Wake structure of circular cylinder in microbubble mixture

Abstract

Injecting microbubbles near wall can reduce the frictional drag. While it has been confirmed by a variety of experiments to date, the fluid dynamics mechanism of drag reduction has not been comprehensively understood so far. We measure the wake structure of a circular cylinder in microbubble mixture to figure out the characteristic of the interaction between microbubbles and liquid flow accompanying high turbulence. The flow field is restricted to be two-dimensional by confining the bubbly two-phase flow into a thin horizontal channel of 2 mm in height. Microbubbles around 100 μm in diameter are generated with water electrolysis at far upstream the measurement section. Experiment is conducted at Reynolds number higher than 104 to clarify the role of microbubbles in highly turbulent situation. We use solid particles as the PTV-tracer in the single-phase flow and assume that microbubbles trace the liquid flow in the microbubble mixture. We measure the turbulent intensity and Reynolds shear stress distribution from the PTV data and demonstrate that microbubbles significantly suppress the turbulence. In addition, the Karman vortex shedding frequency, which is measured from the unsteady stream function, increases when microbubbles are mixed.