  This work describes an experimental investigation on the mixing induced by a
swarm of high Reynolds number air bubbles rising through a nearly homogeneous
and isotropic turbulent flow. The gas volume fraction $\alpha$ and the velocity
fluctuations $u'_0$ of the carrier flow before bubble injection are varied,
respectively, in the ranges $0\leq \alpha \leq 0.93\%$ and 2.3 cm/s $\leq u'_0
\leq 5.5$ cm/s, resulting in a variation of the bubblance parameter $b$ in the
range [0, 1.3] ($b = \frac{V_r^2 \alpha}{{u'}_0^2}$, where $V_r$ is the
relative rising velocity). Mixing in the horizontal direction can be modelled
as a diffusive process, with an effective diffusivity $D_{xx}$. Two different
diffusion regimes are observed experimentally, depending on the turbulence
intensity. At low turbulence levels, $D_{xx}$ increases with gas volume
fraction $\alpha$, while at high turbulence levels the enhancement in $D_{xx}$
is negligible. When normalizing by the time scale of successive bubble passage,
the effective diffusivity can be modelled as a sole function of the gas volume
fraction $\alpha^* \equiv \alpha/\alpha_c$, where $\alpha_c$ is a theoretically
estimated critical gas volume fraction. The present explorative study provides
insights into modeling the mixing induced by high Reynolds number bubbles in
turbulent flows.
