Radoslav Darula

Theoretical and experimental analysis of the stop-band behavior of elastic springs with periodically discontinuous of curvature.

The advanced design of springs with tailored transmission characteristics is proposed. The existence of frequency stop-bands in the transmission characteristics is theoretically predicted and experimentally validated. The underlying mechanism of generating stop-band behavior is the periodic discontinuity of the curvature. The theoretical analysis is performed in the framework of the Floquet theory for an infinitely long spring. Experimental results are obtained for the springs consisting of a small number of repeated periodicity cells.

An Application of Electromagnetic Induction in Vibration Control

Excessive vibration of machines and/or structures can be controlled passively (e.g. introducing resilient elements) and/or actively (e.g. using elements capable to adjust their properties for actual state of the vibration). An electromagnet, as a vibration control element, can be implemented in active (or semi-active) control strategy. This approach is analyzed in the present paper. The electromagnetic induction occurs in a magnetic circuit exposed to variable magnetic flux, which can be obtained e.g. by changing reluctance (magnetic resistance) of the system due to a variable air gap, as the result of armature vibration. The lumped parameter mathematical model of the coupled electro-magneto-mechanical system is formulated. The performance of the model is analyzed, assuming harmonic forced vibration. By the induction, mechanical energy of vibration is converted into electrical one and dissipated in the shunt resistance. Two concepts are investigated further – electromagnet behaves as (a) a spring element (reduction of equivalent system stiffness); (b) a damping element.

Vibro-acoustics of porous materials - Waveguide modelling approach

The porous material is considered as a compound multi-layered waveguide (i.e. a fluid layer surrounded with elastic layers) with traction free boundary conditions. The attenuation of the vibro-acoustic waves in such a material is assessed. This approach is compared with a conventional Biots model and a qualitative agreement in phase velocities as well as damping estimates is found. The waveguide model predicts four waves, out of which two are attenuated when the viscous fluid is considered (while the elastic layer being ideally lossless). One of these waves is found to be significantly controlled by the fluid viscosity, while for the other the effect of viscosity was observed for very small frequencies. The Biots model predicts only one of these attenuated waves, where the latter one is not predicted. Thus the proposed waveguide approach provide additional information about the wave propagation in porous materials.

Mathematical Modelling and Parameter Identification of an Electro-Magneto-Mechanical Actuator for Vibration Control

An electro-magneto-mechanical system, composed of the electromagnet, having an iron core with a coil and a movable yoke, is analyzed in the paper. Exposing the yoke to mechanical motion, the variation of the magnetic flux due to change of the air gap height induces alternating voltage at the coil terminals. If the electrical circuit is closed, the so generated electrical power is dissipated via the internal coil losses, treated in this paper. Thanks to the interaction between the electrical and mechanical system (i.e. via magnetic force), the power dissipation in electrical circuit influences the dynamical response of the mechanical system. And so the mechanical vibrations can be controlled by these means.The mathematical model of the simplified dynamical system, which describes behavior of the experimental set-up, is derived using a lumped parameter approach.The aim of the article is to identify parameters of the derived mathematical model, focused mainly on electrical circuit. Based on measured experimental data, the static constants as well as dynamic losses were analyzed.Copyright