INVESTIGATION OF WINDBREAK SYSTEMS TO CONTROL DUST GENERATION AND DISPERSION IN A RECLAIMED LAND: A NUMERICAL APPROACH
Windbreaks systems play an active role in improving the quality of the natural environment, accelerating the development of agriculture, forestry and animal husbandry, and increasing ecological, social and economical benefits. In a vast reclaimed land, where detrimental dusts are of enormous concern, windbreak's main purpose can be exclusively to control soil erosion, dust generation and diffusion. Artificial and natural windbreaks are very effective in minimizing the wind velocity especially in the dusts source areas, where it is the main catalyst of soil erosion, dust generation and diffusion. In this study, numerical simulation was utilized to evaluate the performance of both artificial and natural windbreaks in minimizing wind velocity. Windbreak fences of varied height and porosity and arranged in an array with different gap distance were investigated. For natural windbreaks, the effectiveness of trees in reducing the wind velocity was measured and quantified. Field experiments as well as wind tunnel studies were initially conducted to establish the validity of the simulation models. Simulation results were evaluated and suitable windbreak design for artificial and natural windbreaks were recommended. The numerical technique utilized in this study has proven its effectiveness in investigating dusts generation and diffusion which saves significant amount of time, labor and cost. INTRODUCTION Amidst the limited land resources of the country, Korea has been reclaiming lands for agricultural and industrial purposes. By 2006, 38% or approximately 1550 km of coastal wetlands had been reclaimed, including a 400 km area known as Saemanguem from three cities namely Gunsan, Buan and Kimje in the province of Chunla book, South Korea (Korea Statistical Information Service, 2006) However, the generation of dust from the ground surface and its diffusion to nearby surrounding areas has become an emerging concern. This usually happens during the very dry months from late February to May where the wind velocity reaches from 2.0 to 7.0 m/s (Bitog et al. 2009) The dusts also contains significant amount of sodium chloride (NaCl) which is very harmful to plants. To minimize these problems, constructing artificial windbreaks within the dusts source areas to significantly reduce the wind velocity are the best options. Artificial windbreaks such as constructing artificial fences, spreading coconets, and planting some plant varieties which can survive on salty soils can be implemented. However, for long term protection, natural windbreaks such as trees are strongly recommended (Zhou, Brandle, Mize, and Tackle, 2004) Therefore, in this work, the effectiveness of artificial fences as well as tree windbreaks in reducing wind velocity was thoroughly investigated. MATERIALS AND METHODS Field experiments, wind tunnel tests and numerical simulations were utilized in this study. In the field experiment, artificial fences were installed in the reclaimed land and the actual wind velocity before and after the fences were measured (Fig. 1) The measured values were used to compute the real turbulence kinetic energy and the turbulent dissipation rate which were utilized in the simulation modeling. Wind tunnel experiments were also conducted to obtain the vertical wind profile comparable with the theoretical logarithmic profile previously discussed by Calvert (2004) The effect of the artificial fences of varied height and varied gap distance between them in reducing the wind velocity were measured. The results obtained were used to validate the simulation models. A schematic diagram of the wind tunnel experiment on artificial fences is shown in Fig. 2. Aside from fences, wind tunnel experiments on tree windbreaks were conducted. However, this time, the actual tree was utilized in the study. The Black pine tree (pinus thunbergii) was chosen as the experimental tree being one of the most typical natural windbreaks popular in the country. The tree under investigation was positioned inside the wind tunnel. The pressure and wind velocity before and after the tree windbreaks were measured. The schematic diagram of wind tunnel experiment on actual trees is shown in Fig. 3. The numerical simulation study utilized the computational fluid dynamics (CFD) technique to quantitatively and qualitatively measure the effect of artificial fences and natural trees in reducing wind velocity. Initially, the CFD code utilized in the models were initially validated from the field experiment as well as wind tunnel studies. RESULTS AND DISCUSSION CFD validation For the artificial fences, field experimental data particularly the actual wind velocity and the computed turbulence presented in Fig. 4 were used in the simulation model and the results were compared with wind tunnel results. The validation of the CFD simulation models on the artificial fences incurred an error approximately 7% when the overall percentage differences of the measured velocities were compared with wind tunnel results. Generally, the error can be considered acceptable among modeling experts. In terms of tree windbreaks, the drag coefficient value presented in Table 1 obtained from wind tunnel experiment was used to check the tree simulation models. The mesh size utilized at the tree region was adjusted in order to obtain similar drag coefficient value from the wind tunnel result. When the similar value was obtained, the mesh size used in the model was chosen and implemented in the subsequent simulations. Effect of fence windbreaks The artificial fences have shown to decrease the wind velocity from approximately 22% to 93% The percentage decrease depends on the height of the fence, the gap distance between the fences and the fence porosity. The maximum percentage decrease was obtained when the fence height, fence porosity, and gap distance between fences was 0.8 m, 0.2 and 2 m, respectively. In terms of fence height and an optimum porosity of 0.2, no significant differences in reducing the wind velocity was observed. However, for solid fences it can observe that the wind velocity can be significantly reduced when the fence height was increased. For the gap distance between fences, the results revealed only a gradual decrease of wind velocity when the gap distance was increased to 2, 4 and 6 m. This would suggest that for maximum used of the fence windbreaks, a gap distance of 6 m is very reasonable and appropriate. Effect of tree windbreaks Visualization of flow as affected by tree windbreaks is shown in Fig. 5. Quantitative analysis of the simulation results 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. CONCLUSIONS In this study, field experiment, wind tunnel test and numerical simulations were utilized. Results of the field experiments were used to complement the simulation modeling techniques. However, the simulation procedure was initially validated from wind tunnel data to establish its reliability. The simulation technique has shown its capability to analyze and investigate wind flow as affected by windbreaks. Results obtained in the study have shown the effectiveness of artificial fences and natural trees as windbreaks. One of their primary roles can be to minimize wind velocity to a value below the threshold velocity for dust diffusion. For the maximum use of artificial fences, porous fence of approximately 0.2 but no higher than 0.4 is highly recommended. In this condition, the wind velocity can be reduced up to 93% The variation of fence height (0.6, 0.8 and 1.0 m) was found to show no significant differences in terms of reducing the wind velocity. However, for very practical reasons, 0.6 m is suggested. In reclaimed lands where dust source areas are usually large, fences in array at 6 m apart is very reasonable and appropriate. For tree windbreaks, the wind velocity at the tree region can be reduced up to 70 to 90% depending on the arrangement of the trees. Maximum reduction on wind velocity can be obtained when two rows of alternately arranged trees are implemented. The wind velocity at the streamwise extent can be reached at 15H where H is the tree height. The percentage reduction at 15H could reach up to 50% For long term consideration especially in reclaimed lands, tree windbreaks are highly recommended. ACKOWLEDGEMENTS Thanks to the Saemanguem Project Office, Korea Rural Community and Agriculture Corporation for financial assistance extended and to the National Institute of Agricultural Engineering of Korea for the use the wind tunnel. Literature Cited Bitog, J.P. Lee, I.B. Shin, M.H. Hong, S.W. Hwang, H.S. Seo, I.H. Yoo, J.I. Kwon, K.S. Kim, Y.H. Han, J.W. 2009. Numerical simulation of an array of fences in Saemangeum reclaimed land. Atmos. Enviro.43: 4612 4621. Calvert, J.B. 2004. Wind: An exploration of the wind. http:/mysite.du.edu/~etuttle/weather/wind.htm. Korea Statistical Information Service, 2006. Zhou, X.H. Brandle, J.R. Mize, C.W. Takle, E.S. 2004. Three dimensional aerodynamic structure of a tree shelterbelt: Definition, characterization and working models. Agroforestry systems 63: 133 147.