A Direct Numerical Simulation (DNS) of the scalar advection equation was used to compute the transfer coefficient for the passive scalar transfer across a shear-free gas-liquid interface. The liquid velocities used in the simulation were derived from a Digital Particle Image Velocimetry (DPIV) technique applied to an experimental channel flow. Essentially, this work combines two well proven tools, DPIV and DNS, to estimate the mass transfer coefficient across a boundary.
Specifically, two-dimensional DPIV experiments were used to provide streamwise and spanwise velocities at the free surface, from which were computed the vertical velocity gradient. These velocities, and their Taylor Series expansion normal to the interface, were used to solve the fully three-dimensional, time instantaneous concentration equation in the liquid, similar to the work in McCready et al. (1986), but not limited to two dimensions and with real velocity data.