3.1 The wave flume
The experiments were carried out in a recirculating open channel flume located at the Hydraulics and water Engineering Laboratory, Higher Institute of Engineering in El Shorouk Academy. The wave channel has a length of 12m, a width of 0.5 m and a depth of 0.6 m. The side walls are made of glass panels to allow a clear view of the wave action. Regular waves were generated with variable height and length by a hydraulic piston wave-maker at one end of the flume. The wave-maker produces 2D waves with no fluid motion normal to the sidewalls. To minimize reflection of the waves, a gravel beach, was installed at the end of the wave channel, as shown in Fig. 1.
The wave heights in the flume were measured using a wave gauge (Sonic Wave Sensor XB) placed at a distance from the wave generator. Ultrasonic travel time is measured as the primary working principle. The wave gauge was connected to the PC through a wireless adapter, and the data was processed using software that plotted data and performed spectral analysis. A definition sketch for the experimental flume is shown in Fig. 2.
Figure (1) view of wave flume, (a) a hydraulic piston wave generator and (b) gravel slope
Figure (2) Definition sketch for the experimental flume.
3.2 Describe the model
The experimental setting of the suggested device is shown in Fig. 2. The device consists of a rectangular base is fixed on it two ramps. One is an inlet ramp with a wooden barrier that catches the incoming waves. The slope angle varies between 33.1, 29.1, and 21 degrees. It has a length of 0.75m (see Fig. 2 -A). 0.43 m above the water's surface and 0.32 m submerged. The other is an exit ramp with a 30-degree slope and barriers in the shape of a funnel that are wide at the top and gradually narrow as they approach the turbine attached to a rotary generator at the bottom of the slope (see Fig. 2 -B). The rectangular base has a length of 61 cm, a height of 10 cm, and a width of 43 cm.
The turbine generator is critical for converting water's kinetic energy into electrical energy. A turbine is a mechanical device that converts the kinetic energy of water into rotational motion. The Pelton wheel turbine is the type of turbine employed in this device. The pelton wheel turbine is composed of plastic and contains eight blades that are 5 cm wide, 18 cm long, and have a 13 cm outer diameter. The pelton wheel turbine has curved blades on the edge and is positioned on a horizontal axis connected to a generator. The rotor is surrounded by a cover, which ensures that no water energy is lost. A definition sketch for the experimental device is shown in Fig. 3.
The draught of the breakwater (D) was changed three times to investigate its influence on wave energy dissipation and electrical generation (D = 3 cm, 5 cm, and 7 cm). For each case of draught, the wave height (\({h}_{i}\)) was varied four times (\({h}_{i}\) ranging from 0.121 to 0.074). The four wave lengths ranged from 1.22 to 2.84 m. Thus, wave steepness conditions (\({h}_{i}\)/L) range from 0.011 to 0.047. The wave period was measured by recording the time taken for two successive wave crests to pass a fixed measuring point. The wave frequency is calculated by taking the reciprocal of the wave period and was varied from 0.35 to 0.78Hz. A total of 36 experimental runs have been conducted at a water depth of 0.3 m to cover all the various parameters concerned in the study. Table 1 summarizes the test parameters in detail.
Table (1) test parameters
Parameters | Range |
Water depth (\({d}_{w}\)) | 0.3 m |
Incident wave height (\({h}_{i}\)) | 0.121, 0.11, 0.091, 0.074 m |
Frequency (1/T) | 0.78, 0.68, 0.57, 0.48 Hz |
Wave length (L) | 2.55, 3.35, 4.86, 6.68 m |
Slope angle (\({\theta }_{r}\)) | 33.1, 29.1, 21 degree |
Model draft (D) | 0.03, 0.05, 0.07 m |
Wave steepness | 0.047, 0.033, 0.019, 0.011 |
Figure (3) the parts of the model in this investigation
Figure (4) sketch of overtopping breakwater model
3.3 Data Analysis
The four wave gauges are installed in the flume to record water level oscillations. The following method was used to calculate the relative distance between the wave gauges, as suggested by (Mansard and Funke 1980):
X12 = L/10; L/6 ⩽ X13 ⩽ L = 3;
X13, ≠L/5; X13 ≠ 3L/10
Where L indicates the wave length, X12 indicates the distance between the first two wave gauges in the propagation line, and X13 indicates the distance between the first and third wave gauges in the wave propagation line.
When evaluating breakwater efficiency, wave transmission is a crucial parameter that should be thoroughly investigated. Wave transmission occurs when wave energy passes over, under, or through a breakwater, resulting in a diminished wave (transmitted wave) on the structure's other side. The transmission coefficient (\({C}_{t}\)), which is defined as the ratio of the transmitted wave height (\({h}_{t}\)) to the incident wave height (\({h}_{i}\)), \({C}_{t}\) = \({h}_{t}\)/\({h}_{i}\), is commonly used to express the amount of wave transmission.
The transmitted wave height \({h}_{t}\) and electric power extraction are mainly affected by the following parameters: the incident wave height (\({h}_{i}\)), the wave length (L), the wave period (T), the water depth (\({d}_{w}\)), the water density ρ and the gravitational acceleration g, the device draught (D) and the inclination angle (\({\theta }_{r}\)). As a result, the following functional relations can be used to describe the breakwater device performance:
\({h}_{t}\) , P =\(f\left({h}_{i},L,T,{\rho }, g, {d}_{w},D, {\theta }_{r}\right)\)
The following dimensionless relationships result from a dimensional analysis using Buckingham theory as follows:
\({C}_{t}\) , P =\(f\left({h}_{i}/L,F,D/{d}_{w}, {\theta }_{r}\right)\)