Experimental and Numerical Investigation on Wave Interaction with Submerged Breakwater

Abstract

In this research work the interaction between waves and rectangular submerged breakwater has been investigated. To investigate the performance of proposed rectangular type submerged breakwater, experimental studies are carried out in a two-dimensional wave flume (21.3 m long, 0.76 m wide and 0.74 m deep) at the Hydraulics and River Engineering Laboratory of Bangladesh University of Engineering and Technology. A set of experiments are carried out at 50 cm still water depth with fixed submerged breakwaters of three different heights ( 30 cm, 35 cm and 40 cm) for five different wave periods (1.5 sec, 1.6 sec, 1.7 sec, 1.8 sec and 2.0 sec) in the same wave flume. For 15 run conditions, data of water surface at different locations are collected manually by providing a vertical scale on the flume side made of glass. Six different locations both in front of and behind the breakwater are selected for data collection. Also the type of wave breaking and position of wave breaking are simultaneously recorded with a digital video camera. Effects of breakwater height and length along the wave direction on wave height reduction are analyzed. Two-dimensional numerical model based on the SOLA-VOF method developed for wave interaction with floating breakwater has been updated in this research to study the wave interaction with fixed submerged breakwater. The SOLA-VOF model allows an efficient tracking of the free surface. It is based on the concept of a fractional Volume Of Fluid (VOF) for tracking free surface boundaries using the donor–acceptor approximation algorithm. The treatment of the free surface configuration using a single VOF function F is computationally very efficient. The SOLA scheme is employed to calculate the pressure and velocities in each time step and the added dissipation zone method is adopted to treat the open boundary. The basic equations used for VOF method are the continuity equation, the Navier-Stokes equation for incompressible fluid and the advection equation representing the behavior of the free surface. Because the wave generation source is placed within the computational domain, these equations involve the wave generation source. The developed numerical model is verified by comparing the model generated wave with the wave as per Stokes 3rd order wave theory. The time series water surface profiles, water particle velocity field, VOF function F, pressure around a breakwater of different heights (20 cm, 25 cm, 30 cm, 35 cm and 40cm) are simulated using the numerical model. The experimentally measured data shows good agreement with the water surface profiles and breaking positions simulated by the developed numerical model. This study is expected to serve as a useful model to analyze wave deformation due to submerged breakwater and will be important for designing submerged breakwater as a coastal protection measure.