Abstract
This paper reports the experimental tests on the behaviour of a commercial MR fluid at high shear rates and the effect of the gap. Three gaps were considered at multiple magnetic fields and shear rates. From an extended set of almost two hundred experimental flow curves, a set of parameters for the apparent viscosity are retrieved by using the Ostwald de Waele model for non-Newtonian fluids. It is possible to simplify the parameter correlation by making the following considerations: the consistency of the model depends only on the magnetic field, the flow index depends on the fluid type and the gap shows an important effect only at null or very low magnetic fields. This lead to a simple and useful model, especially in the design phase of a MR based product. During the off state, with no applied field, it is possible to use a standard viscous model. During the active state, with high magnetic field, a strong non-Newtonian nature becomes prevalent over the viscous one even at very high shear rate; the magnetic field dominates the apparent viscosity change, while the gap does not play any relevant role on the system behaviour. This simple assumption allows the designer to dimension the gap only considering the non-active state, as in standard viscous systems, and taking into account only the magnetic effect in the active state, where the gap does not change the proposed fluid model.
Highlights
The behaviour and the characterization of magnetorheological fluids (MRF) at high shear rate is a topic that is only partially discussed in scientific and technical literature
Rheological behaviour is related to the fluid viscosity, turbulences and fluid flow, while the magnetic behaviour is affected by the intensity of the magnetic field and the materials involved in the magnetic circuit
When a magnetic field is applied, the particles behaves like magnetic dipoles and align along the magnetic flux lines, forming chains able to resist to the shear stress up to an apparent yield stress, called τy
Summary
The behaviour and the characterization of magnetorheological fluids (MRF) at high shear rate is a topic that is only partially discussed in scientific and technical literature. A similar approach was used in other works about the so called squeeze-strengthen effect [8,9,10,11,12,13,14,15,16,17] in which many researchers demonstrates that there is a strong enhancement of the apparent yield stress of the fluid when there is a combination of shear and compression This behaviour is due to the formation of larger particles columns, as stated by [11,18] thanks to the compressive state resulting in a higher yield stress when the magnetic field is applied. Other interesting consideration on the rheological behaviour of the MR fluids, especially considering the fluid
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