A limitation with the current analytical models for predicting the performance of a magnetorheological (MR) damper is that they fail to capture the hysteretic variation of force versus velocity variation correctly. This can significantly underestimate the damper force and overestimate the dynamic range of the device. In this work a transient analytical fluid dynamics model is developed by using a combination of Laplace and Weber transform and Duhamel’s superposition of velocity boundary condition, to overcome these limitations. The solution of the system of nonlinear simultaneous equations, obtained by applying mass flow balance, velocity compatibility conditions and force equilibrium of Bingham plastic plug flow, gives the damper force. This method is shown to generate direct and inverse model of an MR device. The proposed model has been validated against a commercially available MR damper at low speed, to a range of test signals. The mean error using the above model has been shown to be 5% for all the test signals. This compares well with three conventional models which give; transient constant velocity model 35%, quasi static model 35% and phenomenological model 35%. The phenomenological model gives 10% mean error for a sinusoidal input signal. The application of the proposed analytical model has been demonstrated by the design of a novel dual channel damper. The design of the electromechanical components has been shown to be np-hard problem and the optimisation using genetic algorithm has been applied to minimise the volume and electrical time constant. The performance of the dual channel damper has been simulated for various combinations of values of shear yield stress for two channels. Compared to the conventional single channel damper the novel design is shown to give 30% higher damper force, 50% improved dynamic range and limits the effect of transients to within 10% of the damper force. The dual channel damper is an effective solution to resist the onset of turbulent flow in the channels up to 20m/s piston velocity.

A limitation with the current analytical models for predicting the performance of a magnetorheological (MR) damper is that they fail to capture the hysteretic variation of force versus velocity variation correctly. This can significantly underestimate the damper force and overestimate the dynamic range of the device. In this work a transient analytical fluid dynamics model is developed by using a combination of Laplace and Weber transform and Duhamel’s superposition of velocity boundary condition, to overcome these limitations. The solution of the system of nonlinear simultaneous equations, obtained by applying mass flow balance, velocity compatibility conditions and force equilibrium of Bingham plastic plug flow, gives the damper force. This method is shown to generate direct and inverse model of an MR device. The proposed model has been validated against a commercially available MR damper at low speed, to a range of test signals. The mean error using the above model has been shown to be 5% for all the test signals. This compares well with three conventional models which give; transient constant velocity model 35%, quasi static model 35% and phenomenological model 35%. The phenomenological model gives 10% mean error for a sinusoidal input signal. The application of the proposed analytical model has been demonstrated by the design of a novel dual channel damper. The design of the electromechanical components has been shown to be np-hard problem and the optimisation using genetic algorithm has been applied to minimise the volume and electrical time constant. The performance of the dual channel damper has been simulated for various combinations of values of shear yield stress for two channels. Compared to the conventional single channel damper the novel design is shown to give 30% higher damper force, 50% improved dynamic range and limits the effect of transients to within 10% of the damper force. The dual channel damper is an effective solution to resist the onset of turbulent flow in the channels up to 20m/s piston velocity. / © Cranfield University

Dissertation (Ph.D.)--Worcester Polytechnic Institute. / Keywords: MR Dampers; Flexible Structures; Structural Vibration Control. Includes bibliographical references (p.209-216).

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Wire mesh dampers are under investigation because they are seen as replacements for squeeze film dampers as a source of direct stiffness and damping at bearing locations. There are several advantages of wire mesh dampers over squeeze film dampers, including: temperature insensitivity, oil-free operation, and the ability to contain large amplitude vibrations. Furthermore, due to their direct damping and lack of cross-coupled stiffness, the wire mesh reduces the response to imbalance and increases the stability of the system. The objective of this research was to determine the properties of wire mesh dampers under large amplitude vibrations. Impact testing was first conducted on the wire mesh as a means of obtaining the large amplitudes that were of interest. Next, to verify the results, a second methodology was employed using shaker testing. It was found that both the stiffness and hysteretic damping decrease with increasing displacement. However, they both approached asymptotes around 2 mils of displacement, and further increases in displacement had significantly less effect on the properties. Once the results were verified to be consistent, equations were obtained to describe the response of the wire mesh dampers. These equations were then used to create a new design workbook, which would allow an engineer to determine the properties of wire mesh dampers under conditions that they might experience.

Wire mesh dampers are under investigation because they are seen as replacements for squeeze film dampers as a source of direct stiffness and damping at bearing locations. There are several advantages of wire mesh dampers over squeeze film dampers, including: temperature insensitivity, oil-free operation, and the ability to contain large amplitude vibrations. Furthermore, due to their direct damping and lack of cross-coupled stiffness, the wire mesh reduces the response to imbalance and increases the stability of the system. The objective of this research was to determine the properties of wire mesh dampers under large amplitude vibrations. Impact testing was first conducted on the wire mesh as a means of obtaining the large amplitudes that were of interest. Next, to verify the results, a second methodology was employed using shaker testing. It was found that both the stiffness and hysteretic damping decrease with increasing displacement. However, they both approached asymptotes around 2 mils of displacement, and further increases in displacement had significantly less effect on the properties. Once the results were verified to be consistent, equations were obtained to describe the response of the wire mesh dampers. These equations were then used to create a new design workbook, which would allow an engineer to determine the properties of wire mesh dampers under conditions that they might experience.

Structural vibration control (SVC) of large scale structures using the magnetorheological (MR) dampers are studied. Some key issues, i.e. model reduction, suppression of spillover instability, optimal placement of actuators and sensors, modeling of the MR dampers and their applications in SVC system for large scale structures, are addressed in this work. A new model reduction method minimizing the error of a modal-truncation based reduced order model (ROM) is developed. The proposed method is implemented by using a Genetic Algorithm (GA), and can be efficiently used to find a ROM for a large scale structure. The obtained ROM has a finite H2 norm and therefore can be used for H2 controller design. The mechanism of the spillover instability is studied, and a methodology to suppress the spillover instability in a SVC system is proposed. The suggested method uses pointwise actuators and sensors to construct a controller lying in an orthogonal space spanned by the several selected residual modes, such that the spillover instability caused by these residual modes can be successfully suppressed. A GA based numerical scheme used to find the optimal locations for the sensors and actuators of a SVC system is developed. The spatial H2 norm is used as the optimization index. Because the spatial H2 norm is a comprehensive index in evaluating the dynamics of a distributed system, a SVC system using the sensors and actuators located on the obtained optimal locations is able to achieve a better performance defined on a distributed domain. An improved model of MR dampers is suggested such that the model can maintain the desired hysteresis behavior when noisy data are used. For the simulation purpose, a numerical iteration technique is developed to solve the nonlinear differential equations aroused from a passive control of a structure using the MR dampers. The proposed method can be used to simulate the response of a large scale structural system with the MR dampers. The methods developed in this work are finally verified using an industrial roof structure. A passive and semi-active SVC systems are designed to attenuate the wind-induced structural vibration inside a critical area on the roof. The performances of the both SVC systems are analyzed and compared. Simulation results show that the SVC systems using the MR dampers have great potentials in reducing the structural vibration of the roof structure.

A numerical method is used to predict flow fields and forces for squeeze film dampers (SFDs). A two dimensional SFD is modeled with different amplitudes and frequencies of the journal orbiting inside the wall. In addition to the typical circular centered orbit (CCO) motion prescribed in most studies, orbits can vary greatly from circular to linear. The study is divided into two distinctive models including single phase flow model and two phase flow model. The single phase flow model cases including three amplitudes, i.e. 0.002, 0.001, and 0.0005 inches, and three frequencies, i.e. 10, 50, and 200 Hz, of journal motions are conducted to portray flow fields and forces and ultimately determine their relationships. The numerical prediction shows that the journal amplitude and frequency affect flow and consequently force in the SFD. The force is directly proportional to frequency and motion amplitude. Owing to the presence of cavitation in the practical SFD, the two phase flow model is also presented with the journal amplitude of 0.0002 and three frequencies of 10, 50, and 100 Hz, respectively. The ambient pressure condition was set up for numerical processing ranging from 0.001 Mpa to 100 Mpa. The results indicate that the operating pressure has an integral role in suppressing the presence of the cavitation. The caviation disappears if the operating pressure is high enough above the vapor pressure of the lubricant.

An in-house numerical model developed at McMaster University was used in this research to investigate the effect of the wave breaking on the performance of Tuned Liquid Damper (TLD). In this model, the Volume Of Fluid VOF method was used to construct the free surface and the surface tension was taken into consideration to evaluate the wave breaking. The model was implemented on incompressible, 2D flow water within the TLD that was harmonically excited. . The ability of the TLD to cancel out the external excitation was examined via damping effectiveness of the TLD. The damping effectiveness is calculated as the ratio of the net energy experienced by the TLD to the input excitation energy; both energies were calculated as the area under the force-displacement curve. The investigation of the effect of the wave breaking was done through changing the fluid height ratios, amplitude and frequency ratios. The fluid height ratio was changed as h/L= 0.5, 0.35, 0.125 which is above and at and below the critical fluid level for wave breaking occurrence respectively. The critical height is defined as the height at which the waves start to break. It was found that at high fluid ratios wave breaking did not occur, in contrary, at critical level wave breaking did occur and even more breaking waves recorded to have taken place at much lesser levels. The effect of the fluid height ratio on the damping effectiveness of the TLD was investigated, it was seen that the damping effectiveness of the TLD improves as water level becomes shallower. The amplitude ratio was also examined; the behavior of the TLD in general did not change i.e. increasing the amplitude enhances the damping of the TLD. The frequency ratio range was selected to cover the near-resonance region. It was found that the TLD damps most excitation close to the resonance. The wave breaking occurrence was assured via the free surface visualization for several cases and found in agreement with different wave breaking shapes reported experimentally. / Thesis / Master of Applied Science (MASc)