Effects of ventilations on coastal structures with overhangs subject to wave impacts

Abstract

Coastal structures with horizontal overhangs are built due to design constraints, but wave loadings substantially increase under these confined geometries. Vertical structure elements, such as steel gates, are vulnerable to damage caused by impulsive wave impacts, potentially exposing the coastal zone to flooding and erosion. Existing formulas to determine impulsive loadings in engineering practice are limited to purely vertical structures. Research has shown that openings along the surface of structures relieve wave impact pressures, but there are currently no design methods available to quantify this pressure release. The stochastic nature of impulsive impacts and uncertain influence of air adds to the problem complexity. This study aims to investigate the influence of ventilations to reduce wave impact loadings on vertical structures with horizontal overhangs, applying a theoretical pressure-impulse approach and computational fluid dynamics. In this study, the pressure-impulse model is implemented for experimental cases in two and three dimensions using a finite difference numerical scheme and validated against semi-analytical solutions with high accuracy. Boundary conditions are modified to include and assess the influence of venting holes on the pressure-impulse contours. To achieve the largest efficiency in the reduction of pressure-impulses, rectangular ventilations are located at the critical corner between vertical wall and overhang and spaced across the structure width. Based on physical model dimensions, the open source CFD software OpenFOAM is also employed to simulate standing wave impacts on structures. Waves are generated in the CFD model using the waves2Foam toolbox, in conjunction with the OceanWave3D utility. Convergence of the CFD model is achieved by gradually refining the mesh near the wave impact region. Validation between simulated and experimental total wave forces on the vertical wall shows very good agreement for both overhang sizes considered. The distribution of first impact pressure-impulses along the vertical structure show similar trends among the pressure-impulse theory and CFD models. While large discrepancies are observed for predicted maximum pressure-impulses, the relative error of total impulse at wall between both models is low. From both CFD and pressure-impulse model results, empirical relations are derived between relative venting area and total impulse release. The assumption adopted in the theory of zero pressure-impulse at the venting position is not reproduced in CFD results, which leads to overestimation of the impact mitigation effects of ventilations using the pressure-impulse theory. This divergence in the venting boundary condition is possibly linked to the omission of convective acceleration terms in the pressure-impulse model. Two ventilation design methods for vertical structures with overhangs subject to wave impacts are proposed based on the research conducted in this study. The first design method employs the derived empirical relations, standing wave theories and theoretical pressure distributions to determine the total impulse affecting the structure. The second design method applies the splitting approach of measured impulsive forces with low-pass filters to calculate the total impulse, requiring numerical or physical modelling. Further research is needed to support the models with small and large scale experiments using ventilations and expand the validity range of the results.