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| DOI：10.13522/j.cnki.ggps.2019428 |
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| Study on Velocity Distribution of Flow through Rigid Rough Element in a Channel |
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TANG Liangchuan, TANG Zhong, LI Tongshu, HAN Yu
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1.China Agricultural University, Beijing 100083, China；2. Kunming Engineering Corporation Limited, Powerchina, Kunming 650051, China
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| Abstract: |
| 【Background】 Vegetation is found in natural catchments and rivers in different forms and is of utmost importance for hydrologists and hydraulic engineers as related to flood-risk studies, sediment-transport studies and the design of river restoration schemes. However, the riparian vegetation has a great influence on the river resistance, which could raise water level and lead to flood disasters. The analysis of the interaction between fluid flow and vegetation is a fairly complex issue, due to both the different physical mechanisms that play a role in the phenomenon, and the biomechanical properties that characterize the different types of vegetation. Therefore, it’s important to improve the research of rough channels and guide the ecological restoration of rivers.【Objective】We aimed to study the influence of different rough elements in the channel on the flow velocity structure, and determine the distribution law of flow velocity containing rough elements, so as to lay a foundation for further study on the water conveyance capacity of the channel containing rough elements.【Method】In this study, a three-dimensional Acoustic Doppler Velocimetry (ADV) was used to collect velocity at different positions after the flow was stabilized, and ADV instantaneous measurements were noted as time-averaged u for the stream-wise, time-averaged v for the transverse, time-averaged w for the vertical flow velocity, and time-averaged V for the total velocity. In order to render the variables dimensionless, all velocity and energy related variables were normalized with the depth-averaged and time-averaged stream-wise velocity u0 at the farthest upstream, where the flow is considered to be undisturbed. The flow distribution around the rigid roughness element under different discharges, configurations and shapes was analyzed, along with the stream-wise direction and the distribution of longitudinal flow velocity u was studied emphatically. 【Result】①In the emergent cylinder test, distribution of velocity u is the same at different discharges and roughness rows, which may be because the cylinder diameter is small enough and the spacing between cylinders is large. ②The submerged rough element forms a three-layer velocity profile in the y direction. The first layer is located at the lower position behind the rough element, the second layer at the upper position behind the rough element, and the third layer at the upper position above the rough element; Different types of submerged rough elements have significant influence on the flow structure. ③Based on the model experiment, the nonlinear fitting method was adopted for the emergent cylinder test data, and the fitting formulas of the longitudinal velocity distribution related to the upstream and downstream of a single cylinder was obtained; ④The dimensional analysis shows that the downstream longitudinal velocity u of submerged rough element in a channel is related to flow characteristics, channel characteristics and roughness characteristics, like channel slope J, hydraulic radius R, channel width-depth ratio B/h, surface retention Ac of rough elements, submersion degree hc/h, arrangement of rough elements (such as horizontal and longitudinal spacing of rough elements), and distance x behind rough elements, etc. 【Conclusion】The influence of submerged rough elements and emergent rough elements on velocity distribution is different. Under submerged conditions, the longitudinal velocity distribution behind rough elements formed a three-layer velocity profile. Under emergent conditions, the longitudinal velocity distribution behind rough elements formed a “check” mark. |
| Key words: rigid rough elements; velocity distribution; open channel; hydraulic characteristics; dimensional analysis; ADV |
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