LES of suspended particle dispersion in the ocean mixed layer
Responsible: Marcus Herold
Project type: Funded by the ​DAAD within the programme "Hochschulpartnerschaften mit Ostasien" (HOST) and by ​KOSEF
Duration: 01/01/2003 - 31/12/2003

This project examines the influence of particle inertia on mean concentration profiles and settling velocity in the ocean mixed layer (OML). Recently, PALM has been used to examine the effects of wave breaking and langmuir circlulation in the OML (Noh, Min & Raasch, 2004?). In the present study, the code is extended to simulate particle motion in a Lagrangian framework. Particles are released close to the surface to investigate their three-dimensional motion and the effects of wave breaking and Langmuir circulation. Therefore, the simulation setup is the same as in Noh, Min & Raasch (2004). A one-way coupling of fluid and particles is implemented, assuming a sufficiantly low volume fraction and size of the particles. In this case, Stokes drag can be assumed and the particle acceleration is determined by the difference of fluid and particles and the particle time scale tau_p only. tau_p and the terminal particle settling velocity w_s are used as independent parameters to characterize groups of particles.

First analyses show that vertical mixing of particles with slow settling velocities (e.g. w_s / u* = 0.01, 0.1) is dominated by the fluid motion and the significant influence of the Langmuir circulation rather than particle inertia. In this case, the mean profiles of particle concentration differ only slightly from the profiles of passive tracers. The particles become well mixed in the OML instead of sinking through the lower boundary. The results are similar for a whole range of Stokes numbers (St = tau_p / tau_f= 0.01, 0.1, 0.3, 1.0)
However, particles with w_s / u* = 1.0 are more independent of the fluid motion. Some particles are rapidly mixed downward by the Langmuir circulation, but the greater part reaches the terminal settling velocity w_s. Profiles show a distinct concentration maximum which broadens and descends with time.