Wind-Direction Effects on Urban-Type Flows |
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| Authors: | Jean Claus O Coceal T Glyn Thomas S Branford S E Belcher Ian P Castro |
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| Institution: | (1) School of Engineering Sciences, University of Southampton, Highfield, Southampton, SO17 1BJ, UK;(2) Present address: Lockheed Martin Corporation, Palmdale, USA |
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| Abstract: | Practically all extant work on flows over obstacle arrays, whether laboratory experiments or numerical modelling, is for cases
where the oncoming wind is normal to salient faces of the obstacles. In the field, however, this is rarely the case. Here,
simulations of flows at various directions over arrays of cubes representing typical urban canopy regions are presented and
discussed. The computations are of both direct numerical simulation and large-eddy simulation type. Attention is concentrated
on the differences in the mean flow within the canopy region arising from the different wind directions and the consequent
effects on global properties such as the total surface drag, which can change very significantly—by up to a factor of three
in some circumstances. It is shown that for a given Reynolds number the typical viscous forces are generally a rather larger
fraction of the pressure forces (principally the drag) for non-normal than for normal wind directions and that, dependent
on the surface morphology, the average flow direction deep within the canopy can be largely independent of the oncoming wind
direction. Even for regular arrays of regular obstacles, a wind direction not normal to the obstacle faces can in general
generate a lateral lift force (in the direction normal to the oncoming flow). The results demonstrate this and it is shown
how computations in a finite domain with the oncoming flow generated by an appropriate forcing term (e.g. a pressure gradient)
then lead inevitably to an oncoming wind direction aloft that is not aligned with the forcing term vector. |
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