The influence of vegetation structure on saltation threshold was investigated using uniformly spaced arrays of non-erodible roughness elements on a bed of erodible sand in an wind tunnel. Structural variables tested using arrays of solid cylinders included element aspect ratio (height/diameter) and lateral cover (total frontal-silhouette area per unit ground area). In agreement with previous studies, increase in saltation threshold above the value for bare sand was strongly related to lateral cover. Increasing aspect ratio from 0.25 to 4 tended to enhance the increase in saltation threshold at a given lateral cover. An approximate fit to the results could be obtained using a relation proposed by Raupach et al. (Journal of Geophysical Research, 1993, 98D, 3023–3029).



Porous elements were constructed as clusters of narrow, vertically oriented cylinders (model ‘stems’), forming porous model plant bodies that were cylindrical in overall shape. Low-porosity elements were found to be approximately 50 per cent more effective in increasing saltation threshold than either solid or high-porosity elements. The ratio of plant-body frontal-silhouette area (based on overall dimensions) to total stem frontal-silhouette area was found to be a useful measure of plant-body porosity for wind erosion studies. Some conventional measures of vegetation amount fail to account for changes in vegetation structural attributes that may strongly influence aeolian processes.

Wind-Tunnel Modelling of the Influence of Vegetation Structure on Saltation Threshold

Flow structure around perforated cylinders in shallow water

It is well known that perforation of a flat plate reduces its drag when exposed to a flow. However, studies have shown an opposite effect in the case of cylinders. Such a counterintuitive result can have significant consequences on the momentum modelling often used for wind turbine performance predictions, where increased porosity is intrinsically linked to lower drag. Here, a study of the drag of various types of porous cylinders, bars and plates under steady laminar inflow is presented. It is shown that, for most cases, the drag decreases with increased porosity. Only special types of perforations can increase the drag on both cylinders and bars, either by enhancing the effect of the rear half of the models or by organizing the wake structures. These rare occurrences are not relevant to wind turbine modelling, which indicates that current momentum models exhibit the qualitatively correct behaviour.

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The effect of porosity on the drag of cylinders

In this paper we investigate the effect of an inhomogeneous and unsteady velocity field incident on an array of rigid circular cylinders arranged within a circular perimeter (diameter ) of varying solid fraction , where the unsteady flow is generated by placing a cylinder (diameter ) upwind of the array. Unsteady two-dimensional viscous simulations at a moderate Reynolds number and also, as a means of extrapolating to a flow with a very high Reynolds number, inviscid rapid distortion theory (RDT) calculations were carried out. These novel RDT calculations required the circulation around each cylinder to be zero which was enforced using an iterative method. The two main differences which were highlighted was that the RDT calculations indicated that the tangential velocity component is amplified, both, at the front and sides of the array. For the unsteady viscous simulations this result did not occur as the two-dimensional vortices (of similar size to the array) are deflected away from the boundary and do not penetrate into the boundary layer. Secondly, the amplification is greater for the RDT calculations as for the unsteady finite Reynolds number calculations. For the two highest solid fraction arrays, the mean flow field has two recirculation regions in the near wake of the array, with closed streamlines that penetrate into the array which will have important implications for scalar transport. The increased bleed through the array at the lower solid fraction results in this recirculation region being displaced further downstream. The effect of inviscid blocking and viscous drag on the upstream streamwise velocity and strain field is investigated as it directly influences the ability of the large coherent structures to penetrate into the array and the subsequent forces exerted on the cylinders in the array. The average total force on the array was found to increase monotonically with increasing solid fraction. For high solid fraction , although the fluctuating forces on the individual cylinders is lower than for low , these forces are more correlated due to the proximity of the cylinders. The result is that for mid to high solid fraction arrays the fluctuating force on the array is insensitive to . For low , where the interaction of the cylinders is weak, the force statistics on the individual cylinders can be accurately estimated from the local slip velocity that occurs if the cylinders were removed.

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B.S. Piper

Papers

The drag coefficient of finite-aspect-ratio perforated circular cylinders