Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session A18: Vortex Interaction With Objects |
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Chair: Serhiy Yarusevych, University of Waterloo Room: Georgia World Congress Center B305 |
Sunday, November 18, 2018 8:00AM - 8:13AM |
A18.00001: Three-dimensional forcing function and pressure field reconstruction from independent planar PIV measurements for a cylinder undergoing Vortex-Induced Vibration Jeffrey McClure, Serhiy Yarusevych The fluid-induced forcing function operating on a uniform cylinder undergoing Vortex-Induced vibration was reconstructed relying solely on independent planar, two-component Particle Image Velocimetry (PIV) measurements in orthogonal orientations. A comparison of the integrated forcing to simultaneous “ground-truth” total forcing measurements from a load cell indicated high fidelity, with characteristic correlations between 0.8-0.9. Compared to planar force reconstructions, the spanwise distribution of the loading allowed resolution of modulations in total forcing caused by shedding mode phase variations along the cylinder span. Firstly, the analysis procedure depends on the partial three-dimensional reconstruction of the flow field using coherent modes correlated from the shared velocity data at the intersection of the horizontal and vertical plane measurements. From this, pseudo-instantaneous fields were estimated using the time-resolved vertical plane measurement to obtain the flow evolution in the orthogonal spatial direction. The corresponding three-dimensional pressure field evolution was estimated using a Poisson equation solver, and the loading was estimated using a control-volume momentum balance. |
Sunday, November 18, 2018 8:13AM - 8:26AM |
A18.00002: On the transition to 3D motions of wall-mounted flexible plates: insights of the flow and plate interaction with volumetric 4D-PTV Wing Lai, Dan Troolin, jin tae kim, Yaqing Jin, Leonardo P. Chamorro The onset of 3D motions from regular 2D oscillations of wall-mounted flexible plates of low aspect ratios was experimentally inspected for various Reynolds (Re) and Cauchy numbers (Ca) as well as mass ratios and yaw angles under uniform flows. Experiments were performed in an Eiffel-type wind tunnel and a refractive-index-matching (RIM) channel. Combined tracking velocimetry and volumetric particle image velocimetry were used to characterize the unsteady dynamics of the structures and surrounding flow. Complementary force and moment measurements were taken to derive the critical condition triggering the 3D motions of thin plates of rectangular cross sections, which were placed near the inlet of the facilities to minimize the effect of the boundary layer on the motions. Results reveal that the onset to 3D motions occurs at specific dynamical condition, which is satisfied across all cases. The 3D motions of the structures exhibited orbital motions dominated by the structure and torsional natural frequencies. Such motions induced distinctive coherent vortices that, in turn, affected the regular dynamics of the plates leading to unexpected plate velocity patterns. |
Sunday, November 18, 2018 8:26AM - 8:39AM |
A18.00003: Decay of vortex rings in flexible-walled confined domains Milad Samaee, Arvind Santhanakrishnan Vortex rings within radially confined domains (rigid tubes) have been the subject of few recent studies (Stewart et al., Exp. Fluids 53, 2012; Danaila et al., J. Fluid Mech. 774, 2015). However, these studies did not consider the effects of including: 1) wall flexibility, and 2) both radial and axial confinement (tubes were left open at the farthest end from the orifice). An example of confined vortex ring propagation under these complexities is observed during the filling of blood in the human cardiac left ventricle. We experimentally investigated laminar vortex ring propagation in confined domains with passively expanding flexible walls. Vortex ring circulation in rigid tubes of varying diameters were compared with Stewart et al. (2012) to verify our methods. PIV measurements were conducted on flexible-walled silicone models of three geometries imposing radial and axial confinement: hemisphere, semi-prolate spheroid, semi-oblate spheroid. Three levels of wall stiffness were considered for each geometry. Though the formation process was nearly unaffected for a particular geometry, we observed higher rate of decay of circulation with increasing wall stiffness. The effects of varying ejection duration and Reynolds number on the decay of circulation will be discussed. |
Sunday, November 18, 2018 8:39AM - 8:52AM |
A18.00004: Hydrodynamic Regimes for an Oscillating Body Curtis Rusch, Robert J Cavagnaro, Brian Rosenberg, Tim Mundon, Benjamin Maurer, Brian Polagye This work experimentally investigates the hydrodynamics of a vertically oscillating body, tested across a range of frequencies and amplitudes and at multiple geometric scales. These bodies are common features of offshore structures, including floating platforms for renewable energy generation, and their hydrodynamics are traditionally described by the Keulegan-Carpenter (KC) number. In regular waves, KC reduces to a ratio of oscillation amplitude to body diameter. Prior work has shown that added mass and drag coefficients are primarily a function of KC. This work has further explored the effect of non-dimensional frequency, the Roshko number, on the added mass coefficients. These coefficients fall onto one of two planes in a 3D KC-Roshko space, suggesting two, distinct regimes governing the hydrodynamics of an oscillating body. The two regimes are likely dictated by vortex formation and shedding, and a better understanding will improve knowledge of how the hydrodynamics behave across geometric scales. Knowledge of the full scale operating regime will more successfully dictate model tests, allowing for accurate scaling of these results. |
Sunday, November 18, 2018 8:52AM - 9:05AM |
A18.00005: Flow-induced oscillations of prisms with two degrees of freedom Daniel Carlson, Gabrielle Little, Yahya Modarres-Sadeghi Flow-induced oscillations of triangular and square prisms, placed in water are studied for a system with both inline and cross-flow degrees of freedom. The experimental apparatus permits low mass-damping and low structural coupling between the two degrees of freedom. A combination of displacement and force measurements together with flow visualization is used to investigate the prism’s response as the flow velocity is increased. Direct force measurements indicate the formation of higher harmonics in the lift and drag forces. Wake visualization relates the vortex shedding angle to the phase angle shift between lift and velocity and the normal angles of the faces. For the triangular case, alignment between the surface normal and the shedding angle results in a widening of the VIV lock-in region. |
Sunday, November 18, 2018 9:05AM - 9:18AM |
A18.00006: On Passive Control Of Transition To Galloping Of A Circular Cylinder Undergoing Vortex Induced Vibration Using Thick Strips Ashwin Vinod, Andrew Auvil, Arindam Banerjee Experiments exploring modifications to a circular cylinder to enhance vortex-induced vibrations (VIV) and incite galloping oscillations are conducted in the Reynolds number range, 1.5×10^{3 }< Re < 3×10^{4}. These measurements, bound within the TrSL2 (Transition in Shear Layer) Reynolds number regime, complement studies by Michael Bernitsas and group spanning the TrSL3 Reynolds number regime. Pairs of smooth rectangular strips varying in thickness from 1.6% to 31% of the cylinder diameter were attached to a circular cylinder at 60º from the frontal stagnation point. Amplified VIV and galloping oscillations were observed for all strip thicknesses tested, except the least thickness, where reduced VIV amplitudes at the lower flow velocities and high amplitude galloping oscillations at the higher flow velocities were noticed. It was evident that thicker strips led to higher VIV and galloping amplitudes, accompanied by increased steadiness within the transition regime. Thicker strips also led to the earlier initiation of galloping, indicating the potential for increased energy transfer even at the lower flow speeds. Higher mass-damping led to lower vibration amplitudes and frequencies in both modes, however, the potential of smooth strips to incite galloping was evident in all cases tested. |
Sunday, November 18, 2018 9:18AM - 9:31AM |
A18.00007: Decomposition of wake dynamics in fluid-structure interaction via low-dimensional models Tharindu Pradeeptha Miyanawala, Rajeev Jaiman We present a dynamic decomposition analysis of wake flow in fluid-structure interaction (FSI) systems under both laminar and turbulent flow conditions. Of particular interest is to present the interaction dynamics of low-dimensional features to sustain the free vibration of a low mass ratio square cylinder. The snapshot data from high-dimensional FSI simulations are projected to a low-dimensional subspace using the proper orthogonal decomposition (POD). We utilize the corresponding POD modes to detect features e.g. vortex street, shear layer and near-wake bubble. The vortex shedding modes contribute solely to the lift force while the near-wake and shear layer modes contribute to the drag force. We ascertain quantitatively that the shear layer feeds vorticity flux to the wake vortices and the near-wake bubble during wake-body synchronization. We propose an interaction cycle to provide the inter-relationship between high amplitude motion and wake features. This wake-synchronization cycle is also found to be valid for turbulent wake flow. We discover that, for below critical Re flows the bluff body undergoes a high-amplitude vibration due to flexibility-induced unsteadiness. |
Sunday, November 18, 2018 9:31AM - 9:44AM |
A18.00008: Effect of Reynolds number in transversely rotating and translating sphere Aaditya Chandel, Shyama Prasad Das Here we present 2D Particle Image Velocimetry results of flow in the wake of a transversely rotating and translating sphere in quiescent water in wide range of Reynolds number (Re). A roller connects the sphere by a small shaft and assembly moves on a track that is capable of changing the angle of inclination resulting in different velocity of the sphere. For rotation rate of 0.375 (ratio of maximum sphere velocity to free stream velocity) different regimes of wake structures are seen when Re is varied from 250 to 1750. At Re of 250 flow remains steady with strong shear present in the wake. The 2D velocity field shows asymmetry in the velocity field arising due to rotation. With increase in Re to 267 the shear layer becomes unstable and pair of vortices appears in the wake. Vortex shedding continues with the increase in Re. At Re=1500, wake is combination of shear layer vortex (near wake) and vortex shedding (far wake). At Re of 1750 shear layer type of shedding dominates. Possibly this is a 2D representation of hairpin type of vortex in the mid-plane. The velocity profile in the wake at steady as well as in regular vortex shedding regime shows some self-similarity. Self-similarity is destroyed at higher Re due to formation of small-scale structures from the shed vortex rings. |
Sunday, November 18, 2018 9:44AM - 9:57AM |
A18.00009: Accurate critical Reynolds number for flow past a circular cylinder Ke Ding, Paul F. Fischer, Arne J. Pearlstein The critical Rayleigh number for Rayleigh-Benard convection, and the critical Reynolds number for flow between parallel plates are known to high accuracy. But the critical Re for the prototypical bluff-body flow, namely two-dimensional steady flow past a circular cylinder in an unbounded domain, is not. Here, we systematically increase the upstream, downstream, and cross-stream extents of a rectangular domain, and use a spectral-element technique to efficiently achieve spatial convergence on a sequence of such domains. The results show that the upstream extent of the domain is very important, due to the effects of "blockage" on the computed steady base flow. Full domain, spatial, and temporal convergence provides a critical Reynolds number accurate to four significant digits. This result can be used as a special case for validating computations for variants of the "standard" circular cylinder case. |
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