Abstract
This paper presented a direct numerical simulation (DNS) study on the elasticity-induced irregular flow, passive mixing, and scalar evolution in the curvilinear microchannel. The mixing enhancement was achieved at vanishingly low-Reynolds-number chaotic flow raised by elastic instabilities. Along with the mixing process, the passive scalar transportation carried by the flow was greatly affected by the flow structure and the underlying interaction between microstructures of viscoelastic fluid and flow structure itself. The simulations are conducted for a wide range of viscoelasticity. As the elastic effect exceeds the critical value, the flow tends to a chaotic state, while the evolution of scalar gets strong and fast, showing excellent agreement with experimental results. For the temporal changing of scalar gradients, they vary rapidly in the form of isosurfaces, with the shape of “rolls” in the bulk and evolving into “threads” near the wall. That indicates that the flow fields should be related to the deformation of viscoelastic micromolecules. The probability distribution function analysis between micromolecular deformation and flow field deformation shows that the main direction of molecular stretching is perpendicular to the main direction of flow field deformation. It implies they are weakly correlated, due to the confinement of channel wall.
Highlights
Chaotic flow occurs for the viscoelastic fluids at very low Reynolds number (Re)
The objective of this paper is to investigate the passive particles transportation carried by the irregular viscoelastic fluids flow, combining the numerical simulations with the former experimental results
In an early experimental study on the irregular flow field in the curvilinear microchannel, Li et al [20] have shown the velocity distributions at the different planes along depth direction by micro-PIV measurement, indicating that a disordered flow state develops for the viscoelastic fluids even at an ultralow flow rate
Summary
Chaotic flow occurs for the viscoelastic fluids at very low Reynolds number (Re). Since the inertial effect is trivial, such kind of flow state is called purely elastic instability or elastic turbulence which have been identified in a vast range of macroscopic and microscopic flow geometries with curvilinear streamlines, such as Taylor-Couette device, parallel-disk, cone-and-disk, and serpentine microchannels [1,2,3]. Recently there are numerical and experimental evidence of that the nonlinear elastic instability and even elastic turbulence could be generated in the straight microchannels [7, 8] Note that their irregular flow state should be sustained by the outside perturbation, for instance, like numerically introducing a sinusoidal force term into the momentum equation and constitutive equation or placing a variable number of Advances in Mechanical Engineering obstacles in the center microchannel. Perturbation reveals itself in a positive way with the viscoelastic fluid flow, and the flow accelerates the perturbation via the flexible molecular motion As such nonlinearity of elastic effect increases, the flow is proven to be chaotic, which resembles the developed turbulence condition in the statistic characteristics [3, 4]. It behaves stochastic in time, but smooth in space, which differs from the hydrodynamic turbulence
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