Windbreak Effects Of Shelterbelt In Oases Based On Wind Velocity Flow Field

The Ulan Buh Desert Oasis, as one of the most fragile ecosystem, faces serious problems such as vegetation degeneration, wetland shrink, an increasing degree of soil salinization and soil erosion. Windbreak system and shelterbelt of oasis is the most important line of defense to protect the ecological safety and stable development of oasis production construction. There are many years successful experience of shelterbelt construction in the Ulan Buh Desert Oasis, so conducting a comprehensive, systematic and arrurate evaluation to the existing system of windbreak system and shelterbelt mode has significant scientific value and realistic significance for providing the demonstration mode and practice pattern of rebuilding a new system and restoring the damaged ecosystem. The research was aimed to study five different types of the shelter belt (net) models, which were founded in the early stage of Ulan Buh Desert Oasis. Through simultaneously observed of wind speed in different shelterbelts and analyse the wind velocity flow field、statistics parameters of wind velocity、frequency distribution of wind velocity、semivariance function model and wind-protection efficiency so that we could explore different windbreak effects of different shelter belt (net) model and select more effective couple. Based on wind tnunnel experiment, we further conduct the windbreak effects research under different wind speed sequence by using selected shelter belt (net) model, which contains single forest belt, single network and superposition forest network. Then on this basis, we tend to put forward the new construction model.The main conclusions of the research are as follows:(1) The field research indicate that widely spaced large grid shelterbelt 2, the small grid only tree shelterbelt 4 and the small grid shelterbelt 3 comprises both bushes and trees all play an active role in windbreak effects.96.4% of shelterbelt 2 process more than 70% in wind-protection efficiency.92% of shelterbelt 3 process more than 55% in wind-protection efficiency. The effective protection ratio of shelterbelt 3 is 2.5 times of shelterbelt 4.(2) The field research revealed that the wind direction change the distribution of wind field and reduce about 5%,10%,18%,15% and 0% wind-protection efficiency in this 5 different shelterbelts. The influence of wind direction in the large grid shelterbelt is large than that in small grid shelterbelt because of the disturbance of wind direction in small grid shelterbelt is very slight. The opening of "U" type shelterbelt shouldn’t be set towards the harmful wind direction in case of disturbing and diminishing the effect of wind protection in the shelterbelt. The only tree shelterbelt in "□" type can minimize the influence of the wind direction changed. Making full use the protection of long auxiliary forest belt of shelterbelt, the shelterbelt processes more steady wind velocity field and wind-protection efficiency.(3) The wind tunnel experiment indicate that the close spacing forest belt and the high forest belt height play an activity role in windbreak effects. The effective protected area sequence of forest belts is 6 cm height close spacing forest belt> 8 cm height widely spaced forest belt> 6 cm height widely spaced forest belt. The structure of forest belt is one of the main influence factors to wind-protection efficiency of forest belt. Because of the different between forest belt structures, the effective protected area and the effective protection ratio of 6 cm height close spacing forest belt is 2367.9 cm2 (47.3%) larger than 6 cm height widely spaced forest belt. The height of forest belt is one of the other main influence factors to wind-protection efficiency of forest belt. The effective protected area and the effective protection ratio of 8 cm height widely spaced forest belt is 1373.8 cm2 (21.2%) larger than 6 cm height widely spaced forest belt.(4) The wind tunnel experiment indicate that the different shelterbelt models (D=10H) plays an important role in wind-protection efficiency. When the wind-protection efficiency reach at over 50%, the effective protected area and the effective protection ratio sequence of each shelterbelt is 8 cm shelterbelt comprises both bushes and trees 5407.2 cm2 (75.1%)> 6 cm shelterbelt comprises both bushes and trees 5025.6 cm2 (69.8%)> 6 cm shelterbelt interpanting bushes and trees 4816.8 cm2 (66.9%)> 8 cm only tree shelterbelt 4161.6 cm2 (50.3%)> 6 cm only tree shelterbelt 3038.4 cm2 (42.2%). The shelterbelt comprises both bushes and trees and bushes and trees model all play an active role in windbreak effect. When the wind-protection efficiency reach at higher value, the shelterbelt interpanting bushes and trees model have the best windbreak effect, meanwhile this shelterbelt model at the same level of vegetation coverage compare to shelterbelt model comprises both bushes and trees.(5) The wind tunnel experiment indicate that the different shelterbelt models (D=6H) plays an important role in wind-protection efficiency. When the wind-protection efficiency reach at over 50%, the effective protected area and the effective protection ratio sequence of each shelterbelt is 8 cm shelterbelt comprises both bushes and trees> 6 cm shelterbelt comprises both bushes and trees> 8 cm only tree shelterbelt> 6 cm only tree shelterbelt. The effective protection ratio of 6 cm shelterbelt comprises both bushes and trees model is 2.1 times to 6 cm only tree shelterbelt model and the effective protection ratio is 38.1% larger than only tree shelterbelt. The effective protection ratio of 8 cm shelterbelt comprises both bushes and trees model is 1.6 times to 8 cm only tree shelterbelt model and the effective protection ratio is 29.1% larger than only tree shelterbelt. As is shown, the shelterbelt comprises both bushes play an active role than only tree shelterbelt. The heights of shelterbelt have an active infulecnce to each shelterbelt model, the effective protection ratio of 8 cm only tree shelterbelt is 14.3% larger than 6 cm only tree shelterbelt model. The effective protection ratio of 8 cm o shelterbelt comprises both bushes and trees is 5.3% larger than 6 cm shelterbelt comprises both bushes and trees model(6) The wind tunnel experiment indicate that the windbreak effect changes of superimposed only tree shelterbelt (D=6H). The difference between maximum and minimum wind speed value of grid 1 to grid 6 decreases gradually, and the average of it shows a trend of decline as a whole. Windbreak effects increase gradually from the grid 1 and reach at maximum in the grid 3 and 4, after a transit of grid 2. Then it tends to stay at the stable value. When the wind-protection efficiency reach at over 60%, the effective protected area and the effective protection ratio of gird 2 has increased by 72.2%, grid 3, grid 4 has increased by 83.2%, grid 5 has increased by 82.6% and grid 6 has increased by 79.3% compared with the same configuration of single-shelterbelt (16.8%). The average effective protection ratio in each superimposed grid increase 5 times compared with the figure of single-shelterbelt.(7) The wind tunnel experiment indicate that the windbreak effect changes of superimposed shelterbelt comprises both bushes and trees (D=6H). The difference between maximum and minimum wind speed value of grid 1 to grid 6 decreases gradually, and the average of it shows a trend of decline as a whole. Windbreak effective transit in the grid 1, before increasing at the pick of value in grid 2 and 3, then this figure tend to be stable. When the wind-protection efficiency reach at over 70%, the effective protected area and the effective protection ratio of gird 2 and 3 has increased by 76.6%, grid 4 has increased by 72.3%, grid 5 has increased by 74.4% and grid 6 has increased by 68.3% compared with the same configuration of single-shelterbelt (23.3%). The average effective protection ratio in each superimposed grid increase nearly 3times compared with the figure of single-shelterbelt.