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Unsteady Flow and Fluid-Structure Interactions |
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David Sumner, Ph.D., P.Eng., Professor Department of Mechanical Engineering, University of Saskatchewan |
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| Overview |
| In some industrial applications, bluff bodies are immersed in unsteady flow, or may undergo structural motion in response to fluid motion and forces. Examples include the start-up flow in a heat exchanger, an offshore platform experiencing wave motion, and a building experiencing a sudden wind gust. In such fluid-structure interaction problems, the vortex wake modes and the time-dependent forces acting on the bodies are considerably more complex than encountered for non-moving structures in steady cross-flow. Additional complexity occurs when two or more oscillating bodies are in close proximity. |
| Circular Cylinder in Impulsively Started Flow |
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The time evolution of the flow field for a single, isolated circular cylinder (of diameter, D) impulsively set into motion (at constant velocity, U) 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. Below is a schematic of the recirculation zone for approximately Re = 500, t* = 3 (at these conditions, the zone is symmetric) showing the length of the recirculation zone, LR, the streamwise location of the primary eddy centres, a, and the cross-stream spacing of the primary eddy centres, b.
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| Square Prisms in Impulsively Started Flow |
| The flow field of an impulsively started square prism (of side length, D) is similar to that of the impulsively started circular cylinder, with the formation of a recirculation zone in the near-wake containing a pair of stationary primary eddies of equal strength and opposite rotation. Recently, we have completed some of the first experiments of the flow around impulsively started square prisms. |
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Last updated: December 10, 2010 |
