Secondary motions in straight and tapered channels: Experiments and three-dimensional finite element simulation with a multimode differential viscoelastic model

The development of secondary motions due to the nonzero second normal stres s difference is observed and analyzed by tracking the motion of the interfa ce between two batches of the same low density polyethylene, each with a di fferent pigmentation. in this article we focus on straight and tapered chan nels with a square cross section. Three-dimensional numerical simulations a re also performed on the basis of a multimode differential viscoelastic flu id model. These calculations allow the investigation of the development of secondary motions in space. Predictions are compared with the experiments: good agreement is found between the experimental observations and their num erical counterparts. The magnitude and the relevance of secondary motions a re also discussed. A numerical analysis is carried out on the sensitivity o f secondary motions with respect to the nonlinear properties of the viscoel astic model. Finally, we make a brief attempt to explain the mechanism whic h drives the secondary motions. (C) 1999 The Society of Rheology. [S0148-60 55(99)00406-X].