  Experimental observations of high-energy surface melting processes, such as
laser welding, have revealed unsteady, often violent, motion of the free
surface of the melt pool. Surprisingly, no similar observations have been
reported in numerical simulation studies of such flows. Moreover, the published
simulation results fail to predict the post-solidification pool shape without
adapting non-physical values for input parameters, suggesting the neglect of
significant physics in the models employed. The experimentally observed violent
flow surface instabilities, scaling analyses for the occurrence of turbulence
in Marangoni driven flows, and the fact that in simulations transport
coefficients generally have to be increased by an order of magnitude to match
experimentally observed pool shapes, suggest the common assumption of laminar
flow in the pool may not hold, and that the flow is actually turbulent. Here,
we use direct numerical simulations (DNS) to investigate the role of turbulence
in laser melting of a steel alloy with surface active elements. Our results
reveal the presence of two competing vortices driven by thermocapillary forces
towards a local surface tension maximum. The jet away from this location at the
free surface, separating the two vortices, is found to be unstable and highly
oscillatory, indeed leading to turbulence-like flow in the pool. The resulting
additional heat transport, however, is insufficient to account for the observed
differences in pool shapes between experiment and simulations.
