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The properties of bubble-laden turbulent flows at different scales areinvestigated experimentally, focusing on the flow kinetic energy, energytransfer, and extreme events. The experiments employed particle shadowvelocimetry measurements to measure the flow in a column generated by ahomogeneous bubble swarm rising in water, for two different bubble diameters($2.7$ mm $\&$ $3.9$ mm) and moderate gas volume fractions($0.26\%\sim1.31\%$). The two velocity components were measured athigh-resolution, and used to construct structure functions up to twelfth orderfor separations spanning the small to large scales in the flow. Concerning theflow anisotropy, the velocity structure functions are found to differ forseparations in the vertical and horizontal directions of the flow, and thecases with smaller bubbles are the most anisotropic, with a dependence on voidfraction. The degree of anisotropy is shown to increase as the order of thestructure functions is increased, showing that extreme events in the flow arethe most anisotropic. Our results show that the average energy transfer withthe horizontal velocity component is downscale, just as for thethree-dimensional single-phase turbulence. However, the energy transferassociated with the vertical component of the fluid velocity is upscale. Theprobability density functions of the velocity increments reveal that extremevalues become more probable with decreasing Reynolds number, the opposite ofthe behaviour in single-phase turbulence. We visualize those extreme events andfind that regions of intense small scale velocity increments occur near theturbulent/non-turbulent interface at the boundary of the bubble wake.

An experimental study on the multiscale properties of turbulence in bubble-laden flows