Modulation on turbulent puffs in pipe flows by dilute dispersed microbubbles*

We examined mutual interactions between a turbulent puff and microbubbles, where the maximum volume fraction is less than 0.018%, in a horizontal pipe flow at Re = 1,900. Forty trials to investigate flow status were performed at different perturbation amplitudes for both of single-phase and flows with the bubbles. The results indicated that adding bubbles enhances puff creation. The enhancement may be due to increases of local volume fraction. A tiny amount of bubbles is accumulated in vortical structures embedded in a puff, and the accumulated bubbles may reduce the area of the core, which leads to enhancing the vortices. To estimate the accumulation numerically, bubble motions in a puff were calculated by Euler-Lagrange simulations. Motion equations for spherical bubbles were solved by a one-way simulation, which was coupled with velocity fields of a puff obtained by DNS. Using a box counting method for bubbles, we confirmed that bubbles are accumulated in a puff. For indicating that bubble distributions in a puff can be modified relative to in laminar flows experimentally, the distributions are visualized using laser sheet and quantified by image processing.