Interference Effects for Groups of Bluff Bodies

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

Department of Mechanical Engineering, University of Saskatchewan

Groups of cylinder-like structures immersed in cross-flow can be found in designs for heat exchangers, offshore structures, buildings, chimneys, power lines, and other structures.  Problems with flow-induced vibrations and noise are frequently encountered. The most general arrangement of two cylinders is the staggered configuration.  The flow field involves complex interactions between the shear layers, wakes and Kármán vortex streets.  The fluid forces on the cylinders undergo large changes in amplitude and direction as the cylinder positions are changed.  Multiple vortex shedding frequencies may be found. Several critical configurations corresponding to maximum lift force, minimum drag force, and discontinuous behaviour in the forces, have been identified. Complex wake interactions occur between the vortex streets from the individual cylinders; for example, nine distinct flow patterns can be identified for the staggered configuration.

The classic example of a two-dimensional bluff body is the circular cylinder.  By virtue of its common occurrence in many forms and in different applications, the flow of fluid around a circular cylinder has been well studied, perhaps for more than a century.  Although the geometry is relatively simple, the flow is rather complex and undergoes a number of distinct changes with the Reynolds number.  These changes occur in the wake first, followed by the separated free shear layers, and then the boundary layers, as the Reynolds number is increased.  For a wide range of Reynolds number, the flow field is characterized by the periodic, alternate formation and shedding of vortices from opposite sides of the cylinder, and a regular pattern of vortices in its wake known as the Kármán vortex street.  Cylinder-like structures can be found, both alone and in groups, in the designs for heat exchangers, cooling systems for nuclear power plants, offshore structures, buildings, chimneys, power lines, struts, grids, screens, and cables, in both air- and water-flow.  In many of these engineering applications, the strong periodic shedding of Kármán vortices from a cylinder is responsible for problems with flow-induced vibration and noise, the production of highly turbulent and three-dimensional fluid motion in its wake, and structures that experience considerable drag forces.

This research area began with my Ph.D. studies at McGill University, which was focused mainly on flow visualization and vortex shedding experiments at lower Reynolds numbers.  At the University of Saskatchewan, the focus has been on measurements at much higher Reynolds numbers, specifically making extensive wind tunnel measurements of the mean aerodynamic forces, base pressure coefficients, and Strouhal numbers for the staggered configuration of two “infinite” or two-dimensional (2-D) circular cylinders. The work identified staggered configurations linked to sudden changes in, or extreme values of, the mean aerodynamic forces and multiple vortex shedding frequencies, which may be responsible for flow-induced vibrations under certain conditions. Relationships between trends in the measured data and the flow patterns were established.  Related studies have focused on identifying a “universal wake number” for two staggered cylinders.

Shear layer designations for two staggered circular cylinders in cross-flow.

The physical understanding of the flow around groups of two-dimensional bluff bodies (staggered circular cylinders) has matured whereby it can now be applied to the understanding of the flow past groups of three-dimensional bluff bodies (two or more finite circular cylinders or stacks in close proximity).  Recent research has focused on vortex shedding for two finite circular cylinders in a staggered configuration.

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Last updated: July 2, 2010