Simulation the Effect of Hydraulic and of Geometric Parameters on Water Surface Level Variation in Junction Channels by Using Fluent Model

Intersection of two flows have always been a great challenge to hydraulicians in natural or artificial channels. River channel confluences are critical interfaces where intense changesin physical processes occur. These changes affect both the local and downstream characteristics of the river flow and of the bed. There are many parameters affecting on flow patterns in this matter which cause flow complexity. These factors are relevant to not only geometry parameters like cross section, dimension, slope or angle between two channels but also hydraulic parameters like Froude number and discharge ratio in two channels. In addition to mentioned parameters, mobile bed in the form of bed load and suspended load has a major influence on flow pattern in which change geometry and hydraulic design parameters. According to water surface level as an important factor in flow intersections which has not investigated well in previous researches, so the simultaneous effects of geometry and hydraulic parameters on flow pattern and water level variations are discussed in this study. This research was based on numerical study with Fluent model. Geometry creation and meshing were done by Gambit software. Gambit provides a concise and powerful set of solid modeling-based geometry tools. Existing data in hydraulic research center in Iowa University used to validate numerical results. The Reynolds Averaged Navier-Stokes (RANS) turbulence model is used to simulate the turbulent flow field. According to using Volume of Fluid (VOF) method for simulating two-phase flow, the water phase is considered as stable fixed water column in the inlet of the model. Other internal volume of the geometry (10 centimeters above the initial water level) is filled with air. In the inlet of flow field, uniform velocity boundary condition based on experimental condition for water phase and the same velocity boundary condition with trivial velocity value (0.00001 m/s) for air phase are considered. Also, zero-gradient boundary condition is used for pressure in the outlet and wall condition with no slope is considered for channel sides. Finally, symmetric boundary condition is considered to introduce surface flow. Average and maximum error of simulating water surface level in 90 degree confluence was 1% and 3% respectively. Also maximum reduction in water surface level was occurred at lower discharge ratio and by increasing discharge ratio water surface oscillation was decreased . The results showed that with increasing discharge ratio, secondary channel impact will be less on total flow. When secondary channel flow collides to opposite bank of main channel flow at 90 degrees; two flows join together making accelerated flow moves downstream. It can be seen vortex flow after collision of two flows due to high velocity of fluid upper layers. Turbulence and flow mixing will decrease by reducing angle of two flows intersection and flow moves downstream with less energy dissipation causing a reduction in the upstream depth to downstream. The region of flow near the upstream junction corner can be characterized by a zone of relatively slow-moving fluid, with slight water surface super- 1 - Assistant professor of Department of Water Engineering, Bu Ali Sina University, Hamadan, Iran. elevation, that is generated by the stagnation of flow in this region. Flow within this stagnation zone can be recirculating, or exhibit reverse flow from one tributary into the other. Also, increasing the Froude number would strengthen accelerating flow in two channels making mixed flow with less depth and high velocity toward downstream. The effects of angle of intersection, Froude number and discharge ratio on water level variation have been investigated in rectangular channel intersection. The results showed all three mentioned parameters play an important role in flow pattern. Comparison of experimental and numerical results showed model can simulate water level variations very well in channels intersection. The effects of angle of intersection will reduce with increasing in discharge ratio until disregard at a 0.917 ratio. The main result of the study has showed the flow will accelerate quickly with increasing angle of intersection and making vortex with change in flow direction due to collision of two flows. The most important factor was discharge ratio effecting on water level variation trend where Froude number in downstream and angle of intersection stood at the nest place respectively. The highest and lowest changes in the relative depth is observed in the intersection angle of 90 and 30 degrees respectively. Froude number has important role in changing of relative depth so that at intersection angle of 90 degree, the discharge ratio of 0.083 and Froude number 0.9 the depth ratio is reached to 1.76. Investigation of above parameters would be necessary due to water level variation and making bed forms in mobile bed. Also the knowledge of their mechanism can be helpful effectively to