The Experimental Study On The Change Law Of The Flow Structure With Different Vegetation

This paper combines with the project supported by National Natural ScienceFoundation: study on the coupling law of riparian vegetation and river dynamic.Through the field survey to the species distribution of the riparian vegetation, weselect three plastic model vegetation which are similar to the natural hydrophyte asresearch objects(two of them are submerged plant, the other one is emerging plant).The research contents are mainly divided into two parts: one is the characteristic ofvertical velocity distribution without and with plants, another is the change law offlow structure with different vegetation.The measuring instrument for this experiment is the Acoustic DopplerVelocimetry. At first, by analyzing the characteristic of vertical velocity distributionwithout and with plants, we can get: the characteristic of vertical velocity distributionaccord with the logarithmic distribution law theoretically without vegetation, whilepresent to be the shape of ‘S’ with submerged plant and ‘>’ with emerging plant,further more, the ‘S’ distribution is made up of the internal area, transition region andtop area.Then, by analyzing the change law of flow structure with different vegetation wecan find that the impact of the density change of the plants on vertical velocity isobvious. The more the plant density is, the more resistance the flow will get,consequently, the smaller velocity tends to be. When there is no vegetation in theflume, the vertical turbulence intensity and Reynolds stress are relatively small,especially for the Reynolds stress which almost becomes zero except a little increaseat bottom. When there are submerged plants, the vertical turbulence intensity andReynolds stress increase obviously and the distribution between them are similar thatthey both get their maximum at the point of plant canopy which demonstrates thatacutely momentum interaction exists in this area. And the more the plant density is,the bigger they are. When there are emerging pants, the distribution of verticalturbulence intensity and Reynolds stress are similar to the corresponding one withoutvegetation, but bigger in numerical value.Finally, by analyzing the impact of vegetation on flow resistance, it is obviouslythat the water level rises and the flow resistance increases and the more the plant density is, the higher the water level is, the more strong the flow resistance will be.Following with the increasing of the depth of water, the flow resistance decreases forsubmerged plants, while increases for emerging plants.