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 A24: Vortex Dynamics and Vortex Flows: General I |
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Chair: Wim Van Rees, MIT Room: North 224 B |
Sunday, November 21, 2021 8:00AM - 8:13AM |
A24.00001: Adiabatic behavior of a two-dimensional elliptical vortex in a time-dependent external strain flow Noah C Hurst, Pakorn Wongwaitayakornkul, James R Danielson, Daniel H Dubin, Clifford M Surko The motion of a two-dimensional (2D) elliptical vortex in an external strain flow can be complicated when the strain rate varies in time. However, when the external flow changes slowly, adiabatic theory can be employed. We present analytical calculations and experimental measurements of the adiabatic behavior of elliptical vortices in external strain flows where the strain rate is ramped upward in time [N. C. Hurst et. al., Phys. Rev. Fluids 6, 054703 (2021)]. The theoretical work uses a Hamiltonian elliptical vortex patch model, where the adiabatic invariant is interpreted as the amplitude of an oscillation about a stable equilibrium. It is shown that both a driving term and frequency variation contribute to adiabatic breaking, where a WKB method is used to evaluate the latter effect. The experimental work uses pure electron plasmas, which have been shown to closely follow the 2D Euler equations describing ideal fluids. One conclusion is that the adiabatic breaking depends in complicated ways on the time-dependence of the strain rate, with linear ramps resulting in periodically-modulated power-law breaking. Another is that the experimental vortices experience critical-layer damping due to their smooth edges, which decreases the oscillation amplitude. Work supported by US DOE. |
Sunday, November 21, 2021 8:13AM - 8:26AM |
A24.00002: Interaction of a vortex ring impinging on a flat surface William N McAtee, Sarah E Morris, Preet Shah, Vrishank Raghav Vortex rings are ubiquitous in both nature and engineering applications. Some examples in nature include marine animal locomotion, cardiovascular flows, and expiratory events like coughing, sneezing, and speaking. With respect to the COVID-19 pandemic, the fluid dynamic interactions of these vortex-laden expiratory events are paramount to understanding how infectious diseases spread. One strategy to reduce dispersion of expiratory aerosol adopted by essential businesses is the installation of barriers. In this study, the interaction of a vortex-laden expiratory flow with different flat surfaces is investigated via fluorescent dye flow visualization. A programmable syringe pump is used to generate a vortex ring that impinges on a flat surface in a water tank. Various configurations of the flat surfaces are studied to quantify the interaction mechanisms and the ability of the surface to reduce dispersion. It is observed that the generation of secondary and tertiary vorticity at the wall results in vortex-surface interactions that are vastly different from inviscid approximations. A better understanding of these vorticity interactions could lead to improved surface designs that reduce dispersion. |
Sunday, November 21, 2021 8:26AM - 8:39AM |
A24.00003: Experimental study on flow-induced vibration of a flexible triangular cylinder Seyedmohammad Mousavisani, Abdul Raffae, Banafsheh Seyed-Aghazadeh Fluid-structure interactions response of a flexible cylinder with a triangular cross-section, fixed at both ends, placed in the test section of a re-circulating water tunnel, was studied experimentally. Flow-induced vibration (FIV) response was studied in terms of spanwise amplitudes and frequencies of oscillation in both the crossflow and inline directions for angles of attack of 0°, 30°, and 60°. The cylinder had an aspect ratio of 30 and a mass ratio of 0.8. High-speed imaging technique was employed to capture the spanwise oscillations at a reduced velocity range of U* = 0.9-21.7, corresponding to a Reynolds number range of Re= 364-6,500. At low reduced velocities, mono-frequency excitation at the first bending mode of the cylinder was observed, and as the flow velocity was increased, the oscillations transitioned to higher modes with regions of multi-frequency excitation with contributions from both bending and torsional modes. At the angle of attack of 0°, while the onset of oscillation was delayed compared to other angles, as the flow velocity was increased, large-amplitude oscillations were observed. Flow evolution around the cylinder and in its wake was tracked and the three-dimensional vortex wake was studied in correlation with the dynamic oscillations of the cylinder. |
Sunday, November 21, 2021 8:39AM - 8:52AM |
A24.00004: Vortex Dynamics in Axisymmetric Inlet Over a Plane in a Cross Flow Derek A Nichols, Bojan Vukasinovic, Ari Glezer The formation and evolution of a vortex that forms over a horizontal plane due to the suction into an axisymmetric inlet in the presence of a cross flow over the plane and normal to the inlet is investigated in wind tunnel experiments. In these investigations, the speeds of the cross flow and inlet flow and the height of the plane are independently varied and lead to the identification of a critical range at which the shear layer between the inlet (suction) flow and the cross flow leads to the formation of a columnar vortex over the ground plane that is ingested into the inlet. PIV measurements are used to characterize the critical inlet speed required for the onset of vortex formation and it is shown that this speed increases with the height above the plane and/or with cross flow speed. These measurements are used to assess the vortex characteristic cross section and circulation after the formation as the flow parameters continue to vary. It is also shown that the shear that is needed for the plane-normal vortex can be induced in the absence of a cross flow even when the inlet is far from the ground plane. It is anticipated that understanding the development of these vortices and the dependence of their strength and motion dynamics on the formation conditions will allow for development of methods for controlling their evolution and thereby reduce the adverse effects of their ingestion on propulsion systems. |
Sunday, November 21, 2021 8:52AM - 9:05AM Not Participating |
A24.00005: Shocklet formation in vortex reconnection Hamid Daryan, Jean-Pierre Hickey, Fazle Hussain We study the mechanisms of shocklet formation in a transonic viscous vortex reconnection. Shocklets emerge due to spatio-temporal variation of the pressure and induced velocity fields as the vortex tubes approach. The shocklets, if sufficiently strong, can result in early circulation transfer and modify the reconnection dynamics. Due to the strong dilation component, shocklets play an inevitable role in the reconnection noise. A decomposition of the Lighthill's source term is provided and the evolution of all dominant terms through shocklets are investigated. The current study provides a detailed analysis of the shocklets which are estimated to be critical features of the reconnection at high $Re$. |
Sunday, November 21, 2021 9:05AM - 9:18AM |
A24.00006: An extended model for orifice starting jets and its implications on vortex ring formation Raphael Limbourg, Jovan Nedic Starting jets emanating from a straight nozzle and orifices of different orifice-to-tube diameter ratios are investigated using planar particle image velocimetry. The invariants of the motion are measured and compared to the classic slug-flow model. An extension to the slug-flow model is proposed to account for the contraction the fluid is experiencing when being pushed through orifice geometries. The contraction coefficient obtained for two-dimensional jets is applied to the axisymmetric problem. This modified slug-flow model is shown to better predict the invariants of the motion with discrepancies of the order of 10% compared to underpredictions of 130%, 50% and 120% for circulation, hydrodynamic impulse and kinetic energy, respectively, using the classic slug-flow model. Moreover, the new model suggests the existence of a maximum in the production of impulse and energy at an orifice-to-tube diameter ratio of about 0.9, which was also recorded experimentally for the kinetic energy. Finally, the implications of the modified model on vortex ring formation are discussed; the formation time is redefined in terms of the quantities in the vena contracta as T^{*}=U_{*}t/D_{*}, which ultimately unifies the formation number of orifices and straight nozzles with a value of approximately 4. |
Sunday, November 21, 2021 9:18AM - 9:31AM |
A24.00007: Self-similar collapse of point vortices. Sreethin Sreedharan Kallyadan, Priyanka Shukla A linear algebra formulation of self-similar evolution of point vortices in terms of the inter-vortex distances is presented. This formulation can be used to numerically compute initial positions that give rise to self-similar vortex collapse with or without the prior knowledge of circulations. By tending the decay rate to zero, a family of self-similar collapse solutions asymptotically converging to a relative equilibrium can be numerically constructed from any single solution. |
Sunday, November 21, 2021 9:31AM - 9:44AM |
A24.00008: A vortex ring bubble crossing an oil-water interface Eunseong Moon, Minho Song, Seungho Choi, Daegyoum Kim The dynamics of a vortex ring bubble passing vertically through an oil-water interface are experimentally investigated. The vortex ring bubble is generated by compressed air ejected through an orifice in water for a short period of time. The volume of the vortex ring bubble is controlled via the pressure and ejection time of the compressed air. High-speed imaging is used to examine the evolution of the vortex ring bubble and its interaction with the interface. The vortex ring bubble entrains surrounding water, and this entrainment process strongly affects the dynamics of the vortex ring bubble near the interface. The interaction observed at the interface shows notable differences from the interaction between a single-phase vortex ring and a density interface. Throughout the upward motion of the vortex ring bubble, kinematic properties such as the radius and velocity of the bubble are primarily determined by the relative magnitude between the buoyant force of the bubble and the gravitational force acting on the entrained volume of water. We investigate key dimensionless parameters, including the Froude number, that can characterize the dynamics of the interface. |
Sunday, November 21, 2021 9:44AM - 9:57AM |
A24.00009: A theory and analysis that a vortex makes the vorticity lines vortical through vortex stretching Katsuyuki Nakayama The present study shows that the vortex stretching in vortical region with swirling flow gives an effect to swirl vorticity lines or vector, with the formulation of the stretching with respect to the vortex space that is the specific Galilei invariant coordinate system associated with the swirl plane of the local flow. A numerical analysis of bundle of vorticity lines in core region of a vortex clarifies this geometrical phenomenon in the direct numerical simulation of a homogeneous isotropic turbulence. The swirlity andsourcity that represent the unidirectionality and intensity of respective azimuthal andradial flowsin terms of the local flow, and the local axis geometry theory are applied to specify detail geometrical characteristic of the vorticity lines, in the swirl plane of a vortical core region. It shows that the characteristic of the non-azimuthal component of the vorticity lines or vector specifies the swirling feature of the vorticity lines with the stretching, and that the intensity and sign of the sourcity of local vorticity field (vorticity flow) are associated with it. The vortical flow geometry and this particular feature of swirling of the vorticity lines are combined with the vortex stretching in the vortical region. |
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