John C. Ulicny

Mason numbers for magnetorheology

The electric field strength and shear rate dependence of the apparent shear viscosity of electrorheological (ER) suspensions can often be represented by a function of only the Mason number. A Mason number defined for magnetorheological (MR) suspensions by direct substitution of magnetostatic variables for electrostatic variables does not produce a similar collapse of shear viscosity data for MR suspensions. We show that a Mason number defined in terms of the suspension magnetization can be employed to produce a collapse of experimental data at various magnetic field strengths and shear rates. As for ER suspensions, this Mason number can be calculated from experimentally measured quantities.

Dynamic yield stress enhancement in bidisperse magnetorheological fluids

Particle-level simulations are employed to investigate the rheological properties of bidisperse magnetorheological fluids. These suspensions are treated as nonlinearly magnetizable, neutrally buoyant, non-Brownian spheres immersed in a nonmagnetizable Newtonian continuous phase. We examine the effects of particle size ratio, composition, and field strength on the dynamic yield stress. The dynamic yield stress of bidisperse suspensions is larger than that of monodisperse suspensions at the same particle volume fraction. The smaller particles cause the larger particles to form more chainlike aggregates than those formed in monodisperse suspensions.

Enhancing magnetorheology with nonmagnetizable particles

Experimental results illustrate an enhancement in the field-induced yield stress of magnetorheological (MR) fluids caused by the presence of nonmagnetizable particles. Particle-level simulations in three dimensions show similar behavior. However, the enhancement does not appear in simulations in which the spheres are confined to a monolayer. A mechanistic explanation of these observations is currently lacking. Nonetheless, the ability to enhance the MR response by replacing magnetizable particles with nonmagnetizable particles offers several advantages for applications.

Magnetorheological Fluid Fan Drive for Trucks

A magnetorheological fluid (MRF) fan drive prototype for automotive truck application is designed and tested for both performance and durability. A dual concentric gap with drum rotor design is chosen to meet the required torque capacity, packaging, and mass constraints. Finite element magnetostatic modeling is performed to size the electromagnetic circuit and achieve the desired flux density levels in the MR fluid gaps. The clutch is filled with a custom-formulated MR fluid. Performance testing shows excellent speed control and response. The required 40 N-m torque capacity is achieved along with low drag speed, which is a key design characteristic of the fan drive. The clutch successfully passed several 500 h durability tests in a test cell environment. Performance testing indicates that the MRF clutch maintains its required torque capacity with very little increase in drag speed over the duration of the tests indicating no fluid thickening issues. The total dissipated energy in these tests is about 3.8 GJ. The total specific dissipated energy for these tests is more than seven times higher than the 107 J/cm3 upper limit previously suggested in the literature.

Transient response of magnetorheological fluids: Shear flow between concentric cylinders

An experimental investigation of the rheological response of magnetorheological suspensions subjected to step changes in applied magnetic field strength at fixed shear rate is reported. For small applied field strengths, the shear stress increases rapidly to a steady value. Above a critical field strength, the rapid initial increase in shear stress is followed by a slow, transient increase in stress. The critical Mason number corresponding to the critical magnetic field strength at the onset of this transient depends on the particle volume fraction as well as the shear rate. This is in contrast to a previous analysis where the critical Mason number was predicted to depend on only the particle volume fraction. The discrepancy is attributed to colloidal forces that are significant in our experimental system, but were not included in the analysis. Further comparison with the previous analysis requires either including the effects of colloidal forces, or performing experiments with systems in which colloidal ...

Effects of nonmagnetic interparticle forces on magnetorheological fluids

Effects of nonmagnetic interparticle forces on the on- and off-state behavior of magnetorheological fluids are investigated experimentally and with particle-level simulations. Suspensions of iron particles in an aliphatic oil are modified by surface-active species. The modifications significantly alter the off-state properties, but have little impact on the field-induced stresses. Simulations show similar behavior. Off-state rheological properties are strongly influenced by van der Waals forces and modifications of the short-range repulsive forces. Field-induced stresses are less sensitive to the nonmagnetic forces.

Effects of body forces on electro- and magnetorheological fluids

Body forces in electro- and magnetorheological fluids are typically small compared to the magnitudes of the field-induced electric and magnetic forces. Using particle-level simulations, we show that these relatively small forces can have large effects on the rheological response of these fluids in shear flow.

SIMULATION OF BIDISPERSE MAGNETORHEOLOGICAL FLUIDS

A method for simulating the steady-shear behavior of bidisperse, nonlinearly magnetizable MR suspensions is described. Results show that the yield stress of suspensions containing mixtures of large and small particles is larger than that of monodisperse suspensions, in agreement with previous experimental results.

Experimental validation of a magnetorheological energy absorber design analysis

A key challenge when designing linear stroke magnetorheological energy absorbers for high-speed impact is that high piston speeds in linear stroke magnetorheological energy absorbers induce high Reynolds number flows in the magnetic valve of the magnetorheological energy absorber, so that achieving high controllable dynamic range can be a design challenge. So far, the research on magnetorheological energy absorbers has typically assumed that the off-state force increases linearly with piston velocity. But at the higher piston velocities occurring in impact events, the off-state damping exhibits nonlinear velocity squared damping effects. This problem was recognized in our prior work, where it was shown that minor losses are important contributing factors to off-state damping. In this study, a nonlinear analytical magnetorheological energy absorber model is developed based on a Bingham-plastic nonlinear flow model combined with velocity squared dependent minor loss factors. This refined model is denoted as the Bingham-plastic nonlinear flow model with minor losses. From this Bingham-plastic nonlinear flow model with minor losses, an effective design strategy is presented for conventional magnetorheological energy absorbers. The Bingham-plastic nonlinear flow model with minor losses is validated via computational fluid dynamics simulation, so that magnetorheological energy absorber performance can be analytically verified before being manufactured. The magnetorheological energy absorber is fabricated and tested up to an effective piston velocity of 5 m/s by using the high-speed drop tower facility at the GM R&D Center. Comparison of our analysis with measured data is conducted, and the effective design of the magnetorheological energy absorber using the Bingham-plastic nonlinear flow model with minor losses is validated.

Transient behavior of electrorheological fluids in shear flow

The transient response of electrorheological suspensions in shear flow subjected to a suddenly imposed electric field is investigated experimentally. Barium titanate∕silicone oil and alumina∕mineral oil suspensions are employed. The evolution of both the rheological properties and the suspension structure are investigated. Results are compared with predictions from a two-fluid continuum model reported previously. Transient responses appear above a critical field strength, and the critical Mason number for the onset of a transient rheological response is equivalent to the critical Mason number for the onset of lamella formation, within experimental uncertainty. These results are consistent with predictions. The experimentally determined values of the critical Mason number agree with those predicted, with differences of the order of the experimental uncertainty. However, we find that the critical Mason number depends on shear rate, rather than being independent of shear rate as predicted.