| Overland flow is the main force of slope erosion.The water depth is shallow,and the flow pattern is complex due to the influence of vegetation factors.With the existence of vegetation,the flow velocity,water depth,momentum and energy of the upstream flow are adjusted accordingly.However,due to the limitations of field experiment conditions and measurement methods,there are few study on the dynamic characteristics and structure of downhill flow affected by vegetation.In this paper,the flow field characteristics of typical slope flow conditions are obtained by using high-frequency and high-resolution particle image velocimetry(PIV),and Acoustic Doppler Velocimetry(ADV)was used,the flow velocity at a certain point,focuses on the characteristics of the hydraulic parameters of the simulated vegetation in different configurations,and deeply discusses the change rule of horseshoe vortex structure at the base of vegetation and the change characteristics of the local two-dimensional time average flow field of vertical vegetation.Based on the experiment of fixed bed flume in laboratory,the changes of flow dynamics on the slope surface under different densities,slope location and spatial configuration of plants were studied(1)By extracting the time average and instantaneous flow field of the vertical plane in the upstream of the observed cylinder(main cylinder),it was found that the arrangement of auxiliary cylinders at the upstream,parallel and downstream positions of the main cylinder can significantly change the high velocity area in the flow field,and the placement of auxiliary cylinders at the downstream has the best effect on the flow pattern control,and the placement conditions at the downstream correspond to the minimum values of Reynolds stress,shear stress and turbulence intensity in the observation area.Under the same flow rate,the relationship between the rotating strength of horseshoe vortex all the vegetated conditions was:pre-column condition>parallel condition>post-column condition,the number of horseshoe vortex and the distance from the bed surface were:pre-column condition<parallel condition<post-column condition;under different flow rate conditions,the distribution of velocity,shear force,turbulence strength,Reynolds stress and other dynamic fields were:pre-column condition>parallel condition>post-column condition The Reynolds stress shows a single peak characteristic with the flow rate.With the increase of Reynolds number of the cylinder,the separation point moves rapidly upstream,and the rotation intensity of the main horseshoe vortex is negatively related to the water depth;(2)The spatial patterns of 0°,15°and 45° were set with the angle between flow row direction and plant arrangement direction as variables.Under all conditions,the average velocity,Reynolds number,Froude number and flow rate of slope flow have positive correlation and negative correlation with angle.According to t-test,the resistance coefficient of 45° arrangement is significantly greater than 0°,15°.When the flow rate was greater than 1.3l/s,the resistance increasing potential of 45° arrangement was significantly reduced,with the increase of flow rate,the rotating strength of main horseshoe vortex increases and becomes more Add near the base of the plant stem.In addition,the rotation intensity of the plant was 45°>15°>0°,and the rotation intensity of the main horseshoe vortex in the front section is greatly affected by the angle change.For every 10° increase of the angle,the rotation intensity of the main horseshoe vortex increases by 3.2%,Front section accompanying plants,with the increase of the angle,the forward and backward flow velocity was ranked as 45°> 15°> 0°.After the flow velocity passes through the flora,the flow velocity at the base of the column weakens: 0°> 15 °> 45°.45° has the strongest erosion effect on the base of the vegetation,and the flow energy in the water body correspondingly decreases.The flora arranged at 45°can achieve the best water blocking and slow flow effect(3)The total hydrodynamic parameters of bare slopes were significantly different from those of vegetated slopes.The relationship between Re and f illustrated that Re was not a unique predictor of hydraulic roughness on vegetated slopes.In the slope direction,all hydrodynamic parameters on vegetated slopes exhibited fluctuating downward/upward trends due to the clocking effect before the vegetated area and the rapid conveyance effect in the vegetated area,whereas constant values were observed on bare slopes.The performance of hydrodynamics parameters suggested that the dense rearward arrangement(SMD)was the optimal vegetation pattern to regulate flow conditions.Specifically,the vertical profiles of the velocity and turbulence features of the SMD arrangement at different sections demonstrated the significant role of vegetation density in identifying the velocity layers along the water depth.The maximum velocity and Reynolds Stress Number(RSN)indicated the position where local scour was most likely to occur... |
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