Large-Eddy Simulations for Two Nearshore Applications
Abstract
With the growing computational power, Computational Fluid Dynamics (CFD) can be applied to 3D near-prototype scale nearshore simulations up to approximately O(100 m). The most important advantage is the well-resolved intra-wave and vertical flow field, as well as turbulent coherent structures via Large-Eddy Simulation (LES) in a shallow surf zone. LES can resolve large coherent structures and subsequent energy cascade and hence it is more accurate than Reynolds-averaged models, particularly for problems involving flow instabilities and vortices in which artificial separation of scale between a vortex and turbulent coherent structures is ambiguous. In this study, we apply LES to two nearshore applications. The first case is to use LES to study the cross-shore hydrodynamics under irregular waves in the surf and swash zones, and the second case applies LES to scour around a vertical pile under waves. Understanding of the interactions between the cross-shore hydrodynamics and complex beach and dune geometry under energetic random waves remains limited. Thus, we carry out 3D wall-modeled LES with free surface evolution resolved by the Volume of Fluid (VOF) method to investigate the cross-shore wave shoaling, breaking, and the undertow current in the surf and swash zones. The numerical model is validated with a near prototype-scale laboratory experiment (104 m in length, 3.7 m in width, and 4.6 m in height) conducted in the Large Wave Flume at Oregon State University. Model results show good agreement of free-surface elevation and flow velocity time series (index of agreement overall higher than 0.8) even in the swash zone. Other higher quantities, such as undertow, wave skewness and asymmetry are well captured by the model. High spatial resolution simulation data produced by LES can be used to calibrate reduced-complexity models, such as XBeach. Next, we adopt LES to simulate scour using a three-dimensional Eulerian-Eulerian two-phase flow solver, SedFoam. While the Reynolds-averaged SedFoam successfully models sheet flow and ripple evolution, our study shows that SedFoam underpredicts the horseshoe vortex around the vertical pile which leads to the underestimation of scour hole depth. By applying LES, the resolved horseshoe vortex and lee-wake vortex are better resolved, leading to improve the agreement with measured bed shear stress. Consequently, the scour hole depth prediction during the initial development stage is also improved. The two applications show that LES can be applied to near-prototype scales problems with promising results.
Figure Caption: SedFoam simulations of scour around a vertical cylinder driven by an oscillatory flow. Panel (a) shows the LES results, and the turbulent coherent structures are visualized by Q-method using iso-surface of Q=5000 (green iso-surface). The horseshoe vortex can be clearly observed. Panel (b) show the Reynolds-averaged k-ω model results and coherent structures are much weaker. Using a much lower Q value of Q=50 (blue iso-surface), the horseshoe vortex structure can barely be seen.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2022
- Bibcode:
- 2022AGUFMOS45A..04T