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 D9: Vortex Dynamics and Vortex Flows IIVortexes

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Chair: Arne Pearlstein, University of Illinois at UrbanaChampaign Room: 502 
Sunday, November 19, 2017 2:15PM  2:28PM 
D9.00001: Complete Suppression of FullyDeveloped Vortex Shedding and VortexInduced Vibration for a Cylinder in CrossFlow by Release of an Internal Nonlinear Energy Sink. Antoine Blanchard, Arne J. Pearlstein For Reynolds numbers below the critical value for a fixed circular cylinder, we show how fullydeveloped, largeamplitude, vortexinduced vibration of a linearlysprung cylinder can be completely suppressed by release of an internal rotating "nonlinear energy sink," consisting of a mass rotating about the axis of the translating cylinder and a dissipative element that damps the rotational motion of the mass. The approach involves no modification to the cylinder boundary, with the effects being due to inertial coupling of the rotation of the internal mass to the rectilinear motion of the cylinder. The response is characterized in terms of the time required to reduce the vibration amplitude below a certain threshold. Implications for flow control, and for "switching" of mixing are discussed. [Preview Abstract] 
Sunday, November 19, 2017 2:28PM  2:41PM 
D9.00002: Investigation of flowinduced vibration for energy harvesting using a model soapfilm system Wenchao Yang, Mark Stremler One way to extract energy from geophysical flows is to take advantage of flowinduced vibration (FIV) caused by vortices being shed from a bluff body. Wakeinduced vibration of the downstream cylinder in a tandem pair is a promising design for a FIV energy harvesting system, especially suitable for low Reynolds number flows. For this design, the upstream cylinder is fixed in place, while the downstream cylinder is free to oscillate like a pendulum that is driven by interactions with the flow, including the wake of the upstream cylinder. We use a flowing soap film system, with behavior that resembles twodimensional hydrodynamics, to experimentally investigate the wake interaction between a stationary upstream circular disk and a free downstream circular disk, which acts as a swinging pendulum. The wake flow generates thickness variations in the thin soap film, allowing direct observation of wake patterns through visualization of interference fringes. With the ability to tie together the wake structure and the object motion, we investigate the relationship between energy generation and flow structure in the simplified model energy harvesting system for Re$=$150. The research results find the optimal efficiency of the energy harvesting system by a parametric study. [Preview Abstract] 
Sunday, November 19, 2017 2:41PM  2:54PM 
D9.00003: Vortex Shedding in the Wake Induced by a Real Elephant Seal Whisker Jodi Turk, Alexis Omilion, Wei Zhang, JeongJae Kim, JeongJu Kim, WooRak Choi, SangJoon Lee Biomimicry has been adopted to create innovative solutions in a vast range of applications. One such application is the design of sealwhiskerinspired flow sensors for autonomous underwater vehicles (AUVs). In dark, cramped, and unstable terrain AUVs are not able to maneuver using visual and sonarbased navigation. Hence, it is critical to use underwater flow sensors to accurately detect minute disturbances in the surroundings. Certain seal whiskers exhibit a unique undulating threedimensional morphology that can reduce vortex induced vibrations (VIVs) if the major axis of the whisker crosssection is aligned to the inflow. This allows the seal to precisely track prey fish upstream using solely their whiskers. The current study aims to understand the effect of a real seal whisker's morphology on the vortex shedding behavior. Despite extensive studies of wake induced by scaled whiskerlike models, the vortex shedding in the wake of a real seal whisker is not well understood. A series of experiments are conducted with a highspeed Particle Imaging Velocimetry (PIV) system in a water channel to examine the vortex shedding downstream from a smooth whisker and an undulating whisker at a Reynolds number of a few hundred. Results of the vortex shedding induced by real seal whiskers can provide insights on developing highsensitivity underwater flow sensors for AUVs and other whiskerinspired structures. [Preview Abstract] 
Sunday, November 19, 2017 2:54PM  3:07PM 
D9.00004: Stability and dynamics of a stack of flags. Hyeonseong Kim, Daegyoum Kim We theoretically and experimentally study stability for two configurations of flags in a stack: conventional flags with their leading edge clamped and trailing edge free to move and inverted flags with the opposite configuration. In this study, we propose a scaled critical velocity which accounts for the variation of flag thickness, aspect ratio, and the number of flags in a stack. The effects of these parameters on the stability predicted by our theoretical model are in good agreement with our experimental measurements. After bifurcation to a nonlinear regime, while the conventional flags directly show inphase secondmode oscillation in a stacked mode. However, when the freestream velocity further increases and reaches to a certain value, the inverted flags show a largeamplitude flapping motion in a stack state. In the stacked inverted flags, novel nonlinear behaviors such as symmetric clapping motion are also observed in some specific conditions. [Preview Abstract] 
Sunday, November 19, 2017 3:07PM  3:20PM 
D9.00005: Flow Control Behind Bluff Bodies through the Interaction of a Resonant Flexible Tail Samuel Shelley, John Smith, Roy Sambles, Alastair Hibbins, Simon Horsley Steady uniform flow, incident upon a bluff body can separate~causing a wake to form. This can lead to the periodic shedding of vortices~behind the body. In previous work, the effect on both the drag and vortex shedding frequency (VSF) of adding a thin flexible tail to the rear of a bluff body in the Laminar regime was investigated. This was done through modelling with the length, stiffness and inflow velocity being varied. This made it possible to match~the resonant frequencies of the tail to the natural VSF of the~structure. It was found that when one of the resonant frequencies of the tail matched the natural VSF of the body, one of the vibrational modes of the tail would be strongly excited. A sharp increase in both drag and VSF is also predicted. The locations of these jumps could be predicted using an eigenvalue solver. In the present work we conduct experiments to verify these results. Particle image velocimetry measurements are taken as a cylinder, with a neoprene rubber tail, is towed through water. This allows us to extract the vortex shedding frequency and the vibrational motions of the tail. Both the length of the tail and the towing speed are varied to match the resonant frequencies of the tail to the VSF of the body. [Preview Abstract] 
Sunday, November 19, 2017 3:20PM  3:33PM 
D9.00006: The collapse of Tacoma Narrows Bridge: a piece to the puzzle J. H. Walther, D. S. Christensen, M. G. Malthe, M. Roenne, H. J. Spietz, A. Larsen, S. V. Larsen On Nov. 7th 1940 the newly constructed Tacoma Narrows Bridge collapsed due to excessive torsional oscillations caused by the formation and shedding of large coherent vortices. The subsequent wind tunnel tests conducted on both section and full bridge models concluded that the bridge should have collapsed at a wind speed corresponding to approximately half of the wind speed at the day of the collapse. This discrepancy questions our understanding of the phenomena responsible for the failure of the bridge. The present study aims at clarifying this ``mystery'' by considering historical records made available by the US coast guards, and by performing wind tunnel tests and detailed numerical flow simulations. Our findings indicate that the discrepancy is caused by an until now unnoticed yawed wind direction relative to the bridge, which was present at the day of the collapse. [Preview Abstract] 
Sunday, November 19, 2017 3:33PM  3:46PM 
D9.00007: Optimum Energy Extraction from Coherent Vortex Rings Passing Tangentially Over Flexible Plates Alireza Pirnia, Emily A. Browning, Sean D. Peterson, Byron D. Erath Coherent vortical structures can incite selfsustained oscillations in flexible membranes. This concept has recently gained interest for energy extraction from ambient environments. In this study the special case of a vortex ring passing tangentially over a cantilevered flexible plate is investigated. This problem is governed by the KirchhoffLove plate equation, which can be expressed in terms of a nondimensional mass parameter of the plate, nondimensional pressure loading induced by the vortex ring, and a Strouhal (St) number which expresses the duration of pressure loading relative to the period of plate oscillation. For a plate with a fixed mass parameter immersed in a fluid environment, the St number specifies the beam dynamics and the energy exchange process. The aim of this study is to identify the St number corresponding to maximum energy exchange between plates and vortex rings. The energy exchange process between the vortex ring and the plate is investigated over a range of 0.3 \textless St \textless 0.7 for various nondimensional mass parameters. Investigations are performed for both discrete and periodic vortex ring loadings, as well as varying vortex ring to plate distances. The optimum value of St number that maximizes energy transfer is reported in each case and an empirical correlation is provided for predictive purposes. [Preview Abstract] 
Sunday, November 19, 2017 3:46PM  3:59PM 
D9.00008: Vortex dynamics of very low aspect ratio rectangular orifice synthetic jets Joseph Straccia, John Farnsworth The vast majority of prior synthetic jet research has focused on actuators with either circular orifices or rectangular orifices with high aspect ratios (AR), i.e. AR$\ge $25. The results reported in these studies have also been biased towards bulk and time averaged statistics of the jet, viewing them in a steady sense as a source of momentum addition. Recent work has revealed that the unsteady vortex dynamics in a synthetic jet can be very relevant to how the jet interacts with and influences the base flow. In this study the synthetic jet issued into a quiescent fluid by an actuator with low orifice aspect ratios (i.e. AR$=$218) was studied using Stereoscopic Particle Image Velocimetry (SPIV) with a special focus on the vortex dynamics. The progression of vortex ring axis switching is presented and a distinct difference between the axis switching dynamics of very low AR (AR$\le $6) and moderate AR (AR$=$624) vortex rings is discussed. The high resolution SPIV vector fields are also used to extract details of the vortex core structure which are compared to theoretical vortex models. Furthermore, the influence of axis switching on the circulation magnitude around the vortex ring is reported in addition to how circulation varies with time as the ring advects. [Preview Abstract] 
Sunday, November 19, 2017 3:59PM  4:12PM 
D9.00009: On the scaling and dynamics of periodically generated vortex rings Hossein Asadi, Hafez Asgharzadeh, Iman Borazjani Periodically generated vortex rings are observed in nature, e.g., left ventricle or jellyfish, but their scaling and dynamics is not completely well understood. We are interested in identifying the main parameters governing the propagation and dynamics of periodically generated vortex rings. Therefore, vortex rings, generated periodically through a circular cylinder into a tank, is numerically investigated for a range of Reynolds numbers (Re), nondimensional periods (T), and stroke ratios (stroke time to period) for a simple square wave. Based on the results, by using the averaged inflow velocity in definition of Reynolds number and nondimensional period, vortex ring velocity becomes approximately independent of the stroke ratio. The results also show that reducing Reynolds number or increasing nondimensional period increases the translational velocity of vortex ring. Based on our test cases, an empirical relation is proposed to predict the location of vortex cores propagating into domain which shows good agreement with other experimental data. The vortex instabilities and interactions are also visualized and discussed. [Preview Abstract] 
Sunday, November 19, 2017 4:12PM  4:25PM 
D9.00010: Inline flowinduced oscillations of a flexiblymounted square prism Tyler Gurian, Yahya ModarresSadeghi Flowinduced oscillations of a flexiblymounted square prism allowed to oscillate in the inline (parallel to flow) direction were studied experimentally in a recirculating water tunnel. Experiments were first carried out using a circular cylinder to validate the experimental set up. At low angles of attack, 0 \textless $\alpha $ \textless 5 degrees (where 0 degrees represents the case where the flow is perpendicular to a face of the square prism), oscillations are triggered when the shedding frequency reaches one half of the system's natural frequency. The resulting oscillations occur at a frequency close to the system's natural frequency. The frequency of oscillations decreases slightly at higher flow velocities. At midrange angles of attack, 10 \textless $\alpha $ \textless 20 degrees, oscillations are triggered slightly after the shedding frequency reaches one half of the system's natural frequency. Oscillations are initially at the system's natural frequency, but quickly become low frequency oscillations. Amplitudes increase linearly with flow velocity. At higher angles of attack, 25 \textless $\alpha $ \textless 45 degrees, oscillations are triggered at progressively higher flow velocities. These lowfrequency unsteady oscillations increase in magnitude linearly with flow velocity. Maximum amplitudes are close to 20{\%} of the cylinder diameter. [Preview Abstract] 
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