he exchange of momentum, heat, and mass at an air-water interface is generally limited by the aqueous near-interface turbulence structure-where free-surface waves are ubiquitous. A novel measurement technique, digital particle tracking velocimetry (DPTV), was developed and used to make near-surface measurements of the velocity field in wavy and non-wavy open channel flows. These measurements include the wave-induced mean flow, near-surface mean and oscillatory viscous boundary layers, and near-surface turbulence structure.;The measurements of the wave-induced mean flows demonstrate that laboratory waves are rotational waves, inducing a depth decaying negative Eulerian-mean velocity that, near the free-surface, is exactly the negative of the Stokes' drift. Laser Doppler anemometry measurements indicate that this velocity deficit is transported at the waves' group velocity and is not a consequence of retrograde currents induced by the zero-net mass transport requirement. Lagrangian surface drift measurements indicate that the wave-induced surface drift is zero for clean free-surfaces, contradicting Stokes' theory, which predicts that waves induce the Stokes' drift. The measured mean properties of laboratory generated waves are better described as Gerstner waves, a rotational wave exactly satisfying the free-surface boundary condition, than Stokes waves.;The near-surface measurements reveal the existence of a dual viscous boundary layer. The inner layer is oscillatory, extremely thin and satisfies the free-surface stress condition. The outer layer is spatially growing and is driven by the wave amplitude decay and resultant excess wave momentum (radiation stress).Turbulence measurements indicate that this mean stress is balanced by the Reynolds stress.;Turbulence measurements near a quiescent free-surface show the expected decay in
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