Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Fluid Dynamics
Volume 56, Number 18
Sunday–Tuesday, November 20–22, 2011; Baltimore, Maryland
Session L21: Vortex Dynamics V |
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Chair: James Gregory, Ohio State University Room: 324-325 |
Monday, November 21, 2011 3:35PM - 3:48PM |
L21.00001: Effects of Spanwise-Modulated Blowing on the Cylinder Near-Wake James Gregory, Samik Bhattacharya Three-dimensional characteristics of the wake of a circular cylinder at Re=5\begin{math}\times10^{3}\end{math} were controlled using spanwise-modulated forcing from dielectric barrier discharge plasma actuators. Three-dimensional blowing profiles were created by varying the shape of the buried electrode in the spanwise direction. A sinusoidal voltage of 10 kVpp at 5 kHz was applied between the electrodes to create the plasma. Wake surveys conducted with a rake of x-wires were used to understand the impact of the actuation on the time-resolved wake development. Simultaneous transverse profiles at multiple spanwise positions reveal that the actuation has a substantial impact on the spanwise variation of frequency and phase of the K\'{a}rm\'{a}n shedding process, as well as on the mean and fluctuating properties of the wake profile. Cross-correlations indicate a loss of coherence between adjacent spanwise locations, and the amplitude of the dominant vortex shedding frequency was substantially attenuated due to forcing. Further studies were carried out by modulating the actuation with harmonics and sub harmonics of K\'{a}rm\'{a}n shedding and the shear layer instability, with the aim of introducing small-scale structures that enhance break-up of the wake coherence. [Preview Abstract] |
Monday, November 21, 2011 3:48PM - 4:01PM |
L21.00002: A cantilevered flexible cylinder in cross-flow Jessica Shang, Alexander Smits, Howard Stone Biological fluid-structure interactions of high aspect ratio bluff bodies are commonplace: flow around tall plants; flow through arrays of sensory vibrissae, antennae, and hairs. In this study, we seek insight to this class of problems by generalizing the flow configuration to uniform flow past a flexible cantilevered cylinder. Experiments were conducted for $Re_{D}$ = 100-500. Cylinders deflected with the flow and demonstrated multimodal oscillations in both the streamwise and transverse directions. Oscillation frequencies were correlated with vortex shedding frequencies, but low oscillation frequencies (sub-1 Hz), which were not apparently vortex- induced, were also present. Two $Re_{D}$ regimes were noted in which the vortex shedding frequency remained relatively constant with $Re_ {D}$,while the two regimes were separated by an intermediate transition region. This feature results in an apparently linear relationship between $St$ and $Re_{D}$ in each regime. Hydrogen bubble visualization showed strong three-dimensionality in the wake, as well as a diversity of wake structures varying with $Re_ {D}$. [Preview Abstract] |
Monday, November 21, 2011 4:01PM - 4:14PM |
L21.00003: A Case Study of Horseshoe Vortex Dynamics at Laminar and Turbulent Juncture Flows Tayfun Aydin, Adam Blackmore, Alis Ekmekci To better understand the impacts of oncoming boundary-layer characteristics on the horseshoe vortex dynamics forming at a cylinder-wall junction, a case study is conducted in a free-surface water channel involving cylinder-channel floor and cylinder-end plate junctions. Endplates had two different leading-edge geometries, namely a sharp and a super-elliptical shape. The Reynolds number was kept at 10,000, based on the cylinder diameter. Laminar boundary layer forms along the channel floor and on the endplate with the super-elliptical leading edge. The plate with a sharp leading edge, however, involves flow separation at the leading edge, followed by a reattachment then an early transition to turbulence. On this plate, turbulent boundary layer is achieved. Flow visualization was performed on the plane of symmetry via PIV. Vortex trajectories and the velocity spectra depict periodic nature of the laminar horseshoe vortices and a variation of the spatial and temporal characteristics of horseshoe systems with the boundary layer thickness. Turbulent approach boundary layer led to disorganized and complex horseshoe vortex formation. [Preview Abstract] |
Monday, November 21, 2011 4:14PM - 4:27PM |
L21.00004: Self-Excited Fluidic Energy Harvesters with Finite-Length Cylinders Huseyin Dogus Akaydin, Chloe Duquesnois, Niell Elvin, Yiannis Andreopoulos In this experimental work, we explore the possibility of using piezoelectric materials for harvesting electrical energy from fluid flow. Such harvesters may be used for powering small sensors and obviate the need for batteries and/or power lines. Piezoelectric harvesters behave as AC-coupled devices and need oscillatory motion to generate an electrical output. The harvester should be designed to be ``self-excited,'' i.e. capable of initiating and sustaining the necessary oscillations in steady and uniform flows. The present configuration consists of a piezoelectric cantilever beam with a cylindrical tip body which promotes aeroelastic vibrations induced by vortex shedding. The harvester was tested in a wind tunnel and it produced 0.1 mW of electrical power at a flow speed of about 1.19 m/s. Using strain measurements and a distributed parameter model, the harvested electrical power was predicted, and a reasonable agreement is obtained with the measurements. The magnitude and frequency of the driving aerodynamic forces were also estimated. Results were comparable with literature data on flow past oscillating cylinders. Finally, the effect of using various shapes of tip body is presented. [Preview Abstract] |
Monday, November 21, 2011 4:27PM - 4:40PM |
L21.00005: Vortex Induced Vibration of an Inclined Cylinder in Flow Anil Jain, Yahya Modarres-Sadeghi The Vortex Induced Vibration (VIV) of a cylinder inclined to the incoming flow is not studied as extensively as the normal-incidence case. For a rigid inclined cylinder, it is believed that the cylinder behaves similarly to a vertical one if the component of the flow velocity normal to the cylinder axis is considered. We investigate the extent to which this assumption is valid by conducting a series of experiments on a flexibly mounted cylinder placed inclined to the incoming flow with various angles of inclination in a Reynolds number range of 500 - 10,000. The cylinder, mounted on springs, is placed in the test section of a recirculating water tunnel, and for each angle of inclination, we increase the flow velocity gradually, and measure the displacement and frequency of the resulting vibrations. We examine how various angles of inclination result in various lock-in regions (large-amplitude vibrations) and we determine the critical angle of inclination beyond which the axial component of the flow velocity will have a non-negligible influence on the observed vibrations. [Preview Abstract] |
Monday, November 21, 2011 4:40PM - 4:53PM |
L21.00006: Vortex dislocations in the wake of a circular cylinder Serhiy Yarusevych, Christopher Morton Cross flow over complex cylindrical geometries, such as cylinders with discontinuities in diameter, results in non-uniform wake vortex shedding with complex vortex interactions. In the present study, the development of vortex dislocations in the wake of a circular cylinder is investigated. A dual-step circular cylinder geometry is employed, which consists of a large-diameter cylinder placed at the midspan of a small-diameter cylinder. In a uniform flow, wake development depends primarily on the Reynolds number (Re$_{D})$, the ratio of the large cylinder diameter (D) to the small cylinder diameter (d), and the aspect ratio of the large cylinder (L/D). Experimental investigations are performed for Re$_{D}$=1080, D/d=2, and 0.2$\le $L/D$\le $2.0 in a water flume facility utilizing flow visualization, laser Doppler velocimetry, and particle image velocimetry. Also, direct numerical simulations are performed for a laminar vortex shedding regime, where, based on previous studies, interactions between large-scale vortical structures are expected to be similar to those observed in turbulent wakes. The results show that the large cylinder induces vortex dislocations that are manifested by half-loop vortex connections forming between consecutive small cylinder vortices. The remaining small cylinder vortices form vortex connections across the wake of the large cylinder. The dislocations occur at a distinct frequency, which decreases with decreasing L/D. [Preview Abstract] |
Monday, November 21, 2011 4:53PM - 5:06PM |
L21.00007: Nonlinear restoring forces in vortex-induced vibration A.W. Mackowski, C.H.K. Williamson When studying vortex-induced vibration of a rigid circular cylinder, almost all experimental and computational studies involve the cylinder being supported by linear springs. However, there are cases in which we may be interested in the VIV response of a cylinder supported by nonlinear springs. A system with nonlinearities in the restoring force has the potential to increase the amplitude response envelope, critical to the success of aero-vibrating energy harvesters. On the other hand, designing nonlinear restoring forces to decrease the amplitude response may lead to structures more able to withstand flow-induced vibration. In addition, adding nonlinear terms to the restoring force on a rigid cylinder might be used to simulate higher-order dynamics of long, elastic marine cables. To experimentally observe the effects of nonlinear springs on flow-induced vibration, we apply a novel approach that lets us parametrically control the nature of the springs and the strength of the nonlinearities. The technique, called Cyber-Physical Fluid Dynamics, uses a force-feedback control system to simulate arbitrary forces on a submerged body [the details of this system were shown in the APS presentation of Mackowski \& Williamson (2010)]. We present results using this technique to explore the amplitude response of a circular cylinder in a crossflow. [Preview Abstract] |
Monday, November 21, 2011 5:06PM - 5:19PM |
L21.00008: The flow around the node to anti-node transition of an oscillating flexible cylinder Francisco Huera-Huarte, Zafar Bangash The vortex dynamics in the wake of cylinders is a topic of interest, for its implications in engineering since the forties. Most of the work has been done with stationary cylinders or with oscillating rigid cylinders when they are elastically mounted. If the cylinders are flexible, not only the structural dynamics become more complex, but the fluid dynamics around the cylinder are much more complicated. We study the flow around a flexible cylinder when oscillating in its first or second structural mode. An experimental set-up allows producing forced oscillations on the model, in a way that mode number, oscillating frequency and amplitude can be controlled. Experiments were conducted in a motorised towing tank, for a wide range of towing speeds, frequencies and amplitudes. Digital Particle Image Velocimetry (DPIV) was used to quantify the flow and to investigate the flow structures and vortex modes around the cylinder. The interest was given to the extreme amplitude regions and to the node to anti-node transitional regions. [Preview Abstract] |
Monday, November 21, 2011 5:19PM - 5:32PM |
L21.00009: The Effect of Shape on the Wake of Low-Aspect-Ratio Wall-Mounted Obstacles Seyed Hajimirzaie, James Buchholz Wall-mounted bodies in boundary layer flows are ubiquitous in nature and engineering applications. We evaluate the role of shape on the wakes around three different low-aspect-ratio wall-mounted obstacles in shallow boundary-layer flow: semi-ellipsoids with the major axis of the base ellipse aligned in the transverse and streamwise directions, and a sphere. Despite their geometric simplicity, the obstacles create extremely complex, highly three-dimensional and unsteady flow fields for which the transport mechanisms of momentum and scalars are still not well-understood. All three obstacles have the same height and the aspect ratios considered are 0.67, 0.89 and 1, respectively. DPIV was used to interrogate the flow. Streamwise structures observed in the mean wake include tip, base, and horseshoe vortex pairs, which vary significantly in strength with changes in obstacle geometry. Significant variation in the strength of these structures with streamwise location suggests a complex connectivity with the mean spanwise arch structure in the near wake. The three-dimensional topology of the mean wake will be discussed. [Preview Abstract] |
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