  Recent complex-chemistry direct numerical simulations of lean hydrogen-air
flames propagating in forced turbulence in a box were continued by
switching-off the turbulence forcing. Results show that a decrease in burning
velocity U_T(t), caused by the turbulence decay, is reversed when the
turbulence becomes weak and a peak of U_T(t) appears, with the peak magnitudes
and associated Karlovitz numbers being similar in two different cases. These
results (i) are attributed to activation of laminar flame instabilities, which
have been suppressed by intense turbulence, and (ii) are argued to indicate
that the instabilities can substantially affect U_T in sufficiently weak
turbulence only.
