The Effect Of Vegetation Rigidity On Flow Resistance

In order to study the mechanism of flow resistance exerted by vegetation in open channel, vegetation rigidity has been researched and a dimensionless number, F, was introduced to describe vegetation ability to resist bending in this paper. Physical experiment, Particle Image Velocimetry (PIV) measurement and numerical simulation were used to study the effect of vegetation rigidity on flow resistance in submerged conditions.In PIV measurement, 2-D data collection techniques were used to gain vector field of flow around vegetation with different vegetation rigidity under similar flow conditions. The result shows that when vegetation exists in channel, flow structure is more complex, with abundant whirlpools signifying violent turbulence. And as the decrease of vegetation rigidity, flow field intends to be stable.In physical experiment, the correlation between F and vegetation drag coefficient, as well as vegetation roughness, were analyzed. The result indicates that, under the same condition of vegetation density and arrangement, when F is smaller than 12.2371, the vegetation behaves absolute erectness and the vegetation roughness coefficient and drag coefficient have no relation with F. When F is larger than 12.2371, the vegetation roughness coefficient and drag coefficient decrease as F increases.In numerical stimulation, flow field, flow velocity distribution and total pressure distribution on vegetation surface were studied around vegetation of three different statuses, i.e. vertical, acclivitous and lying. The result demonstrates that the turbulence effect of vertical vegetation is larger than that of acclivitous vegetation, and much larger than lying vegetation. The pressure on vegetation front surface is larger than that on back surface. And the pressure difference decreases with the lying of vegetation, while viscous force increases.