Masoud Ansari

Longitudinal dynamics of freight trains

A comprehensive parametric study is performed on the longitudinal dynamics of a freight train based on different nonlinear time-domain models. The effect of different parameters, such as the couplers stiffness and damping, train speed, train acceleration, load distribution pattern, on the longitudinal train dynamics are investigated. Moreover, the effect of an empty wagon and its position, on the overall longitudinal train dynamics is discussed. A computer simulation model is developed to determine the optimum placement of the second locomotive in order to obtain the lowest longitudinal forces. Further study is carried out to determine the best type of automatic couplers.

Exact Frequency Analysis of a Rotating Cantilever Beam With Tip Mass Subjected to Torsional-Bending Vibrations

An exact frequency analysis of a rotating beam with an attached tip mass is addressed in this paper while the beam undergoes coupled torsional-bending vibrations. The governing coupled equations of motion and the corresponding boundary condition are derived in detail using the extended Hamilton principle. It has been shown that the source of coupling in the equations of motion is the rotation and that the equations are linked through the angular velocity of the base. Since the beam-tip-mass system at hand serves as the building block of many vibrating gyroscopic systems, which require high precision, a closed-form frequency equation of the system should be derived to determine its natural frequencies. The frequency analysis is the basis of the time domain analysis, and hence, the exact frequency derivation would lead to accurate time domain results, too. Control strategies of the aforementioned gyroscopic systems are mostly based on their resonant condition, and hence, acquiring knowledge about their exact natural frequencies could lead to a better control of the system. The parameter sensitivity analysis has been carried out to determine the effects of various system parameters on the natural frequencies. It has been shown that even the undamped systems undergoing base rotation will have complex eigenvalues, which demonstrate a damping-type behavior.

Internal Resonance of Finite Beams on Nonlinear Foundations Traversed by a Moving Load

Vibration analysis of beams traversed by moving load is an old and well known topic in structural mechanics, and has been of great interest for researchers of different fields, such as mechanical, railway and civil engineering. Many researchers have conducted different investigations in this field. In the present research, the nonlinear vibration of the system is studied and consequently the response of the system to a moving load is determined as a closed form solution. Furthermore, the effects of load amplitude on the response of the system are investigated. Galerkin’s method is first utilized to truncate the governing equation of motion and then MMS (Method of Multiple Scales) perturbation method is applied to study the nonlinear vibration of the system, in the presence of the internal resonance. Effects of damping of the foundation as well as magnitude of the moving load on the frequency responses are investigated. The proposed methodology and obtained results can be used to investigate the behavior several systems among which railway system shows a good compatibility.Copyright

Effects of Coupler Specifications and Operational Conditions on the Longitudinal Freight Train Dynamics

The longitudinal dynamics of railway vehicles is studied in this paper. The model is a time domain and nonlinear one. Model includes a train-consist containing one locomotive, nine wagons and nine automatic couplers between them. The effects of different parameters, (such as stiffness and damping of automatic couplers, train speed and train acceleration during both accelerating and braking process) on the longitudinal train dynamics are investigated in a parametric study. It is found that increase of the train operational speed has no effect on the maximum tractive forces while it results in increasing the maximum pressing forces as well as the RMS (Root mean square) value of the coupler forces. Higher acceleration during accelerating leads to higher maximum tractive and pressings forces and also higher RMS value of the coupler forces. Although increase of acceleration during braking results in higher pressing coupler forces, but it has no effect on the maximum tractive force.Copyright

Optimal Level Set Vibration Control of Plate Structures

This research is motivated by the need for control of flexural vibrations of lightweight plates. It addresses application of the level set method in optimal control of vibrations in plate-like structures. One of the most commonly practiced methods in control of vibration is to apply constrained layer damping patches to the surface of a structure. In order to consider the weight efficiency of the structure, the best shape and locations of the patches should be determined to achieve the optimum vibration suppression with lowest amount of damping patch. A novel topology optimization approach is proposed that is capable of finding the optimum shape and locations of the patches simultaneously. A 2D cantilever plate, undergoing flexural vibrations, will be considered. The optimal damping set will be found in the structure, such that the lowest modal energy in the fundamental vibration mode of the system is achieved. The proposed level set topology optimization method shows capability of determining the optimum damping set in structures accurately.Copyright

Optimal Viscous Damper Placement Using Level Set Topology Optimization Method

Vibration control has always been of great interest for many researchers in different fields, especially mechanical and civil engineering. One of the key elements in control of vibration is damper. One way of optimally suppressing unwanted vibrations is to find the best locations of the dampers in the structure, such that the highest dampening effect is achieved. This paper proposes a new approach that turns the conventional discrete optimization problem of optimal damper placement to a continuous topology optimization. In fact, instead of considering a few dampers and run the discrete optimization problem to find their best locations, the whole structure is considered to be connected to infinite numbers of dampers and level set topology optimization will be performed to determine the optimal damping set, while certain number of dampers are used, and the minimum energy for the system is achieved. This method has a few major advantages over the conventional methods, and can handle damper placement problem for complicated structures (systems) more accurately. The results, obtained in this research are very promising and show the capability of this method in finding the best damper location is structures.Copyright

Parameter Sensitivity Analysis of Rotating Beams in Frequency Domain

Many mechanical rotating systems can be modeled as a cantilever beam attached to a rotating substrate. Vibratory beam gyroscopes are good examples of such systems. They consist of a rotating beam with a tip mass, attached to a rotating base. Due to the base rotation, the governing partial differential equations of the system are coupled, and hence, the system undergoes coupled torsional-bending vibrations. The coupling effect complicates the frequency analysis of the system, especially in determining the system characteristic equation. Many investigators have chosen to use the assumed mode method in their analysis of such systems instead of extracting the exact mode shapes of the system. In spite of all these difficulties, this paper addresses the exact frequency analysis of such systems and presents a closed-form frequency characteristic equation and evaluates the accurate values of the natural frequencies. The application of the proposed method is not limited to the system at hand, as it can be utilized for analyzing general systems with coupled governing equations of motion. Having analyzed a closed-form frequency equation has two valuable advantages: a) it can serve as the basis for the subsequent time-domain analysis; and b) it can be very essential in developing control strategies. In this study a thorough sensitivity analysis is performed to determine the effects of different parameters on the natural frequencies of the coupled vibrating system. The proposed method reveals some interesting findings in the systems which were difficult, if not impossible, to be revealed by the assumed mode method commonly utilized in many research work reported recently in literature.Copyright

On Coupled Flexural and Torsional Oscillations of a Vibrating Beam Gyroscopic System

Gyroscopes are commonly used to measure the angle of rotation and its rate of change in several critical systems like airplanes. Therefore, there is a never-ending desire for researchers to increase measurement precision of these devices. In order to achieve this goal, some new gyroscopes have been invented recently. Especially, advent of micro manufacturing has appeared some sophisticated to more precised gyroscopic systems. The widely-used gyroscopes are vibrating beam gyroscopes; however they face a very important drawback, called cross-coupling error. In presence of the secondary base rotations, significant errors will be produced in measurement of the gyroscope output. In order to deal with this issue, this paper addresses a novel gyroscopic system, called rocking-mass gyroscope. It is consist of four beams attached to a rigid substrate, undergoing coupled flexural and torsional vibrations with a finite mass attached in the middle. This configuration is such that, it does not encounter the same problems as vibrating beam gyroscopes. This configuration makes the vibration analysis very complicated. Despite this fact, a thorough analysis is performed in this paper. Using Extended Hamilton’s principle, eight governing partial differential equations of motion along with their corresponding boundary conditions are derived. Further attempt is made to find the closed-form frequency equation of the system. Solving this equation needs high computational costs and gives the natural frequencies of the system. In spite of this fact, the system is analysed in the frequency domain using an exact method in full detail, for two cases of fixed and rotating base support. Furthermore, a detailed parameter sensitivity analysis is carried out to determine the effects of different parameters on the natural frequencies of the system. The contributions of this research are very important from two viewpoints. Firstly, determination of natural frequencies and resonance conditions are essential for design of the system, and design of appropriate control strategies. Secondly, frequency domain analysis forms the basis of time domain analysis, followed by exact mode superposition method.Copyright

Effects of the Load Distribution Patterns on the Longitudinal Freight Train Dynamics

A comprehensive parametric study is carried out on the longitudinal dynamics of a freight train having different loading patterns. A nonlinear time domain model, with one locomotive and nine wagons, is considered. In another simulation the train model has two locomotives and eight wagons, and in both models, every two cars are connected to each other through an automatic coupler. The effects of different load distribution patterns on the coupler forces for the cases of ascending, descending, constant, ascending-descending and descending-ascending are investigated through a parametric sensitivity study. In order to investigate how an empty wagon and its position in a train-consist model may affect the overall longitudinal dynamic behavior of freight trains a second computer simulation model has been developed. Moreover, the best possible position for the second locomotive with the objective of reaching to the lower longitudinal forces, in the case that an additional locomotive is included will be discussed. Finally, an investigation is carried out to determine the kind of couplers with their relevant specifications that must be installed in different positions of a train-consist in order to improve the longitudinal train dynamic behavior.Copyright