Coherent structures in the mixing layers of a non-Newtonian fluid

Abstract

Numerical simulations were carried out to investigate the coherent structures in the mixing layers of a non-Newtonian fluid, which includes the viscoelastic second-order fluid, FENE-P fluid and fibre suspensions. The results indicate that: (1) for the viscoelastic second-order fluid, viscoelasticity suppresses the development of the fundamental perturbation and results in a longer life time of the coherent structures; (2) for the FENE-P fluid, the evolution of fundamental and subharmonical perturbations are restrained, accompanied by a stronger diffusion of vorticity and a slower rotatory motion of neighbouring vortices. This effect becomes less and more remarkable with an increase in Weissenberg number and the extensibility of the polymer chain, respectively; (3) for the fibre suspensions, the existence of fibres makes the coherent structure evolve quickly, and the coherent structures cause the additional stress induced by the fibres to be heterogeneous. The degree of heterogeneity increases with an increase in Stokes number (

, where ρ _p is the density of the fibres, a is the fibre radius, ΔU is the velocity difference in the mixing layer, μ is the viscosity of the fluid and θ₀ is the momentum thickness of the mixing layer) and fibre aspect ratio. The maximum and minimum additional stresses occur on the edge of the coherent structures.