Ardeshir Karami Mohammadi

This study investigates the free and forced nonlinear vibrations, along with the static bifurcation behavior, of a fluid transmission hyperelastic pipe supported by clamped ends. Both geometrical and material nonlinearities are taken into account. The mathematical model is grounded in Euler–Bernoulli beam theory, incorporating von Kármán nonlinearity to capture the complexities of the system. For the hyperelastic materials, the strain energy function proposed by Yeoh is employed. Considering that the lowest critical velocity of the fluid flow is associated with the first mode, after deriving the governing equation using Hamilton’s principle, Galerkin discretization is applied to obtain the reduced order model in the first mode. Analysis of static bifurcation and stability is performed and critical velocity values of fluid flow are determined. The analysis of free and forced vibrations of the hyperelastic pipe is conducted using the method of multiple time scales, specifically focusing on the sub-critical velocity range of fluid flow, which corresponds to the unbuckled state of the pipe. This approach allows for a comprehensive examination of how fluid flow velocity influences the frequency characteristics of free vibrations, as well as the frequency response in the vicinity of the main resonance. The results obtained from this study can significantly enhance our understanding of the design and application of hyperelastic pipes in various fluid transfer scenarios. By elucidating the dynamic behavior and critical parameters associated with these pipes, the findings can inform engineering practices and contribute to the optimization of systems that utilize hyperelastic materials for fluid conveyance.

A Galerkin-based nonlinear normal mode approach based on the concept of invariant manifolds is adopted to analyze the effect of thermoelastic damping on the nonlinear vibrations of a microbeam with mid-plane stretching and electrostatic actuation. The derivation of the mathematical model starts with two coupled equations: a general Euler–Bernoulli equation for microbeam vibrations, which includes thermal moment and thermal force terms due to the thermoelastic damping effect; and a thermal equation describing the generation of the thermal moment due to bending vibrations. Given the available information about the material properties and geometry of conventional microbeams, and a physical reasoning based on it about the time constant of the thermal equation, the two governing equations are integrated into a single nonlinear partial differential equation. Galerkin discretization is used to transform the governing equation into a system of coupled nonlinear ordinary differential equations that are uncoupled at linear order. The modal analysis is then followed by finding an invariant manifold leading to a nonlinear ordinary differential equation that governs the vibration behavior of the system. The results of the case study showed that the amount of thermoelastic damping is dependent on the excitation voltage, and the damping increases with increasing voltage. Furthermore, in the presence of thermoelastic damping, by applying the step voltage and starting the vibrations, the frequency increases over time until it reaches a certain limit. With a higher excitation voltage, the frequency increases at a higher rate, but to a lower final limit. In an undamped system, the frequency of oscillations is constant over time, but a higher excitation voltage causes lower frequency vibrations. The results of pull-in analysis showed that thermoelastic damping has no significant effect on static or dynamic pull-in voltage.

In this paper, a nonlinear energy sink is used to improve the dynamic behavior of a square plate in supersonic flow. The plate is examined with different boundary conditions. The effect of changing the parameters and installation location of nonlinear energy sink on improving the dynamic behavior of the plate has been studied. The governing equations are first obtained using Von Kármán’s plate theory and piston theory, and then discretized using the Rayleigh–Ritz process. These equations are then solved using the fourth-order Runge–Kutta method. Time history curves, phase portraits, Poincaré maps, and bifurcation diagrams were used to investigate the dynamic behavior and impact of nonlinear energy sink. The results show that the dynamic behavior of the plate is very complex in some cases, but with proper use of nonlinear energy sinks, this behavior can be improved.

In this paper, the molecular gas lubrication model was used to analyze the nonlinear dynamic behavior of two-lobe non-circular gas-lubricated micro-bearings. The effects of temperature rise are taken into account. At high temperatures, in addition to gas rarefaction, its viscosity and friction will also change, and slip across boundaries will occur. The rarefaction of the lubricating gas film caused by the microscale effect at high temperatures was considered. The effects of temperature and rotation speed (with and without rarefaction effect) on the dynamic behavior of the non-circular micro gas bearing were studied. The nonlinear equation governing the gas behavior is discretized using the finite-element method and then solved simultaneously with the dynamic equations of rotor motion using the fourth-order Runge–Kutta method. Center orbit diagrams, phase portraits, Poincare maps, power spectrum, and bifurcation diagrams are used to investigate the dynamic behavior of two-lobe non-circular gas-lubricated micro-bearings. Some results show that with increasing temperature, the rotor behavior changes from T-periodic to quasi-periodic. It was also observed that at high temperatures, with increasing rotational speed, the behavior of the system changes from T-periodic to quasi-periodic, but if the gas is rarefied, this change occurs at a slower speed.

Numerous applications have been found in various microelectromechanical systems for shape memory alloys. This study analyzes the nonlinear free vibrations of a sandwiched microbeam, consisting of a shape memory alloy core and two elastic layers on either side. The behavior of the shape memory layer is modeled with the one-dimensional Brinson model. The equation of motion is derived from Hamilton’s principle and the modified couple stress theory for the Euler–Bernoulli beam and then truncated into a reduced-order model through Galerkin’s technique. An approximate analytical solution is obtained using the Lindstedt–Poincare method for different temperatures. The effects of temperature, material length scale parameter, aspect ratio, and maximum vibration amplitude on nonlinear normalized frequency have been investigated. The results have been compared with those of some previous studies and can provide a better understanding of the design of sandwiched microbeams, which have a shape memory alloy layer.

A new augmented Biderman model inspired by the modified couple stress theory has been introduced to investigate the size effect in addition to nonlinear material effects. Then, this model is used to investigate free vibration of a hyper-elastic microbeam. Classical Biderman strain energy does not...

In this paper, an Euler–Bernoulli model has been used for nonlinear vibration, stability, and bifurcation analysis of spinning twisted beams with linear twist angle, and with large transverse deflections, near the primary and parametric resonances. The equations of motion, in the case of pure single mode motion are analyzed by two methods: directly applying multiple scales method and using multiple scales method after discretization by Galerkin’s procedure. It is observed that the same final relations are obtained in the two methods. Effects of twist angle, damping ratio, longitudinal to transverse stiffness ratio, and eccentricity on the frequency responses are investigated. Then, the results are compared with the results obtained from Runge–Kutta numerical method on ODEs in a steady state, and confirmed with some previous research. Finally, the results show a good correlation, and it shows that with increasing the twist angle from 0 to 90°, the natural frequencies increase in the first two modes.