Presented in this paper is a mathematical model to calculate the probability of the sediment incipient motion, in which the effects of the fluctuating pressure and the seepage are considered. The instantaneous bed she...Presented in this paper is a mathematical model to calculate the probability of the sediment incipient motion, in which the effects of the fluctuating pressure and the seepage are considered. The instantaneous bed shear velocity and the pressure gradient on the bed downstream of the backward-facing step flow are obtained according to the PIV measurements. It is found that the instantaneous pressure gradient on the bed obeys normal distribution. The probability of the sediment incipient motion on the bed downstream of the backward-facing step flow is given by the mathematical model. The predicted results agree well with the experiment in the region downstream of the reattachment point while a large discrepancy between the theory and experiment is seen in the region near the reattachment point. The possible reasons for this discrepancy are discussed.展开更多
The presence of submerged vegetation on river beds can change the water flow structure and alter the state of sediment motion. In this study, the incipient motion of sediment in the presence of submerged flexible vege...The presence of submerged vegetation on river beds can change the water flow structure and alter the state of sediment motion. In this study, the incipient motion of sediment in the presence of submerged flexible vegetation in open channels was investigated in a laboratory experiment. The vegetation was simulated with flexible rubber cylinders arranged in parallel arrays. The effect of the vegetation density, water depth, and sediment grain size on the incipient motion was investigated. The experimental results indicate that the incipient motion velocity of sediment increases as the vegetation density decreases and the water depth and sediment grain size increase. With flexible plants, the incipient motion velocity of sediment is lower than it is without vegetation, and is larger than it is with rigid vegetation. A general incipient motion velocity equation was derived, which can be applied to both flexible and rigid vegetation conditions.展开更多
Sediment incipience under flows passing a backward-facing step was studied. A series of experiments were conducted to measure scouring depth, probability of sediment incipience, and instantaneous flow velocity field d...Sediment incipience under flows passing a backward-facing step was studied. A series of experiments were conducted to measure scouring depth, probability of sediment incipience, and instantaneous flow velocity field downstream of a backward-facing step. Instantaneous flow velocity fields were measured by using Particle Image Velocimetry (PIV), and an image processing method for determining probability of sediment incipience was employed to analyze the experimental data. The experimental results showed that the probability of sediment incipience was the highest near the reattachment point, even though the near-wall instantaneous flow velocity and the Reynolds stress were both much higher further downstream of the backward-facing step. The possible me- chanisms are discussed for the sediment incipience near the reattachment point.展开更多
Incipient sediment motion plays a key role in scouring and bed load transport. However, the incipient sediment motion in the vegetated open-channel flows has yet to be fully understood. This study aims to quantify the...Incipient sediment motion plays a key role in scouring and bed load transport. However, the incipient sediment motion in the vegetated open-channel flows has yet to be fully understood. This study aims to quantify the critical conditions of the sediment particle movement in the presence of emergent and submerged vegetation. A new formula of the critical flow velocity was proposed to predict the incipient sediment motion based on the force balance equation of a sediment particle and the assumption that the velocity distribution in the bed roughness boundary layer fits the logarithmic law. Analysis of the derived formula revealed that the critical flow velocity for incipient sediment motion decreases with the increase in vegetation density. The proposed formula agrees well with the experimental data in the literature, thereby implying that the critical flow velocity can effectively quantify the incipient sediment motion in the vegetated open channel flows.展开更多
Flow over a backward-facing step was studied to investigate the effect of large-scale vortex structures on sediment incipience. The transient flow velocity field at the downstream of the backward-facing step was obtai...Flow over a backward-facing step was studied to investigate the effect of large-scale vortex structures on sediment incipience. The transient flow velocity field at the downstream of the backward-facing step was obtained using the technique of Particle Tracking Velocimetry （PTV）. The optical amplification technique was employed to measure the instantaneous flow velocities near the bed and the instantaneous bed shear stress was given. The experimental observations revealed a new insight into the oscillation of the large-scale structure and the three-dimensional characteristics of the flow. In particular, very high turbulence intensity, instantaneous horizontal velocity near the bed and the bed shear stress near the reattachment point were observed. The sediment incipient probability obtained from the sequent images of sediment particles near the bed indicates that the critical instantaneous shear stress of the sediment incipience is independent of flow conditions.展开更多
基金supported by the National Natural Science Foundation of China (Grant No.10602017)
文摘Presented in this paper is a mathematical model to calculate the probability of the sediment incipient motion, in which the effects of the fluctuating pressure and the seepage are considered. The instantaneous bed shear velocity and the pressure gradient on the bed downstream of the backward-facing step flow are obtained according to the PIV measurements. It is found that the instantaneous pressure gradient on the bed obeys normal distribution. The probability of the sediment incipient motion on the bed downstream of the backward-facing step flow is given by the mathematical model. The predicted results agree well with the experiment in the region downstream of the reattachment point while a large discrepancy between the theory and experiment is seen in the region near the reattachment point. The possible reasons for this discrepancy are discussed.
基金supported by the China National Funds for Distinguished Young Scientists(Grant No.51125034)the State Key Program of the Nationa Natural Science Foundation of China(Grant No.51239003)the Nationa Natural Science Foundation of China(Grants No.51109066 and 51109065)
文摘The presence of submerged vegetation on river beds can change the water flow structure and alter the state of sediment motion. In this study, the incipient motion of sediment in the presence of submerged flexible vegetation in open channels was investigated in a laboratory experiment. The vegetation was simulated with flexible rubber cylinders arranged in parallel arrays. The effect of the vegetation density, water depth, and sediment grain size on the incipient motion was investigated. The experimental results indicate that the incipient motion velocity of sediment increases as the vegetation density decreases and the water depth and sediment grain size increase. With flexible plants, the incipient motion velocity of sediment is lower than it is without vegetation, and is larger than it is with rigid vegetation. A general incipient motion velocity equation was derived, which can be applied to both flexible and rigid vegetation conditions.
基金National Natural Science Foundation of China (No.10602017)Maritime Research Center and DHI-NTU Center of Nanyang Technological University, Singapore
文摘Sediment incipience under flows passing a backward-facing step was studied. A series of experiments were conducted to measure scouring depth, probability of sediment incipience, and instantaneous flow velocity field downstream of a backward-facing step. Instantaneous flow velocity fields were measured by using Particle Image Velocimetry (PIV), and an image processing method for determining probability of sediment incipience was employed to analyze the experimental data. The experimental results showed that the probability of sediment incipience was the highest near the reattachment point, even though the near-wall instantaneous flow velocity and the Reynolds stress were both much higher further downstream of the backward-facing step. The possible me- chanisms are discussed for the sediment incipience near the reattachment point.
基金supported by the National Natural Science Foundation of China(Grant Nos.52020105006,11872285).
文摘Incipient sediment motion plays a key role in scouring and bed load transport. However, the incipient sediment motion in the vegetated open-channel flows has yet to be fully understood. This study aims to quantify the critical conditions of the sediment particle movement in the presence of emergent and submerged vegetation. A new formula of the critical flow velocity was proposed to predict the incipient sediment motion based on the force balance equation of a sediment particle and the assumption that the velocity distribution in the bed roughness boundary layer fits the logarithmic law. Analysis of the derived formula revealed that the critical flow velocity for incipient sediment motion decreases with the increase in vegetation density. The proposed formula agrees well with the experimental data in the literature, thereby implying that the critical flow velocity can effectively quantify the incipient sediment motion in the vegetated open channel flows.
基金Project supported by the National Natural Science Foundation of China(Grant No.10602017).
文摘Flow over a backward-facing step was studied to investigate the effect of large-scale vortex structures on sediment incipience. The transient flow velocity field at the downstream of the backward-facing step was obtained using the technique of Particle Tracking Velocimetry （PTV）. The optical amplification technique was employed to measure the instantaneous flow velocities near the bed and the instantaneous bed shear stress was given. The experimental observations revealed a new insight into the oscillation of the large-scale structure and the three-dimensional characteristics of the flow. In particular, very high turbulence intensity, instantaneous horizontal velocity near the bed and the bed shear stress near the reattachment point were observed. The sediment incipient probability obtained from the sequent images of sediment particles near the bed indicates that the critical instantaneous shear stress of the sediment incipience is independent of flow conditions.
