The Effect Of Porosity And Spatial Distribution Patterns On The Windbreak Efficacy Of Shelterbelt

The ecological environment is harsh in arid area of Northwest of China,where desert and desertification land are widespread,and serious wind sand disaster.Through the construction of ecological protection system and sand control project,the wind speed of earth's surface can be reduced and the movement of sand blown by the wind,which is the main measures to control desertification.This text,the wind control effectiveness of 7 kinds of porosity and 2 kinds of biomass spacial distribution pattern were measured with the help of wind tunnel test.The influence of wind prevention effectiveness caused by porosities and biomass special distribution pattern was analyzed,and explored the research results was applied in wind control and sand fixation.The main conclusions are as follows:(1)Ranged from 0cm to 30 cm after the shelterbelt(2h),the windbreak efficacy of the shelterbelt can be divided into two different regions: the effective protection region(1cm-13 cm,where wind speed reduction efficacy is positive)and acceleration region(20cm-30 cm,where wind speed reduction efficacy is negative).In this two regions,the totally different influence of airflow caused by protected zone and also the influence is enhanced with the increasing of the porosity.(2)Influenced by the longitudinal structure of the model,wind speedreduction efficacy at the position of 2h after the shelterbelt,ranged from 1cm to 13 cm,was not changed obviously.With increasing of the distance,the windbreak reduction efficacy is gradually declining at the height of 5cm in the shelterbelt.And the porosity is becoming smaller,the value of wind speedreduction efficacy isgreater.The location is getting close to 4h,wind speed reduction efficacy is declining obviously ranged from 2h-24 h,At the height of 30 cm in the shelterbelt,wind speed accelerated equally with different extent ranged from 2h to 12 h,the more close to the shelterbelt,porosity of shelterbelt is getting smaller and wind speed acceleration trend is more obvious.(3)The average wind speed reduction efficacy ranged from 2h to 24 h,which w as decreased with the increasing of porosity of the shelterbelt,and when the porosity changes from 1% to 100%,wind speed reduction efficacy has three obviously vari ation interval.From 1% to 40%,wind speed reduction efficacy decrease slowly,and from 45% till 66%,wind speed reduction efficacy decrease rapidly,and when the p orosity of the shelterbelt larger than 66%,the wind speed reduction efficacy decreas e slowly with the increasing of porosity again.(4)Below the height of the shelterbelt,with wind speed of entrance has been in creased from 6 m/s to 12 m/s,the discrepancy of the wind reduction change obvious ly with the variation of porosity and distance to the shelterbelt,the more closer to th e shelterbelt or smaller porosity,the greater discrepancy.Above the height of the she lterbelt,the wind reduction discrepancy of the shelterbelt with the porosity of 1% ?40% are decrease speedy with the decreasing of height,and become similar at the he ight of 30 cm,and the variation of the shelterbelt with the porosity of 66% is smaller than the first two.(5)Changing of the spatial distribution pattern of biomass will have a significant impact on the wind speed reduction.To change the space arrangement of the lower porosity in the shelterbeltefficiency andenlarge transverse thickness and longitudinal porosity(33%)Compared with original lower horizontal porosity and longitudinal thickness biomass spatial distribution pattern,the wind speed reduction rate is obviously declining(0-13cm).At the height of 5cmranging from 2h to 24 h,wind speed reduction rateis obviously decliningwhich is higher than the single high porosity and lower than longitudinal thickness biomass space arrangement pattern(66%),it felt in between the two kinds of shelters.(6)Under the condition of maintaining biomass and total coverage coincidently in the region,change the smaller distance,multiply belts,disperse and bigger porosity shelterbelts into single belt,centered and lower porosity,get better protection efficiency with the same investment and water consumption.Under the condition of consider cost,the optimum porosity of the single strip shelterbelt should be controlled about 45%,and decreased the porosity of shelterbelt from 66% to 45%,efficiency earnings of single investment will be maximum.