Development of Secondary Stream in Oscillatory Flow through Curved Pipes.

An experimental study has been made to investigate the secondary stream in an oseillatory laminar flow in the entrance region of curved pipes. The experiments were performed under the conditions of the Womersley numbers α-5.5∼30, the Dean numbers D=100∼300, and the curvature radius ratios of Rc=4 and about 10. The secondary flow was visualized and was recorded photo-graphically at several sections near the bend inlet. Development of the secondary flow is discussed mainly for D=300 and influences of the Womersley number on the characteristics are made clear. The classification found by authors to describe the secondary flow pattern in the fully-developed region is also useful in doing the problem in the entrance region. For α2/D<0.655, the secondary flow forms a Dean type and grows up at Ω=20∼50°around the curved turn at the inflow phase, particularly its intensity showing drastic increase for a moderate a of α2/D≒0.41, while the secondary flow in the tangent at the outflow phase almost breakes down at the section of four times diameter from the bend inlet. For high a of α2/D>0.655, the secondary flow in a kind of Lyne type, which consists of two pairs of vortices, is induced even at the bend inlet. The steady streaming is generated near the bend inlet and the fluid drifts in the axial direction depending on the secondary flow patterns.