The paper presents results of Direct Numerical Simulations of bubbles rising in a vertical channel flow configuration for a dilute and for a denser swarm. The bubbles, considered as spherical objects, are simulated by means of an Immersed Boundary Method and the channel dimensions are chosen in order to address large-scale flow features. The interaction between the bubbles and the fluid phase is investigated with instantaneous flow visualizations and a detailed statistical analysis for both phases addressing single-point statistics, two-point correlation functions and pair correlation functions. Elongated flow structures in the streamwise direction induced by the bubbles are found in both cases but are somewhat larger and present a higher turbulence level in the denser swarm. For the chosen parameter range turbulence enhancement due to the bubbles is observed. The small-scale interaction of bubbles is investigated yielding different results for the two cases: in the dilute swarm, the aspiration mechanism of a trailing bubble yields a preferential vertical alignment while in the denser swarm, a pressure-related mechanism dominates due to the smaller bubble distance yielding a preferential horizontal alignment. The results reported here provide detailed and reference data for model development and validation. (C) 2015 Elsevier Ltd. All rights reserved.