Experimental and numerical studies of squeeze mode behaviour of magnetic fluid

Abstract

Magneto-rheological (MR) fluids are suspensions of micron-sized ferromagnetic particles in a non-magnetic carrier fluid. The essential characteristic of MR fluid is the rapid and reversible transition from the state of a Newtonian-like fluid to the behaviour of a stiff semi-solid by applying a magnetic field of ∼0.1–0.4 T. This feature can be understood from the fact that the particles form chain-like structures aligned in the field direction. The MR fluid offers three modes of operation, namely the direct shear mode, the valve mode, and the squeeze mode. The latter is of particular interest due to its highly non-linear behaviour, which is still not fully understood and therefore expected to give rise to new industrial applications. A test rig for the exploration of the MR-fluid behaviour was designed for experimental purposes. The present article describes the results of measurements under sinusoidal loading modes. Special emphasis was posed on the dependence of the MR-fluid response with respect to parameter variations of the applied static magnetic field, the cyclic loading amplitude, and frequency values. Cavitation effects have been investigated and partially suppressed by pre-pressurizing the MR fluid, which enables a more thorough insight into particle chain disruption and segregation effects. Well-pronounced hysteresis loops are observed and exhibit characteristic kinks, which cannot be understood within the frame of elementary constitutive laws such as for Bingham fluids. To describe the squeeze mode phenomenon numerically, adequate constitutive laws were applied, checked numerically by utilizing finite-element simulations, and validated against experimental data. New perceptions attained so far provided reason to design an adaptive MR-fluid bearing in squeeze mode behaviour for industrial applications.