Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session A32: Focus Session: Vortex Dynamics in Fluid-Structure Interactions I |
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Chair: Stefan Llewellyn Smith, University of California, San Diego Room: 33C |
Sunday, November 18, 2012 8:00AM - 8:13AM |
A32.00001: Dynamics of Multiple Cylinders in an Inviscid Fluid Anthony Leonard, Andrew Tchieu We consider a collection of circular cylinders moving in an unbounded inviscid fluid. The force on one particular cylinder will be determined by the accelerations, velocities, positions, and radii of all cylinders. We approximate this interaction by taking, in the vicinity of a given cylinder, only the first two terms in a Taylor series expansion of the velocity field induced by the remaining cylinders, i.e. the induced velocity at the center of the given cylinder plus the linearized induced strainrate field. We then correct the potential for this field to satisfy the no through-flow constraint on the cylinder in question. Using Bernoulli's equation, we then derive a coupled system of equations for the forces on all the cylinders. As a result, the added masses are computed to order $a^2/d^2$ and the force coefficients, not dependent on accelerations, are computed to order $a^3/d^3$, where $a$ is a typical cylinder radius and $d$ is a typical separation distance. An additional correction for close interactions is proposed. Several examples of cylinder dynamics will be presented. [Preview Abstract] |
Sunday, November 18, 2012 8:13AM - 8:26AM |
A32.00002: Seal whisker-inspired circular cylinders reduce vortex-induced vibrations Heather Beem, Michael Triantafyllou Recent work [1] shows that the undulatory, asymmetric geometry of harbor seal whiskers passively reduces vortex-induced vibration (VIV) amplitudes to less than 0.1 times the whisker diameter. This reduction holds in frontal flows, but due to the elliptical cross-section of the whisker, flows that approach from large angles of attack generate significant vibrational response. The present study investigates the possibility of extending the vibration reduction to unidirectional bodies, such that flows from all angles cause reduced VIV. A method for developing a new geometry that incorporates the ``whisker'' features into bodies with uniform, circular cross-section is presented. This geometry and multiple variations on it are fabricated into rigid models. Forces are measured on the models while they undergo imposed oscillations and are towed down a water tank. Contour plots of $C_{L,v}$ show peak VIV amplitudes to decrease as much as 28\% from that of a standard cylinder. This result holds promise for applications where vibration reduction is desired, regardless of the angle of oncoming flow. \\[4pt] [1] Beem, H., et al., ``Harbor Seal Vibrissa Morphology Reduces Vortex-Induced Vibrations,'' American Physical Society 24th Annual DFD Meeting, Nov. 2011. [Preview Abstract] |
Sunday, November 18, 2012 8:26AM - 8:39AM |
A32.00003: An experimental study of vortex induced vibrations of inclined flexible cylinders Banafsheh Seyed-Aghazadeh, Yahya Modarres-Sadeghi When a flexible cylinder is placed in flow, vortices are shed downstream of the cylinder. If the vortex shedding frequency is close to one of the natural frequencies of the structure, the cylinder undergoes vortex induced vibrations. If the cylinder is placed with an angle with respect to the incoming flow (an inclined flexible cylinder) the flow will have a component normal to the long axis of the cylinder and a component in the axial direction. For small angles of inclination, the influence of the axial flow component can be neglected and the cylinder can be treated as a cylinder placed normal to the flow. In the current research, we investigate the degree to which this assumption is valid experimentally. A flexible cylinder with different values of tension along the cylinder is placed at various angles of inclination in the test section of a recirculating water tunnel. The cylinder is clamped at both ends and the transverse displacement of the cylinder at the first mode of vibration is measured using a non-contacting displacement sensor. These measurements together with the flow visualization results are used to investigate the influence of the axial component on the cylinder response at various angles of inclination. [Preview Abstract] |
Sunday, November 18, 2012 8:39AM - 8:52AM |
A32.00004: Interaction of vortices with flexible piezoelectric beams Oleg Goushcha, Huseyin Dogus Akaydin, Niell Elvin, Yiannis Andreopoulos A cantilever piezoelectric beam immersed in a flow is used to harvest fluidic energy. Pressure distribution induced by naturally present vortices in a turbulent fluid flow can force the beam to oscillate producing electrical output. Maximizing the power output of such an electromechanical fluidic system is a challenge. In order to understand the behavior of the beam in a fluid flow where vortices of different scales are present, an experimental facility was set up to study the interaction of individual vortices with the beam. In our set up, vortex rings produced by an audio speaker travel at specific distances from the beam or impinge on it, with a frequency varied up to the natural frequency of the beam. Depending on this frequency both constructive and destructive interactions between the vortices and the beam are observed. Vortices traveling over the beam with a frequency multiple of the natural frequency of the beam cause the beam to resonate and larger deflection amplitudes are observed compared to excitation from a single vortex. PIV is used to compute the flow field and circulation of each vortex and estimate the effect of pressure distribution on the beam deflection. [Preview Abstract] |
Sunday, November 18, 2012 8:52AM - 9:05AM |
A32.00005: VIV experiments with a semi-immersed vertical flexible cylinder driven by top motion Rodolfo Goncalves, Guilherme Franzini, Celso Pesce, Andre Fujarra, Pedro Mendes An experimental investigation of the VIV phenomenon with a long and flexible cylinder was carried out at a recirculating water channel facility and intends to better understand regarding risers non-linear dynamics. The cylinder total length is L=2520mm with an immersed length L$_{i}$=720mm and the diameter D=22.2mm, current velocities up to 0.4m/s are carried out in the immersed portion to promote vortex-shedding and consequently VIV. The mass parameter, m* is close to 10. Prescribed vertical and monochromatic harmonic motions were imposed at the top aiming at causing geometric stiffness modulation and, therefore, eigen frequencies oscillations, together with current excitation. The motion imposed at the top has amplitude A$_{t}$/L=1{\%} and was imposed at three frequencies ratios, with respect to the first natural frequency in still water: f$_{t}$:f$_{N,1}$=1:3,1:2,1:1. Cartesian displacement time-series were obtained for 44 tracking-points, marked with reflective targets placed all along the model, by using a Qualisys optical motion capture system. As expected, a much richer spectral content is revealed under imposed motions than in its absence, i.e. only current condition. The presence of subharmonic components in the response spectra is analogous to the response of a classic parametrically excited system. It was also observed a marked increase of the maximum response amplitude in the case f$_{t}$:f$_{N,1}$=1:1 [Preview Abstract] |
Sunday, November 18, 2012 9:05AM - 9:18AM |
A32.00006: Collision of a flexible filament with a point vortex Silas Alben A flexible filament is attracted to a point vortex when they move together as a coupled system. The point vortex collides with the filament at a finite time, with the separation distance tending to zero as a square root of time. The filament curvature, velocity, and pressure loading diverge near the collision. We explain the power laws describing these behaviors. [Preview Abstract] |
Sunday, November 18, 2012 9:18AM - 9:31AM |
A32.00007: Simulation of the structural response of a cable immersed in a uniform flow: comparison of three different methodologies Bruno Carmo, Rafael Gioria, Alessandro Lima, Julio Meneghini The structural response of slender structures immersed in a fluid flow are very difficult to predict and costly to calculate because of the nonlinear nature of the flow-structure interaction. The search for less expensive, but yet sufficiently accurate methodologies to carry out these calculations has been subject of intense research in the last decades, mainly due to interest of offshore engineering companies. Here we present a comparison of three different methodologies that can be used to predict the structural response of a flexible cable immersed in a uniform flow. The first is the so-called strip approach, in which two-dimensional simulations of the flow at different points along the cable axis are coupled through the cable motion. In the second approach, the flow simulation is three-dimensional considering only domains of few diameters in the spanwise direction, distributed along the cable axis and coupled through the cable displacements. The third is the full three-dimensional calculation of the cable response, considering the entire flow field. We compare the results obtained for a few different Reynolds numbers and cable tensions, analyzing both the accuracy of the results and computational cost. [Preview Abstract] |
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