  The hydrodynamics of bubble columns in the heterogeneous regime is
investigated from experiments with bubbles at large particle Reynolds numbers
and without coalescence. The void fraction field $\varepsilon$ at small scales,
analyzed with Vorono\"i tessellations, corresponds to a Random Poisson Process
(RPP) in homogeneous conditions but it significantly differs from a RPP in the
heterogeneous regime. The distance to a RPP allows identifying meso-scale
structures, namely clusters, void regions and intermediate regions. A series of
arguments demonstrate that the bubble motion is driven by the dynamics of these
structures. Notably, bubbles in clusters (respectively in intermediate regions)
are moving up faster, up to 3.5 (respectively 2) times the terminal velocity,
than bubbles in void regions those absolute velocity equals the mean liquid
velocity. Besides, the mean unconditional relative velocity of bubbles is
recovered from mean relative velocities conditional to meso-scale structures,
weighted by the proportion of bubbles in each structure. Assuming
buoyancy-inertia equilibrium for each structure, the relative velocity is
related with the characteristic size and concentration of meso-scale
structures. By taking the latter quantities values at large gas superficial
velocities, a cartoon of the internal flow structure is proposed. Arguments are
put forward to help understanding why the relative velocity scales as
$(gD\varepsilon)^{1/2}$ (with $D$ the column's diameter and $g$ gravity's
acceleration). The proposed cartoon seems consistent with a fast-track
mechanism that, for the moderate Rouse numbers studied, leads to liquid
velocity fluctuations proportional to the relative velocity. The potential
impact of coalescence on the above analysis is also commented.
