Numerical investigation on yielding phenomena of magnetorheological fluid flowing through microchannel governed by transverse magnetic field

C. Shao, J. Zhou, S. Zhang
American Institute of Physics
Physics of Fluids
Computational fluid dynamics, Deformation, Discrete element method, Electromagnetism, Lattice Boltzmann methods, Magnetic fields, Magnetic fluids, Microchannel, Velocity gradient tensor, yield-stress

To investigate the yielding phenomena during the start-up process of a magnetorheological fluid (MRF) flowing through a microchannel under a transverse uniform magnetic field, a three-dimensional and two-phase numerical simulation method is established based on the lattice Boltzmann method, united with the immersed boundary method and the discrete element method. Affected by the transverse uniform magnetic field, the randomly distributed magnetic nanoparticles (MPs) in MRF form some chains parallel to the direction of the magnetic field, and driven by the carrier fluid, the straight chains become curved due to the velocity gradient of the carrier fluid. It is revealed that the yielding process of MRF with water being the carrier fluid is essentially the deformation of the chains composed of MPs. The averaged shear force on the MPs at the ends of chains is taken to characterize the yielding status of an MP cluster, and there exists the maximum shear force during the deformation of chains. The Reynolds number has obvious influence on the characteristic shear force in the single chain, while for a multi-chain system, the chains are entangled with each other during the course of yielding and the characteristic shear force presents a complicated regularity. When the intensity of the magnetic field is relatively small, it has influence on the yielding force; once it is larger than a critical value, the yielding force remains constant. The yielding of MRF flow in a microchannel is a transient experience, and after yielding, the chain-like structure can maintain its integrity along with the channel flow.

Keywords: Discrete element method, Computational fluid dynamics, Yield stress, Lattice Boltzmann methods, Magnetic fluids, Microchannel, Electromagnetism, Velocity gradient tensor, Deformation, Magnetic fields

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