Shallow DOF-based particle tracking velocimetry applied to horizontal bubbly wall turbulence

Abstract To investigate the effect of microbubble injection into a turbulent boundary layer, we measure the velocity fields of gas–liquid two-phase flow in a horizontal water channel with a Particle Tracking Velocimetry (PTV) technique. In this study, we focus on a thin shear layer where the Reynolds shear stress is the highest. The effect of the shallow focal depth-of-field (DOF) in a lens system assists to improve the spatial resolution of the PTV results. Considering the distribution of luminous flux, we first enhance the illumination intensity to label the objects on images. With their assistance together with an image processing filter, we can then improve the in-depth resolution of tracer particles and determine their positions more accurately. We apply this shallow DOF-based PTV technique to measure the streamwise and spanwise velocity components and visualize the motion of bubbles relative to continuous phase in the near-wall region at three downstream positions. The result shows that the averaged velocity right downstream of the bubble injector clearly decreases due to microbubbles. The local instantaneous flow structure around bubbles shows no clear coherence because of highly turbulent background; however, the frequency spectrum analysis finds that the bubbles reduce the kinetic energy in a frequency range higher than the bubble passing frequency.

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