A comprehensive laboratory investigation of Langmuir turbulence forced by a range of wind and wave conditions

A comprehensive study on the properties of Langmuir turbulence (LT) and its response to a variety of wind (U₁₀: 2 – 20 m/s) and wave (ak: 0 – 0.27, λ = 3.8 m) forcing conditions was conducted at the SUSTAIN facility, University of Miami, RSMAS. The dimensions of SUSTAIN wind-wave-current tunnel are 18L x 6W x 2H, uniquely allowing for freely forming LT unconstrained by side walls. The grid of investigated input parameters included multiple constant wind speeds, as well as multiple wave steepness levels (mechanically generated) within each constant wind speed, allowing for the separation of purely wave-driven effects. The response of LT to each pair of wind-wave forcing conditions was measured in steady state conditions (after 20+ minutes of settling time) by a suite of non-intrusive turbulence visualization techniques, located at ~10 m fetch. These included 3D visualization of water tracing dye entrainment by turbulence, underwater particle image velocimetry, as well as passive and active thermal imagery resolving surface skin temperature and velocity fluctuations. The resulting dataset includes both qualitative structural 3D view of Langmuir turbulence, as well as precise quantitative turbulent kinetic energy (TKE) measurements mapped out in response to the grid of input parameters. In slower winds, TKE was found to grow substantially in response to increasing wave steepness, in line with the expected effect of increasing vortex force. However, in higher winds the effect of increasing wave steepness on TKE was found to be negative. This unexpected behavior is attributed to the rise of airflow separation, effectively sheltering much of the water surface from the direct TKE production by wind friction.