Interference and Proximity Effects
Severe aeroelastic problems arise in Wind Engineering as a result of close spacing between parallel, slender structures such as stacks, towers and overhead electric power cables. Oscillation amplitude of the structure can be considerably greater than for
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R.L. Wardlaw National Research Council, Ottawa, Canada
ABSTRACT Severe aeroelastic problems arise in Wind Engineering as a result of close spacing between parallel, slender structures such as stacks, towers and overhead electric power cables. Oscillation amplitude of the structure can be considerably greater than for single isolated structures. The motion can be caused by vortex shedding, buffeting or aerodynamic instabilities. The effect of proximity on the flow around the structures and on their dynamic response is described. Analytical considerations are discussed and case histories from wind tunnel and full scale observations are presented.
H. Sockel (ed.), Wind-Excited Vibrations of Structures © Springer-Verlag Wien 1994
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R.L. Wardlaw INTRODUCTION
The dynamic behaviour of structures as a result of wind action can be dramatically altered by their proximity to neighbouring structures. Mechanisms that lead to aerodynamically induced motion are present that do not.exist for a single isolated structure. Slender, parallel structures are particularly sensitive and there are weU-documented examples of severe engineering problems that have arisen and there has been extensive related research reported in the literature in the fields of coastal and off-shore engineering, electrical engineering, and civil engineering. Much research has been motivated by problems encountered with the closely spaced tubes in heat exchanger bundles and with bundled conductors used in high voltage electric power transmission lines. In the Wind Engineering field there are several examples of proximity effects relating to closely spaced slender towers and chimneys. In reviewing the interference and proximity effects it is not possible here to do justice to the vast and diverse literature that is available.
3 No interlerence
Wake IOterlerence
Fig. 1 Regions offlow interference for two parallel circular cylinders: bistable flow regions are shown hatcbed (after Zdravkovich [5]) The flow around closely spaced structures is very complex and there are several vibration mechanisms that arise. Therefore at the outset it is useful to attempt to break down the problem into some simple categories. This has been extensively reviewed by Zdravkovich in aseries of papers [1,2,3,4,5]. He identifies tbree interference regions as shown in Figure I, where T is the cross-stream separation and L the streamwise separation of two cylinders, and D is the cylinder diameter: i) a "wake interference" region where one cylinder is partially or wholly immersed in the wake ofthe other, or is in close contact with the wake ii) a "proximity interference" region where the cylinders are not in or near the wake of their neighbour iii) an overhipping region where proximity and wake interference effects are combined Outside ofthese regions the cylinders bebave as isolated structures. More detailed versions of Figure 1 are given in the referenced publications. For beat exchanger tube bundles, regions (ü) and (iü) are important. Vibration problems that have been report
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