On wave damping occurring during source-based generation of steep waves in deep and near-shallow water

摘要

Generation of steep waves (H/lambda > 0.03) in a NSE-based NWT is a challenging task and is seldom attempted in near-shallow water (kh < 1.0). Failure towards attainment of the target steepness is characterized by an under prediction of the wave height (H) along the length of the NWT. The issue of height damping has received limited attention in literature. In this context, a NSE based NWT has been developed using a PLIC-VOF formulation and a mass source-based wave generator. Wave damping at the NWT boundaries has been achieved using sponge layers. A total of nine wave designs have been simulated in three categories each of steepness (low, moderate and large) and relative depth (near-shallow, intermediate and deep). Criteria for selecting spatio-temporal resolution within the tank have been determined through parametric analysis. For H/lambda > 0.03, it is seen that wave height reduction in excess of 5% occurs in near-shallow and deep water. It is found that height reduction in deep water is largely attributable to free-surface damping as predicted by Airy theory. However, numerical height damping in near-shallow water is observed to be much stronger, especially near the source. A deeper analysis reveals that mass source-based wave generation induces strong and persistent vortical activity in near-shallow water which in turn leads to dissipation of source-injected momentum. Appropriate modifications in source design, that are aimed at reducing wave-vorticity interactions in the near-field, are proposed for steep wave generation in kh < 1 and kh > 2.5. Substantial improvement in the prediction of far field wave height is hence reported.