  Deflagration-to-detonation transition (DDT) can occur in environments ranging
from experimental and industrial systems to astrophysical thermonuclear (type
Ia) supernovae explosions. Substantial progress has been made in explaining the
nature of DDT in confined systems with walls, internal obstacles, or
pre-existing shocks. It remains unclear, however, whether DDT can occur in
unconfined media. Here we use direct numerical simulations (DNS) to show that
for high enough turbulent intensities unconfined, subsonic, premixed, turbulent
flames are inherently unstable to DDT. The associated mechanism, based on the
nonsteady evolution of flames faster than the Chapman-Jouguet deflagrations, is
qualitatively different from the traditionally suggested spontaneous reaction
wave model, and thus does not require the formation of distributed flames.
Critical turbulent flame speeds, predicted by this mechanism for the onset of
DDT, are in agreement with DNS results.
