  The effects of pressure waves on the tulip flame formation in closed and
semi-open tubes were studied using numerical simulations of the fully
compressible Navier-Stokes equations coupled to a detailed chemical model for
stoichiometric hydrogen air mixture. Rarefaction waves generated by the
decelerating flame are shown to be the primary physical process leading to the
flame front inversion and the tulip flame formation for spark and for planar
ignited flames in closed tubes. In the case of a spark ignited flame, the first
rarefaction wave is generated by the flame, which is decelerating due to the
reduction in flame surface area as the flame skirt touches the tube walls.
Flame collisions with pressure waves in a closed tube result in additional
deceleration stages and rarefaction waves that shorten the time of the tulip
flame formation.
