Results from an experimental study investigating the turbulent structure beneath the air-water interface during natural convection are reported. The two-dimensional velocity field beneath the surface in a plane perpendicular to the surface was measured using digital particle image velocimetry. The results show that the waterside flow field undergoes three-dimensional flow interactions forming complex flow patterns, which appear to be random. The magnitude of the turbulent velocities and turbulent kinetic energy increases with the heat flux. The profiles of the turbulent velocities are self-similar and appropriately scaled by the parameters proposed for the natural convection above a heated wall. The wave number and frequency spectra exhibit −3 slopes providing the evidence that during natural convection the buoyancy subrange exists within the inertial subrange where the energy loss is due to the work against buoyancy.

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We report on an experimental study conducted to investigate the airside flow structure above an evaporative water surface during natural convection. Two-dimensional airside velocity fields were measured using particle image velocimetry for three different surface heat flux conditions. Detailed analysis of the turbulent velocity fields shows a complex flow structure due the local interactions of fluid motions in vertical, horizontal, and normal directions. The trends of turbulent intensity profiles on airside and waterside are found to be similar. However, the airside turbulent intensities are approximately 20 times stronger than that on the waterside. The spectral analysis of the turbulent velocities showed the existence of two distinct power law regimes. In low wavenumber range, the buoyancy subrange is observed with a slope of −3 whereas, in high wavenumber range, the inertial subrange with the classical slope of −5 / 3 is observed. The results also indicate that the airside turbulent velocity fields control the local evaporation rate, which in turn influences the water surface temperature field and the waterside velocity field.

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