Some experiences with the slip boundary condition in viscous and viscoelastic flows

• Slip boundary condition (SBC) is tested for benchmark viscoelastic problems of the K-BKZ type.
• Test problems include: Poiseuille flow, viscoelastic flows in tapered contractions.
• Application of partial slip is compared with full slip along the boundaries.
• SBC formulation is given in some detail for Picard-type schemes.

A slip boundary condition (SBC) has been known to occur at solid boundaries in a variety of materials processing including polymer processing. In the present work, implementation of the SBC has been tested for viscous and viscoelastic fluids obeying integral constitutive equations of the K-BKZ type. The Finite Element Method (FEM) is used to provide numerical results for tapered dies where sometimes slip is neglected in the reservoir and the converging entry section and applied only in the die. The present results show that this is valid for small values of a dimensionless slip coefficient Bsl (Bsl < 1). However, slip must be applied to all the walls when Bsl > 1, or the contraction angle or the contraction ratio is small; otherwise the pressure drops in the system are severely overpredicted. Viscoelasticity enhances those trends.