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An experimental investigation was carried out on the development of oscillatory laminar flow in curved tubes with a turn angle of 180 (i.e., U-shaped tubes) and curvature radius ratios Rc of 4 and 10. Experiments were conducted under Womersley numbers  of 18 to 30 and Dean numbers D of 150 and 300, giving  (2/D) above 0.655, for which the characteristics of the oscillatory flow in the fully developed region of the curved tubes are roughly divisible into two. The secondary flow induced in the cross section was visualized by a solid tracer method with a stroboscopic light and was recorded photographically at several stations upstream and downstream from the entrance of the curved tube. The development of the secondary flow and transitions between the straight-tube and curved-tube oscillatory flows were systematically investigated in detail. Two pairs of vortices, the so-called Lyne circulation, for high  values appear even at the entrance. The classification of the secondary-flow pattern observed for the fully developed region is applicable to the region developing downstream from the entrance. Accompanying the secondary-flow development, in the straight section immediately before the entrance, a steady streaming occurs in the symmetrical horizontal plane. Furthermore, the steady streaming forms an elliptical trajectory with an increase in . Both the streamings generated in the oscillating boundary layer near the entrance show very complicated behaviors. .

Steady Streaming Motion in Entrance Region of Curved Tubes during Oscillatory Flow