For submerged vegetated flow, the velocity profile has two distinctive distributions in the vegetation layer in the lower region and the surface layer in the upper non-vegetated region. Based on a mixing-layer analogy...For submerged vegetated flow, the velocity profile has two distinctive distributions in the vegetation layer in the lower region and the surface layer in the upper non-vegetated region. Based on a mixing-layer analogy, different analytical models have been proposed for the velocity profile in the two layers. This paper evaluates the four analytical models of Klopstra et al., Defina & Bixio, Yang et al. and Nepf against a wide range of independent experimental data available in the literature. To test the applicability and robust of the models, the author used the 19 datasets with various relative depths of submergence, different vegetation densities and bed slopes (1.8 × 10?6 - 4.0 × 10?3). This study shows that none of the models can predict the velocity profiles well for all datasets. The three models except Yang’s model performed reasonably well in certain cases, but Yang’s model failed in most the cases studied. It was also found that the Defina model is almost the same as the Klopstra model, if the same mixing length scale of eddies (λ) is used. Finally, close examination of the mixing length scale of eddies (λ) in the Defina model showed that when λ/h = 1/40(H/h)1/2, this model can predict velocity profiles well for all the datasets used.展开更多
The flow structure and geomorphology of rivers are significantly affected by vegetation patterns. In the present study, the effect of vegetation in the form of discontinuous and vertically double layered patches parti...The flow structure and geomorphology of rivers are significantly affected by vegetation patterns. In the present study, the effect of vegetation in the form of discontinuous and vertically double layered patches particularly on the resulting flow turbulence was examined computationally in an open channel. A k-ɛ model was implemented in this research work which was developed using 3-D numerical code FLUENT (ANSYS). After the validation process of numerical model, the impact of discontinuous layered vegetation patches on the flow turbulence was investigated against varying vegetation density and patch length. The mean stream-wise velocities at specified positions showed larger spatial fluctuations directly upstream and downstream of vegetation elements, whereas sharp inflections in the profiles were witnessed at the top of smaller submerged elements i.e. z/hs= 1 (where z is the flow depth and hs is the smaller vegetation height). The reduction in flow velocity due to tall vegetation structure was more as compared with that of short vegetation. The mean velocity in the patch regions was visibly higher than that in the gap regions. The profiles of turbulent flow properties showed more rise and fall within the patches with a high vegetation density i.e. Ss/d= 4;and St/d= 8 (where Ss/d and St/d are the smaller and taller vegetation spacing, while d is the vegetation diameter) as compared with low vegetation density i.e. Ss/d= 8;and St/d= 16. The turbulent flow structure in the large patch and gap regions was found to be more stable than that in the small patches and gaps;whereas, due to the variation in distribution form of the patch, turbulence is relatively unaffected, and the flow structure variation is low. Turbulence was observed to be large, followed by a saw-tooth distribution within the patches;whereas, low turbulence is observed in the non-vegetation regions. The turbulent intensity acquired maximum of 13% turbulence for dense vegetation arrangement as compared to that of sparse arrangement having maximum of 9% turbulent intensity. A noteworthy rise in turbulent kinetic energy and turbulent intensity was witnessed as the flow passed through the vegetated regions. Hence, a non-uniform flow was observed through discontinuous and double layered vegetation patches.展开更多
Aquatic vegetation can influence the transport of sediment and contaminants by changing the mean velocity and turbulent flow structure in channels. It is important to understand the hydraulics of the flows over vegeta...Aquatic vegetation can influence the transport of sediment and contaminants by changing the mean velocity and turbulent flow structure in channels. It is important to understand the hydraulics of the flows over vegetation in order to manage fluvial processes. Experiments in an open-channel flume with natural vegetation were carried out to study the influence of vegetation on the flows. In a half channel with two different densities of vegetation, the flow velocity, Reynolds stresses, and turbulence intensities were measured using an Acoustic Doppler Velocimeter (ADV). We obtained velocity profiles in the lateral direction, Reynolds stresses in the vertical direction, and the flow transition between the vegetated and non-vegetated zones in different flow regimes. The results show that the streamwise velocity in the vegetated zone with higher density is almost entirely blocked. Reynolds stress distribution distinguishes with two different regions: inside and above the vegetation canopies. The turbulence intensities increase with increasing Reynolds number. The coherent vortices dominate the vertical transport of momentum and are advected clockwise between the vegetated zone and non-vegetated zone by secondary currents (a relatively minor flow superimposed on the primary flow, with significantly different speed and direction), generated by the anisotropy of the turbulence.展开更多
文摘For submerged vegetated flow, the velocity profile has two distinctive distributions in the vegetation layer in the lower region and the surface layer in the upper non-vegetated region. Based on a mixing-layer analogy, different analytical models have been proposed for the velocity profile in the two layers. This paper evaluates the four analytical models of Klopstra et al., Defina & Bixio, Yang et al. and Nepf against a wide range of independent experimental data available in the literature. To test the applicability and robust of the models, the author used the 19 datasets with various relative depths of submergence, different vegetation densities and bed slopes (1.8 × 10?6 - 4.0 × 10?3). This study shows that none of the models can predict the velocity profiles well for all datasets. The three models except Yang’s model performed reasonably well in certain cases, but Yang’s model failed in most the cases studied. It was also found that the Defina model is almost the same as the Klopstra model, if the same mixing length scale of eddies (λ) is used. Finally, close examination of the mixing length scale of eddies (λ) in the Defina model showed that when λ/h = 1/40(H/h)1/2, this model can predict velocity profiles well for all the datasets used.
文摘The flow structure and geomorphology of rivers are significantly affected by vegetation patterns. In the present study, the effect of vegetation in the form of discontinuous and vertically double layered patches particularly on the resulting flow turbulence was examined computationally in an open channel. A k-ɛ model was implemented in this research work which was developed using 3-D numerical code FLUENT (ANSYS). After the validation process of numerical model, the impact of discontinuous layered vegetation patches on the flow turbulence was investigated against varying vegetation density and patch length. The mean stream-wise velocities at specified positions showed larger spatial fluctuations directly upstream and downstream of vegetation elements, whereas sharp inflections in the profiles were witnessed at the top of smaller submerged elements i.e. z/hs= 1 (where z is the flow depth and hs is the smaller vegetation height). The reduction in flow velocity due to tall vegetation structure was more as compared with that of short vegetation. The mean velocity in the patch regions was visibly higher than that in the gap regions. The profiles of turbulent flow properties showed more rise and fall within the patches with a high vegetation density i.e. Ss/d= 4;and St/d= 8 (where Ss/d and St/d are the smaller and taller vegetation spacing, while d is the vegetation diameter) as compared with low vegetation density i.e. Ss/d= 8;and St/d= 16. The turbulent flow structure in the large patch and gap regions was found to be more stable than that in the small patches and gaps;whereas, due to the variation in distribution form of the patch, turbulence is relatively unaffected, and the flow structure variation is low. Turbulence was observed to be large, followed by a saw-tooth distribution within the patches;whereas, low turbulence is observed in the non-vegetation regions. The turbulent intensity acquired maximum of 13% turbulence for dense vegetation arrangement as compared to that of sparse arrangement having maximum of 9% turbulent intensity. A noteworthy rise in turbulent kinetic energy and turbulent intensity was witnessed as the flow passed through the vegetated regions. Hence, a non-uniform flow was observed through discontinuous and double layered vegetation patches.
基金supported by the National Basic Research Program of China (973 Program, 2008CB418203)the National Natural Science Foundation of China (Grant No. 50709009)the Elitist Support Project of Ministry of Education (Grant No. NCET-07-0254)
文摘Aquatic vegetation can influence the transport of sediment and contaminants by changing the mean velocity and turbulent flow structure in channels. It is important to understand the hydraulics of the flows over vegetation in order to manage fluvial processes. Experiments in an open-channel flume with natural vegetation were carried out to study the influence of vegetation on the flows. In a half channel with two different densities of vegetation, the flow velocity, Reynolds stresses, and turbulence intensities were measured using an Acoustic Doppler Velocimeter (ADV). We obtained velocity profiles in the lateral direction, Reynolds stresses in the vertical direction, and the flow transition between the vegetated and non-vegetated zones in different flow regimes. The results show that the streamwise velocity in the vegetated zone with higher density is almost entirely blocked. Reynolds stress distribution distinguishes with two different regions: inside and above the vegetation canopies. The turbulence intensities increase with increasing Reynolds number. The coherent vortices dominate the vertical transport of momentum and are advected clockwise between the vegetated zone and non-vegetated zone by secondary currents (a relatively minor flow superimposed on the primary flow, with significantly different speed and direction), generated by the anisotropy of the turbulence.