Alireza Farjoud

Magneto-rheological fluid behavior in squeeze mode

Little published data exists on the behavior of MR fluids in squeeze mode. Many of the basic properties of MR fluids in squeeze mode are still unknown. In squeeze mode, MR fluids can generate a large range of force associated with a small displacement. As a result, squeeze mode has recently received more attention. This research focuses on modeling and testing MR fluids in squeeze mode. A novel squeeze mode rheometer is designed and built. MR fluid is tested in squeeze mode to evaluate its performance and behavior. The rheometer can test MR fluid under different conditions (gap size, magnetic field density, speed, etc). It utilizes a Gauss meter for direct measurement of the magnetic field density. MR fluid squeeze test results show that MR fluid can deliver a large range of force that is comparable in magnitude to the force in shear mode. The tests also indicate a clumping effect of the fluid when tested in repeated cycles that does not appear to have been documented previously. This paper describes, in detail, the clumping effect and provides possible reasons for this phenomenon. A non-dimensional mathematical model is developed and validated experimentally. The non-dimensional model directly compares the squeeze mode force to the shear mode force. The results indicate that MR fluid in squeeze mode can be used in many applications requiring a large range of controllable force in envelopes that can only accommodate small strokes.

Experimental Investigation of MR Squeeze Mounts

Based on results obtained from testing MR fluid in a squeeze mode rheometer, a novel compression-adjustable element has been fabricated and tested, which utilizes MR fluid in the squeeze mode. While shear and valve modes have been used exclusively for MR fluid damping applications, recent modeling and testing with MR fluid has revealed that much larger adjustment ranges are achievable in the squeeze mode. Utilizing the squeeze mode, an MR squeeze mount was developed and tested. Test results show that the device was capable of varying the compression force from less than 8 lb 35.6 N to greater than 800 lb 3560 N, when the pole plates were 0.050 in. 1.3 mm apart. Test results showed that the mount tends to achieve higher forces after each repeat of test. This behavior, called ‘clumping’, was studied and solutions to minimize this effect are discussed.

Dynamic Testing and Modeling of an MR Squeeze Mount

MR fluid squeeze mode investigations at CVeSS have shown that MR fluids show large force capabilities in squeeze mode. It was found that MR fluids in squeeze mode may be used in a wide range of applications such as engine mounts and impact dampers. In these applications, MR fluid is flowing in a dynamic environment due to the transient nature of inputs and system characteristics. The research presented in this article undertakes the problem of dynamic testing and modeling of MR fluid squeeze mounts. Dynamic tests of an MR mount are studied for different applied currents, initial gaps, frequencies, and excitation amplitudes. An effective mathematical model of the MR squeeze mount for steady-state testing was built which includes the effect of the inertia of the fluid. The results show that the compression force and the area of the hysteresis loop increase with the increase of excitation amplitude or applied current and it will decrease with the increase of frequency or initial gap. Also, the inertia effect becomes more significant for higher displacement frequencies. The mathematical model agrees with the test data very well during the compression process and it can be used for the dynamics analysis and the real-time control.

MR-fluid yield surface determination in disc-type MR rotary brakes

Magneto-rheological (MR) fluids are currently attracting a great deal of attention because of their unique rheological behavior. Many devices have been designed using MR fluids, and of potential interest here are disc-type MR rotary brakes. The plug flow region in MR devices is defined as the region where the fluid is not flowing. The plug flow region plays an important role in design and analysis of MR devices. In MR dampers, the damping coefficient is a function of the plug thickness. In MR valves, the plug thickness is used to control the flow rate through, and the pressure drop across, the MR valve. A MR clutch is performing at the highest efficiency when the entire MR gap is the plug region. For an MR rotary brake, the highest restraining torque is obtained when the entire gap is the plug region as far as there are no wall slip effects. In this paper, using the Bercovier and Engelman constitutive model, the MR fluid flow in disc-type MR brakes is modeled to determine the plug flow region. The resulting system of equations is solved numerically. It is shown that the existence of a plug flow region in the brake will affect the control torque ratio. Better estimation of the plug flow region results in better estimation of the viscous torque.

Rheometer characterization of MR fluids in squeeze mode

This study provides a comprehensive analysis of the characteristics of MR mounts in squeeze mode which is the least commonly-analyzed aspect of MR fluids. The results of the study are based on a novel rheometer that is designed and fabricated for the purpose of better understanding the characteristics of MR fluids in squeeze mode. The paper describes the details of the rheometer design. It further provides the test results for MR fluids tested in squeeze mode. The tests indicate a clumping effect of the fluid when tested in repeated cycles that does not appear to have been documented previously. The paper describes, in detail, the clumping effect and provides possible reasons for this phenomenon.

Non-dimensional modeling and experimental evaluation of a MR squeeze mode rheometer

Squeeze mode flow in MR fluids has recently become an interest to many researchers. In squeeze mode, MR fluids can generate a wide range of force associated with a small displacement. The generated force is controllable by means of a variable magnetic field. This is promising for applications that require variable forces along short strokes. There are not many published data on the behavior of MR fluids in squeeze mode. Many of the basic properties of MR fluids in squeeze mode are still unknown. The presented research here focuses on modeling and testing MR fluids in squeeze mode. A novel squeeze mode rheometer was designed and built. The results show that MR fluid can deliver a force that is comparable in magnitude to the force in shear mode. A non-dimensional mathematical model was developed and validated experimentally.

NONLINEAR MODEL OF SQUEEZE FLOW OF FLUIDS WITH YIELD STRESS USING PERTURBATION TECHNIQUES

This research is focused on mathematical modeling of fluids with yield stress in the squeeze mode. The focus of the research is on magneto-rheological (MR) fluids but the results can be extended to any non-Newtonian fluid exhibiting a yielding behavior. There is no universally accepted mathematical model of squeeze flow of MR fluids to date because of the complexity of the flow and lack of theoretical and experimental studies. In this research, the squeeze flow problem of MR fluids is solved using perturbation techniques and squeeze force and flow field are determined. The model is verified and validated using experimental test data and it is shown that the model can be used as a design tool for design of MR devices that operate in the squeeze mode.

Shim stack deflection analysis in hydraulic dampers using energy methods

This paper presents a detailed analysis of the deflection of the shim stacks used in hydraulic dampers. In hydraulic dampers, a stack of circular disks (shims) is mounted on each side of the main piston to create a pressure drop as the hydraulic oil is passed through the piston from one side to the other. A stiff shim stack creates a high pressure drop across the piston, resulting in high damping. A softer shim stack creates less pressure drop and smaller damping. In practice, shims can be added or removed from the shim stack assembly to tune the damper and generate the desired damping force characteristics as a function of velocity. Tuning a damper requires taking the damper apart, making the changes to the shim stack assembly, and putting the damper back together. This takes a considerable amount of time and effort. Therefore, mathematical modeling of the shim stack assembly becomes a crucial part of the analysis of hydraulic dampers. The goal of the study presented here is to provide a model of the shim stack assembly in order to accurately predict the level of damping for different configurations of the shim stack. The shims that are stacked on each other will deflect under the pressure created by the hydraulic oil, and at the same time, slide against each other. This important characteristic of the shim stack needs to be accounted for in the mathematical model and makes the analysis complicated. For the sake of simplicity, in past studies the shim stack is approximated by the deflection of a single disk and formulas for a single disk are used. This, however, introduces a significant amount of error in the damper hydraulic model. In this paper, the deflection of shim stacks is analyzed and compared with the single disk approximation. It is found that this approximation fails to agree with the more accurate model of representing the shims individually. Therefore, a more detailed and accurate model is necessary for better simulating the damping characteristics of hydraulic dampers as a function of relative velocity across the damper.