FLUID DYNAMIC FORCES ON A SPHERE IN THE WAKE OF FENCE

Abstract

The knowledge of fluid dynamic forces experienced by bodies of various shapes under various conditions of flow is of great interest to engineers. It has wide applications in field viz, analysis and design of structures submerged in water or air; forces causing motion of sediments in shelter belts behind a vertical wall type barrier etc. fan be modelled experimentally by placing a sphere in the region of wake of a two dimensional fence. The present work is an experimental study of drag and lift forces on a sphere in the wake of a solid two-dimensional fence with a uniform approach flow. The study was carried out in an open-circuit wind tunnel. 9000 mm long, having a test section of 510 mm x 510 mm. Two Aluminium fences of 70 mm and 50 mm heights spaninq over full width, were fixed on to the floor of the tunnel. A single hollow plastic table tennis ball of 38 mm external diameter was placed downstream of the fence at various positions on the, floor as well as away from it. The results indicate that the fluctuations in drag and lift coefficients follow the normal Gaussian distribution. Further, the mean and r.m.s. drag and lift coefficients of the sphere placed on the fthe fence height upto the reattachment point. However, beyond the reattachment point the mean drag and lift coefficients attain the same values as obtained under the'same approach flow but without the fence. Moreover the variation in the mean -nd r.m.s drag and lift coefficients are limited to the lower left hand corner of the fence wake, characterized by the limits x/h=0-12.5 and yih=0-2.0. Outside this zone, else-where in the region of the wake, they practically have constant small values. The variation of r.m.s. drag and lift coefficients within the wake of the fence away from the boundary'are more or less identical to each other. And the maximum r.m.s. drag is about 600 percent of its mean occuring at x/ht.,,:,.5 and y/h,',..=-1.68, while the maximum r.m.s lift is about 3000 percent of the mean lift which occurs at x/h\%."2.5 and y/h-',1.2loor in the wake region vary with