No-Feedback Control of Kármán Vortex Streets at Low Reynolds Numbers

A discussion is given of the current state of knowledge of vortex streets behind circular cylinders in the Reynolds number range 50 to 160. This was prompted by Gaster's (1969) report that he could not find the transition at a Reynolds number of about 90 observed by Tritton (1959) and Berger (1964a). A further brief experiment confirming the existence of the transition is described Reasons for rejecting Gaster's interpretation are advanced. Possible (mutually alternative) explanations of the discrepant observations are suggested.

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Mechanism of determination of the shedding frequency of vortices behind a cylinder at low Reynolds numbers

Journal of Fluid Mechanics ◽

10.1017/s0022112078002451 ◽

1978 ◽

Vol 89 (1) ◽

pp. 49-60 ◽

Cited By ~ 72

Author(s):

Michio Nishioka ◽

Hiroshi Sato

Keyword(s):

Stability Theory ◽

Linear Growth ◽

Flow Direction ◽

Reynolds Numbers ◽

Linear Growth Rate ◽

Low Reynolds Numbers ◽

Direct Measurements ◽

Vortex Streets ◽

Low Reynolds ◽

The Stability

Two kinds of experiment were made in the wake of a cylinder at Reynolds numbers ranging between 20 and 150. One was a close look at the structure of the vortex street with a stationary cylinder at Reynolds numbers greater than 48. The other experiment was made at lower Reynolds numbers with a cylinder vibrating normal to the flow direction. In this case an artificially induced small-amplitude fluctuation grows exponentially with the rate predicted by the stability theory. Because of the similarity between the two kinds of wake, we postulate that the shedding of the vortex at low Reynolds numbers is initiated by the linear growth, namely, the fluctuation with the frequency of maximum linear growth rate develops into vortex streets. By using the measured width of the wake at the stagnation point in the wake and the result of the stability theory, we could calculate the Strouhal number for Reynolds numbers ranging from 48 to 120. The predicted Strouhal numbers agree well with the values from direct measurements.

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Feedback control of vortex shedding at low Reynolds numbers

Journal of Fluid Mechanics ◽

10.1017/s0022112093000771 ◽

1993 ◽

Vol 248 ◽

pp. 267-296 ◽

Cited By ~ 178

Author(s):

Kimon Roussopoulos

Keyword(s):

Feedback Control ◽

Vortex Shedding ◽

Single Channel ◽

Water Channel ◽

Open Water ◽

Reynolds Numbers ◽

Circular Cylinders ◽

Low Reynolds Numbers ◽

Wake Instability ◽

Low Reynolds

This paper describes experiments undertaken to study in detail the control of vortex shedding from circular cylinders at low Reynolds numbers by using feedback to stabilize the wake instability. Experiments have been performed both in a wind tunnel and in an open water channel with flow visualization. It has been found that feedback control is able to delay the onset of the wake instability, rendering the wake stable at Reynolds numbers about 20% higher than otherwise. At higher flow rates, however, it was not possible to use single-channel feedback to stabilize the wake - although, deceptively, it was possible to reduce the unsteadiness recorded by a near-wake sensor. When control is applied to a long span only the region near the control sensor is controlled. The results presented in this paper generally support the analytical results of other researchers.

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