Proper orthogonal decomposition analysis on longitudinal streaks in channel flow laden with micro-bubbles

Abstract

The streaky structures in the fluid laden with and without micro-bubbles of channel turbulent boundary layer flow were experimentally studied by using particle image velocimetry technique at a Reynolds number of Re = 9.3 × 104. Micro-bubbles were produced by the electrolysis of water through platinum wires in a low speed circulating water tunnel. The velocity fields in the streamwise-spanwise plane were obtained at three different heights (i.e. y = 3 mm, 5 mm and 7 mm) along the wall-normal direction. The two-point autocorrelation of velocity fluctuations was carried out to identify the scale of the streaky structures in the physical space statistically. The proper orthogonal decomposition (POD) method was employed to analyze the turbulent kinetic energy contents and the streaky structures in mode space. The results show that the streak intensity is decreased and the structures become indistinct in the presence of micro-bubbles at a relatively lower height of the boundary layer in physical space. The ratios of the energy corresponding to leading-order modes to the total energy of the turbulent for the mixed fluid (fluid laden with micro-bubbles) are smaller than those for the single-phase fluid (fluid without micro-bubbles) at these three heights. The effect on the energy content is decayed with the increase of POD mode number as well as the normal height in POD mode space for the mixed fluid. Through comparing the streaky and non-streaky turbulent activities in the presence of micro-bubbles, it is found that the streaky structure turbulent activities are weakened while the non-streaky turbulent activities are enhanced for the mixed fluid.