Simulation Analysis of Wind Collecting Device for A Vertical Axis Wind Turbine

In this paper, a wind collection device (WCD) is designed for vertical axis wind turbine, and the WCD and vertical axis wind turbine are modelled by SolidWorks software and numerically simulated by CFD software FLUENT for the WCD and vertical axis wind turbine before and after installing the WCD. The wind amplification effect of the WCD is demonstrated from different angles and the final results show that the WCD can amplify the wind speed from 5m/s to 8m/s and double the rotational speed of the wind turbine in a certain area, which is of certain reference significance in the field of wind power generation using traffic wind energy.


Introduction
As new energy technology is more and more valued by many countries and enterprises, all kinds of new energy technology begin to develop rapidly, and the application of green energy in the city is also emerging gradually[1-2].At the same time, various technologies in urban transport are also developing rapidly [3][4][5].This paper is devoted to the study of wind power generation technologies that can be applied to traffic scenarios.
In the field of wind power generation, many scholars have been working for the better use of wind energy.Some scholars have studied the application scenarios of wind turbines in cities. Aravindhan N [6] et al. found that the output power of ducted wind turbines can be significantly increased as compared to open rotors and discussed the installation of such wind turbines on the on top of or around a building and discusses the power generated by installing such a wind turbine on top of or around a building.In addition, Aravindhan N [7] has conducted a study on the performance of a shrouded INVELOX small wind turbine in two scenarios: a low-rise building and the top of the tallest building in order to investigate the effect of the shroud profile on wind speed.Aravindhan N [8] et al. examined the potential productivity of wind turbine installations in urban areas and investigated the feasibility and success of small and medium-sized turbines in community residences.Some scholars have looked at the optimisation of turbine structures.Aravindhan N [9] et al. found that the standard profile performs better than the modified profile because of the reduced velocity at the tip of the impeller and the sharp increase in pressure on the tip of the impeller.Pan B [10] et al. analysed and optimized the structure of vertical axis wind turbine and discussed some key safety issues.Khamlaj T A [11] et al. significantly improved the power output coefficient by optimising the design of shroud, turbine shape and flange height.Chiu P K [12] et al. optimised the biplane blades to be more than 45% lighter than similarly optimised monoplane blades, which improved the performance and economics while reducing the mass.Some scholars have researched in the direction of designing wind gathering systems to increase wind speeds.Bramantya M A [13] et al. investigated a wind energy harvesting device called diffuser based on a wind turbine and found that an increase in the number of inlet nozzles leads to an increase in the maximum total power produced.Hosseini S R [14] et al. studied the performance of Invelox wind turbine under the influence of geometrical variation of nozzle-diffuser cross section using finite volume method and found that the channel flow velocity increases when the ratio of nozzle length to throat diameter rises to an optimum value and then starts to decrease.Tian D [15] et al. designed an improved model with six different booster plate configurations for a wind energy concentrator and identified the best performing wind energy concentrator among them.
We apply wind power technology to traffic scenarios to make use of the traffic wind energy generated by vehicles, especially on motorways where the wind energy generated is more abundant.Small wind turbines are divided into horizontal axis wind turbines and vertical axis wind turbines, of which vertical axis wind turbines are more suitable for traffic wind scenarios because they are simple in structure and are not affected by the direction of the incoming wind.This paper mainly designs a wind collecting device (WCD) that can amplify the wind speed to promote the rotation of the vertical axis wind turbine and studies its wind speed amplification effect through simulation, and finally discusses the application scenarios of this device from the simulation results.

Principle of wind speed amplification
Since the traffic wind generated by vehicle travelling belongs to low-speed wind, both the internal and external flow fields of the wind energy collection device can be regarded as continuous incompressible fluids, and therefore all the flow fields involved in this paper are regarded as continuous and uniform constant flow.According to the continuity equation, the cross-section average flow velocity of a constant flow is inversely proportional to the cross-section area, and the wind speed amplification effect of our designed wind energy collecting device is based on this.

Modelling and Simulation
In this paper, the 3D modelling software SolidWorks is used to model the wind energy collection device and the vertical axis wind turbine, respectively, and then Fluent in ANSYS is used to simulate the wind field.The ANSYS Workbench software (version 2021 R1, ANSYS Inc. Canonsburg, PA, USA) used in this paper is a large-scale general-purpose finite element analysis software developed by ANSYS Inc. in the United States, of which Fluent is the Computational Fluid Dynamics (CFD) simulation module, which is applicable to a variety of fields such as fluids, heat transfer, chemical reactions, etc., while we Numerical simulation of a self-designed wind energy harvesting device using ANSYS to study its amplification of wind speed.

Modelling
The structure of the wind energy collection device is shown in the figure 1(a), which consists of six panels, including the upper and lower panels and four wind guiding panels on four directions.Four wind guiding panels are mounted vertically on the diagonal of the four directions of the upper and lower panels and at 45°to the side lines, where the dimensions of the upper and lower panels are 0.01m×1m×1m, and the dimensions of the wind guiding panels are 0.01m×0.3m×0.8m.In addition, we perform a simplified modelling of a resistance vertical-axis wind turbine, shown in figure 1(b), which consists of two fan blades, with a wind turbine diameter of 0.192m, a height of 0.5m and a thickness of 0.015m.

Computational mesh sensitivity study
In order to save computational resources and improve computational efficiency, our target mesh size needs to be able to ensure the accuracy of the computational results while not taking up too much computational resources because of the dense size, so a computational mesh sensitivity study was carried out.We added a 0.15m×0.15m×7mfluid field region outside the vertical axis wind turbine model and drew a cylinder region to locally encrypt the area where the turbine is located, and three sets of meshes with different densities were obtained through dimensional control.The number of mesh cells is shown in table 1 and a comparison of the meshes delineated is shown in figure 2.  For all three sets of meshes, the same treatment and boundary conditions were performed, and then the sensitivity of the meshes was verified by comparing the velocity-time curves at one point on the wind turbine.As shown in figure 3(a), we can see that the velocity-time curves of the three sets of meshes basically coincide with each other, and in order to get the magnitude of the error between the three, we plotted the velocity differential-time curves between mesh 1 and mesh 3 as well as mesh 2 and mesh 3 respectively.As shown in the figure 3(b), we find that the two errors remain below 0.01 and 0.008, respectively.Therefore, we believe that the meshing method is consistent with the mesh sensitivity study.

Boundary condition setting
For the setting of boundary conditions, we set the turbulence model as the k-epsilon standard model, the inlet wind speed as 5m/s, the outlet as the outflow boundary, the dynamic mesh setup as 6-dof and the properties of the wind turbine as 0.1kg mass, 1kg/m^2 moment of inertia, and rotation around the Z-axis, in addition, the wind turbine and its interface are set as rigid bodies, while the fluid and the WCD are set as stationary.

Variation of wind speed in the wind collecting device
We intercepted the plane at the height of the centre of the wind collection device and generated a topview cloud image of this cross-section, as shown in figure 4, which shows that the wind speed in the inner area of the four wind guiding panels was significantly amplified by the wind collection device's wind gathering effect.The wind inlet velocity is 5m/s, while the amplified maximum wind speed reaches 8.6m/s, which is a 72% improvement.
Figure 4. Overhead cloud view of the WCD.In order to know more clearly how much the wind speed has been enlarged, we intercepted four line segments of 1m length (the same as the length of the upper and lower panels of the WCD) in the upper part of the wind collection device area in the above plane, as shown in the figure 5, and set up a coordinate system with the centre point of the WCD as the origin, and the straight lines in which the four segments are located are Y=0m, 0.09m, 0.18m and 0.27m, respectively, and we take 50 points on each of these segments and plot their changes in wind speed in the X-axis direction.The point line diagram of wind speed variation obtained is shown in figure 6.We can see that when Y=0.27m, the wind flow in the X direction from the inlet of the WCD to the wind guiding panel is increased by the aggregation effect, but as the wind continues to converge towards the centre of the WCD at 45°with the wind guiding panel, the wind speed starts to decrease in the X direction at the line segment behind the wind guiding panel (X>-0.287m)until it is lower than 2m/s.When Y=0.18m, the overall wind speed trend is similar to that of Y=0.27m, however, since the wind flow is farther away from the wind guiding panel compared to Y=0.27m, the amplification area is longer in the X direction, and the wind speed starts to decrease at X>-0.16m until it reaches about 5m/s.At Y=0m and 0.09m, the wind speed reaches a maximum value of 8m/s at the centre of the plenum and continues more steadily to the outlet of the WCD.

Comparison of wind turbine blade speed clouds at different moments
After installing the WCD, we conducted transient simulation of the wind turbine for a total of 58s, and found that the velocity distribution of this form of vertical-axis wind turbine is shown in figure 7. Taking the moment of 58s as an example, the velocity of the wind turbine blade is centred at the centre of the rotation and increases with the increase of the distance from the radius, and the velocity on the wind turbine is the same at the same radius but at different heights.Therefore, it is considered to compare the top-view clouds of wind turbine blades before and after the installation of the WCD at different moments in order to better visualise the difference in wind turbine speed between the two operating conditions.We selected the top-view velocity maps of the wind turbine at three moments of 19s, 38s and 57s under two operating conditions, as shown in figure 8, we set all the clouds' wind speed scales to the default scales after the installation of the WCD and at the time of 57s, i.e., 0~2.65m/s.By comparison, we found that the maximum velocities at each point in time without the installation of the wind collection device were 0.48m/s, 0.91m/s and 1.27m/s, and the maximum velocities at each point in time after the installation of the WCD were 1.10m/s, 1.97m/s and 2.65m/s., 0.91m/s and 1.27m/s, and the maximum velocities at each time point after the installation of the WCD are 1.10m/s, 1.97m/s and 2.65m/s, respectively.The latter were 129%, 116% and 109% respectively in terms of speed enhancement.Although the increase in speed was reduced with the increase in time, the increase was more than double, so it can be seen that the wind speed amplification effect of the WCD has a better promotion effect on the rotational speed of the wind turbine.

Comparison of rotational speed-time variation curves at the point of maximum speed on the turbine blades
The point on the wind turbine blade furthest from the centre of rotation is selected and the change in velocity at that point over time is plotted, and then we convert the velocity into rotational speed.The obtained wind turbine rotational speed-time curves are shown in figure 9.By comparison we find that the overall rotational speed of the wind turbine increases significantly after installing the WCD, and the average rotational speed of the wind turbine after installing the WCD is about doubled compared with that of the uninstalled wind turbine in the period of 0~57s.

Discussion and conclusion
The wind energy collection device (WCD) has a significant amplification effect on the wind speed, and the wind speed amplification area is concentrated in the area within a certain distance from the wind guiding panels, and the structure of the vertical axis wind turbine is very suitable for this kind of scenario.At the same time, the symmetrical structure of the WCD enables the wind speed amplification effect to be realised for the winds coming from four directions, which is also very suitable for the characteristics of the vertical axis wind turbine to adapt to all kinds of wind directions.However, since the wind speed will be lower than the original wind speed in the area close to the wind guiding panels due to the shading effect, when considering the real application, we should take into account the structural form of the wind turbine and place the wind-facing part of the turbine blade completely in the wind speed amplification area, and adjust the dimensions of the WCD and the wind turbine according to the application scenarios, so as to better utilise the amplified wind energy.In addition, we envisioned the application scenarios for the designed WCD, one of which is on a motorway divider, where the lanes on both sides of the divider are fast lanes, and therefore generate more wind energy from traffic compared to the other lanes.However, since the wind energy generated by vehicle traffic is not continuous, the wind amplification system is sited in a high traffic highway barrier.Another application scenario is in motorway tunnels, where traffic wind is more likely to gather because the tunnel is a semi-enclosed area.However, this scenario requires a higher size of the wind amplification equipment, as the installation space inside the tunnel is smaller.In addition, there are still some shortcomings in this paper, through the above simulation results, we can understand that the amplification effect of WCD on wind speed is significant, but at present, no relevant test has been carried out to further verify it, which needs to be perfected in the subsequent research.In addition, the parameters of the wind turbine are set in favour of adapting to the conditions of simulation convergence, and its mass, rotational moment and other parameters are set without taking into account its material characteristics, so the simulation results are more suitable to be regarded as a comparison of two working conditions of the wind turbine before and after installing the WCD under the same conditions.By comparing the different cloud diagrams and curves before and after installing WCD through the above simulation methods, we get the following three conclusions: Firstly, the wind energy collection device (WCD) can steadily increase the wind speed from 5m/s at the inlet to about 8m/s at a certain distance from the wind guiding panel, with an increase of 60%, and this amplification effect will be weakened as it gets closer to the wind guiding panel.
Secondly, through comparison, we found that the overall speed of the wind turbine was significantly increased after installing the WCD, and the speed of the turbine before and after installing the WCD in 0~57s was increased from 0rpm to 67.3rmp and 140.6rpm respectively, which doubled the average rotational speed.
Finally, we chose three moments, 19s, 38s and 57s, to compare the top-view cloud images of the wind turbine under the two conditions, and found that the maximum speeds of the wind turbine at each moment without WCD installation were 0.48m/s, 0.91m/s and 1.27m/s respectively, and that the maximum speeds of the wind turbine at each moment after installing the WCD were 1.10m/s, 1.97m/s and 2.65m/s, with an increase in speed of 129%, 116% and 109% respectively.
(b) Drag type vertical axis wind turbine model.

Figure 1 .
Figure 1.Schematic diagram of the three-dimensional models.

Figure 2 .
Figure 2. Comparison of three sets of meshes.
(a) Velocity-time curves at a point on the wind turbine for different meshs.(b) Error-time curves between different meshes.

Figure 3 .
Figure 3. Evaluation of standard curves for mesh irrelevance analysis.

Figure 5 .
Figure 5. Illustration of the monitoring line segments of the WCD.

Figure 6 .
Figure 6.The velocity change curve of the monitoring line segment.

Figure 7 .
Figure 7. Cloud view of wind turbine at 58s after installing the WCD.
(a) Before installing the WCD.(b) After installing the WCD.

Figure 8 .
Figure 8. Overhead cloud view of the wind turbines at different moments.

Figure 9 .
Figure 9.Comparison of wind turbine speed-time curves before and after installing the WCD.

Table 1 .
Comparison of the quantity in each region of the mesh.