Obstacles in the path of a flame lead to a significant increase of the turbulence in the expansion flow in front of the flame and therefore, have a strong influence on the combustion process itself. The present paper deals with the aspect of flame quenching due to the turbulence induced by one highly blocking obstacle in hydrogen-air mixtures and the resulting acceleration of the flame. In case of sensitive mixtures it is possible to obtain a transition from the defiagrative to the detonative combustion mode (DDT). Several experiments were performed in facilities of different dimension in order to investigate the physical phenomena for the jet ignition behind the obstacle. Non-intrusively optical measurement techniques were used to obtain information on the flow field, the turbulence intensity in the expansion flow directly in front of the flame, and the structure of the flame itself.