The effect of meshing and comparing different models of turbulence in topographic prediction of bed and amplitude of flow around the groin in 90-degree arc with movable bed

Abstract

Centrifugal force at the bend causes the terrain slope at water surface that increases water surface in outer arc and reduces in internal arc. This phenomenon creates adverse pressure gradient inside the arc of cross. As a result, a flow in the transverse direction within the cross is shaped that is called secondary flow. Secondary flow and its interaction with the inertia of the flow, which leads to create a spiral flow causes changes in the topography of the Bed. Understanding the topography of Bed in arcs is essential for specifying scour positions and sediments to determine the best location for the construction of hydraulic structures such as the groin, pond and rivers organizing management. Groin is a structure that used for river training and prevent the erosion of the outer wall of arcs. Realistic estimation of scouring and also the flow velocity amplitude around the groin in alluvial rivers, for using in certain and economical designs of their foundation has very high importance. Also in the numerical models, one of the most important stages of doing the work is the optimal mesh to reach the most precise results and on the other hand, the least time of the model analysis. Since, studying the sediment transmission numerically has been a very difficult work and there are a few researches in this field, in this research, at first, the most optimal mesh in numerical model has been studied and then, it has been attempted that scouring phenomenon in a 90-degree arc with groin to be simulated in a 45° situation. For this purpose, the results of topographic changes of bed have been gained in the range of channel arc by using of FLOW3D software and different models of turbulence and they have been compared with laboratory results. It is necessary to be mentioned that due to the disability of software in giving the results in the sections perpendicular on the flow and in the direction of the flow in arc, the neural network was used for extraction of numerical results in the points accordant on the laboratory results. In the continuation, the flow amplitude in the arc has been also compared with laboratory results. The results indicate high ability of numerical model FLOW3D in simulation of channel with movable bed. Also comparison of the turbulence models indicates the superiority of the Turbulence model LES in the bed changes in arc-shaped channel over other turbulence models.