Experiments were carried out at the hydrodynamics laboratory of the University of Glasgow with smooth rectangular cylinders of five different aspect ratios in steady flow. The cylinders were tested vertically as surface piercing and horizontally submerged to determine the drag coefficient CD for varying Reynolds numbers (up to 8 105) and investigate the effect of the aspect ratio on the drag coefficient. It was found that the aspect ratio has to some extent a certain effect on CD values. The values of CD were generally found to remain sensibly unchanged when changing the orientation of a cylinder from vertical to horizontal.
The main feature of a flow past a bluff body is the phenomenon of flow separation from the body surface and the resulting formation of a large wake behind the body. The presence of the wake alters the flow and the pressure distribution on the body resulting in a deficit of pressure on the downstream side, the rear side, of the body and an excess on the upstream side, the front side, of the body. This difference of pressure between the front and the back of the body gives rise to a force, the pressure drag. With regard to flow around rectangular cylinders, the separation at extremely low Reynolds number is known to occur at the trailing edges rather than the leading edges where the separation is indiscernible owing to immediate reattachment. As Reynolds number increases, the flow separation at the leading edges will develop and the steady reattachment becomes impossible. This vortex system determines the hydrodynamic (or aerodynamic) forces acting on these bluff bodies. In steady flow, the character of the vortices shed immediately behind the cylinder and in the wake further downstream is strongly dependent of Reynolds number.