Characteristics of mean flow and turbulence in bubble-in-chain induced flows
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2019 American Physical Society. This paper presents the experimental results from an air bubble-in-chain induced flow at different bubble release frequencies, resulting in a range of 1510-2318 for the Reynolds number based on bubble parameters. Mean flow and turbulent statistics were measured using planar particle image velocimetry. The bubble rise velocities were validated to be the superposition of the mean vertical velocity of water and the terminal velocity of the isolated bubbles. We tested the scaling parameters for the bubble-in-chain induced flow using those typically for bubble plumes, which demonstrates the intrinsic similarities of these two bubbly flows. We found that the radial profiles of normalized mean vertical velocity for bubble-in-chain induced flows and bubble plumes collapse well using the center-line velocity and the half-width of the velocity profile. We found the half-width correlated with the characteristic length scales of the bubble wakes. The normalized Reynolds stresses using the local velocity square show the same shape of profiles for both bubbly flows, but they do not collapse onto universal curves. The normalized turbulent kinetic energy is a function of normalized gas flow rate and approaches an asymptotic value of 0.05. The dynamic length scale, a function of initial kinematic buoyancy flux, was found to govern turbulent dissipation rates in both bubbly flows. The normalized one-dimensional velocity energy spectra collapse in the regime of kv>0.1 where kv is the wave number and is the Kolmogorov length scale, showing a -3 slope as the result of energy balance between production from bubble motion and dissipation in the bubble wakes. The wake length scale was found to define the lower-end wave number of the -3 slope.
