Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session L12: Vortex Dynamics and Vortex Flows VII 
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Chair: Jonathan Naughton, University of Wyoming Room: 336 

L12.00001: ABSTRACT WITHDRAWN 
Monday, November 25, 2013 3:48PM  4:01PM 
L12.00002: Harbor seal whiskers synchronize with frequency of upstream wake Heather Beem, Michael Triantafyllou Harbor seals are able to use their whiskers to track minute water movements, such as those left in the wake of a fish [1]. The current study is a simple representation of what the whiskers experience as the seal chases a fish. A scaled whisker model (average crossflow diameter: $d_{w}$) is first tested in a towing tank by itself and then towed behind a larger cylinder ($d_{c} = 2.5 d_{w}$), which serves as a wake generator. A flexing plate attached to the model base allows the whisker to freely vibrate in response to the flow. Measurements from strain gages on the plate are calibrated to tip deflections. While in the cylinder wake, the whisker vibrates with an amplitude up to ten times higher than it does on its own ($A/d_{w} = 0.15$). Also, the whisker synchronizes with the vortex shedding frequency ($f_{s} = \frac{0.2 U}{d_{c}}$) of the upstream cylinder over the range of reduced velocities tested, whereas on its own, the whisker oscillates around its own natural frequency in water. Seals may use the difference in vibration amplitude and frequency between these two cases to help detect the presence of a vortex wake. \\[4pt] [1] Dehnhardt, G., et al. (1998). ``Seal whiskers detect water movements,'' Nature, 394(6690), 2356. [Preview Abstract] 
Monday, November 25, 2013 4:01PM  4:14PM 
L12.00003: VortexInduced Vibration (VIV) Reduction Properties of Seal WhiskerLike Geometries Hendrik Hans, Jianmin Miao, Michael Triantafyllou Biological studies have shown that harbor seal whiskers are capable of reducing VortexInduced Vibrations (VIV). As the whiskers have convoluted geometry, it is necessary to evaluate the parameters that define their VIV reduction properties. WhiskerLike Geometries (WLGs) consisting of all but one feature on the true whisker geometry are designed. Comparison of VIV on these WLGs with VIV on circular and elliptical cylinders at Re $=$ 500 is performed. Threedimensional simulations of flow past these geometries, which are allowed to freely vibrate in crossflow, are performed with the Implicit Large Eddy Simulation as the turbulence model. The results indicate that the existence of axial undulations is the most dominant feature that affects the VIV reduction. The smallest VIV is observed on WLGs with dualaxial undulations and the largest VIV is observed on the circular cylinder. Variations in the features of the WLGs result in noticeable changes in their VIV. The circular cylinder is observed to response as a steady system while the WLGs with dualaxial undulations are observed to respond as a chaotic system. The response of WLGs with singleaxial undulations is found to depend on their detailed features. [Preview Abstract] 
Monday, November 25, 2013 4:14PM  4:27PM 
L12.00004: Higher Harmonic Forces in Purely Crossflow VortexInduced Vibrations Yahya ModarresSadeghi, Banafsheh SeyedAghazadeh, Remi Bourguet, George Karniadakis, Michael Triantafyllou In vortexinduced vibrations (VIV) of flexiblymounted rigid cylinders free to oscillate both in the inline and crossflow directions, higher (3$^{\mathrm{rd}})$ harmonic forces have already been observed in the crossflow direction. In the present work, we report higher harmonic force components for a flexiblymounted rigid cylinder with only one degree of freedom in the crossflow direction. We show that the inline displacement is not necessary to observe higher harmonic components in the crossflow force spectrum. Due to the relative velocity of the cylinder with respect to the oncoming flow, the lift and drag forces make an angle with respect to the crossflow and inline directions, and the contribution of the components of each of these forces in the crossflow direction results in a 3$^{\mathrm{rd}}$ harmonic force component. These higher harmonic components have been observed in selfexcited VIV experiments, performed in a water tunnel for a Reynolds number range of Re$=$4001000, as well as in numerical simulation results at Re$=$100. We also find that the maximum ratio of the 3$^{\mathrm{rd}}$ harmonic to the 1$^{\mathrm{st}}$ harmonic occurs when the phase between the crossflow force and displacement changes from 0 to 180 degrees, resulting in a small first harmonic component. [Preview Abstract] 
Monday, November 25, 2013 4:27PM  4:40PM 
L12.00005: Flowinduced vibrations of a rotating cylinder Remi Bourguet, David Lo Jacono The flowinduced vibrations of a circular cylinder, free to oscillate in the crossflow direction and subjected to a forced rotation about its axis, are studied by means of two and threedimensional numerical simulations, at a Reynolds number equal to 100. This problem serves as a paradigm to investigate the impact of symmetry breaking on the phenomenon of vortexinduced vibrations (VIV), previously described in the nonrotating case. The cylinder exhibits free oscillations up to a rotation rate close to 4. Under forced rotation, the vibration amplitude reaches 1.9 diameters, i.e. three times the maximum amplitude in the nonrotating case. Contrary to galloping responses, the free vibrations of the rotating cylinder are found to involve a condition of wakebody synchronization similar to the lockin condition driving nonrotating cylinder VIV. A variety of flow patterns including novel asymmetric wake topologies is identified; it is shown that free oscillations may develop in the absence of vortex shedding. The symmetry breaking substantially alters the fluid force spectra and phasing mechanisms. The flow threedimensional transition is found to occur at high rotation rates; its influence on the fluidstructure system behavior is analyzed. [Preview Abstract] 
Monday, November 25, 2013 4:40PM  4:53PM 
L12.00006: VortexInduced Vibrations of a FlexiblyMounted CyberPhysical Rectangular Plate Kyohei Onoue, Benjamin Strom, Arnold Song, Kenneth Breuer We have developed a cyberphysical system to explore the vortexinduced vibration (VIV) behavior of a flat plate mounted on a virtual spring damper support. The plate is allowed to oscillate about its midchord and the measured angular position, velocity, and torque are used as inputs to a feedback control system that provides a restoring torque and can simulate a wide range of structural dynamic behavior. A series of experiments were carried out using different sized plates, and over a range of freestream velocities, equilibrium angles of attack, and simulated stiffness and damping. We observe a synchronization phenomenon over a wide range of parameter space, wherein the plate oscillates at moderate to large amplitude with a frequency dictated by the natural structural frequency of the system. Additionally, the existence of bistable states is reflected in the hysteretic response of the system. The cyberphysical damping extracts energy from the flow and the efficiency of this harvesting mechanism is characterized over a range of dimensionless stiffness and damping parameters. [Preview Abstract] 
Monday, November 25, 2013 4:53PM  5:06PM 
L12.00007: Vortexinduced vibration of a curved cylinder Banafsheh SeyedAghazadeh, Collin Budz, Yahya ModarresSadeghi Vortexinduced vibration of a curved circular cylinder free to oscillate in the crossflow direction is studied experimentally. Both concave and convex orientations (with respect to the oncoming flow direction) were considered. The system had a mass ratio of 3.6 and a structural damping of 0.01. The amplitudes and frequencies of oscillations were measured in a Reynolds number range of Re$=$5002700. It was found that the amplitude of oscillations in both configurations was decreased compared with a vertical cylinder with the same mass ratio. The lockin range was also wider in both cases compared with the lockin range of a vertical cylinder. Higher harmonic components in the crossflow force were observed in both cases. In the entire lockin range, the crossflow displacement and force stayed in phase, however, the contribution of the higher harmonic force components became more significant at higher reduced velocities. Dye flow visualizations showed that the vortices were shed in parallel to the curved cylinder, when the cylinder was free to oscillate. [Preview Abstract] 
Monday, November 25, 2013 5:06PM  5:19PM 
L12.00008: Suppression of Vortex Induced Vibrations by Fairings Yue Yu, Hongmei Yan, Yiannis Constantinides, Owen Oakley, George Karniadakis Fairings are nearlyneutrally buoyant devices, which are fitted along the axis of long circular risers to suppress vortex induced vibrations (VIV) and possibly reduce the drag force. Here we study numerically how VIV can be practically eliminated by using freetorotate fairings. Since the mass ratio and rotational inertia are both low for the fairings, direct numerical simulations based on standard flowstructure interaction algorithms fail because of the socalled added mass effect. To resolve this problem we introduce fictitious methods and successfully stabilize the simulations. In particular, we investigate the effect of rotational friction $C_f$ on the stabilization effect of the fairings. We found that there exists a critical value for the rotational friction, and when $C_f$ is close to this value, large oscillations and unsymmetrical trajectories can be observed for the riser but for smaller $C_f$ values VIV are suppressed substantially. [Preview Abstract] 
Monday, November 25, 2013 5:19PM  5:32PM 
L12.00009: Flapping dynamics of an inverted flag Daegyoum Kim, Julia Cosse, Morteza Gharib The dynamics of an inverted flag are investigated experimentally in order to find the conditions under which flapping can occur. In contrast to a general flag with a fixed leading edge and a free trailing edge, the inverted flag of our study has a free leading edge and a clamped trailing edge. The behavior of the inverted flag can be classified into three regimes based on its bending stiffness. Two quasisteady regimes, straight mode and fully deflected mode, are observed, and limitcycle flapping mode with large amplitude appears between the two quasisteady regimes. Bistable states are found in both straight to flapping mode transition and flapping to deflected mode transition. The effect of mass ratio, relative magnitude of flag inertia and fluid inertia, on the bending stiffness range for flapping is negligible unlike the instability of the general flag. Because of unsteady fluid force, a flapping sheet can produce several times larger elastic strain energy than a sheet of the deformed mode, improving the conversion of fluid kinetic energy to elastic strain energy. According to the analysis of leadingedge vortex formation process, the time scale of optimal vortex formation correlates with efficient conversion to elastic strain energy during bending. [Preview Abstract] 
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