  The early stages of hydrogen-air flame dynamics and the physical mechanism of
tulip flame formation were studied using high-resolution numerical simulations
to solve the two-dimensional fully compressible Navier-Stokes equations coupled
with a one-step chemical model, which was calibrated to obtain the correct the
laminar flame velocity-pressure dependence. The formation of tulip flames was
investigated for a flame ignited by a spark or by a planar ignition and
propagating to the opposite closed or open end. For a flame ignited by a spark
on-axis at the closed end of the tube and propagating to the opposite closed or
open end, a tulip flame is created by a tulip-shaped axial velocity profile in
the unburned gas flow near the flame front caused by the rarefaction wave(s)
created by the flame during the deceleration stage(s). It is shown that, in a
tube with both closed ends, this mechanism of tulip flame formation also holds
for flames initiated by planar ignition. The deceleration stages in the case of
planar ignition are caused by collisions of the flame front with pressure waves
reflected from the opposite end of the tube. In the case of a flame initiated
by planar ignition and propagating toward the open end, the mechanism of tulip
flame formation is related to the stretching of the flame skirt edges backward
along the side wall of the tube due to wall friction, which leads to the
formation of bulges in the flame front near the tube walls. The bulges grow and
finally meet at the axis of the tube, forming a tulip-shaped flame. Regardless
of the method of flame initiation at the closed end, no distorted tulip flame
is formed when the flame propagates to the open end of the tube.
