Comparison of oil slick and drifter motion at OHMSETT wave tank

In the aftermath of the Deepwater Horizon oil spill, thousands of ocean drifters were deployed in order to better understand the pathways and dispersion processes that drive oil slicks from the deep ocean to the coastline. Most of the drogued drifters released had a 0.60 m draft, hundreds of undrogued drifters had a 0.05 m draft, and thousands of drifting bamboo plates had a draft of 0.017 m. But typical slick thickness is 0.01 m or less, which raised an important question to interpret the ocean datasets correctly: how drifters move compared to oil slicks? The experimental study presented here addresses this issue by quantifying the differences, in terms of advection and diffusion, between the motion of oil slicks and the three groups of drifters mentioned above. Oil and drifters were released side-by-side in a vertically sheared flow controlled by waves produced in the National Oil Spill Response Research and Renewable Energy Test Facility (OHMSETT) wave-tank in April 2016. Trajectories were recorded by an aerial camera with spatial accuracy of 1.5 cm. The trajectories of each material patch were analyzed in details to determine how fast they were advected, but also how fast they spread and in which direction.

深海地平线（Deepwater Horizon）漏油事故发生后，为更精准地解析原油浮油从深海向海岸运移的路径与扩散过程，研究人员部署了数千台海洋漂流浮标（ocean drifters）。本次投放的载体中，多数带锚定装置的漂流浮标（drogued drifters）吃水深度为0.60米，数百台无锚定装置的漂流浮标（undrogued drifters）吃水深度为0.05米，另有数千片漂流竹板的吃水深度为0.017米。但原油浮油的典型厚度仅为0.01米及以下，这一参数差异为海洋观测数据集的正确解读提出了关键问题：漂流浮标的运动与原油浮油的运动存在何种差异？本项实验研究针对这一问题展开，通过量化平流（advection）与扩散（diffusion）维度的差异，对比了原油浮油与前述三类漂流载体的运动特征。2016年4月，研究人员于美国国家溢油应急响应研究与可再生能源试验设施（National Oil Spill Response Research and Renewable Energy Test Facility，缩写OHMSETT）的波浪水槽中，在由波浪调控的垂直剪切流环境内，同步投放了原油与三类漂流载体。实验采用空间精度达1.5厘米的航空相机记录各载体的运动轨迹。研究人员对每一类物质斑块的轨迹展开了精细化分析，以明确其对流运移速率、扩散速率及扩散方向。