Hydrodynamic performance study of shore-based oscillating water column wave energy conversion device

To cope with the increasing energy demand, countries have started to actively develop renewable energy. Therefore, the study of wave energy generation devices is an important issue. In this paper, we investigate the performance and characteristics of shore-based oscillating water column wave energy generators concerning structural dimensions such as front wall draft and water depth variation. The results show that the front wall draft have significant effects on the internal flow characteristics of the oscillating water column wave energy generator. When the draft of the front wall changes, the reflection coefficient of the air chamber changes accordingly and the performance changes significantly. The variation of water depth has less effect on the performance of the low-frequency region of the oscillating water column wave energy generation device.


Introduction.
The ocean is widely distributed on the earth, which contains huge energy, and wave energy is an important part of it [1].
Wave power generation is a promising clean energy source because it requires relatively small land area compared to other power generation methods.Our country has a vast sea area, with the overall sea area of 4.7 million square kilometers.According to the national special survey, the total amount of ocean energy available in the coastal waters of our country has reached nearly 1.58×10 9 kW.It is calculated that the theoretical average annual generation total amount of ocean energy available in the offshore waters of our country can reach 3.94×10 13 kW• h [2][3][4].Since 2006, the development and utilization of ocean energy in our country has entered an unprecedented period of vigorous development, and the development of ocean energy has been explicitly mentioned in a number of important national plans [5][6][7][8].In the latest "14th Five-Year Plan for Renewable Energy Development", it is clearly pointed out that scientific and technological innovation must adhere to the combination of "bringing in" and "going out", and actively promote the development and utilization of ocean energy, so as to make our country lead the world by making a breakthrough in the development and utilization of ocean energy.Actively promoting the development and utilization of Marine energy can not only promote the realization of the "double carbon" goal in our country, improve the ecological environment around the sea area of our country, but also give a shot in the tail for the economy of the relevant regions in our country and provide more jobs.As an important part of the ocean energy in offshore waters of our country, wave energy is also an important part of the development of ocean energy in our country [9][10][11][12].Therefore, it is particularly important to actively promote the development of wave energy generation technology.

Numerical wave flume.
In this paper, the air chamber of an OWC wave energy generation device is studied.The model used in this numerical simulation is shown in figure 1. Figure 1 shows the schematic model of the numerical wave flume, whose structural dimensions are shown in Table 1.The numerical wave tank is mainly divided into three areas.The first area is the wave making area.The motion equation of the pushing plate can be written based on CEL language to realize the regular movement of the pushing plate to output the wave, and the wave can also be created by adding the fluid velocity function at the coordinate origin.The second area is the research area, and the air chamber is placed in the middle and back section of the area, where d w is the width of the air chamber,  is the draft of the front wall,   is the thickness of the front wall, and  1 , 2 , 3 is set in front of the air chamber to detect the wave surface data, and pressure monitoring points  1 ,  2 are set in the air chamber, and pressure monitoring points  1 ,  2 are set in and outside the bottom of the front wall [13].The third area is the wave cancellation area, and the damping cancellation method is used to cancel the waves.

Calculation method.
The controlling equation for a viscous incompressible fluid is the continuity equation.
Where,   is the average velocity,  is the time average pressure,  is the air density,  is the hydrodynamic viscosity,   is the body force, −  ′   ′ is the Reynolds stress.

Free surface.
The VOF method was used to track the free liquid level.By setting the volume fraction of water   , it can be divided into three cases: As a function of its volume fraction as: Where   and   are velocity components in different directions.

Push plate wave making.
The wave making method is the principle of the wave-making machine in the simulation of physical model test, and the wave formation is realized through the round-trip movement of the boundary.Its equation of motion is:

Damped wave elimination.
The wave attenuation is realized by adding a damping term to the rear section of the numerical flume.Its equation is Where the damping function  is defined as.
Where  0 is left vertex of the damping region,  1 is the right vertex of the damping region,  1 is the damping coefficient.

Numerical model.
The mesh division is very important in numerical simulation calculation, and the mesh quality affects the result of numerical simulation calculation to a great extent, so the reasonable mesh division is particularly important.In this paper, ICEM software is used to divide each component into structured grids, and the positions such as the wave propagation area and the rotation Angle of the air chamber are grid encrypted to ensure the quality of the grid, as shown in Figure 3-2.In this study, the model is selected as the turbulence model for simulation simulation, which combines the advantages of the model and the model, and has high accuracy, fast speed and wide application range.CEL language is written for the boundary so as to realize the round-trip movement of the boundary, whose formula is Equation (5).

Figure 2.Model grid.
In order to ensure the calculation accuracy of the simulation calculation, four groups of grids were selected by dividing 40, 60, 80, and 100 grids on a single wave height and 500, 800, 1000, and 1400 grids on a single wavelength, as shown in Table 1.
Table 1 The simulation results are shown in Figure 3. Comparing the differences between the waveforms under the four groups of grids and the theoretical waveforms, it can be seen that when the grid number is low, the generated waveform is closer to the theoretical waveform in the middle stage of propagation than the initial stage and the later stage of propagation.However, the overall waveform is much different from the theoretical waveform compared with the group with higher grid number, but M2, M3, the overall waveform difference of M4 is small, which is similar to the theoretical waveform, and the simulation accuracy is high.Therefore, considering the calculation time, calculation accuracy and economy, M2 is selected as the grid for subsequent calculation in this study, and the overall calculation time is 20 wave cycles.

Influence of front wall draft depth.
In order to study the influence of this parameter on the air chamber,   = 1m, 3m, 5m was selected for research.
To study the effect of this parameter on the performance of the air chamber, the OWC wave energy conversion device with the front wall draft depth of 1m,3m and 5m was selected for research.
is the pressure coefficient: As shown in figure 4-6, as the   decreases, an resonance period appears in the OWC device.At the resonance period, the RC(relative amplitude) of the gas chamber and the energy conversion efficiency of the OWC device increase significantly.However, when the   increases, the energy conversion efficiency of the OWC device is flat with the RC of the gas chamber, and the high efficiency area of the gas chamber gradually shifts from the low period area to the high period area.The main reason for this change may be that the wave reflection coefficient at the front wall increases with the depth of the front wall draft.

Effect of water depth.
In order to study the effect of seawater depth on the hydrodynamic performance of the gas chamber, the OWC wave energy conversion device with seawater depth of 7.5m,10m and 13m was selected for research.
As shown in Figure 17-18, in the short-period area, the RC shows a downward trend with the decrease of the sea depth, and the decrease amplitude increases significantly with the decrease of the sea depth; while in the long-period area, with the increase of the sea depth, the RC shows a slight increase.The RC is at a relatively high level.The main reason for this change may be that for deep water waves, the propagation of wave energy decreases exponentially as the depth of the water increases, which means that the wave energy is mainly concentrated at the shallow surface of the water.When the sea depth is small, the wave energy is concentrated, which is more conducive to the wave energy being captured by the air chamber, thus improving the energy conversion efficiency of the OWC device.

Conclusion.
In this paper, the effects of water depth and the draught depth of the front wall on the internal flow characteristics and hydrodynamic performance of the air chamber are analyzed in depth by relative amplitude, RC and pressure coefficient.The main results are as follows: (1) With the decrease of the   , the OWC device has an resonance period, while with the increase of the current draft depth of the wall, the rising trend of the RC and energy conversion efficiency of the gas chamber slows down due to the increase of the wave reflection coefficient, resulting in the shift of the low period area to the high period area.
(2) When the depth of sea water increases, due to the principle of deep water wave, its energy dissipation increases sharply, and the wave reflection coefficient increases obviously.

Figure 3 .
Figure 3.The wave surface under different grids.