2.10 Solution Strategies

The fiber model solves sets of differential equations. It provides its own numerical algorithms showing their own numerical difficulties. Therefore it is highly recommended to first achieve a numerical solution for the pure fibers without any coupling to the surrounding flows. If the fiber model doesn't converge for a given flow field of the surrounding fluid it will not converge for changing fields of the surrounding fluid.

When you start with a fiber simulation, chose the appropriate models needed for the fibers. For the first simulation, disable options like Include lateral drag, Fiber Radiation Interaction, and Fiber Viscous Heating to reduce possible interactions.

When specifying the grid of the fibers be sure to refine the grid in the area where large gradients of the velocity appear. This is mainly near the injection point where the fiber is released and near the point of solidification. Since this point is not known a priori, you have to refine the grid during subsequent steps.

If the fiber grid seems to be well suited, you can influence the convergence behavior by starting the iteration with low under-relaxation factors in the Fiber Solution Controls dialog box.This may help in most situations where the species and energy equations are strongly coupled (e.g., dry spinning applications), or if the solvent has a very high latent heat of vaporization (e.g., water).

Be sure to increase the under-relaxation factor of the momentum equation to 1, when doing a melt spinning case to achieve a converged solution. This should be done after a numerically stable solution has been set up.

When the solution process of the pure fiber equations show a numerically stable behavior, the user can increase the complexity of the models by activating viscous heating, or radiation interaction, if such effects are important in their application. After this, the user can proceed with a coupled solution by solving the fiber equations and the fluid flow equations alternately. If the solution algorithm of the fluid flow equations diverges, the user must investigate the source terms computed by the fiber model, see Section  3.9.2.

The user can damp strong changes of the source terms with a low under-relaxation factor. Another choice is to increase the number of ANSYS FLUENT iterations between two subsequent fiber computations.

If the coupled solution process converges, the user can increase the under-relaxation factor of the source terms and decrease the number of ANSYS FLUENT iterations between two subsequent fiber computations.

The user may also want to consider underrelaxing the fluid flow equations. This helps especially for the energy and species equations.