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