Bulletin of the American Physical Society
2005 58th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 20–22, 2005; Chicago, IL
Session AN: Separated Flows I |
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Chair: George Haller, Massachusetts Institute of Technology Room: Hilton Chicago PDR 2 |
Sunday, November 20, 2005 8:00AM - 8:13AM |
AN.00001: Predicting the response of a cylinder undergoing vortex-induced vibration using controlled vibrations Timothy L. Morse, Charles H.K. Williamson In this study, we measure the fluid forces on a cylinder that oscillates with a sinusoidal motion transverse to a free stream. We generate contour plots of the fluid force, phase, and energy transfer in the plane of normalised amplitude and frequency (and compare with Hover, et al., 1998; Carberry et al., 2001). Interestingly, the regime boundaries of force and phase in our study correspond well with the boundaries separating vortex wake modes in the Williamson-Roshko (1988) map of modes. Using these measurements, we are able to predict the response of an elastically mounted cylinder, and we find good agreement with the free vibration response plots of Govardhan \& Williamson (2000). We have been especially careful to match the experimental conditions of both the controlled and free vibration cases. Further studies of the energy transfer between fluid and body motion suggest conditions under which one might expect hysteretic jumps or intermittent switching between free vibration modes. We also predict the effects of mass and damping on the response and the regime of synchronization of a freely vibrating cylinder. [Preview Abstract] |
Sunday, November 20, 2005 8:13AM - 8:26AM |
AN.00002: Dynamics of rising and falling cylinders and spheres Matthew Horowitz, Charles H.K. Williamson We investigate the dynamics of cylinders and spheres allowed to rise or fall freely through a fluid. In our studies, all falling bodies are found to descend rectilinearly. However, for both rising cylinders and spheres, we show the existence of a critical mass ratio, below which the body suddenly begins to vibrate vigorously. We obtain a critical mass ratio of 0.54 for the cylinder, which agrees closely with the value found for the elastically mounted cylinder experiments of Govardhan and Williamson (2000, 2002; JFM). This surprisingly close agreement is not yet fully understood, as the fluid-structure interactions would appear to be somewhat different. For rising spheres, we find a critical mass of 0.61, near the approximate value for an elastically mounted or tethered sphere (Govardhan and Williamson, 2004, JFM). It is significant, and perhaps coincidental, that the values of the critical mass ratio of cylinders and spheres are relatively close, despite the remarkably different vortex dynamics which cause the vibrations to occur. In the case of the cylinder, the vortices causing vibration are normal to the flow, whereas for the sphere they are streamwise to the flow. [Preview Abstract] |
Sunday, November 20, 2005 8:26AM - 8:39AM |
AN.00003: On the effects of damping on the amplitude and frequency response of vortex-induced vibrations J.T. Klamo, A. Leonard, A. Roshko We have studied the effects of controlled damping on the amplitude and frequency response profile of an elastically mounted cylinder in cross flow. The dimensionless damping parameter, b* = 2b/$\rho$LDU, which is closely related to the traditional ``mass-damping'' parameter, was varied over a wide range of values (typically 0.10 $<$ b* $<$ 1.50) through the use of a variable magnetic eddy current damping system. It is generally believed that only two types of amplitude response profiles, the ``low mass-damping'' type and the ``high mass-damping'' type, exist. In the former one has a large amplitude, three-branch (initial, upper, lower) response profile and in the latter a small amplitude, two-branch (initial and lower) response profile. In our experiments, as damping was systematically increased, we see a blending of these profiles characterized by a gradual erosion and eventual disappearance of the large amplitude section (upper branch) and the scaling down of the lower branch region. Of equal importance are the changes in the frequency response profile of the system that is connected with the changes in the amplitude profile. We also discuss why the traditional labels of high and low mass-damping systems are misleading with regard to predicting a large or small amplitude response profile because of the influence of Reynolds number. [Preview Abstract] |
Sunday, November 20, 2005 8:39AM - 8:52AM |
AN.00004: Cyclical wake mode-switching for a heated cylinder oscillating in cross flow Tait Pottebaum, Mory Gharib For an unheated, transversely oscillating circular cylinder in cross flow, the wake mode is determined by the period and amplitude of the oscillations [1]. Other than a possible single transition associated with startup conditions [2], the wake mode remains fixed if the oscillation amplitude and period are constant. In contrast, experiments with heated cylinders have revealed cyclical switching between distinct wake modes. The mechanism of this mode-switching has been identified, with temperature induced variations in the boundary layer viscosity playing a critical role. This discovery exposes the role of viscosity in determining wake mode and may lead to an improved understanding of vortex formation and pinch-off processes for wakes in general. \newline \newline References: \newline [1] C.H.K. Williamson, A. Roshko, 1988. ``Vortex formation in the wake of an oscillating cylinder,'' Journal of Fluids and Structures, 2, pp. 355-381. \newline [2] J. Carberry, J. Sheridan, D. Rockwell, 2001. ``Forces and wake modes of an oscillating cylinder,'' Journal of Fluids and Structures, 15(3-4), pp. 523-532. [Preview Abstract] |
Sunday, November 20, 2005 8:52AM - 9:05AM |
AN.00005: Modification on the wake of a cylinder: Effect of small amplitude surface modification Alis Ekmekci, Donald Rockwell Substantial changes in the structure of the near-wake of a stationary and oscillating cylinder at a Reynolds number of 10,000 can be induced by a wire(s) having a diameter two orders of magnitude smaller than the cylinder diameter, and oriented parallel to the axis of the cylinder. A technique of high-image-density particle image velocimetry allows characterization of the instantaneous and averaged patterns of the flow structure. As the placement angle of the wire is altered, relative to the forward stagnation point of the cylinder, ordered patterns of asymmetry of the near-wake structure are induced over a defined range of angles. Existence of the wire produces significantly higher values of Reynolds stress in the separating shear layer, which is the genesis of the asymmetry. Moreover, the vortex formation length of vortices from both sides of the cylinder can be substantially altered when the wire is placed on only one side. These findings provide a basis for interpreting the wake structure for the more complex case of a helical wire pattern about the surface of the cylinder. [Preview Abstract] |
Sunday, November 20, 2005 9:05AM - 9:18AM |
AN.00006: Controlling the Flow around a Swept Back Circular Cylinder Using Periodic Excitation Lutz Taubert, Israel Wygnanski Periodic excitation emanating from one or two slots located on the surface of a circular cylinder was used to affect the forces acting on the cylinder and the flow around it, with particular attention being paid to the unsteady structures in the near wake. Experiments of this nature were carried out earlier on a cylinder whose axis was normal to the flow. Currently, research is being focused on a highly swept back cylinder (mostly at 60$^{\circ}$ sweep angle) that is bound between two parallel walls (infinitely yawed configuration) or has a finite aspect ratio. The sweep back enables the decoupling of the natural vortex shedding frequency from the excitation frequency, because the former decreases as a result of the sweepback while the optimum excitation does not change significantly. Three-dimensional PIV was used to map the flow field while surface and wake pressure measurements enabled the estimation of forces acting on the cylinder. [Preview Abstract] |
Sunday, November 20, 2005 9:18AM - 9:31AM |
AN.00007: PIV Measurements in the Wake of a Magnetically Supported Axisymmetric Bluff-Body Hiroshi Higuchi, Hideo Sawada, Hiroyuki Kato Flow over a short cylinder suspended in the axial free stream was studied using a PIV system. Two length-to-diameter (fineness) ratios, 1.31 and 1.68, were selected. Between these fineness ratios, the drag coefficient reaches the minimum as the separated shear layer from the leading edge starts to reattach near the trailing edge. (Higuchi et al 2005.) The cylinder model was supported magnetically in the wind tunnel free stream at ReD=100,000. Performance of the optical sensors used to monitor and control the position of the model was not affected by the PIV laser sheet. Free of any physical support protruding into the flow, an excellent flow axisymmetry was achieved both in the mean velocity and turbulence profiles, while instantaneous flowfield was fully three-dimensional. Unsteady longitudinal large-scale vortical structures travel across the entire wake but were on the average organized axisymmetrically to exhibit maximum (for fineness ratio 1.31) or minimum (for fineness ratio 1.68) turbulence energy at the center of the immediate wake, which depended on the shear layer reattachment. Leading edge shear layer vortices and downstream helical vortex structure were also observed. The base pressure was recorded via telemetry (Sawada et al, 2005) and its results will be also discussed with the measured flowfield and drag coefficient. [Preview Abstract] |
Sunday, November 20, 2005 9:31AM - 9:44AM |
AN.00008: Separation Control over a Surface-Mounted Hemisphere D. Brzozowski, B. Vukasinovic, A. Glezer High-frequency control of the separated flow over a hemispherical, surface-mounted shell is investigated experimentally at Re$_{D}$ up to 700,000. Actuation is effected by an array of surface mounted synthetic jet actuators and the control effectiveness is characterized using high-resolution particle image velocimetry, hotwire anemometry, and surface pressure distributions. Actuation results in a substantial reduction in the extent of the recirculating flow domain through separation delay and concomitant decrease in the attachment length. Consequently, the recirculating vortex within the separated domain is displaced toward the hemisphere-surface juncture and its cross-sectional area is substantially diminished. Direct coupling to small-scale motions within the separated domain leads to a substantial reduction in the magnitude of the turbulent stresses and kinetic energy over all flow scales and the suppression is particularly effective at the large-scale motions. It is also shown that large scale coherent motions within the recirculating domain can be induced and controlled at desired frequency and phase by amplitude modulation of the actuation waveform. [Preview Abstract] |
Sunday, November 20, 2005 9:44AM - 9:57AM |
AN.00009: Computing Bluff Body Flows Using Commercial CFD Software Sujit Kirpekar, David Bogy Commercial CFD codes are increasingly being used to simulate complex engineering flows. Three commercial codes: CFD-ACE v2004, Fluent 6.2.16 and CFX 5.7.1 are examined for their ability to compute the separated flow over a square cylinder. Large Eddy Simulation (LES) results are presented using four SGS models implemented in these commercial codes: the Smagorinsky's model, the dynamic model (Germano et al., 1991), the localized dynamic model (Kim and Menon, 1995) and the WALE model (Nicoud and Ducros, 1999). Global simulation results, time averaged quantities and phase averaged quantities are benchmarked against the experimental results of Lyn and Rodi (Journal of Fluid Mechanics, 1994). All simulations predict the Strouhal number accurately, and simulations employing the dynamic model are excellent in predicting the mean recirculation length and the r.m.s. of the lift coefficient on the cylinder. In terms of flow fluctuations, all simulations over-predict the streamwise component, but under-predict the vertical component. Velocity fluctuations in the wake correlate well with the fluctuation of forces on the cylinder. An examination of the streamlines of the flow indicates that CFD-ACE and Fluent's implementation of the dynamic model offers the best prediction of the vertical displacement of the wake and the size of the shed vortex. Finally, the addition of 10{\%} upwind differencing to the convective terms is also investigated. [Preview Abstract] |
Sunday, November 20, 2005 9:57AM - 10:10AM |
AN.00010: Mechanical model of separation phenomena Rouslan Krechetnikov, Jerrold E. Marsden, Hassan M. Nagib In this talk we discuss the low-dimensional modelling of separated flows. First, we recall the essential physics and identify the governing variables, which highlight a non-Eulerian character of the main dynamical entity -- the separation bubble. The extraction of this most essential information demonstrates a universality of the separation bubble behavior and its striking similarity to the dynamics of real bubbles or drops in interfacial physics. This suggests the existence of a low-dimensional model that captures the main features of separation phenomena. Guided with this observation, we develop a mechanical analog for the behavior of separated fluid flows. In view of the complexity of the dynamics, the low-dimensional model at this stage is intended to capture only some of the essential features of the phenomena, namely the primary bifurcation and the hysteresis. The construction of the model also provides a deeper look at the physical mechanisms which govern the separation bubble. Our study is motivated by the problem of active flow control with the goal that the resulting low-dimensional model can serve as a model-based observer for a closed-loop control. [Preview Abstract] |
Sunday, November 20, 2005 10:10AM - 10:23AM |
AN.00011: Local time-dependent (``dimple'') forcing of laminar channel flow Henrik Koberg, Beverley J. McKeon, Jonathan F. Morrison, Spencer J. Sherwin Laminar channel flow over smooth time-dependent depressions of sinusoidal and Gaussian shapes is investigated by means of direct numerical simulation using the spectral/hp element solver Nektar. In previous simulations the dimple was approximated by a linearized boundary condition which does not permit separation and so is only valid for very shallow depressions ($\epsilon/D= 5\times10^{-6}, Re_D\approx 100$ and Strouhal numbers (based on dimple diameter), $St=0.1-1.0$). These results showed that the dimples generate `lobes' of contra-rotating streamwise vorticity, with the vorticity roughly in antiphase with the dimple acceleration. For St of order one, the magnitude of the vorticity is at least two orders of magnitude larger than that for the static case in which vorticity generation proceeds by the action of pressure gradients alone. In the present work, a fully non-linear solution is obtained by use of a time- dependent, curvilinear transformation of the channel with time- dependent wall perturbations, permitting a Fourier representation in the spanwise direction. A detailed analysis of the results is presented for both sinusoidal and Gaussian dimple shapes for a range of St and Re$_D$: these include the effects of local, time-dependent separations. [Preview Abstract] |
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