Wave overtopping flow striking a human body on the crest of an impermeable sloped seawall. Part II: Numerical modelling

Abstract

The present paper is the second of two companion papers on the investigations of wave overtopping flow striking a cylinder, which is the schematisation of a human body, on the crest of an impermeable sloped seawall with a deep foreshore. This paper numerically examines the detailed characteristics of the overtopping flow and the force on the cylinder in-line with the flow direction by using a volume-of-fluid (VOF) based Reynolds-Averaged Navier-Stokes (RANS) model. The numerical model is successfully validated against the experimental data on the wave overtopping flow depth and the inline force on the cylinder. The characteristics of the overtopping flow velocity are analysed using the validated numerical model. It is observed that the maximum depth-averaged flow velocity usually occurs before the maximum flow depth during an overtopping event, and decay of the overtopping velocity is much slower than the flow depth along the seawall crest. For plunging-breaker cases, the overtopping force on the cylinder usually comprises a cycle of a first impact peak, a main peak and a secondary peak. However, for surging-breaker cases, it is largely dominated by the main peak. The first impact peak is due to the impact of the tip of the overtopping flow on the cylinder, which usually has a higher flow velocity than the main stream. The main peak is generated by the asymmetric pressure distribution around the cylinder, which co-occurs with the maximum local momentum flux of the overtopping flow. As the force decreases after the main peak, there sometimes exists a secondary peak, which is formed by the complex free surface motion locally behind the cylinder. Additional numerical experiments are presented to demonstrate the applicability of a simple maximum force predictor developed in Part I. A prototype-scale simulation of irregular waves overtopping a sloped seawall is conducted to obtain the maximum force. The predictor also gives the maximum force in a wave-by-wave manner using the incident wave condition. These two approaches produce very similar estimates, suggesting that the predictor can be used for engineering applications.