Large Scale CFD Modelling of Wave Propagation into Mehamn Harbour

Abstract

Ocean wave propagations into harbours are large scale phenomena with complex wave transformations. Computational fluid dynamics (CFD) has the advantage of capturing most physics with few assumptions. It has also been successfully applied in marine engineering and coastal engineering. Altogether, CFD is considered to be an ideal tool to analyse the wave propagation at harbours. The most prominent limitation of CFD application is the high requirement of computational resources. However, with increasing computational resources, CFD is becoming an attractive alternative. One of the challenges in large scale CFD simulation is to generate the realistic waves. An extended flat-bottom wave generation zone tends to interrupt the continuity of the sub-sea terrain and introduces unrealistic wave transformations. To locate the wave generation zone far from the topography is one remedy, but it demands more computational resources. Therefore, generating waves over an irregular bottom is of particular interest in CFD simulations. In this paper, the three dimensional large scale numerical simulation of wave propagation into Mehamn harbour is performed with waves generated over an irregular bottom using the open source CFD model REEF3D. The relaxation method is used for the wave generation and absorption. A modified wave generation method considering the local water depths and wave numbers is used in this paper. A study case is performed to demonstrate the effect of the irregular bottom wave generation. Then, the large scale wave propagation into Mehamn harbour is simulated with two different waves generated over the real topography. REEF3D simulates the Mehamn harbour wave propagation by solving Navier-Stokes equations on a staggered grid with the finite difference method. The level-set method is applied to capture the free surface. A fifth-order WENO scheme is applied to the convection terms and a third-order TVD scheme is applied on the transient terms. The topography of the harbour is modelled using the local inverse distance interpolation method. The Mehamn harbour simulations show good wave transformation results and indicate a successful application of wave generation over irregular bottoms.