Two-dimensional particle image velocimetry (PIV) was applied to study hydrodynamics in a mixing tank at medium Reynolds numbers. Post-processing of PIV data was based on proper orthogonal decomposition (POD) which enabled to reconstruct organized motion induced by impeller blades without performing angle-averaged sampling. Energy transfers between POD modes were analyzed. In particular, a local analysis of the modal interactions was developed in terms of energy transfer between the first mode, i.e., mean flow, and successive modes, i.e., organized motion. Eventually, derivation of transfer of energy between the second mode and fourth or fifth ones reveals a double pulsation.