  The paper examines the mechanism of the tulip flame formation for the flames
propagating in closed tubes of various aspect ratios and in a half-open tube.
The formation of tulip flames in 2D channels is studied using high resolution
direct numerical simulations of the reactive Navier Stokes equations coupled
with a detailed chemical model for a stoichiometric hydrogen/air flame. It is
shown that rarefaction waves generated by the flame during the deceleration
stage play a key role in the tulip-shaped flame formation. The interaction of
the reverse flow created behind the rarefaction wave with previously produced
be accelerating flame flow in the unburned gas, results in the decrease of the
flow velocity in the near field zone ahead of the flame and in the increase of
the boundary layer thickness. The profile of the axial velocity close ahead of
the flame takes the form of an inverted tulip. Therefore, the flame front
acquires a tulip shape repeating to a large extent the shape of the of the
axial velocity profile in the upstream flow.
