| Summary: | 碩士 === 國立臺灣大學 === 應用力學研究所 === 105 === In this study, we employ large eddy simulation (LES) along with ghost cell immersed boundary method (GCIBM) and arbitrary Lagrangian-Eulerian scheme (ALE) to simulate local scour induced by flow over a wall-mounted structure. In order to simulate the instantaneous evolution of the bed form, we use LES to resolve the detailed flow features. The GCIBM is used to model the geometry associated with the complex solid boundaries without much computational effort. Moreover, the ALE scheme incorporates the grid velocity such that the domain boundary is able to follow the time-dependent bed form geometry.
The numerical study comprises four parts. The first part presents the steady current passing an idealized structure with the different shape. Our simulation results show that the sharp-nosed body effectively reduces the bed erosion. In the second part, simulations of the bed scour under field conditions are presented, by means of the law of similarity. The main erosion mechanism within the range of the field Keulegan-Carpenter number (KC) is due to lee-wake vortices. The third part considers the bed scour around a cylinder with the riprap protection. Results show that due to asymmetric motion of lee-wake vortices, scour is more vigorous at the left side of the riprap. The fourth part presents the simulation of scour during a long-wave period. When the wave period increases, the formation of horseshoe vortices around the cylinder is evident. As a result, the scour pattern is different from that in the small KC case. In the latter case, erosion mainly occurs at the left and right sides of the cylinder. In the former case, the scour hole forms around the cylinder, which appears to be a feature similar to the case of the steady current.
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