Bulletin of the American Physical Society
60th Annual Meeting of the Divison of Fluid Dynamics
Volume 52, Number 12
Sunday–Tuesday, November 18–20, 2007; Salt Lake City, Utah
Session KT: Separated Flows II |
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Chair: Tait Pottebaum, University of Southern California Room: Salt Palace Convention Center Ballroom FH |
Tuesday, November 20, 2007 8:00AM - 8:13AM |
KT.00001: Dynamics and wake patterns of freely rising and falling spheres at Re = 500 M. Horowitz, C.H.K. Williamson We study the dynamics of spheres rising or falling freely at \textit{Re} = 500. All falling spheres, whose mass ratio (or relative density) $m*$, exceeds 1, descended rectilinearly. For rising spheres, there exists a critical value of the mass ratio below which the sphere undergoes large-amplitude oscillations, $m*_{crit }$= 0.36. This motion occurs in a vertical plane; no helical trajectories are observed. Initial wake visualizations showed that rather than the two alternately signed vortex loops found in the flow past stationary spheres, the wake of a vibrating freely rising sphere comprised four vortex structures per cycle of oscillation. However, due to the small size and high oscillation frequency of the freely rising spheres, the exact nature and formation of these structures remained unclear. Further studies were performed in a towing tank, prescribing the motion of the sphere based on the measured displacement of the rising spheres. We are able to use much larger, slower-moving bodies while matching the Reynolds numbers of the rising spheres. These experiments result in the same vortex pattern, and reveal that the four structures found in the wake of the rising sphere are vortex rings. What previously appeared to be unusually sharp bends in the counter-rotating vortex pairs are very weak loop-shaped structures, delivering a total of six vortical structures per cycle. Immediately preceding these structures, the two vortices in the pair cross over one another, providing a mechanism for the change in sign of the streamwise vortex pair as the body moves from one half cycle to the next. [Preview Abstract] |
Tuesday, November 20, 2007 8:13AM - 8:26AM |
KT.00002: An investigation of wake mode transitions and amplitude jumps in vortex-induced vibration using controlled vibration T.L. Morse, C.H.K. Williamson In this study, we have made extensive measurements of the fluid forces on a cylinder that is controlled to oscillate transverse to a free stream at Re = 4000. These measurements were used to create extremely high-resolution contour plots (based on nearly 6000 experimental runs) of the magnitude of the fluid force, and the phase angle between the forces and body motion, in the plane of normalized amplitude and frequency. We find transitions in certain regions of this plane where the character of the fluid forces changes between distinct modes. Interestingly, the mode regime boundaries we find based solely on force measurements correspond well with boundaries separating different vortex shedding modes in the Williamson-Roshko (1988) map. Using particle image velocimetry (PIV) to examine the wake of the cylinder, we have confirmed the existence of the vortex shedding patterns suggested by the fluid forces. A further fascinating characteristic, which is only observable with very high-resolution data, is the existence of regimes in this amplitude-frequency plane where two modes overlap. The peak amplitude of motion for a cylinder undergoing vortex-induced vibration occurs inside an overlap region, so understanding the vortex dynamics that occur in this region is essential to understanding the worst-case, peak amplitude of vortex-induced vibrations. As part of our ongoing research, we plan to extend our high-resolution data to higher Re. [Preview Abstract] |
Tuesday, November 20, 2007 8:26AM - 8:39AM |
KT.00003: Structural sensitivity of the wake behind a rotating cylinder Luca Brandt, Jan Oscar Pralits, Flavio Giannetti The stability of the flow around a rotating circular cylinder is considered for rotation rates between zero and six and Reynolds numbers below 200. Newton iterations, eigenvalue analysis and direct numerical simulations are used to compute the base flows, the instability modes and to investigate the nonlinear shedding frequencies. Besides the classic vortex street, quenched at rotation rates of about 2 (defined as the ratio between the velocity of the cylinder surface and the free-stream velocity), a second shedding, of lower Strouhal number, is observed in agreement with previous numerical studies for rotation rates between 4.4 and 4.8, the values indicated being slightly dependent on the Reynolds number. The computation of the direct and adjoint linear instability modes allow to identify the region in the flow which most affect the instability. This is located on the back of the cylinder and follows its surface in the sense of rotation. Multiple solutions, with one stable branch, are found at the highest rotation rates, thus explaining the second flow stabilisation. [Preview Abstract] |
Tuesday, November 20, 2007 8:39AM - 8:52AM |
KT.00004: Dynamics of recirculating flows around a surface-piercing rectangular block mounted at the side of an open channel Mehran Parsheh, Jeff Marr, Joongcheol Paik, Fotis Sotiropoulos, Fernando Porte-Agel The instantaneous velocity field of the flow upstream and downstream of a long rectangular obstacle mounted on both the side and the bottom walls of a large aspect ratio open channel ($L/H$ = 2.2, $L$: obstacle length, $H$: depth) at Fr = 0.2 and Re =150,000 is measured using ADV and PIV. The rich dynamics of the flow is visualized by time-averaging the gray-scale of the seeding particles imaged at the free-surface. The resulting flow structure is broadly consistent with the DES computations by Paik and Sotiropoulos [\textit{Phys.~ Fluids} 17, 2005] who studied a shallower case ($L/H$ =27). A new and persistent feature of the surface flow in the upstream region is the presence of an aperiodic source-like flow which acts to deflect tracers away from the obstacle wall and triggers the emergence of a strong jet-like flow along the side wall, associated with bimodal histograms of velocity fluctuations at the region. In the downstream recirculating region, the source flow occurs less frequently and acts to disorganize the main recirculation eddy, subsequently replenished by vorticity extracted from outer shear layer. [Preview Abstract] |
Tuesday, November 20, 2007 8:52AM - 9:05AM |
KT.00005: Can vortex pinch-off explain wake modes? Tait Pottebaum Experiments with heated, transversely oscillating cylinders have shown that the surface temperature can play a significant role in determining the wake mode. Temperature induced variations in the boundary layer viscosity, and the resulting changes in the velocity profile of the attached boundary layer, are believed to produce this effect. By viewing the wake mode from the perspective of vortex pinch-off, the significance of the velocity profile becomes evident. Pinch-off is generally modeled in terms of the non-dimensional kinetic energy of the forming vortex and the corresponding flux in the trailing shear layer, which is set by the velocity profile at the separation point. The evolution of the vorticity, momentum and kinetic energy fields in the shear layers and vortices throughout the shedding process was measured from DPIV data for two different wake modes at the same oscillation amplitude and period. Lagrangian coherent structures were used to determine the boundary between the shear layers and the forming vortices, allowing for integration over the vortices and calculation of fluxes. The ability of pinch-off models to explain the observed wake modes was then tested. By testing these models, the understanding of general vortex formation and pinch-off processes is advanced. [Preview Abstract] |
Tuesday, November 20, 2007 9:05AM - 9:18AM |
KT.00006: The drag on a square cylinder from the impact of a gravity current Esteban Gonzalez-Juez, Eckart Meiburg, George Constantinescu The drag on a square cylinder from the impact of a gravity current is studied using high-resolution two-dimensional direct numerical simulations. The calculated drag from the simulations agrees well with that from previous experimental measurements. A simplified model is proposed for the drag variation with time during the impact stage using Morrison's equation. Differences between the drag predicted with Morrison's equation and that calculated from the simulations are attributed to the formation of vortical structures, whose effect on the drag cannot be captured with Morrison's equation. The effect of these structures on the drag variation with time is discussed in detail in this work. The fluctuations of the drag are seen to increase as the Reynolds number increases. [Preview Abstract] |
Tuesday, November 20, 2007 9:18AM - 9:31AM |
KT.00007: LES of flows around a circular cylinder at the critical Reynolds numbers Yoshiyuki Ono, Tetsuro Tamura The recent advancement of numerical techniques has made it possible to simulate a bluff body wake accompanied with unsteady vortices at relatively high Reynolds numbers. However, even now it is not easy to accurately simulate the flow around a circular cylinder at higher Reynolds numbers especially above the critical Reynolds number. In order to simulate very high Reynolds number flows, the adequate numerical model which has a sophisticated SGS model, sufficient grid resolution and appropriately-controlled numerical dissipation is required. On the other hand, for realizing the simulation of the critical Reynolds number flow, one strategy is the usage of oncoming turbulence. Oncoming turbulence can sensitively affect the flow characteristics around the separation point and lead to reduce the critical Reynolds number.In this research, the LES method is applied to the flow around a circular cylinder at high Reynolds number. We control the numerical dissipation introduced by higher-order upwind scheme. The applicability and the limitation of the present LES model to predict the flow around a circular cylinder at very high Reynolds numbers are investigated. [Preview Abstract] |
Tuesday, November 20, 2007 9:31AM - 9:44AM |
KT.00008: Frequency and Symmetry Characteristics of Laminar Vortex Shedding from a Sphere Dongjoo Kim Numerical simulations are conducted for laminar flow past a sphere in order to investigate the variation in the frequency and symmetry characteristics of vortex shedding with respect to the Reynolds number. The Reynolds numbers considered are between 300 and 475, covering unsteady planar-symmetric and unsteady asymmetric flows. Results show that unsteady planar- symmetric flows, where hairpin vortices are periodically shed in a fixed orientation, can be divided into two different regimes: single-frequency regime and multiple-frequency regime. The former has a single frequency due to regular shedding of vortices with the same strength in every shedding cycle, while the latter has multiple frequency components due to cycle-to- cycle variation in the strength of shed vortices. The multiple- frequency planar-symmetric flow, newly found in the present study, occurs at $Re=340, 350$, and $360$. On the other hand, the asymmetric flow occurs at $Re \ge 365$, where vortices shed from the sphere show variation both in strength and azimuthal angle unlike the planar-symmetric flows. It is also found that the breaking of planar symmetry is closely related to the imbalance of vortical strength between a pair of streamwise vortices. [Preview Abstract] |
Tuesday, November 20, 2007 9:44AM - 9:57AM |
KT.00009: Flow past an Inclined Square Cylinder Dong-Hyeog Yoon, Kyung-Soo Yang, Choon-Bum Choi Crossflow past an inclined square cylinder is numerically studied in the laminar range of Reynolds number (Re). We consider both the internal flow (Case A) where the cylinder is located inside a channel and the external flow (Case B) where the cylinder is positioned in an open domain. Flow characteristics change depending on Re and the inclination angle (IA) with respect to the main flow direction. For Case A, we show how the vortex shedding from the inclined square cylinder can enhance heat transfer from the channel walls, and present the optimum inclination angle for each Re to obtain the most effective heat transfer. For Case B, we present the contour diagrams for force coefficients on the cylinder as well as for Strouhal number of the vortex shedding on an Re-IA plane. The diagrams are useful in estimating flow-induced forces and the frequency of vortex shedding not only for a fixed inclined square cylinder but also for a slowly-rotating square cylinder. [Preview Abstract] |
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