SIMULATION AND COMPARISON OF FLOW CHARACTERISTICS IN 180° CONVERGENT AND UNIFORM OPEN-CHANNEL BENDS BY 3D NUMERICAL MODEL

摘要

Few rivers running on straight paths are seen in nature; most of them have meandrous form. For meandering rivers, the flow pattern is highly complex and flow mechanics has specific characteristics at bends, not observed on straight paths. Numerical models can be used as an effective tool for predicting such flow fields. Since all river bends do not have a uniform width, in this research the numerical model SSIIM 3-D was used to simulate two 180° bends, one with a uniform (0.6 m) width and another with a convergent (0.6 to 0.45 m) width. Flow characteristics such as streamwise and vertical velocity profiles, primary and secondary flows, streamwise and spanwise slopes of water surface, and helical flow strengths were compared. Verification using experimental measurements on a uniform bend showed that the numerical model can successfully simulate flow fields in bends. Results indicate that in a convergent bend, the path of maximum velocities at a plane near the water surface crosses the channel's central line at about 30°～40° while in uniform bends, this occurs at about 50°. The range of changes in height of water surface is wider in the convergent channel and the strength of helical flow is larger in the uniform bend. Also, the uniform bend exhibits an additional secondary circulation cell at 135°, which is not present in the convergent bend.