Quadrant analysis for fluctuations of turbulent shear stress and void fraction in a horizontal bubbly channel flow

Abstract

We measured fluctuations of turbulent shear stress in a horizontal bubbly channel flow. In this research, simultaneous measurement of wall shear stress by a new shear stress sensor using laser Doppler principle and visualization for bubble image by high-speed camera were performed. In order to investigate the relationship between the instantaneous change of shear stress and the passage of bubbles, we evaluated the variation of time series data of the shear stress and the passage distribution of bubbles by analyzing high-order moment and quadrant classification. As a result, it was found that the relationship between the fluctuation of shear stress and the frequency of bubble passage was altered as injection method of bubbles was changed from continuous to repetitive injection. We have confirmed a phenomenon peculiar to repetitive bubble injection, i.e. the shear stress decreased during no bubble passes by. It was also found that the relationship was reversed as the injection frequency of bubbles ranged from 2 to 4 Hz.