  The properties of bubble-laden turbulent flows at different scales are
investigated experimentally, focusing on the flow kinetic energy, energy
transfer, and extreme events. The experiments employed particle shadow
velocimetry measurements to measure the flow in a column generated by a
homogeneous 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 at
high-resolution, and used to construct structure functions up to twelfth order
for separations spanning the small to large scales in the flow. Concerning the
flow anisotropy, the velocity structure functions are found to differ for
separations in the vertical and horizontal directions of the flow, and the
cases with smaller bubbles are the most anisotropic, with a dependence on void
fraction. The degree of anisotropy is shown to increase as the order of the
structure functions is increased, showing that extreme events in the flow are
the most anisotropic. Our results show that the average energy transfer with
the horizontal velocity component is downscale, just as for the
three-dimensional single-phase turbulence. However, the energy transfer
associated with the vertical component of the fluid velocity is upscale. The
probability density functions of the velocity increments reveal that extreme
values become more probable with decreasing Reynolds number, the opposite of
the behaviour in single-phase turbulence. We visualize those extreme events and
find that regions of intense small scale velocity increments occur near the
turbulent/non-turbulent interface at the boundary of the bubble wake.
