Benthic Boundary Layer/Sediment/Mud

The benthic boundary layer (BBL) is a layer above the bottom of water bodies.  The BBL plays an important role in the cycling of biogeochemically important elements, suspension of sediment, and deformation of the water bed.  As such, the BBL is regarded as an interdisciplinary endeavor among biologists, sedimentologists, and oceanographers.  However, these processes linking to the BBL are controlled by the fluid dynamics within the BBL.  We focus on the studies of the fluid dynamics of the BBL, sediment transport, and mud deformation beneath water waves using our high-fidelity numerical simulation.  

The interaction between wind-driven currents and shallow-water waves leads to the generation of full-depth Langmuir circulations, which are pairs of counter-rotating vortices along the streamwise direction.  Full-depth Langmuir circulations can significantly alter flow properties of the BBL, and hence challenge the laws and turbulence models in traditional turbulent BBL.  Using our high-fidelity large-eddy simulation, we study the BBL coexisting with full-depth Langmuir circulations.  We provide application ranges of the traditional log law of mean velocity in the BBL with full-depth Langmuir circulations and develop a new turbulence model for further simulations of full-depth Langmuir circulations based on Reynolds-averaged Naiver-Stokes equations.

LC and sediment
Full-depth Langmuir circulations
Full-depth Langmuir circulations (Deng, Yang, Xuan, and Shen, 2019, 2020, JFM).

Sediment within the BBL plays an important role in environmental sciences, physical oceanography, and coastal engineering.  The transport of sediment in coastal areas leads to the destabilization of sand bed, erosion of beaches, and decrease of navigation depth.  Wave breaking is known to be highly effective in suspending and transporting sediment.   While substantial progresses have been made in recent studies, it is still challenging to obtain a complete picture of the sediment transport, especially the small-scale dynamics associated with wave breaking.  We perform LES of sediment suspension and transport under plunging breaking waves.  The coupled level-set and volume-of-fluid method is used to capture the geometry of wave surface.  Our research reveals the underlying mechanism of the wave breaking suspending and transporting sediment. 

The sediment concentration beneath breaking waves
Sediment concentration beneath breaking waves around one quarter and one half of the wave period (Yang et al., Computers & Fluids,2017).

The interaction between water waves and muddy seafloor is of great importance in remoting sensing of nearshore regions and foundation of coastal structures.  The forcing of water waves can deform the lutoclines, and the Stokes drift can cause a horizontal transport of the mud, while a muddy layer can significantly attenuate water waves within several wavelengths.  We perform direct simulations of the Navier–Stokes equations for non-breaking water waves with finite amplitudes propagating over viscous fluid mud.  The air, water, and mud are treated as a coupled fluid system with different physical properties.  The air-water and water-mud interfaces are captured using level-set method. Our high-fidelity simulation provides details of the flow field, based on which the energy transport and transfer processes in the wave-mud interaction are elucidated.

The streamwise and vertical velocities in the water and mud
Streamwise and vertical velocities in the water and mud (Deng et al., Computers & Fluids,2017).

Selected Publications:

  • 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.
  • 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.
  • Yang, Z., Lu, X., Guo, X., Liu, Y. & Shen, L. (2017), “Numerical simulation of sediment suspension and transport under plunging breaking waves,” Computers and Fluids, Vol. 158, pp.57-71.
  • Deng, B., Hu, Y., Guo, X., Dalrymple, A. & Shen, L. (2017), “Numerical study on the dissipation of water waves over a viscous fluid-mud layer,” Computers and Fluids, Vol. 158, pp.107-119.