The Wind-Tunnel Experiment Research Of Dynastic Mechanism About The Formation Of Vegetation Dunes In Hetian River

Vegetation dunes, commonly distributed in arid and semi-arid areas, are mounds composed of wind-borne sediment that trapped by shrubs. The dunes are extensively distributed in China, but little attention is put to it. This text chooses Hetian river valley as research area ,according to field study and measurement to the vegetation dunes's appearance,the open country prognosticate and indoors emulation experiment to the surface current of air,the analysis to the sample of vegetation dunes and the dune land,this article mainly elaborates the surface dynamic process of the vegetation dunes,the shape characteristic and the distribution regulation,the charicterristic of vegetation dunes's surface current of air and the sand rate and grain degree characteristic of vegetation dunes.The conclusions are made as followed:1. The wind velocity profile was influenced by the vegetation condition greatly. When the bottom is covered by vegetation, the wind velocity decreases at the perpendicular height near the ground for the vegetation's resistance, which breaks the logarithms regulation of the wind velocity varies with the height near the ground at the condition without vegetation. The wind velocity profile of semi-ball vegetation dune has the same variety trend with the cone vegetation dune. It indicates that different shape of vegetation dunes has little influence to the wind velocity profile.2 .The flow patterns structure of the semi-ball vegetation dune are divided into three parts: reflected vortex accelerating area at the bottom of the upwind position in front of semi-ball dune and the strong reflected vortex area on the top of the dune and the weak reflected vortex area in the back of the vegetation dune. The obvious reflected vortex area in front of semi-ball sand mound is the basic flow patterns for the foundation of the reflected sand arc. The flow patterns structure of the cone vegetation dune can also be divided into three parts: airflow accelerating area in front of at the bottom of the upwind position of cone-ball sand mound; the weak reflected vortex area at the top of sand mound and the strong reflected vortex area in the back of the vegetation dune. Reflected vortex area in front of dune is not obvious. However, reflected sand arc was not found in open country. The shape of the vegetation dune has great influence to the atmosphere flow pattern. Comparing the semi-ball vegetation dune atmosphere flow pattern. with the cone vegetation dune, the reflected vortex is strong in front of he upwind position of semi-ball sand mound, and a strong wind eroding area exits at the bottom. At the same time , the vortex is weak in the back of the vegetation dune. However, an opposite result has proved in the cone vegetation dune. According to the outdoors observation , a wind- eclipse pit always appears firstly at the top of the semi-ball vegetation dune and the phenomenon is not obvious in cone-ball vegetation dune. A shape dynamic equilibrium of the atmosphere flow is attained easily by cone-ball vegetation dune. The result of field study has proved that the conclusion is right.3 .The flow patterns structure of the semi-ball Tamarix dunes can be divided into five parts: reflected vortex area in front of the dunes; the airflow increasing area attached to the dune; airflow gathering super-speed area at the top of the plant; airflow separating area and the weak reflected vortex area in the back of vegetation dune; airflow instauration area. The reflected vortex area in frontof the Tamarix dunes have no relevance with the vegetation. The flow patterns structure of the cone-ball Tamarix dunes can also be divided into five parts: airflow increasing area attached to the dune in front of the dune; airflow gathering super-speed area at the top of the plant, airflow separating area and the weak reflected vortex area in the back of vegetation dune, airflow instauration area. There is no obvious reflected vortex area in front of the cone-ball Tamarix dune at the bottom of the upwind position .When the wind velocity<6m/s,there is always weak reflected vortex in the height of 0.5H-1.83H from the ground.4. Surface pressure of the semi-ball dunes without vegetation can be divided into four parts: the positive pressure increasing area in front of a semi-ball sand; the high pressure area at the top of the semi-ball dune ;negative pressure area in the back of the semi-ball dune and the high and low pressure alternating to distribute area between the two sides. Surface pressure of the cone-ball dunes without vegetation can be divided into three parts: positive pressure increasing area in front of a semi-ball sand; negative pressure area in the back of the semi-ball sand; the high pressure area between the two sides. Cone vegetation dunes has no obvious positive and negative area. The compression and gather function is not obvious to the air flow in front of dune.5. The surface pressure of the semi-ball dunes with vegetation can be divided into four parts: positive pressure increasing area in front of a semi-ball sand; high pressure slices area at the top of dunes; negative pressure area in the back of the semi-ball dune; the high and low pressure alternating to distribute area between the two sides. Vegetation made the high pressure break up on the top of the dunes, Pressure of being erosion is wakened on the top of the dune. The surface pressure of the cone-ball dunes with vegetation can be divide into four parts: the changing positive pressure area in front of semi-ball sand; negative pressure area at the top of the dunes; negative pressure area in the back of the dunes and the high pressure between two sides. Negative pressure area was formed by the influence of the vegetation. The wind pressure increases evenly and then decrease rapidly, which weakened the negative pressure of reflected vortex in the back of the dunes. 6. The shape of dune has a great influence to the characteristic of the surface pressure. Comparing the surface pressure of semi-ball vegetation dunes with cone vegetation dunes, a conclusion could be gained that the equilibrium pressure line at the bottom in front of the semi-ball vegetation dunes is sparser, which make the air current gather in front of the dunes and reflected vortex always formed in this area. There is a close high pressure area at the top of the dunes, so this part meets with erosion easily. At the same time, the wind pressure is weak in the negative pressure area in the back of the dunes, so the reflected vortex is weak. However, cone-ball vegetation dunes has a opposite result, the equilibrium pressure lines increase evenly from sparse to dense , the reflected vortex is weak in front of the dunes. Otherwise, the negative pressure worth is much bigger in the back of the dunes, which make the vortex stronger. There is no obvious high pressure area at the top of the dunes and the dunes are not eroded intensely by strong wind.7. The result of transporting sand rate with and without vegetation indicates that vegetation influence the wind erosion transporting sand rate and the perpendicular distribution of the transporting sand rate greatly. The total transporting sand rate has decreased rapidly for the influence of vegetation and the total transporting sand rate occupy 50.34% to the total transporting sand rate without vegetation. It indicates that vegetation has an effect of preventing the ground from being eroded.8. Both the semi-ball vegetation dune and cone vegetation dune has the same grain size variety pattern. The finest grain size all appears at the top of the sand mounds. The grain size at the back of the dune is finer than which in front of the dunes. But the average grain size of cone vegetation dune is finer than semi- ball vegetation dune. Grain size variety pattern of vegetation dune has a good relation with the surface flow construction.9. On the base of much field measurements in the field and according to the extant condition and the degree of development, the process of the formation and evolution of Vegetation dunes can be divided into embryonic stage, the stage of growth, stability and declining. But in each stage, the shapes of Vegetation dunes vary because of different plant species.