Friction Drag Characteristics in Horizontal Air-Silicone Oil Bubbly Channel Flow

(1st report; Measurement of Frictional Drag and Gain Evaluation)

Abstract

Experiments on a bubbly channel flow utilizing silicone oil as the test fluid were performed to elucidate mechanisms of frictional drag reduction by bubbles: Using silicone oil overcomes problems of contaminations on bubble interfaces and allows large deformations of bubbles even at small Reynolds numbers (Re_m) because of its small surface tension. The local friction drag coefficient measured by a shear stress sensor indicated that the bubble injection introduces drag reduction even at low Reynolds number turbulent flows (Re_m ~ 1900) at which the reduction has never been observed in water channel. We also introduced evaluations of the drag reduction using a gain factor that includes energy spent for bubble injections. The gain factor for an actual ship was estimated on the experimental results to investigate applicability of the drag reduction technique for vessels.