Investigation of the Dynamics Bed Shear Stress Distribution around a Circular Cylinder Using Various Turbulences Models

Abstract

Scouring around cylinders is greatly affected by the value of bed shear stress entered on sediment materials of the bed. Regarding the importance of the bed shear stress fluctuations and their influence on the initiation and development of scouring around cylinders, in this paper, the dynamic bed shear stress distributions around a circular cylinder in high Reynolds numbers flow is investigated using various turbulence closures. Since the bed shear stress can be regarded as one of the key parameters in scouring and erosion predictions, then the accurate calculation of this parameter would be the first step in scour prediction and calculation. As the vortex shedding at the downstream of a cylinder depends on the turbulence intensity of flow, a similar behavior may be anticipated for the dynamic bed shear stress. To evaluate the bed shear stress distribution, several numbers of numerical simulations are conducted to predict the bed shear stress using the CFD solver FLUENT. Various turbulence models of time-and space-averaged types are used to simulate the turbulence effects. Regarding the importance of the turbulence models in accurate simulation of the cylinder wake region, three models of k - ε, RSM and LES are evaluated using a comparison of the flow pattern and the fluctuation of near wake vortices. Results revealed that in the simulation with LES turbulence model, the wake remains very symmetrical, consisting primarily of two large vortices in the near wake at two sides of the cylinder. With time passing, a transition occurs between the two states, from the initially symmetrical to the asymmetrical. Also, comparing the critical bed shear stress pattern in different phases show that the bed shear stress is repeated again after a certain time period. This is in good agreement with the fluctuation time period of wake vortices.