Stochastic approach to free-surface turbulence

Abstract

Free surface turbulence is important to the transfer of heat and mass across an air-liquid interface, and to the remote sensing capabilities for ship movement. Velocity measurements near a free surface, however, are complicated by limitations on probe resolution and fluctuations of the free surface itself New approaches and techniques are used for the quantifications of free-surface turbulence. Free surface velocities were measured using an adaptation of streakline photography and particle image velocimetry (PIV) in a flume and an oscillating grid chamber. Quantitative analysis of the free-surface turbulence data requires a stochastic approach. However, auto-correlations revealed a pattern to the free-surface turbulence in an open-channel flow, on the scale of the outer dimensions of the flow field. These patterns are qualitatively related to turbulence patterns in the bulk of the flow. Turbulent frequency spectra in the oscillating grid chamber revealed the predominance of a −2 slope on a log-log plot. The spectra also have a transition to a −3 slope at higher frequencies, indicative of viscous dissipation. Ramifications of these measurements for air-water mass transfer are also discussed.