Magnetorheological (MR) fluids are suspensions of fine micron-sized magnetizable particles in a suitable carrier liquid. The rheological properties of the fluid can be controlled by application of a suitable magnetic field and can be used in a variety of applications where variable damping and stiffness characteristics are required, based on the requirements of the rotor dynamic system. In this work, the stiffness and damping characteristics of MR fluid long squeeze film damper operating at low eccentricity ratios are calibrated in terms of Reynolds number of the squeeze film for different clearance and L/D ratios. A theoretical constant magnetic field viscosity model is developed, based on the literature, and is subsequently used to evaluate the theoretical stiffness and damping coefficients, at a particular excitation frequency. The results indicate that the stiffness and damping coefficients decrease with increasing Reynolds number of the squeeze film and is found to be abysmally low, indicating that the flow has ceased and the film has solidified. This is in accordance with the literature that predicts the formation of chain-like semi-solid structures, restricting the flow, and consequently increasing the viscosity, under the influence of the magnetic field. This change in viscosity, in turn, influences the stiffness and damping coefficients and the Reynolds number of the squeeze film. The stiffness and damping coefficients are found to increase with decreasing clearance, increasing L/D ratio, and eccentricity ratio. The results of the investigation assist the designer in obtaining the stiffness and damping characteristics of the squeeze film damper based on the Reynolds number of the squeeze film. Conversely, the stiffness and damping characteristics of the squeeze film damper are calibrated in terms of the Reynolds number of the squeeze film for different damper configurations.
Squeeze film dampers are novel rotor dynamic devices used to alleviate small amplitude, large force vibrations and are used in conjunction with antifriction bearings in aircraft jet engine bearings to provide external damping as these possess very little inherent damping. Electrorheological (ER) fluids are controllable fluids in which the rheological properties of the fluid, particularly viscosity, can be controlled in accordance with the requirements of the rotor dynamic system by controlling the intensity of the applied electric field and this property can be utilized in squeeze film dampers, to provide variable stiffness and damping at a particular excitation frequency. The paper investigates the effect of temperature and electric field on the apparent viscosity and dynamic (stiffness and damping characteristics) of ER fluid (suspension of diatomite in transformer oil) using the available literature. These characteristics increase with the field as the viscosity increases with the field. However, these characteristics decrease with increase in temperature and shear strain rate as the viscosity of the fluid decreases with temperature and shear strain rate. The temperature is an important parameter as the aircraft jet engine rotors are located in a zone of high temperature gradients and the damper fluid is susceptible to large variations in temperature.
The present investigation is aiming to get better squeeze film dampers which are normally used in high speed jet engines to minimize small amplitude large force vibrations. The investigation was started with squeeze film dampers employing conventional lubricating oils. Magneto rheological fluids are then used to enhance the viscosity characteristics of the fluid under the influence of magnetic fields in order to improve the damper performances. It is observed that the dynamic characteristics of the damper with magneto rheological fluids are enhanced. Further to improve the damper performance, few modifications in the damper assembly are carried out in this research work. A good amount of reduction in the amplitude of vibrations is observed in these modified squeeze film dampers coupled with magneto rheological fluids. This research work discusses dual and triple clearance squeeze film dampers and bump foil squeeze film damper, also subjected to variation in temperature. Dynamic characteristics are found to be decreasing as the viscosity of the fluid decreases with rise in temperature of the fluid.
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