Turbulent transport of a high-Schmidt-number scalar near an air-water interface

We measure solute transport near a turbulent air-water interface at which there is zero mean shear. The interface is stirred by high-Reynolds-number homogeneous isotropic turbulence generated far below the surface, and solute transport into the water is driven by an imposed concentration gradient. The air-water interface is held at a constant concentration much higher than that in the bulk of the water by maintaining pure CO2 gas above a water tank that has been initially purged of dissolved CO2. We measure velocity and concentration fluctuations below the air-water interface, from the viscous sublayer to the middle of the 'source region' where the effects of the surface are first felt. Our laboratory measurement technique uses quantitative imaging to collect simultaneous concentration and velocity fields, which are measured at a resolution that reveals the dynamics in the turbulent inertial subrange. Two-point statistics reveal the spatial structure of velocity and concentration fluctuations, and are examined as a function of depth beneath the air-water interface. There is a clear dominance of large scales at all depths for all quantities, but the relative importance of scales changes markedly with proximity to the interface. Quadrant analysis of the turbulent scalar flux shows a four-way balance of flux components far from the interface, which near the interface evolves into a two-way balance between motions that are raising and lowering parcels of low-concentration fluid.