Physical Oceanography

Physical oceanography is a major sub-division of oceanography that focuses on the study of physical processes in oceanic water systems. We use highly advanced CFD tools to conduct numerical and theoretical study in this area. Our numerical models can capture many of the key processes at the ocean surface.

Turbulence in the upper ocean is crucial to the transport of momentum, mass, and heat. Notably, Langmuir turbulence, often present when the wind blows over surface waves, is considered one of the most common types of upper-ocean turbulence.  Langmuir turbulence is characterized by the presence of an array of long and counter-rotating vortical structures under the water surface. First studied by Langmuir (1938), these underwater motions are referred to as Langmuir circulations. We study the effects of water surface waves on the vorticity in the turbulence underneath for Langmuir turbulence using wave-phase-resolved large-eddy simulation. The simulations are performed on a dynamically evolving wave-surface-fitted grid such that the phase-resolved wave motions and their effects on the turbulence are explicitly captured. Our study focuses on the vorticity structures and dynamics in Langmuir turbulence driven by a steady and co-aligned progressive wave and surface shear stress. For the first time, the detailed vorticity dynamics of the wave–turbulence interaction in Langmuir turbulence in a wave-phase-resolved frame is revealed. We can also predict the aggregation of surface particles due to Langmuir cells generated from the interaction between nonlinear water waves and underwater turbulence

Video file
Vortices and windrows induced by Langmuir circulation

 

Video file
Formation of windrows at the sea surface due to Langmuir circulation under JONSWAP waves

Langmuir circulations (LCs) generated by the interaction between wind-driven currents and surface waves can engulf the whole water column in neutrally stratified shallow water and interact with the turbulence in the bottom boundary layer. We also perform a mechanistic study using wall-resolved large-eddy simulations (LES) based on the Craik–Leibovich equations to investigate the effects of LCs on turbulence statistics in the bottom half of shallow water. It is found that the logarithmic layer disrupted at Reτ = 395 reappears at Reτ = 1000, but the von Kármán constant is slightly different from the traditional value 0.41.

Langmuir cells in shallow water
Langmuir cells engulfing the whole water column in shallow water
LC_mean
Effect of Reynolds number on (a) mean velocity  and (b) diagnostic function in the bottom half of the shallow-water Langmuir turbulence with Reτ = 1000, 700 and 395, respectively. 

 

Submesoscale oceanic structures with horizontal scales of O(0.1–10 km) and vertical scales of O(0.01–1 km), such as fronts and filaments, play an important role in air-sea interactions, affecting the turbulent transport and fluxes of momentum, energy, heat, and materials.  Despite the importance, the modeling ability of the submesoscale processes is still limited. We have used the high-fidelity LES tool developed in house to simulate the upper-ocean turbulence modified by the submesoscale structures.  The figure below shows an example of our LES of a submesoscale cold filament in a domain of 10 km × 2 km.  The water temperature shows a warm-cold-warm distribution in the surface layer.  Streak-like turbulent structures can be seen from the surface velocity.  The density of the streaks is distinctly different across the filament, and the characteristic scales of turbulence are also found to vary with time, indicating the importance of inter-scale interaction in the dynamics of the turbulence in the front region. 

Front
LES of a cold filament.

 

Selected Publications:

  • Xuan, A., Deng, B. & Shen, L. (2020), “Numerical study of effect of wave phase on Reynolds stresses and turbulent kinetic energy in Langmuir turbulence,” Journal of Fluid Mechanics, Vol. 904, A17.
  • Deng, B., Yang, Z., Xuan, A. & Shen, L. (2020), “Localizing effect of Langmuir circulations on small-scale turbulence in shallow water,” Journal of Fluid Mechanics, Vol. 893, A6.
  • Xuan, A., Deng, B. & Shen, L. (2019), “Study of wave effect on vorticity in Langmuir turbulence using wave-phase-resolved large-eddy simulation,” Journal of Fluid Mechanics, Vol. 875, pp.173-224.
  • Deng, B., Yang, Z., Xuan, A. & Shen, L. (2019), “Influence of Langmuir circulations on turbulence in the bottom boundary of shallow water,” Journal of Fluid Mechanics, Vol. 861, pp.275-308.