  In this study, we focus on Langmuir turbulence in the deep ocean with the
presence of a large macroalgal farm using a Large Eddy Simulation method. The
wave-current interactions are modelled by solving the wave-averaged equations.
The hydrodynamic process over the farm is found to drive a persistent flow
pattern similar to Langmuir circulations but is locked in space across the
farm. These secondary circulations are generated because the cross-stream shear
produced by the rows of canopy elements leads to a steady vertical vorticity
field, which is then rotated to the downstream direction under the effect of
vortex force. Since the driving mechanism is similar to the CraikLeibovich type
2 instability theory, these secondary circulations are also termed as attached
Langmuir circulations. We then apply a triple decomposition on the flow field
to unveil the underlying kinematics and energy transfer between the mean flow,
the secondary flow resulting from the farm drag, and the transient eddies. Flow
visualizations and statistics suggest that the attached Langmuir circulations
result from the adjustment of the upper ocean mixed layer to the macroalgal
farm, and they will weaken (if not disappear) when the flow reaches an
equilibrium state within the farm. The tripledecomposed energy budgets reveal
that the energy of the secondary flow is transferred from the mean flow under
the action of canopy drag, while the transient eddies feed on wave energy
transferred by the Stokes drift and energy conversion from the secondary flow.

Fluid Dynamics;Atmospheric and Oceanic Physics