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.

Much attention has recently been given to high-efficiency cooling of pistons in internal combustion engines by cooling channels because of improved thermal efficiency. Cooling a piston efficiently requires a grasp of the gas-liquid multiphase flow state. However, because the magnitude and direction of the inertial force applied to the piston change depending on the crank angle, the flow field in the cooling channel that forms a complex gas-liquid multiphase flow remains a problem. Therefore, we developed a rig test apparatus simulating the reciprocating motion of a piston and visualized internal flows in a clear acrylic channel using a high-speed camera. This paper examines the effects of the Reynolds number of the oil jet and oscillation frequency of the reciprocating motion on flow characteristics in a right circular cylindrical channel. The Reynolds number and oscillation frequency were tested in the ranges 1000 to 2500 and 0 to 8.33 Hz, respectively. We found that the oil flow pattern in the channel forms the complex air-oil multiphase flow via air entrainment caused by the collision between the oil jet and the channel oil at the inlet. The gas phase area ratio in the channel increases with increasing Reynolds number, but its fluctuation is dominated by oscillation frequency. The fluctuation of the gas phase center of gravity becomes larger because of increases in inertial force with increases in oscillation frequency. The average bubble diameter in the channel decreases with increasing Reynolds number and oscillation frequency. We found that bubbles of small diameter are generated because the interfacial fluctuation of the oil jet increases as the jet goes downstream.