An experimental investigation of the end effects on the wake of a circular cylinder towed through water at low Reynolds numbers

Slaouti, Arezki; Gerrard, J. H.

doi:10.1017/s0022112081000414

Cited by 108 publications

(49 citation statements)

References 10 publications

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“…Dye visualization showed that with this clearance the end effects are significantly reduced and do not reach more than two chord lengths across the span. These observations are in qualitative agreement with similar experiments on cylinders by Slaouti & Gerrard (1981).…”

Section: Three-dimensional Effectssupporting

confidence: 92%

Oscillating foils of high propulsive efficiency

et al. 1998

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Thrust-producing harmonically oscillating foils are studied through force and power measurements, as well as visualization data, to classify the principal characteristics of the flow around and in the wake of the foil. Visualization data are obtained using digital particle image velocimetry at Reynolds number 1100, and force and power data are measured at Reynolds number 40 000. The experimental results are compared with theoretical predictions of linear and nonlinear inviscid theory and it is found that agreement between theory and experiment is good over a certain parametric range, when the wake consists of an array of alternating vortices and either very weak or no leading-edge vortices form. High propulsive efficiency, as high as 87%, is measured experimentally under conditions of optimal wake formation. Visualization results elucidate the basic mechanisms involved and show that conditions of high efficiency are associated with the formation on alternating sides of the foil of a moderately strong leading-edge vortex per half-cycle, which is convected downstream and interacts with trailing-edge vorticity, resulting eventually in the formation of a reverse Kármán street. The phase angle between transverse oscillation and angular motion is the critical parameter affecting the interaction of leading-edge and trailingedge vorticity, as well as the efficiency of propulsion. IntroductionFish and cetaceans employ their oscillating tails to produce propulsive and maneuvering forces. The tails of some of the fastest swimming animals closely resemble high-aspect-ratio foils. Because of the presumed optimal propulsive performance of fish, oscillating foils have been studied extensively using theoretical and numerical techniques (Lighthill 1975;Wu 1961Wu , 1971Longvinovich 1971;Cheng & Murillo 1984;Karpouzian, Spedding & Cheng 1990;McCune & Tavares 1993), and experimentally (Scherer 1968;DeLaurier & Harris 1982;Lai, Bose & McGregor 1993).A foil in steady forward motion and a combination of steady-state harmonic heaving and pitching motion produces thrust through the formation of a flow downstream from the trailing edge, which when averaged over one period of oscillation has the form of a jet. This average jet flow is unstable, acting as a narrow-band amplifier of perturbations. The harmonic motion of the foil causes unsteady shedding of vorticity from the trailing edge, while there are conditions when leading-edge vortices form as well. The interaction between the unsteady vorticity shed by the foil and the inherent dynamics of the unstable wake result in the formation of patterns of

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Section: Three-dimensional Effectssupporting

confidence: 92%

Oscillating foils of high propulsive efficiency

et al. 1998

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“…We can see that this amplitude is strongly diminished near the free ends of the cylinder as reported in the numerical work of Dauchy et al (1997). We also note that, from #ow visualizations, Slaouti & Gerrard (1981) state that the free-end e!ect is to suppress the vortex shedding in the vicinity of the ends.…”

Section: Periodic Regimesupporting

confidence: 76%

“…Free-end cylinder wakes have also been investigated numerically by Dauchy et al (1997) and experimentally by Slaouti & Gerrard (1981); they report that the vortex shedding is diminished or suppressed near the free ends.…”

Section: Introductionmentioning

confidence: 99%

Periodic Wakes of Low Aspect Ratio Cylinders With Free Hemispherical Ends

Schouveiler

Provansal

2001

Journal of Fluids and Structures

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“…This 'grazing' end condition, coupled with a clean free surface at the upper end, was recommended by Slaouti & Gerrard (1981) and Lisoski (1993) for achieving predominantly two-dimensional flow along the span of a model in a towing tank. For this experiment (plotted as a dash-dotted line in figure 2), there is instead a C D minimum centred on T = 5.…”

Section: Drag Force Measurementsmentioning

confidence: 99%

Role of the tip vortex in the force generation of low-aspect-ratio normal flat plates

2007

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We investigate experimentally the force generated by the unsteady vortex formation of low-aspect-ratio normal flat plates with one end free. The objective of this study is to determine the role of the free end, or tip, vortex. Understanding this simple case provides insight into flapping-wing propulsion, which involves the unsteady motion of low-aspect-ratio appendages. As a simple model of a propulsive half-stroke, we consider a rectangular normal flat plate undergoing a translating start-up motion in a towing tank. Digital particle image velocimetry is used to measure multiple perpendicular sections of the flow velocity and vorticity, in order to correlate vortex circulation with the measured plate force. The three-dimensional wake structure is captured using flow visualization. We show that the tip vortex produces a significant maximum in the plate force. Suppressing its formation results in a force minimum. Comparing plates of aspect ratio six and two, the flow is similar in terms of absolute distance from the tip, but evolves faster for aspect ratio two. The plate drag coefficient increases with decreasing aspect ratio. IntroductionWork on the flapping-wing propulsion of insects and birds has increased greatly in recent years, motivated by its potential application to the design of micro air vehicles. However, there is a corresponding lack of fundamental research on unsteady threedimensional vortex flows at the appropriate low to moderate Reynolds numbers. The present study focuses on understanding the effect of the wingtip vortex on the overall wing force of a hovering insect. This tip effect is dependent on the wing aspect ratio (AR).Flying insects have wing ARs between 2.75 and 6 (Ellington 1984; Dickinson, Lehmann & Sane 1999), while hummingbird wings have ARs of about 4 (Dhawan 1988). Aspect ratio is defined here as b 2 /S, where b is the span of a single wing and S is the single-wing planform (top-view) area. Hovering wing kinematics typically include a segment of downward, plunging motion which tilts the drag force upward so that it contributes to supporting the insect's weight; for dragonflies and hoverflies, drag is the primary wing force keeping the insect aloft (Wang 2004). Here, we focus on drag-based hovering. † Present address:

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An experimental investigation of the end effects on the wake of a circular cylinder towed through water at low Reynolds numbers

Cited by 108 publications

References 10 publications

Oscillating foils of high propulsive efficiency

Oscillating foils of high propulsive efficiency

Periodic Wakes of Low Aspect Ratio Cylinders With Free Hemispherical Ends

Role of the tip vortex in the force generation of low-aspect-ratio normal flat plates

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