Experimental characterization of the agitation generated by bubbles rising at high Reynolds number

An experimental investigation of the flow generated by a homogeneous population of bubbles rising in water is reported for three different bubble diameters (d = 1.6, 2.1 and 2.5 mm) and moderate gas volume fractions (0.005 <= alpha <= 0.1). The Reynolds numbers, Re = V(0)d/v, based on the rise velocity V-0 of a single bubble range between 500 and 800. Velocity statistics of both the bubbles and the liquid phase are determined within the homogeneous bubble swarm by means of optical probes and laser Doppler anemometry. Also, the decaying agitation that takes place in the liquid just after the passage of the bubble swarm is investigated from high-speed particle image velocimetry measurements. Concerning the bubbles, the average velocity is found to evolve as V-0 alpha(-0.1) whereas the velocity fluctuations are observed to be almost independent or a. Concerning the liquid fluctuations, the probability density functions adopt a self-similar behaviour when the gas Volume fraction is varied, the characteristic velocity scaling as V-0 alpha(0.4). The spectra of horizontal and vertical liquid velocity fluctuations are obtained with a resolution of 0.6 mm. The integral length scale Lambda is found to be proportional to V-0(2)/g or equivalently to d/C-d0, where g is the gravity acceleration and C-d0 the drag coefficient of a single rising bubble. Normalized by using Lambda, the spectra are independent on both the bubble diameter and the volume fraction. At large scales, the spectral energy density evolves as the power -3 of the wavenumber. This range starts approximately from A and is followed for scales smaller than Lambda/4 by a classic -5/3 power law; Although the Kolmogorov microscale is smaller than the measurement resolution, the dissipation rate is however obtained from the decay of the kinetic energy after the passage of the bubbles. It is found to scale as alpha V-0.9(0)3/Lambda. The major characteristics of the agitation are thus expressed as functions of the characteristics of a single rising bubble. Altogether, these results provide a rather complete description of the bubble-induced turbulence.