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Langmuir turbulence consists of Langmuir circulation (LC) generated at the surface of rivers, lakes, bays, and oceans by the interaction between the wind-driven shear and surface gravity waves. In homogeneous shallow water, LC can extend to the bottom of the water column and interact with the bottom boundary layer. Large-eddy simulation (LES) of LC in shallow water was performed with the finite volume method and various forms of subgrid-scale (SGS) model characterized by different near-wall treatments of the SGS eddy viscosity. The wave forcing relative to wind forcing in the LES was set following the field measurements of full-depth LC during the presence of LC engulfing a water column 15 m in depth in the coastal ocean, reported in the literature. It is found that the SGS model can greatly impact the structure of LC in the lower half of the water column. Results are evaluated in terms of (1) the Langmuir turbulence velocity statistics and (2) the lateral (crosswind) length scale and overall cell structure of LC. LES with an eddy viscosity with velocity scale in terms of S and Ω (where S is the norm of the strain rate tensor and Ω is the norm of the vorticity tensor) and a Van Driest wall damping function (referred to as the S-Omega model) is found to provide best agreement with pseudo-spectral LES in terms of the lateral length scale and overall cell structure of LC. Two other SGS models, namely the dynamic Smagorinsky model and the wall-adapting local-eddy viscosity model are found to provide less agreement with pseudo- spectral LES, for example, as they lead to less coherent bottom convergence of the cells and weaker associ ated upward transport of slow downwind moving fluid. Finally, LES with the S-Omega SGS model is also found to lead to good agreement with physical measurements of LC in the coastal ocean in terms of Langmuir turbulence decay during periods of surface heating
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Lagrangian subgrid-scale modeling applied to evolving firebrand particle transport
The transport and deposition of firebrand particles is an important fire spread mechanism in wildland fires. These particles can be transported by wind over large distances and can ignite secondary fires upon landing. The transport of firebrands by wind is a complex, multiscale process that is largely controlled by interactions between the firebrand particles and the atmospheric wind. To account for the complex temporal evolution of atmospheric turbulence over large scales, the use of large-eddy simulation (LES) techniques is necessary. However, filtering of subgrid-scale (SGS) turbulence in LES hinders the accuracy of particle transport models. In this work, we employ a Lagrangian SGS model in an LES framework to investigate the effects of small-scale turbulence on the transport of mass- and size-changing firebrand particles. The impact of SGS turbulence was analyzed by comparing landing and trajectory statistics for firebrand and regular (fixed size and mass) particles under different Stokes numbers. It was found that the presence of SGS turbulence modifies the particle transport behavior, which is characterized by smaller spanwise dispersions but larger travel distances along the streamwise direction compared with particles under no SGS turbulence. As expected, the enhanced velocity field produced by the SGS model has larger influence on the statistics of firebrand particles compared with regular particles due to the time-evolving reduction in particle mass and size induced by pyrolysis.
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- Award ID(s):
- 2043103
- PAR ID:
- 10397584
- Publisher / Repository:
- Stanford University
- Date Published:
- Journal Name:
- Center for Turbulence Research Proceedings of the Summer Program 2022
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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