Entrainment of Oil Droplets, View Looking Downward on Ocean Surface

Описание: This animation shows the entrainment of the oil droplets looking down at the ocean surface. The results are based on Case 1. The numerical results are periodically extended to double the lengths of the computational domain along the x- and y-axes such that the animation is four wavelengths long and two wavelengths wide. The wind is moving from left to right along the x-axis. The frame of reference is moving with the linear phase speed of the primary wave.

The entrainment of oil droplets due to a 20m spilling breaking wave is considered. At t = 1000, the free surface is seeded with 10 classes of droplets that are uniformly distributed. The diameters of the droplets are D=1, 10, 50, 100, 200, 300, 400, 500, 600, and 1000 microns. There are 8192 droplets in each class. Red and blue colors are used to denote the smallest and largest droplets, respectively. The Lagrangian motion is tracked for 100 non-dimensional units of time (approximately 16 linear wave periods). The Reynolds number of the large-scale flow is Re = 1.49x10^7. The density of oil to water is 0.84. The ratio of the kinematic viscosity of air to water is 12.2.

The oil droplets are initially entrained in streaks beneath the ocean surface. Some of the smallest droplets aerosolize as they get launched by spilling breaking waves. The streaks slowly merge to form a single plume of droplets in the unextended computational domain, which in not shown in this view. The plume is slightly inclined to the axis of the wind. The angle of inclination is 14 degrees, which is within Smith’s (2001) observed range of 5-15 degrees. Here, Coriolis effects are not modeled that could otherwise trigger the angle of inclination one way or the other.

The plume of oil droplets forms in downwelling regions, whereas the droplets are cleared out in up-welling regions. The formation of a single Langmuir cell is consistent with the final state of an inverse energy cascade in a finite-size domain (Burgess and Scott, 2017; Boffetta and Ecke, 2012).