Effect of the Free Surface on the Drag Forces on a Flat Plate Translating Normal to the Flow

A series of experiments were carried out with a flat plate towed normal to the flow in quiescent fluid. The focus was given to the analysis of the drag force seen by the plate as a function of its aspect ratio and hydraulic diameter. The effect of towing the plate near the water free surface was also investigated thoroughly. Plates of aspect ratio ranging from 0.25 to 4 were towed in a still water tank at different Reynolds numbers in the range from 15000 to 60000. Submergence depth was measured from the upper edge to the free surface and varied from zero to the centre of the tank. Forces on the plates were measured using a submersible bending beam load cell and the carriage motion was monitored by a rotary potentiometer. It was found that the drag increases abruptly prior subsiding with increasing submergence depth, with this effect being more dominant in lower aspect ratio plates. The abrupt rise in the drag is due to the interaction of the upper edge of the plate with the free surface resulting in a large shrinkage of the recirculation zone. The non-unit low aspect ratio plates also showed another drag peak around 50% depth, especially at lower speeds. Overall, the trends were Reynolds number independent, except when the aspect ratios was in the range from 0.75 to 1.33 and the plate was near the free surface.