Pressure drag and friction drag for truncated pyramids in a turbulent open channel flow

The purpose of this study is to establish a method that more efficiently reduces drags for a three-dimensional object whose surfaces are shaped in some kind of wavy form. We use a truncated pyramid that is a relatively simple three-dimensional object. Experiments were carried out for truncated pyramids covered with flat surfaces or wavy surfaces in a turbulent open channel flow. The total drag was obtained by measuring the force acting on the pyramids. The local wall shear stress was estimated from the local mean velocity profile in the direction normal to the surfaces. The difference between the pressure on the uphill surface and that on the downhill surface of the pyramid was also measured with a pressure difference gauge. It was found that the total drag and the pressure difference for the truncated pyramid covered with the wavy surfaces were at their highest 7.9% and 13.7% lower respectively than the equivalent figures for the truncated pyramid covered with flat surfaces. The reductions of the pressure difference and total drag were caused by the recirculation flow intermittently occurring near the top face and the downhill face of the truncated pyramids.