NUMERICAL-SIMULATION OF THE TRANSITION FROM ADHESION TO SLIP WITH FRICTION IN GENERALIZED NEWTONIAN POISEUILLE FLOW

The axisymmetric Poiseuille flow of a purely viscous generalized Newto nian fluid under rate of flow controlled conditions is studied with a change in the boundary conditions at a transition point from an adhesi ve to a slip condition with friction at the wall. The friction law use d originates from an experimental study by (J.M. Piau and N. Fl Kissi, J. Non-Newtonian Fluid Mech. 54 (1994) 121-142) using a capillary mad e of steel and a silicone fluid, and is based also on a molecular dyna mics theory by (Yu B. Chernyak, A.I. Leonov, Wear, 108 (1986) 105-138) . It gives a non-linear multivalued dependance of the wall shear stres s to the velocity at the wall. Moreover, wall shear stress Values may become smaller than values obtained when adhesion prevails in the capi llary. The shear stress must over-step some limiting stress level to t rigger the wall slip. After checking slip boundary condition implement ation for the case of Poiseuille flow with slip along the entire wall, the convergence and the validity of the computation was studied. Impo rtant morphologic changes of the flow field and the stress field appea r around the transition point from adhesion to slip boundary condition . Slip at the wall allows the principal stress difference to be drasti cally reduced, except in the vicinity of the transition point where th is difference is maximum. A peak in shear stress located upstream of t he transition, and a peak in elongational stress located downstream of the transition, are observed at the wall. Fully developed near plug-l ike flows are obtained within about 1D only downstream of the transiti on point. It is concluded that the effect of slip on extrudates distor sion should appear clearly even when the exit slippery zone is reduced to 1D. (C) 1998 Elsevier Science B.V. All rights reserved.