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We investigate generating mechanisms of atrial fibrillation (AF) based on numerical solutions of the FitzHugh-Nagumo equations. In particular, the interaction of reentrant wavefronts with obstacles, modeled as tissue with gradually reduced excitability, is presented. We show that with increasing modification strength, the spatio-temporal characteristics of the wave changes from functional to anatomical reentry. With decreasing distance of the obstacle to a non-conducting boundary, a transition is observed from a stable spiral to a transient reentrant wave with two to three reentries up to a suppression of reentries. We further study the possibility to generate irregular, fibrillatory patterns by the perturbation of regularly paced waves by a second pacemaker. The irregularity depends on the perturbation frequency and the geometry of the simulation area and is quantified in terms of a Shannon entropy.