Rimantas Barauskas

Finite element modeling of cooled-tip probe radiofrequency ablation processes in liver tissue

Finite element model of radiofrequency ablation (RFA) with cooled-tip probe in liver has been developed by employing COMSOL Multiphysics software. It describes coupled electric, thermal and sodium chloride solution infiltration flow phenomena taking place during ablation processes. Features of hydraulic capacity, saturation of the tissue by infiltration, and dependency of electrical conductivity on the damage integral of the tissue have been supplied to the model. RFA experiments have validated the model. Physical parameters describing hydraulic capacity and hydraulic conductivity in the tissue, as well as, the relation of electrical conductivity against the value of damage integral have been determined.

A model for numerical simulation of heat and water vapor exchange in multilayer textile packages with three-dimensional spacer fabric ventilation layer

The work presents a structural computational model developed for investigation of heat and air—water vapor mass exchange between the multilayer textile packages and the human body. Each fabric layer is represented by a single element, characterized by its air and water vapor permeability, thermal resistance and heat capacity, as well as, water vapor absorption properties. While characteristics of each layer are obtained experimentally, the computational model enables the prediction of the variation in time of temperature and humidity in the gaps between each pair of adjacent layers. The finite element, which represents the forced ventilation layer made of three-dimensional textile material, is created and may be used as a structural element within the overall structural model of the textile package. The element equations are derived on the basis of ideal gas state equation. The results of the measured textile layers characteristics and the numerical data are presented. Though the condensation of water vapor was not considered in this model, the time point of initiation of sweating at given heat and water vapor generation rates could be predicted.

Line Scale Calibration in Non-Ideal Measurement Situation

The precision line scale calibration in dynamical mode of operation is considered. A new interferometer-controlled comparator with moving microscope has been developed and optimised in order to reduce both the measurement uncertainty and calibration process duration. Modal analysis performed and measurements conducted of the spatial vibrations of comparator structure revealed that dynamically-induced errors can noticeably contribute to the measurement uncertainty budget. They can be prominently reduced, in particular, by proper improvement and optimisation of the carriage structure and elimination of the dry friction in the carriage drive.

Software based control techniques for precision line scale calibration

A precision line scale calibration method in a dynamic regime enables to gain the calibration throughput and simplify the comparatorpsila s structure and control. However, the errors featured by geometrical deviations, static and dynamic loads as well as temperature induced deformations should be considered and incorporated into uncertainty budget. These errors contain random and systematic components and can be minimized by increasing the accuracy of system design and implementing numerical error compensation methods. The paper discusses advances of precision line scale calibration system design gained by implementing new measurement and software based control techniques.

Estimation of thermo-elastic damping of vibrations in micro-electro-mechanical systems resonators: finite element modeling

Abstract. The authors investigated finite element (FE) analysis of damped modal vibrations in complex geometries of micro-electromechanical (MEM) resonators. Q-factor values were determined by taking the thermo-elastic damping into account. The basic model created is presented as a system of partial differential equations, which describe the elastic and thermal phenomena in the MEM structure. Mathematically the problem is formulated as a complex eigenvalue problem. Modal properties of square- and ring-shaped bulk-mode MEM resonators were investigated by taking into account the layered structure of the MEM system and the influence of the geometry of the clamping zone. The calculations were performed by employing the COMSOL Multiphysics FE software. The solution method was verified by comparing numerically and analytically obtained damped modal properties of a MEM cantilever resonator.

Investigation of the thermal properties of spacer fabrics with bio-ceramic additives using the finite element model and experiment:

The heat resistance of fabric enhanced by bio-ceramic additives (BCAs) is investigated theoretically and experimentally in order to determine the influence of modification of the infrared (IR) absorption property of the fabric. The enhanced IR sensitivity of textiles improves the thermoregulatory processes when worn in cold environments. The finite element model has been developed by taking into account the coupled phenomena of heat conduction, surface convection and the interaction of the fabric with IR power flux by employing heat transfer differential equations and the Stefan–Boltzmann law. Evaluations of IR absorptivity, reflectivity and transmissivity, the temperature transients during the hot plate chamber test and heat retaining properties of the fabric heated by an IR lamp have been obtained experimentally and simulated by means of the developed finite element model. The values of model parameters have been found, which provided a satisfactory match between the computation and the experiment in all considered cases. Simultaneously, the obtained values were reasonably close to rough theoretical estimations. Efforts have been made to distinguish from each other the influence of diffusive and radiative components of heat transfer, which affect the results of thermal resistance tests. The comparative analysis of contributions of different heat exchange mechanisms allows a better understanding of the peculiarities of standard heat resistance measurement procedures applied to BCA-enhanced fabrics and facilitates the validation of the computational models.

Minimization of Numerical Dispersion Errors in Finite Element Models of Non-homogeneous Waveguides

The paper presents the approach for the reduction of numerical errors, which are inherent for simulations based on wave propagation models in discrete meshes. The discrete computational models always tend to generate errors of harmonic wave propagation velocities in higher frequency ranges, which can be treated as numerically-induced errors of dispersion curves. The result of the errors is the deterioration of the shapes of simulated waves as the time of simulation increases. The presented approach is based on the improvement of the matrices of elements of the finite element model by means of correction of the modal frequencies and modal shapes of an individual element. The approach developed by the authors earlier and proved to work in the case of a uniform waveguide now has been demonstrated to be valid for simulations of waves in networks of waveguides. The non-reflecting boundary conditions can be implemented by combining synthesized and lumped mass elements in the same model. The propagating wave pulses can be satisfactorily simulated in comparatively rough meshes, where only 6-7 finite elements per wavelength are used.

Finite element modelling and simulation of thermo-elastical damping of MEMS vibrations

The contribution is directed to providing accurate simulation and approximation of the Q-factor determined by thermalelastic damping in complex micro-electromechanical (MEM) resonators. The base model created is presented as a system of partial differential equations, which describe the elastic and thermal phenomena in the MEM structure. The FEM calculations were performed by using COMSOL Multiphysics software. The model was verified by comparing numerically and analytically obtained damped modal properties of a MEM cantilever resonator. The comparison of calculated and experimentally obtained resonant frequencies and Q-factor values indicated a good agreement of tendencies of change of the quantities against temperature. Investigation of longitudinal and bending vibration modes in 3D of a beam resonators was accomplished by taking into account the layered structure of the resonator and the influence of the geometry of the clamping zone. Modal properties of rectangle- and ring-shaped bulk-mode MEM resonators were examined, too.

Dynamic Mode of Line Scale Calibration

The need for high-resolution and high-speed detection of line scale graduations is a result of increasing throughput requirements of precision scale calibration systems. These new demanding requirements can be met by exploiting the advantages of the dynamic mode of calibration. The state-of-the-art platform-based approach to improving calibration systems performance will allow both functional extension to already deployed systems and the design of new ones, capable of adapting to different application patterns and user profiles. An interferometer-controlled comparator setup has been designed to measure the performance of the dynamic calibration process. A novel 3D finite element model has been developed in order to both investigate thermo-mechanical processes in the comparator structure and evaluate possible temperature influence on geometrical dimensions of the line scale. A series of measurements has been conducted at PTB and MIKES length calibration laboratories in order to evaluate the capabilities of the dynamic calibration of graduated line scales.

Multi-scale finite element modeling of 3D printed structures subjected to mechanical loads

Purpose The purpose of this paper is to investigate the influence of geometrical microstructure of items obtained by applying a three-dimensional (3D) printing technology on their mechanical strength. Design/methodology/approach Three-dimensional printed items (3DPI) are composite structures of complex internal constitution. The buildup of the finite element (FE) computational models of 3DPI is based on a multi-scale approach. At the micro-scale, the FE models of representative volume elements corresponding to different additive layer heights and different thicknesses of extruded fibers are investigated to obtain the equivalent non-linear nominal stress–strain curves. The obtained results are used for the creation of macro-scale FE models, which enable to simulate the overall structural response of 3D printed samples subjected to tensile and bending loads. Findings The validation of the models was performed by comparing the computed results against the experimental ones, where satisfactory agreement has been demonstrated within a marked range of thicknesses of additive layers. Certain inadequacies between computed against experimental results were observed in cases of thinnest and thickest additive layers. The principle explanation of the reasons of inadequacies takes into account the poorer quality of mutual adhesion in case of very thin extruded fibers and too-early solidification effect. Originality/value Flexural and tensile experiments are simulated by FE models that are created with consideration to microstructure of 3D printed samples.