Session: 06-02-02 Coastal Engineering - II

Submission Number: 175214

Large-Eddy Simulation of Wave Overtopping Flow 

Wave overtopping over a seawall is a violent natural phenomenon that can cause the seawall to fail and pose risks to pedestrians. The overtopping flow is a typical post-breaking flow in which the water surges to the crest and passes over it.
Characterised by a thin, fast-moving layer of water, it exhibits highly turbulent bore-like behaviour and is often subjected to rapid free surface deformation. Therefore, understanding wave overtopping under complex, highly turbulent flow conditions remains a challenging research topic, and gaining deeper insights into the underlying mechanisms is essential for advancing both scientific knowledge and engineering practice.

Historically, in coastal engineering, much research on wave overtopping has focused on the average discharge and the maximum overtopping volume, which are the primary parameters used to quantify wave overtopping. However, our recent studies on wave overtopping impacts on human bodies in~ suggest that the key flow parameters defining the hazard are flow depth and flow velocity, which primarily determine the inline force exerted on the human body. These findings underscore the need for in-depth investigations into the kinematics of overtopping flows to enhance the prediction of overtopping loads on residential structures in coastal areas and on human bodies, which motivates the present study on high-fidelity large-eddy simulation of wave overtopping flows.

A numerical wave tank is set up in OpenFOAM using a two-phase wall-modelled LES to reproduce the experiments. The domain measured 6.5 m in length from the inlet to the seawall toe, 0.6 m in width, and 1.2 m in height, with a seawall of 1 m height and 3 m length. Regular waves were generated and absorbed with waves2Foam using stream function theory, with a relaxation zone from the inlet to 1.25 m upstream of the seawall toe and a second zone 0.3 m at the seawall crest to dissipate overtopping. No-slip conditions with wall functions were applied to the seabed and seawall. The top boundary was set as free-atmosphere, and cyclic side boundaries were used to reduce confinement effects. The structured mesh, refined via a convergence study, used uniform spanwise spacing and vertical stretching towards the seabed. Offshore, coarser horizontal resolution captured wave propagation efficiently, while the mesh was significantly refined near the seawall toe to resolve shoaling, breaking, and turbulent overtopping flows.

We thoroughly compare the numerical results against the experimental data from the PIV test on the velocity contour. The comparison is based on the first stabilised cycle in the central plane, with both datasets synchronised to the first arrival of the overtopping flow tip at the leading edge. In addition, we validate the LES model against the instantaneous horizontal velocity profiles u(z). In general, the numerical results show good overall agreement with the experimental data. While a perfect point-for-point match of turbulent fluctuations is not expected, the model accurately reproduces the boundary layer shape and bulk velocity, particularly at the later stages of the overtopping event. More noticeable deviations are observed at the peak flow time during the early stage, which may be attributed to significant air entrainment during this phase.

Presenting Author: Hao Chen Newcastle University

Presenting Author Biography: Dr. Chen has devoted his efforts to research into coastal and marine hydrodynamics, with emphasis on numerical modelling of free surface waves, including wave breaking, wave overtopping, wave-wave and wave-structure interactions. He develops computational fluid dynamics-based numerical solvers to describe the physical processes and phenomena, support sustainable extractive activities related to energy generation and food, and enhance the resilience of the coastal community.

Authors:

Deping Cao Tongji University
Tianze Lu Tongji University
Hao Chen Newcastle University