Free-surface turbulence and air-water gas exchange

Abstract

This thesis investigates the physical mechanisms of air{water gas transfer through direct measurements of turbulence at the air{water interface. To enable this study, a new approach to the particle image velocimetry (PIV) technique is developed in order to quantify free- surface °ows. Two aspects of this work are innovative. First, the use of a three-dimensional laser light cone and optical ¯ltering of the camera allow for the motion of °uorescent °ow tracers at the water surface to be isolated and measured. Validation experiments indicate that this measurement re°ects the °uid motion within the upper few hundred microns. A key bene¯t to this approach is the ability to deal with deforming surfaces, provided the amplitudes are not prohibitively large. This feature was used in this thesis to explore the surface °ow induced by mechanically generated waves. Second, a new hybrid PIV image processing algorithm was developed that provides high accuracy velocity estimation with improved computational e±ciency. This algorithm combines the concepts of dynamic Fourier-domain cross-correlation with a localized direct multiplicative correlation. In order to explore relationships between free-surface hydrodynamics and air{water gas transfer, an oscillating grid-stirred tank was constructed. By its design, this tank can be managed for chemical cleanliness, o®ers an unobstructed free surface, and is suited for turbulent mixing and air{water gas-exchange studies. A series of acoustic Doppler velocimeter, PIV, and infrared imaging experiments are presented that characterize the °ow for the grid forcing conditions studied. Results indicate that the °ows are stationary and reasonably repeatable. In addition, the °ows exhibit near-isotropic turbulence and are quasi- homogeneous in horizontal planes. Secondary circulations are revealed and investigated. Finally, PIV measurements of free-surface turbulence are performed with concurrent measurements of gas transfer in the grid tank for a range of turbulent mixing and surface conditions. Surface turbulence, vorticity, and divergence are all a®ected by the presence of a surface ¯lm, with signi¯cant e®ects realized for relatively small surface pressures. Results show that while a relationship between surface turbulence and the gas-transfer velocity is an obvious improvement over that found using an estimate of the bulk °ow turbulence, this relationship is dependent on the °ow regime. This is revealed through additional surface wave studies. However, the data from both the wave experiments and the grid turbulence experiments can be reconciled by a single relationship between the gas-transfer velocity and the 1/2-power of the surface divergence, which agrees with previous conceptual models. These results (1) further our understanding of interfacial transport processes, (2) demonstrate the important role of surface divergence in air{water gas exchange, and (3) relate, in a physically meaningful way, the interactions between surface renewal, surfactants, and gas transfer.