Localized disturbances in channel flow of a viscoelastic fluid

Linear and non-linear growth of a localized disturbance in polymeric channel flow is investigated using Direct Numerical Simulations. The polymeric stress is represented by the FENE-P model. When the amplitude of the intial disturbance is sufficiently small, the presence of the polymer reduces the linear amplification of the disturbance energy, and this stabilizing influence depends on the Weissenberg number, maximum polymer extensibility and the ratio of the solvent viscosity to the total viscosity. When the initial disturbance amplitude is increased, the same trend is identified in the early linear phase. In the subsequent non-linear phase, the behaviour of the Newtonian and polymeric flows are substantially different. In the Newtonian case, non-linear growth is followed by an ultimate decay of the disturbance energy due to viscosity. On the other hand, in the polymeric flow, the non-linear terms due to the solvent contribution are reduced. However, a new energy growth mechanism is present and leads to bypass transition.