The Structure of Langmuir Circulation in the Rapid Deepening Stage of the Mixed Layer
| Autor: | Chen, Shi-Ming, 陳世明 |
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| Rok vydání: | 2015 |
| Druh dokumentu: | 學位論文 ; thesis |
| Popis: | 103 An oceanic mixed layer of an observed storm event that is featuring growing wind/surface waves and a rapidly deepened mixed layer is realized with threedimensional large-eddy simulations to study interactions between Langmuir circulation and the mixed layer. Langmuir circulation in the simulations is parameterized with Craik-Leibovich equations that a term of vortex force in the momentum equations, representing the interaction of wind-induced shear currents and the wave fields, controls the strength of Langmuir circulation. Numerical experiments are conducted as well by varying the vortex force and strength of stratification for the reason to understand how the strength of vortex force and stratification influence the structures of Langmuir circulation, including an extreme scenario for a no wave condition. The simulation results show that strong vortex force induces more homogeneous mean velocities through the mixed layer, suggesting enhancement in turbulent mixing. In the perspective of turbulent kinetic energy (TKE), Langmuir circulation could intensify the TKE crosswind component near the water surface and also the vertical component at around 0.1 times mixed-layer depth. By analyzing TKE budget, reduced shear production and increased Stokes production are revealed near the water surface. It suggests the primary source of near surface TKE for the flows influenced by Langmuir circulation is Stokes shear, rather than wind-driven shear which is the main source for pure shear boundary layer flows. Langmuir circulation also enhances deepening of the mixed layer, and the extent of influence of Langmuir circulation on mixed-layerdeepening is negatively correlated to the strength of stratification. One predominant feature of Langmuir circulation is the streak of crosswind convergence at the water surface, which is generated by the underlying structure of counter-rotating vortex cells. Through the spectral analysis in terms of crosswind velocity near the water surface, the dominant streak spacing is quantified. Also a method combining the surface streak detection and conditional average algorithm is employed to evaluate the lateral width and penetration depth of a conditional-averaged Langmuir cell. For the cases of Langmuir-turbulence-dominant flows (turbulent Langmuir number Lat ranges from 0.3 to ∞ ), the dominant streak spacing approximates twice the lateral width of the Langmuir cell, indicating a pair of cells reside between successive dominant streaks. The quantified cell width and depth correlate to the mixed-layer depth since the pycnocline below the mixed layer can hinder penetration of the Langmuir cell. The Langmuir cell exhibits similarity of the cell width and depth when the mixed layer is deep from blocking the vertical extending of the cell. Other than the unconstrained condition, the cell could be squashed vertically by a very shallow mixed layer with extremely strong stratification. Thus it leads to a laterally stretched Langmuir cell with an aspect ratio (width to depth) larger than one. From the analyses of the numerical data, this study depicts the connection of surface streak signatures and the underlying Langmuir circulation structures, and the relation between Langmuir cells and the mixed layer in distinct conditions. It therefore suggests that there is no specific aspect ratio of the Langmuir cell and that the shape of the cell depends on the mixed-layer depth and the stratification in the pycnocline. |
| Databáze: | Networked Digital Library of Theses & Dissertations |
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