The impact of air saturation on the flow structure beneath air–water interface during natural convection

We report on results from an experimental study conducted to investigate the impact of air saturation on the turbulent structure beneath air–water interface. Particle image velocimetry (PIV) was used to measure the two dimensional velocity fields beneath the water surface. The results show that air saturation has a significant impact on the waterside turbulent structure. As the air becomes saturated, the magnitudes of the horizontal and vertical turbulent velocities are decreased by factors of 5 and 2.5, respectively. It was argued that in addition to the surface heat flux, the waterside flow field is also influenced by the airside velocity via the air–water interface. The latent heat flux at the saturation state is not equal to zero. The film condensation on the tank walls at the saturation state is found to be responsible for the latent heat flux. The spectral analysis shows that both the horizontal and vertical turbulent motions are much weaker than that for the unsaturated case. For the saturated case, the range of the turbulent motions responsible for working against buoyancy forces is smaller than that for the unsaturated case. It was concluded that during natural convection, when the turbulence is relatively weak, in addition to the buoyancy subrange, another subrange exists within the inertial subrange, where the turbulent motions are simply convected by the buoyancy forces and that the energy interaction is highly dependent on the energy of the turbulent motions and the magnitude of the buoyancy forces.