Transient response of magneto-rheological fluids in high shear rate regime

Magnetic field-responsive fluids are functional materials whose rheological properties can be controlled by an external magnetic field. The magneto-rheological (MR) fluid consists of magnetically soft (magnetizable) micron-sized particles suspended in a low-viscosity base fluid. Without an external magnetic field, MR fluids behave as a Newtonian fluid. On the other hand, the applied magnetic field introduces a rapid and reversible transition of an MR fluid from a free-flowing state to a solid-like behaviour.
Understanding the transient characteristics of MR fluids, particularly the hydrodynamic response time of the fluid, is integral to the design of MR dampers. However, there is a scarcity of studies in the literature examining the hydrodynamic response time of magnetorheological fluids (MRFs) and their correlation with fluid properties. The response time of MRFs depends on several factors, such as the magnitude of the applied magnetic field, the dimensions of an MR valve, or the corresponding shear rates of MRFs. Thus, in this work, we investigate the transient behaviour of MR fluid for a wide range (very low to high) of shear rates based on advanced constitutive relations for continuum-based fluid models.
Computational fluid dynamics (CFD) analysis has been carried out, and the results indicate that the higher the Bingham number, the lower the hydrodynamic response time of MRF. However, this only applies to low to moderate shear rate regimes, as recently reported in the literature [1, 2]. In the high shear rate regime, for the first time, we report that the response of MR fluid becomes much faster below a critical Bingham number. The observed phenomenon can be described by considering the notion of dwell time [2]. When the dwell time decreases significantly, especially falling below the response time under conditions of high shear rates, the MRF doesn't exhibit resistance to flow as expected for an applied magnetic field but rather only partial resistance. Our analysis implies that one must consider the effect of the dwell time on the response of MR fluid while designing an MR damper dealing with high velocities.