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Abstract: The research project presented below gives a detailed explanation of the phenomenon of an increased relative velocity between gas and liquid phase in bubbly flows. It points out the conditions under which classical models for the description of two-phase flows tend to predict higher gas hold-ups than those determined experimentally, and explains the underlying physical relations. The cause for the increased relative velocity is an interactive effect of the bubble wakes on the shapes and trajectories of other bubbles. The experimental facilities of this research project which allowed the simultaneous observation of integral and local effects under a broad range of operating conditions are described in detail. p]A model for the calculation of the relative velocity in homogeneous bubbly two-phase flows is presented. It overcomes deficiencies of classical models which do not consider the mutual effects of swarm turbulence, gashold-up,bubble shape and size on the relative velocity. Based on observations of the shape and trajectories of single bubbles under swarm conditions ane xtension to the model, which includes further parameters of the liquid phase, is proposed.