Two-dimensional numerical simulations of detonation deflagration transition (DDT) in hydrogen-air mixtures are presented and compared with experiments. The investigated geometry was a 3 meter long square channel. One end was closed and had a single obstacle placed 1 meter from the end, and the other end was open to the atmosphere. The mixture was ignited at the closed end. Experiments and simulations showed that DDT occurred within 1 meter behind the obstacle. The onset of detonation followed a series of local explosions occurring far behind the leading edge of the flame in a layer of unburned reactants between the flame and the walls. A local explosion was also seen in the experiments, and the pressure records indicated that there may have been more. Furthermore, local explosions were observed in the experiments and simulations which did not detonate. The explosions should have sufficient strength and should explode in a layer of sufficient height to result in a detonation. The numerical resolution was 0.5 mm per square cell, and further details of the combustion model used are provided in the paper.