Experimental and numerical investigations of premixed flame propagation behaviour associated with vortex interactions due to planar pressure waves crossing a curved flame front have been carried out. The resulting "tulip flame" formation in such a closed tube has been studied by Schlieren visualization. The "tulip flame" phenomenon was observed only in closed tubes, while cellular flame Fronts appeared in half-open tubes. A physical model has been developed and implemented in a discrete vortex method combined with a flame tracking algorithm. The numerical method has been applied to model and understand the processes that cause the flame to change from a curved to a tulip shape. The results of the simulation are in good agreement with the experimental observations. We find that the rotational flows causing the tulip formation in our experimental case originate from the baroclinic effect - an interaction of non-parallel density and pressure gradients. Pressure waves were generated ahead of the accelerating and highly turbulent flame front. In closed tubes the pressure waves were reflected and crossed the curved flame Front. As a result we saw the "tulip flame". Within 0.5 ms, the flame front velocity reversed from about 50 m/s to about -20 m/s.