水平二次元拡大チャネル内の気液二相流に現れるボイド波の周波数特性

Abstract

Void wave, which is a spatio-temporal fluctuation of the void fraction of air bubbles in turbulent flow of water, appears beneath the bottom of a ship with an air-lubrication system when the frictional drag is reduced. Controlling the void wave has a potential to improve the drag reduction rate by air-lubrication. However, it is hard to generate the void wave in a normal channel facility generally used for laboratory-scale experiments on air lubrication. As the reason for this, it is expected that there is the boundary layer without spatially development in such channel flow. We, hence, designed an expanded channel facility where the boundary layer thickness developed and realized the generation of the void wave. To estimate the spatial development of the generated void wave in our channel, we observed the void wave at several positions in the streamwise direction by a high-speed camera and then, analyzed the frequency spectrum of the void fraction by Walsh transformation. Two dominant frequencies of the void wave were revealed at the midstreamposition regardless of the superficial velocity of gas phase, and the frequencies decreased in the downstream-position.