Implementation of a flexible fiber model in a general purpose CFD code

Licentiate thesis, 2012

This work is related to the process of making pulp mats for use in hygienic products. One part of that process is the transportation of flexible cellulose fibers suspended in flowing air. The fibers should be evenly distributed on the substrate, and it is thus of high importance to avoid the formation of fiber flocks during the transportation. The purpose of the present work is to implement a flexible fiber model in a general purpose Computational Fluid Dynamics (CFD) code, for detailed studies of fiber-fiber and fiber-flow interaction in real flow situations. The fibers are modeled as chains of cylindrical segments, and the translational and rotational degrees of freedom of each segment are taken into account. Each segment is tracked individually, using Lagrangian Particle Tracking (LPT), and the equations of fibermotion are derived from the conservation of momentum for each segment. The segment inertia is taken into account and the one-way coupling with the fluid phase is considered. The fiber integrity is ensured through connectivity forces acting between the adjacent fiber segments. The implemented model has been applied both with imposed flow fields, and in a flow field simultaneously predicted by the CFD solver. The results show that the fibers are transported by the flow and are deformed due to flow gradients. Further, a generic test case is described and used to validate the energy conservation and response time of the fiber model concept. This work is the foundation for further improvements of the fibermodel through the addition of bending and twisting forces, as well as the inclusion of interaction (e.g. collision) forces between individual fiber segments. These features, together with a two-way coupling with the flow, will lead to a more complete fiber model.