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Aerodynamic and Sand Trapping Properties of Porous Mesh 3-Dimensional Roughness Elements

John Gillies 1, Vicken Etyemezian2, George Nikolich2, William Nickling3
1Desert Research Institute, Reno, Nevada, USA, 2Desert Research Institute, Las Vegas, Nevada, USA, 3Department of Geography, University of Guelph, Guelph, Ontario, Canada

Large spatial arrays of solid three-dimensional roughness elements can be used to reduce sand transport to specified targets for control of wind erosion through the effect of drag partitioning and interaction of the moving sand with the large (>0.3 m high) roughness elements.  Observations made in wind tunnel experiments involving roughness elements and sand transport reveal that areas of enhanced erosion in the element lee are associated with vortical structures shed by the roughness element. Deflation of sand at the sides of the element, due to higher shear stress there, forms depressions in the bed that propagate towards the front where they merge to form a continuous curved depression.  In pursuit of a more optimal form for applying the roughness array method to control wind erosion and dust emissions, a wind tunnel experiment was undertaken to evaluate the performance of porous roughness elements.  It was hypothesized that porous elements could increase aerodynamic drag as compared with solid elements, improve sand-trapping potential, and reduce the problem of scour and deflation.

The porous forms were constructed of screen with a porosity of 0.525 and included a cube, four rectangular cylinders and five round cylinders.  The forms of smaller dimension and similar shape could be nested inside the largest.  The drag coefficient (Cde) versus Reynolds number (Reh) relationship for both the cubic/rectangular and round cylinder nests consistently show Cde increasing with increasing Reh to at least Reh ~70,000.  A drag curve for a solid cube or round cylinder over the Reh range tested is expected to be constant.  Over the Reh range tested the round mesh cylinder forms have lower Cde values than the cubic/rectangular forms, and at independence from Reh both shapes have values greater than their solid form equivalents.  The dependence of Cde on Reh over this range can be explained by the flow inside the forms creating different zones that can have turbulent, transitional, and perhaps even laminar flow within them, which arise from effects caused by the permeability (K) of the mesh as well as the permeability of the 3-dimensional form (K').  Deposition of sand in the lee of the porous forms as well as the sand trapping efficiency of the forms also scales with K' for up to four nested forms.  It is also dependent on element shape with the cube/rectangular cylinder forms more effective at trapping sand for similar values of K' than the round cylinders.