مطالعه شبیه سازی عددی باد شکن درخت

In this study, computational fluid dynamics (CFD) was utilised to investigate the flow characteristics around tree windbreaks. The efficiency of windbreaks depends on many factors which can be investigated in field experiments, though this is limited due to several reasons such as unstable weather conditions, few measuring points, etc. Fortunately, the investigation is possible via computer simulations. The simulation technique allows the trees to be modelled as a porous media where the aerodynamic properties of the trees are utilised in the model. The trees employed are Black pine trees (Pinus thunbergii) with a drag coefficient value of 0.55. The simulation provides analysis of the effect of gaps between trees, rows of trees, and tree arrangements in reducing wind velocity. The simulations revealed that 0.5 m gap between trees was more effective in reducing wind velocity than 0.75 and 1.0 m. The percentage reduction in velocity at the middle of the tree section for 0.5, 0.75 and 1.0 m gap distance was found to be 71, 65 and 56%, respectively. Two-rows of alternating trees were also found to be more effective than one-row and two-rows of trees. The reduction at the middle of the tree region for one-row and two-rows of trees and two-rows arranged alternately was 71, 88 and 91%, respectively. Results revealed that the percentage reduction in wind velocity measured at distance 15H, where H is the tree height, for one-row, two-rows of trees and two-rows arranged alternately was approximately 20, 30 and 50%, respectively.

South Korea is mostly surrounded by bodies of water with approximately 2413 km of coast line along three seas. To the west is the Yellow Sea, to the south is the East Sea, and to the east is Ulleung-do and Liancourt Rocks (Dokdo) in the East Sea. The country has geographical land mass of approximately 100,032 km2 with limited land resources. This prompted the country to implement several land reclamation projects especially near the coastlines. By 2006, 38% or 1550 km2 of coastal wetlands had been reclaimed, including 400 km2 in the Saemangeum area (Korea Statistical Information Service, 2006). The wind velocities in the coastal areas and in the reclaimed lands are usually higher because of the sea wind. In these areas, the recorded average wind velocity at 5 m height can reach up to 7.0 m s−1 especially during the dry months of the year from February to May (Hwang et al., 2006). This has caused generation and diffusion of dusts to nearby areas, such as agricultural and animal farms. In addition, the dusts from the reclaimed lands contain significant quantities of sodium chloride (NaCl) which is very detrimental to plants, animals and humans (Bitog et al., 2011). These problems can be reduced by minimising the wind velocity especially in the dust source areas, where it is the main catalyst of dust generation and diffusion. Constructing artificial barriers or planting natural windbreaks such as trees to control the wind velocity are the best options. However, for long term protection, tree windbreaks are strongly recommended (Zhou, Brandle, Mize, & Takle, 2004). Natural windbreaks, especially trees, direct winds over or around protected areas such as agricultural farms, and livestock farms. They are very effective in reducing wind speed in the protected area. Windbreaks operate by creating pressure at the windward side of the trees as wind blows against them, with the direction of large air flows shifted in direction over the top or around the ends of the windbreaks (Bitog et al., 2011). Especially in coastal areas and in reclaimed lands, windbreaks can control the wind velocity to a level usually lower than the threshold velocity required for the generation and diffusion of dust. The amount of wind speed reduction depends on several factors such as the tree height, density, width, shape, arrangement, porosity, etc. (Bitog et al., 2011). However, several studies have already shown that, among the factors, tree porosity has the most influence on windbreak efficiency (Bitog et al., 2009, Cornelis and Gabriels, 2005 and Heisler and Dewalle, 1988). Determining the actual tree porosity is complex, considering the irregular size and shape of the trees as well as the varied distribution of the gaps. However, this can be well represented by the drag coefficient (CD). As mentioned by Jacobs (1985), the resistance to wind flow or the drag coefficient of the windbreak can provide information on its effectiveness and efficiency in reducing high velocity winds. Therefore, knowing the dimensionless drag coefficient value of the tree windbreak is very important index to evaluate the wind protection effect of the tree (Bitog et al., 2011). In principle, the flow characteristics around windbreaks are being disrupted because of a net loss of momentum as the tree exerts a drag force on the incoming winds (Raine & Stevenson, 1977). Performing computer simulation has been in the forefront for studying the effectiveness and efficiency of natural and artificial windbreaks including the relevant flow mechanisms around barriers. This is evidenced by the numerous published papers. A number of small-scale windbreaks studies were also conducted in wind tunnels (Dong et al., 2008, Dong et al., 2006 and Gromke and Ruck, 2008; Guan, Zhang, & Zhu, 2003) since the results could be applied to full scale model or actual field conditions with reliable scale-up procedures. The use of correct aerodynamic and engineering equations to resolve the scaling up technique, especially the dimensional difference of the models, must also be carefully managed. The continuing development of both wind tunnel and numerical simulation techniques has paved the way for more accurate and reliable laboratory investigations and computer simulation studies (Li, Wang, & Bell, 2007). The analysis of the flow characteristics of these simulation studies was based on the objectives for the windbreaks, with the main emphasis on the effect to the leeward. However, most of the simulations were still limited to 2-dimensional models because of insufficient computer memory to process the computations. The simulation study presented by Raine and Stevenson (1977) measured and analysed the wind velocity and energy spectra to leeward of a modelled fence. Similar studies by Castro and Garo (1998) and Judd, Raupach, and Finnigan (1996) were conducted on a porous barrier where the mean velocity and turbulence stress downstream of the barrier were investigated. A numerical model has been developed by Wilson, 1985 and Wilson, 1987 and Wilson and Yee (2003) to accurately simulate the wind flow characteristics around a single fence and multi-array windbreaks. These simulation studies proved their reliability by showing similar results when compared to wind tunnel or field experimental results. Computer simulation, particularly computational fluid dynamics (CFD), has now become very popular for studying wind flow characteristics around artificial and natural windbreaks. The most recent studies have been conducted by Bitog et al., 2009, Gromke and Ruck, 2008 and Rosenfeld et al., 2010, and Santiago, Martin, Cuerva, Bezdenejnykh, and Sanz-Andres (2007). These studies have exploited the power of CFD in the investigation and analysis of wind flows over a terrain or an area protected by windbreaks. Because of rapid development of more powerful and high memory personal computers, simulations of 3-dimensional models can now be easily achieved. A study by Rosenfeld et al. (2010) established the significance and extent of 3-dimensional flow patterns across tree windbreak comprising of individual cypress trees. Their study was validated by comparing their simulation results with experimental data and showed good agreement. In this study, a 3-dimensional model was designed adopting the general shape and dimension of Black pine trees (Pinus thunbergii). The study was conducted with an attempt to simulate the tree windbreaks in the natural environment. The drag value of the experimental tree, which had been determined earlier (Bitog et al., 2011), was utilised as input in setting up the boundary conditions of the tree, which was designed as a porous media. With the simulation technique, numerical analysis on the effect of the windbreaks in reducing wind velocity, especially to leeward, can be thoroughly investigated. The effect in reducing wind velocity of gap distance between trees, number of rows of trees and their arrangement were determined, and the horizontal extent of the effect of the tree windbreak at varied wind velocities was also measured. The results obtained will be used in the design of an effective windbreak system for use in the reclaimed lands and in the coastal areas of Korea, and can also utilised in future experimental and simulation studies of wind flows as affected by artificial or natural windbreaks, considering complex topographies.

CFD has proved to be an effective tool to simulate flow around tree windbreaks. Exploiting the power of CFD could allow quantitative visualisation and velocity reduction investigation. However, for the accuracy of the simulation study, significant values required in the simulation process such as the drag coefficient of the tree must be precisely determined. In this study, computer simulation was performed for the Black pine tree (Pinus thunbergii) windbreak, one of the most typical tree windbreaks that are very appropriate in the coastal areas and in most of the reclaimed lands in South Korea. The drag coefficient of the tree, which was earlier obtained from wind tunnel experiment, was employed in the simulation studies for the tree windbreaks to predict the effect of gap distance between trees, rows of trees and arrangement in decreasing wind velocity. The horizontal extent of its effect was also quantitatively determined. Results of the simulation studies have shown that the gap distance is a very important factor for the effectiveness of the tree windbreak especially when the effect in the lateral direction is one of the main goals of the windbreak system. The velocity reduction of the tree windbreak with a gap distance of 0.5 m could reach up to 20% at a streamwise distance of 15H. For 0.75 and 1.00 m gap distance, the reductions were approximately 11 and 4% respectively. Two-rows of trees arranged alternately were found to be the most effective in reducing the wind velocity compared to one-row and two-rows of trees. The reduction of wind velocity determined along the middle of the tree zone for one-row is approximately 71%, two-rows, 88%, and two-rows alternately arranged, 92%. Considering the streamwise extent of the effect of the windbreak at 15H, the reduction of wind velocity for two-row of trees arranged alternately could reach up to 50%. For one and two-row of trees, the quantitative effect had lowered to approximately 20 and 30%, respectively. Findings of this study should be very useful in planning and setting up natural windbreak system for a long term control of the generation and diffusion of dust especially in the reclaimed lands and the coastal areas of Korea.