Analysis and control of complex flows in U-bends using computational fluid dynamics

Güden, Yiğitcan
Analysis and control of flow structure is crucial in the sense that the increase in the ratio of inertial and centrifugal forces to viscous forces destabilizes the flow and creates a three-dimensional complex flow consisting of stream wise parallel counter-rotating vortices, so-called Dean vortices. In addition, due to the curvature in U-bends, in line with these vortices, a high level of turbulence is detected, which is quite critical in considering noise problems and structural failures. In this thesis, computational fluid dynamic (CFD) models are developed using ANSYS FLUENT to analyze and to control these complex flows patterns in square sectioned U-bend with a radius of curvature Rc/D=0.65. The predictions of velocity profiles on different angular positions of the U-bend are compared against the experimental results available in the literature and previous numerical studies. The performances of six different turbulence models are evaluated, namely: the standard k-ε, the k-ε Realizable, the k-ε RNG, the k-ω SST, the Reynolds Stress Model (RSM) and the Scale-Adaptive Simulation Model (SAS), to propose the best numerical approach that has high accuracy with reduced computation time. Numerical results of the present thesis show improvements with respect to the previous numerical studies and very good agreement with the available experimental results. In addition, a flow control technique with using airfoils inside the bend is applied to regulate the flow inside the duct. With this control technique, the elimination of Dean vortices along with significant reduction in turbulence levels in different cross flow planes are successfully achieved.


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Citation Formats
Y. Güden, “Analysis and control of complex flows in U-bends using computational fluid dynamics,” M.S. - Master of Science, Middle East Technical University, 2014.