Experimental and numerical investigation on wave interaction with horizontal slotted submerged porous breakwater

Abstract

In this research work the interaction between waves and horizontal slotted submerged porous breakwater has been investigated experimentally as well as using numerical model. To investigate the performance of proposed horizontal slotted 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 the Bangladesh University of Engineering and Technology. A set of experiments are carried out at 50 cm still water depth with fixed horizontal slotted submerged breakwaters of three different porosities ( n = 0.4, 0.5 and 0.6) for interaction with regular waves of four different wave periods (T = 1.6 sec, 1.7 sec, 1.8 sec and 2.0 sec) in the wave flume. For 12 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. At each measuring location, water surface data were recorded for 1 min, at 5 sec time interval. Also the position of wave breaking is simultaneously recorded with a digital video camera. Different hydrodynamic co-efficient such as transmission co-efficient (Kt), reflection co-efficient (Kr) and wave energy loss co-efficient (KL) are determined from the measured water surface data for various run conditions. These co-efficient values were then, analyzed with respect to relative breakwater width (k.B), [where, k = wave number (2π/L), B = breakwater width] and porosity of breakwater. Experimental results reveal that, for transmitting smaller part of wave energy through the breakwater, minimum transmission co-efficient, Kt = 0.526 was obtained for breakwater with the lowest porosity (n=0.4) for the shortest wave, i.e. T = 1.6 sec. Minimum reflection co-efficient, Kr = 0.03448 was obtained for breakwater with the highest porosity (n=0.6) for the longest wave, i.e. T = 2 sec. It is also seen that wave energy loss co-efficient (KL) decreases from 0.68 to 0.47 with increasing porosity. Two-dimensional numerical model based on the SOLA-VOF (SOLution Algorithm-Volume Of Fluid) method developed for wave interaction with fixed submerged breakwater has been updated in this research to study the wave interaction with horizontal slotted submerged porous breakwater. 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 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 porosities (n=0.4, 0.5 and 0.6) are simulated using the numerical model. The water surface profiles and breaking positions simulated by the developed numerical model show good agreement with the experimentally measured data. This study is expected to serve as a useful model to analyze wave deformation due to horizontal slotted submerged porous breakwater and will be important for designing submerged porous breakwater as a coastal protection measure.