  Effect of surface tension reduction on wind-wave growth is investigated using
direct numerical simulation (DNS) of air-water two-phase turbulent flow. The
incompressible Navier-Stokes equations for air and water sides are solved using
an arbitrary Lagrangian-Eulerian method with boundary-fitted moving grids. The
wave growth of finite-amplitude and non-breaking gravity--capillary waves,
whose wavelength is less than 0.07 m, is simulated for two cases of different
surface tensions under a low--wind speed condition of several meters per
second. The results show that significant wave height for the smaller surface
tension case increases faster than that for the larger surface tension case.
Energy fluxes for gravity and capillary wave scales reveal that, when the
surface tension is reduced, the energy transfer from the significant gravity
waves to capillary waves decreases and the significant waves accumulate more
energy supplied by wind. This results in faster wave growth for the smaller
surface tension case. Effect on the scalar transfer across the air-water
interface is also investigated. The results show that the scalar transfer
coefficient on the water side decreases due to the surface tension reduction.
The decrease is caused by suppression of turbulence in the water side. In order
to support the conjecture, the surface tension effect is compared with
laboratory experiments in a small wind-wave tank.
