimental data and quantify the predominant forces acting on the particle trajectory. |
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C31.00005: Vibrissa-inspired geometries exhibit enhanced sensitivity to wake-induced vibrations Eva Erickson, Joel W Newbolt, Eric Edward Handy-Cardenas, Kenneth S Breuer We report on experiments designed to characterize the wake-induced vibration (WIV) experienced by bluff bodies immersed in unsteady flows. A sinusoidally pitching and heaving hydrofoil upstream is used to generate vortices of different strength and frequency which advect downstream and impinge on a bluff body that is free to oscillate in the flow. Using a real-time Cyber-Physical System (CPS) we systematically prescribe the virtual mass, spring constant, and damping of the elastically mounted models. We characterize the forces and displacements of the WIV response of (i) a circular cylinder, (ii) an elliptical cylinder and (iii) a seal whisker inspired vibrissa model with an undulating elliptical geometry. We find that the reduced aspect ratio objects, known to suppress vortex-induced vibrations (VIV) in a clean flow, experience WIV with the highest amplitude oscillations occurring when the structural frequency of the test object matches the frequency of the upstream foil. Further, the vibrissa has a lower damping ratio than the elliptical cylinder, suggesting that the vibrissa geometry may serve as a high sensitivity flow disturbance sensor. |
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C31.00006: Numerical study of singularity formation in two vortex sheets Takeshi Gotoda, Robert Krasny The evolution of vortex sheets is described by a nonlinear singular integrodifferential equation called the Birkhoff-Rott equation. The initial-value problem for the Birkhoff-Rott equation seems to be ill-posed due to the Kelvin-Helmholtz instability. The preceding result by Krasny has numerically shown that a single vortex sheet forms a singularity at a finite time by using the point-vortex approximation. In this study, we investigate the formation of singularities in two vortex sheets with Krasny's method. We consider the initial condition perturbed by solutions of the linearized Birkhoff-Rott equations and see how the process of the singularity formation varies depending on the amplitude of the perturbation and the distance of two vortex sheets. |
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C31.00007: Investigation of impulse enhancement of a train of vortex rings via the vortex nozzle effect Eva Liu, Arthur Monnier, Sean D Peterson The vortex nozzle effect, wherein the impulse of a vortex ring is enhanced via passive interaction with a wall with co-axial aperture, has been previously demonstrated for an isolated ring. For ring-to-aperture radius ratios near one, the hydrodynamic impulse of the ring is enhanced due to fluid entrainment during the collision increasing ring volume and minimized circulation loss due to the low-velocity core region of the ring enveloping the aperture tip, which reduces opposite-sign vorticity production. The vortex nozzle was shown to increase impulse up to 11% compared to a freely advecting ring for a ring Reynolds number of 3000. What remains unknown is whether the same impulse enhancement arises for a train of vortex rings, wherein inter-ring interactions may alter the dynamics near the aperture. In this study, the dynamics of a train of vortex rings advecting towards a wall with a co-axially aligned aperture is explored numerically and experimentally for source Reynolds numbers from 1000 to 3000 (based on generating orifice diameter and time averaged orifice exit velocity) and various inter-ring spacings. Ring geometry, circulation, and impulse are examined to assess the viability of the vortex nozzle phenomenon as an impulse enhancement method for synthetic jets. |
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C31.00008: Dynamics of circular vortex dipoles Michael J Wadas, Axel Haydt, Tim Colonius Extensive research has examined colliding vortex rings as a way to isolate fundamental dynamical interactions comprising more complex flows. Initiating such collisions with vortex ring generators, however, constrains what types of rings are produced and requires precisely aligned generators. We address these limitations through a framework for generating circular vortex dipoles congruent to systems of colliding vortex rings following their initial approach. Such dipoles are generated computationally by selectively applying a non-conservative body force within the domain, enabling near-arbitrary dipole geometries that isolate mechanisms of interest. Experimentally, dipoles are generated with a radial starting jet in a water tank. |
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C31.00009: Tripolar Instability of a Strained Lamb-Oseen Vortex Aditya Sai Pranith Ayapilla, Yuji Hattori, Stephane Le Dizes Our goal is to investigate the stability of a Lamb-Oseen vortex under tripolar straining. The motivation for our study is that the hub vortex formed in turbomachinery with three blades experiences tripolar straining from the surrounding vortices and this can potentially lead to instability and the onset of turbulence. Although the curvature and elliptic instabilities of a Lamb-Oseen vortex and the multipolar instability of a Rankine vortex are well-studied, the multipolar instability of a Lamb-Oseen vortex remains under-explored numerically and theoretically. In this study, we present the evidence for the tripolar instability through direct numerical simulations (DNS) by performing linear stability analysis for a basic flow of a hub Lamb-Oseen vortex strained by three satellite Lamb-Oseen vortices. We demonstrate that for a specific combination of resonant azimuthal wavenumbers of 1 and -2, instability occurs only for some resonant axial wavenumbers. Unstable growth rates are also computed theoretically and compared to the DNS results. |
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C31.00010: Optimizing the Performance of Flapping Foil Wave-Assisted Propulsion Systems: Insights from Computational Fluid-Structure Interaction Models Harshal Suresh Raut, Jung-Hee Seo, Rajat Mittal The wave glider is one example of a watercraft that converts wave energy into propulsion using a series of oscillating hydrofoils. The trailing foils in the wave glider can utilize the wake of the leading foil, improving thrust generation. Additionally, constraining the pitching motion of the foils with torsional spring or angle limiter can enhance the performance of these flapping foils. To understand the dynamics and optimize the performance of the wave glider system, we conduct a computational study of flow-induced pitch oscillations of sinusoidally heaving foils at a chord-based Reynolds number of 10,000. We first examine the flow physics of multi-foil systems. We find that the trailing foil can be optimally placed to enhance its leading-edge vortex (LEV) by utilizing the wake of the leading foil, resulting in a thrust improvement of around 80%. We then investigate the effect of constraining the pitching motion of the foil using torsional spring and angle limiter in different sea-states. Our results indicate that angle limiters are a simple and effective method for generating thrust under varying sea conditions. Additionally, the LEV-based model is used to uncover the role of kinematics of the foil on thrust generation. |
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C31.00011: Transition from Vortex Shedding to Jet Formation in the wake of Kirigami Sheets in Flow Daniela Caraeni, Adrian G Carleton, Yahya Modarres-Sadeghi In this study, we explore the transition from vortex shedding to jet formation in the wake of kirigami sheets placed in flow at low Reynolds numbers using computational fluid dynamics simulations. In these simulations, we use multiple walls to obstruct the flow from the inlet, allowing it to flow only through small orifices of a kirigami sheet into a domain of quiescent fluid. This setup enables a detailed examination of how variations in the dimensions—length, height, and gap—of these obstructing walls influence the flow dynamics in the wake of the kirigami sheets. This study is conducted for several cases of ribbon-cut Kirigami patterns, where experimental results have already shown that the incoming flow results in buckling or elongation of the sheet. A comparison between experiments and simulations sheds light on the mechanisms for the formation of jets and vortices at multiple scales in the wake of kirigami sheets. |
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C31.00012: A weak coupling between a Near-Wall Eulerian solver and a Vortex Particle-Mesh method for the efficient simulation of 3D incompressible external flows. Guillaume Querinjean, Paul F Fischer, Ananias Tomboulides, Philippe Billuart, Philippe Chatelain, Gregoire Winckelmans We report on a new hybrid numerical method for the simulation of 3D incompressible external flows, coupling a Near-Wall (NW) Eulerian solver, Nek5000, with a Multi-Resolution Vortex Particle-Mesh (VPM) solver, Murphy. |
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C31.00013: A high fidelity numerical study on the vortex induced vibration of a rigid cylinder in 103<Re_D<106 Zhicheng Wang, Dixia Fan, George Em Karniadakis, Michael Triantafyllou We developed a high fidelity spectral element solver for the simulation of the vortex induced vibration (VIV) a smooth cylinder at high Reynolds number (ReD). The coordinate transformation method is employed to deal with the moving boundary due to the vibrating cylinder. The entropy viscosity method based on the residues of the vorticity transport equations is employed to deal with the turbulence colsure. The solver is implemented on the spectral element code NekRS. With the solver, we have simulated VIV of a rigid cylinder in the ReD range [103, 106], reduced order velocity (Ur) range [2,30]. In particular, for the cylinder only allowed to vibrate in crossflow only, in range 8×104<ReD<2.6×105 and 8<Ur<25, large amplitude up to 2.3D is observed. To the best of our knowledge, it is first time that a numerical simulation is able to reproduce both the unusual large amplitude. |
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C31.00014: Sensing capabilites of bent Harbor Seal Whiskers Winston Jiang, Geng Liu, Xudong Zheng, Qian Xue, Biao Geng, Luke Ingraham Harbor seals utilize uniquely shaped whiskers to detect, locate, and follow prey, a remarkable adaptation that offers promising insights for advancing underwater sensing technologies. Our research has demonstrated that harbor seals' whiskers generate hairpin vortices, which reduce self-induced vibrations and enable the detection of small disturbances in the water. In nature, whiskers are not always perfectly straight, and this effect has long been overlooked in past studies. In this study, we aim to determine whether curvature in these whiskers affects the harbor seals' sensing capabilities. We conducted direct numerical simulations of flow past segments of both straight and progressively bent whiskers. Our findings revealed that bends in the whiskers disrupt the formation of hairpin vortices, increasing lift oscillation and therefore noises. Future studies are necessary to confirm that the observed effects are indeed due to the curvature of the whiskers and to quantify the effect of bends on lift oscillation and drag. Verifying these results will be crucial for understanding the precise role of whisker morphology in harbor seals' sensory abilities. |
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C31.00015: Stability of the Leray scaling solution for vortex reconnection in Euler flows II Yoshifumi Kimura, Philip J Morrison As a model for studying the evolution towards finite-time singularity of the Navier-Stokes equation, a dynamical system was proposed for describing the behavior of vortex reconnection of two vortex rings placed symmetrically on two tilted planes [1][2][3]. For the Euler limit, it was shown that this dynamical system can be written in noncanonical Hamiltonian form with Hamiltonian, H, and a Casimir invariant, C, and that a solution is obtained as the intersection of two surfaces, H=const. and C=const. [4]. The special case where both H and C vanish gives a singular solution, obtained by quadrature, which has the exact scaling proposed by Leray for studying the self-similar solutions of the Navier-Stokes equations. We investigate the stability of this Leray solution by employing time dependent stretched coordinates that transform the singular solution into a fixed point, and then examining a linear perturbation about the fixed point. Among the three eigen solutions, there are two stable and one unstable modes. It is verified that the eigenvector of the unstable mode is parallel with the trajectory, and thus the Leray solution is linearly stable. |
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C31.00016: On Galilean Invariance of Mean Kinetic Helicity Dina Soltani Tehrani, Hussein Aluie While kinetic helicity is not Galilean invariant locally, it is known [1] that its spatial integral quantifies the degree of knottedness of vorticity field lines. Being a topological property of the flow, mean kinetic helicity is Galilean invariant. We provide a direct mathematical proof and discuss that kinetic helicity is Galilean invariant when spatially integrated over regions enclosed by vorticity surfaces, i.e., surfaces of zero vorticity flux. We also discuss so-called ``relative'' kinetic helicity, which is Galilean invariant when integrated over any region in the flow. |
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C31.00017: The origin of vorticity in viscous incompressible flows Tianrui Xiang, Gregory L Eyink, Tamer A Zaki In inviscid incompressible flows, vortex lines are material lines that can be traced forward and backward in time without ambiguity. For viscous Navier-Stokes, this symmetry is no longer present. A stochastic interpretation of vorticity was recently introduced to account for the influence of viscosity, where particles are tracked back-in-time along stochastic trajectories. The terminal vorticity at the target location is then evaluated as the expectation of the initial vorticity being stretched, tilted/twisted along the particle paths. We present a new Eulerian approach to determine the origin of vorticity in backward time, and we prove mathematically that it is exactly equivalent to the stochastic interpretation. With our new equation, we can dissect the contributions to the target vorticity in terms of stretching and tilting of the interior vorticity and a wall contribution. We apply our approach to turbulent channel flow to study the back-in-time origin of high-stress events on the wall. |
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C31.00018: ABSTRACT WITHDRAWN
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C31.00019: Experimental Investigation of Starting Flow in the Wake of a Perforated Plate Scott Bollt, Manoochehr M Koochesfahani, Morteza Gharib The flow over a perforated or porous plate is commonly encountered in civil engineering, and its practical importance is growing. Since the seminal work of Castro in 1971, it has been recognized that the steady state mean flow produced by a perforated plate contains an attached vortex dipole. The dipole moves further from the plate and its strength decreases (as does vortex shedding) for plates with larger open area fractions until it disappears entirely. Despite renewed interest in the steady perforated plate problem, almost no attention has been paid to a perforated plate’s starting flow. This leaves many questions unanswered. Does the disappearance of attached vortices in the steady case have a correspondence to behavior in the startup flow? How is the vortex formation process effected by perforation? |
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C31.00020: Aerodynamic Forces on a Wing Surfing in a Two-dimensional Vortex Wake Siyang Hao, Pedro Costa Ormonde, Kenneth S Breuer “Surfing” on wakes, such as birds riding each other’s wake in flocks, is a well-observed natural phenomenon. The goal of this study is to understand the aerodynamic force variation on a wing surfing in an unsteady 2-D wake. Wind tunnel experiments were conducted using Particle Image Velocimetry (PIV) and force measurements with a fixed wing immersed in the wake of a pitching airfoil. Key results show that the force response of the surfing wing is aligned with the impingement of flow structures and can be effectively predicted using classic unsteady aerodynamics (Wagner, Kussner and Sears) based on measured unsteady local flow conditions (instantaneous angle of attack and speed). The theoretical predictions compare well with direct force sensor measurements. The dependance of the overall lift fluctuations on the upstream flapping kinematics are scaled as a function of the reduced amplitude and reduced frequency of the flapping motion. Good collapse of the data is found and deviations from scaling are explained in terms of the wake characteristics. |
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C31.00021: Dynamics of melting ice cylinders in a cross-flow Kari Perry, James Luo, CHK Williamson, Sarah E Morris The formation and release of icebergs into the ocean accounts for half of the total freshwater discharged from ice sheets (Depoorter, 2013). As icebergs float and melt far from their sources, they impose adverse effects on both local and global marine ecosystems. Despite this, present-day ice melt models frequently underpredict iceberg melt dynamics and the spread of meltwater, necessitating further research into the fundamental fluid mechanics of icebergs. In this work, we study the bluff body dynamics of ice to understand the effect of a cross-flow on ice melt and resultant meltwater spread. Experiments are completed in a closed-loop water channel for initial Reynolds numbers (Re0=UD0/ν) ranging from Re0=0 to 885. Flow structures are measured using flow visualization techniques and particle image velocimetry (PIV), and melt rates are found from surface area measurements. It is found that the melt rate increases with higher Re0; however, across all Re0 it is found that as the ice melts, the ice shape does not remain symmetric. Rather, the resultant vortex wake leads to different local melt rates between the front and back faces, creating significant shape changes. As a result of the time-varying ice cylinder shape, results show that the formation length (Lf), wake width (Wy) and Strouhal number (St=fD/U) vary through the melt process as well as with Re0. The results are compared to non-melting cylinders that are representative of initial and terminal ice shapes. |
Sunday, November 24, 2024 11:20AM - 12:50PM |
C31.00022: INTERACT DISCUSSION SESSION WITH POSTERS: The Dynamics of Vortices After each Flash Talk has concluded, the Interact session will be followed by interactive poster or e-poster presentations, with plenty of time for one-on-one and small group discussions. |
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