Unsteady Flow around Bluff Bodies

David Sumner, Ph.D., P.Eng., Associate Professor

Department of Mechanical Engineering, University of Saskatchewan

Circular Cylinder in Impulsively Started Flow

The time evolution of the flow field for a single, isolated circular cylinder impulsively set into motion may be considered to be one of the classic examples of unsteady fluid dynamics.  Its study has provided physical insight into a number of time-dependent fluid dynamic processes, including two-dimensional unsteady boundary layer formation and separation, and also vortex formation and vortex shedding.  The flow field is primarily characterized by the formation of a symmetrical recirculation zone in the cylinder near wake, containing a pair of stationary eddies of equal strength and opposite rotation.  Eventually, these eddies are shed from the cylinder and the familiar steady flow pattern of periodic vortex shedding is initiated, provided the Reynolds number is sufficiently high.  The impulsively-started flow field of the circular cylinder is also marked by the formation of small regions of secondary vorticity, located downstream of the point of boundary layer separation.  These regions are not observed for a circular cylinder under steady cross-flow conditions.

Groups of Circular Cylinders in Impulsively Started Flow

The behaviour of multiple-cylinder configurations in impulsively started flow has not been extensively studied.  The tandem and side-by-side configurations have been examined using a towing tank, flow visualization, and PIV.  An example of the side-by-side configuration, and the effect of varying the transverse pitch ratio, T/D, is shown in the figure below.  For configurations with T/D > 1.0, strong flow through the gap between the cylinders and vortices formed alongside the gap dominate the fluid behaviour. This strong gap flow quickly initiates breakup of the recirculation zones that form behind each of the cylinders, and vortex shedding.  Near-wake regions of individual cylinders are asymmetrical and biased towards this gap flow.  There is strong interaction between the gap vortices, particularly for small T/D.  In many cases, the far-wake region is marked by the formation of a single counter-rotating vortex pair, the strength of which is independent of T/D  when T is used to non-dimensionalize the circulation (vortex strength).

Sumner, D., Hemingson H.B., Deutscher, D.M., Barth, J.E., 2007, PIV measurements of the flow around oscillating cylinders at low KC numbers, IUTAM Symposium on Unsteady Separated Flows and their Control, June 18-22, 2007, Kerkyra (Corfu), Greece.

Sumner, D., Barth, J.E., Dansereau, O.J.P., Heseltine, J.L., Crane, M.G., 2005, X-Y towing tank for unsteady fluid mechanics experiments, Proceedings of the 20th Canadian Congress of Applied Mechanics (CANCAM 20005), Montreal, Canada, pp. 353-354.

Sumner, D., Price, S.J., Païdoussis, M.P., 1999, Tandem cylinders in impulsively started flow, Journal of Fluids and Structures, 13, 955-965.

Sumner, D., Price, S.J., Païdoussis, M.P., 1999, Tandem circular cylinders in impulsively started flow, Proceedings of the 17th Canadian Congress of Applied Mechanics, Hamilton, Canada, pp. 115-116.

Sumner, D., Price, S.J., Païdoussis, M.P., 1997, Investigation of impulsively started flow around side-by-side circular cylinders: application of particle image velocimetry, Journal of Fluids and Structures, 11, 597-615.

Sumner, D., Price, S.J., Païdoussis, M.P., 1997, Investigation of impulsively started flow around side-by-side circular cylinders: application of particle image velocimetry, Proceedings of the 4th International Symposium on Fluid-Structure Interactions, Aeroelasticity, Flow-Induced Vibration & Noise, Dallas, USA, ASME Aerospace Division, Vol. 53-1, pp. 93-102.

Sumner, D., Price, S.J., Païdoussis, M.P., 1997, Side-by-side circular cylinders in impulsively started flow, Proceedings of the 16th Canadian Congress of Applied Mechanics, Quebec City, Canada, pp. 271-272.

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Last updated March 5, 2009