The effects of the wall boundary layer thickness on the development of an a
xisymmetric gas jet injected into a confined vertical water how were invest
igated. The variations in the wall boundary layer were made by using suctio
n at the wall through rectangular profiled slots. The water velocity around
the two-phase jet was studied for several boundary layer thickness values
by laser Doppler velocimetry. The gas jet outline was extracted by image pr
ocessing applied on visualisations for a wide variety of water, gas and suc
tion conditions. These comparisons shelved that the boundary layer has no i
nfluence on jet development. The data showed that the interactions between
flows near the injection do not develop downstream, which accounts for the
absence of the two classical contrarotating vortices in the medium field of
the jet. The influence of the gas pocket, and then of buoyancy, is predomi
nant over other phenomena.