| In this work, Direct Numerical Simulation (DNS) is used to qualitatively and quantitatively investigate the structure of turbulence near an interface, and how these structures promote mass transfer.; The canonical flow used in this work is that of open channel flow, that is, gravity-driven flow over a slightly inclined flat plate, bounded above by a free surface (e.g. a gas phase). To clarify the affect of these assumptions on channel flow, three different DNS were performed, each differing only by the boundary conditions employed at the free surface. In the first, or base, case, the clean, rigid slip-surface was imposed. In the second simulation, insoluble surfactants were introduced to the rigid slip-surface. In the last simulation, the rigid surface was replaced with a linearized version of the actual continuity of stress conditions. Surfactants were not present in the ‘linearized’ free surface.; In order to assess the affect of these boundary conditions on the liquid-side mass transfer coefficient, a passive scalar concentration field was coupled to these three simulations. The average diffusional flux at the surface was used to compute the mass transfer coefficients.; The passive concentration field was also simulated using experimental velocity data, measured by Kumar et. a1.[22]. The experimental velocity data consisted of a time series of velocities measured from an experiment, and was used to drive the passive scalar equation. This numerical experiment spans the gap between the ‘fully’ simulated case (i.e. both velocity and passive scalar concentration) and the experimental results found in the literature.; The main result of this thesis is that the rigid slip-surface does not significantly change the characteristics of turbulence near the interface, especially at length scales important for mass transfer. This result is limited to flows without wind shear and with waves of low steepness.; The effect of surfactant concentration on the near interface turbulence is much more dramatic. The turbulent intensities at the interface are damped, and there is a modification of the turbulent energy transfer at the interface. For a clean surface, normal motions (i.e. upwellings), are redistributed into surface-parallel motions near the interface. For a contaminated surface, this energy transfer process is shifted further below the interface, resulting in less surface renewal. (Abstract shortened by UMI.)... |
