This thesis aims to investigate the capability of different RANS based turbulence models to simulate turbulence induced secondary motions in a rectangular shaped straight open duct flow. Secondary flows are of very small amplitude and capturing their effects in a simulation is a difficult task. Nevertheless, they are of great importance, as these fluid flows have a significant effect on the flow, despite their small magnitude. Three RANS models that hold different degrees of complexity are applied to simulate this kind of flow. The models reach from a simple two-equation eddy-viscosity model over a non-linear two-equation model with additional cubic functional terms to a much more complex Reynolds stress model. To evaluate the performance, each of the RANS model’ output is compared to available DNS (direct numerical simulation) data, which yields a many times higher computational effort but in return also delivers very realistic and high quality results. This enables a very accurate foundation on which the obtained outcomes can be compared to. In order to validate the trustworthiness of the simulations a convergence study was performed, which involves convergence towards a steady-state solution, as well as grid-convergence. The flow possesses a bulk Reynolds number of Reb = 2205, which corresponds to a friction Reynolds number of Reτ = 150. For each applied turbulence mode the analysis commences with the flow field, hence the streamwise-, spanwise- and free-slip plane normal velocity component, vorticity and streamlines of the secondary velocities are investigated. Next, all six different entries of the Reynolds stress tensor are analyzed. Finally, the individual terms of the streamwise vorticity budget equation are investigated. Especially the gradient terms of the normal and shear stresses of this equation are said to generate secondary motions. It was observed that the eddy-viscosity model was, as expected, unsuited to simulate this kind of flow and fails to generate any results of considerable magnitude. The non-linear model delivered surprisingly often superior results in terms of qualitative shape of the single fields, while the Reynolds stress model was able to achieve the best output regarding the quantity. It was furthermore detected that capturing the features near the free surface appropriately requires a much finer grid resolution as it does for modelling the bed region.     «

This thesis aims to investigate the capability of different RANS based turbulence models to simulate turbulence induced secondary motions in a rectangular shaped straight open duct flow. Secondary flows are of very small amplitude and capturing their effects in a simulation is a difficult task. Nevertheless, they are of great importance, as these fluid flows have a significant effect on the flow, despite their small magnitude. Three RANS models that hold different degrees of complexity are appli...     »