Drag reduction of turbulent flows in a curved pipe due to the swirling instability

The effect of the introduction of swirling motion into turbulent flow in a curved pipe is studied experimentally. The friction of the flow in a straight pipe is known to increase as the introduced swirling motion intensifies. In the present study, it is found that the friction in a curved pipe is reduced when weak swirling motion is provided. In order to investigate this drag reduction, velocity distributions are measured at some cross sections in the curved pipe by use of the rotating probe technique with an inclined hot-wire. Detailed measurements of the velocity field reveal that interesting swingback phenomena appear as the vortical flow structure develops from the entrance of the pipe, which may cause the drag reduction in case of weak swirls.