Date of Award

2014

Degree Type

Thesis

Degree Name

Master of Science in Oceanography

Department

Oceanography

First Advisor

Tetsu Hara

Abstract

At high wind speeds the drag coefficient, characterizing the momentum transfer at the ocean surface, is known to be lower than extrapolation of the existing bulk parameterizations, which were derived at low to medium wind speeds. We hypothesize that sea spray may be responsible for the reduction of the drag, and investigate its effect through direct numerical simulations (DNS). The Lattice Boltzmann method (LBM) is coupled to a Lagrangian particle tracking approach to model numerically the dispersion of sea spray droplets in air turbulence near the air-sea interface during hurricanes and other strong wind events. Our results suggest that the turbulent vortices present near the boundary are damped and/or broken down by the passage of the particles, and that the turbulent Reynolds stress and the production of the turbulent kinetic energy are decreased by the particles. Our results generally agree with previous studies on the turbulence modulation by particles. The streamwise component of the velocity fluctuations is increased, while the spanwise and wall-normal components are decreased compared to the clean channel case. The Reynolds force and the viscous force are reduced and are replaced by the particle feedback force in the force balance. These findings suggest that the sea spray may play an important role in modifying the near surface turbulence during high wind speed events. Our results show that the mean streamwise velocity of the carrier phase is slightly reduced in the logarithmic layer when particles are added to the flow, contrary to the findings in previous studies. Therefore, the particle effect on the drag coefficient remains unclear.

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