جستجوی مقالات مرتبط با کلیدواژه "سرعت بحرانی" در نشریات گروه "آب و خاک"

In order to understand the mechanism of flow patterns in vegetated channels, the flow located in a rectangular channel was numerically investigated by using OpenFOAM software. Firstly, two solvers of that software (i.e. icoFoam and pimpleFoam) were used to calculate the velocity profiles in both longitudinal and cross-sectional directions for four selected sections in a rectangular channel with a square cylinder. By comparing the simulation results with the available data, the icoFoam solver with a better performance (six Percent Error) was selected for the next developed model. A new model was then created with two tandem square cylinders with spacing ratios of two and a half and five. Flow patterns, velocity distribution and pressure characteristics in the channel with different inlet flow velocities were investigated for two cases. It was observed that a flow field disturbance occurred in all simulations and the current changed from steady state to unsteady one at a critical velocity. This instability occurred in a distance between the cylinders for the spacing ratio of five at an average Reynolds number of eight, while for the ratio of two and a half it is occurred at an average Reynolds number of 32. The maximum values ​​of longitudinal and transverse velocity timelines in a period of 200 seconds for four states (including two Reynolds numbers and two different spacing ratios) were plotted in two spatial ranges and fully investigated. According to the results, it can be said that the overlap has an important role on the flow characteristics in tandem arrangements and by increasing the distance ratio between the cylinders by 55 percent, the critical velocity value decreases by 74 percent.

Solid particles transport in fluid flow is a common phenomenon in nature, and its typical example in hydraulic science is sediment transport in rivers. In a channel with a bed of sedimentary materials, there are no moving particles in the streams with a low discharge and remain steady in their place. As the particle velocity increases, the bed particles begin to move, which is called the threshold of motion of the sediment particles. In this study, using a numerical model based on OpenFOAM software, a two-dimensional simulation of the threshold of group motion of sediment particles in turbulent flow has been investigated. The approach used in the simulation is Eulerian-Lagrangian, in which the continuous phase is emulated in a continuous manner in the form of an Eulerian with finite volume method and sediment particles are simulated in a Lagrangian and particle tracing method. In the current model, the effect of fluid on the particle, the particle on the fluid, and the forces that the particles of the sediment enter to each other are considered. In order to consider the effect of sediment particles inner interacting, a soft contact approach and a friction damper spring impact model were used. The results of the numerical model have been compared with other researchers in the form of critical velocity method and a relation has been proposed for the particle motion threshold criterion. Also, Incipient motion of sediment particles with different criteria has been studied. The results show that the present model can simulate the phenomenon well with proper accuracy.

Rigid boundary channels are widely used as water conveyors in surface drainage، sewers and other systems. In order to do optimum and economic design، consideration of the movement of sediment particles is quite important. In this research، experimental data of incipient deposition and incipient motion of sediment particles are used. Applying critical velocity approach، new equations for incipient deposition of sediment particles in rigid boundary channels having circular، rectangular and U-shaped cross sections and also a new incipient motion equation for a V-shaped cross section are presented. These equations are compared with available equations given by critical velocity approach for incipient motion of sediment particles in rigid and loose boundary channels. The present research reveals the difference between incipient deposition and incipient motion، indeed. Herein، the effect of cross sectional shape on incipient deposition and incipient motion of sediment particles in rigid boundary channels are also considered. Analysis of the results using the estabilished equations for incipient deposition of the sediment particles in channels with different cross section shapes shows that rectangular channels require a lower critical velocity and U-shaped channels need a higher amount. This result clearly states the effect of cross sectional shape on incipient deposition for sediment particles in rigid boundary channels. Because of the lack of available data، the present analysis can be usefull for developing numerical models in rigid boundary channels.

Different methods have been proposed to prevent or reduce scouring around bridge piers. Making use of collars as countermeasures has recently been noticed by researchers. Collar dimensions have great impact on reducing local scour around bridge piers. This study showed that rectangular collars around circular piers reduced both intensity of scouring and depth of scour hole. In this study the effects of collar length in up and down-stream of the pier model and also on its width were investigated. The dimensionless length of rectangular collar in upstream (Lu/D) and downstream (Ld/D) found to be 0.92 and 1.42, respectively, and the width of collar was estimated 3 times greater than the pier diameter. It is noteworthy to mention that with these dimensions scouring process was rarely observed around the pier model after elapsing 62 hours.