Investigation of Drag Force on Fibres of Bonded Spherical Elements using a Coupled CFD-DEM Approach

Clogging in wastewater pumps is often caused by flexible, stringy objects. Therefore, simulation of clogging effects in wastewater pumps entails simulation of such flexible objects and the interaction between these objects and fluid in the pump. Using a coupled CFD-DEM approach, the flexible object can be modelled as a multi-rigid-body system using bonded spherical DEM particles. However, the flexible objects are not resolved by the CFD mesh, and therefore modelling of fluid forces on the flexible object becomes a key issue. This study investigates the modelling of fluid forces on a rigid fiber as the first step towards simulation of clogging effects using CFD-DEM. The drag force on cylinders of finite aspect ratios 2 < beta < 40 for Reynolds numbers in the range 0.1 Re 1000 at an angle normal to the flow direction are investigated. The drag is examined along the span of the cylinders and end effects are quantified. The drag coefficient on the resolved cylinder is compared to a CFD-DEM simulation of a flexible fiber made by a chain of bonded spherical discrete elements, using a free steam drag formulation on each particle. Based on the results, a drag force model can distinguish between the outermost segments and the segments in the center of the fiber.