Volume 5, number 1 (2011)
Permanent URI for this collection
Browse
Recent Submissions
Item Use of electromyography measurement in human body modeling(University of West Bohemia, 2011) Valdmanová, Linda; Čechová, HanaThe aim of this study is to test the use of the human body model for the muscle activity computation. This paper shows the comparison of measured and simulated muscle activities. Muscle active states of biceps brachia muscle are monitored by method called electromyography (EMG) in a given position and for given subsequently increasing loads. The same conditions are used for simulation using a human body model (Hynˇc´ık, L., Rigid Body Based Human Model for Crash Test Purposes, EngineeringMechanics, 5 (8) (2001) 1–6). This model consists of rigid body segments connected by kinematic joints and involves all major muscle bunches. Biceps brachia active states are evaluated by a special muscle balance solver. Obtained simulation results show the acceptable correlation with the experimental results. The analysis shows that the validation procedure of muscle activities determination is usable.
Item Bending of a nonlinear beam reposing on an unilateral foundation(University of West Bohemia, 2011) Machalová, Jitka; Netuka, HorymírThis article is going to deal with bending of a nonlinear beam whose mathematical model was proposed by D. Y. Gao in (Gao, D. Y., Nonlinear elastic beam theory with application in contact problems and variational approaches,Mech. Research Communication, 23 (1) 1996). The model is based on the Euler-Bernoulli hypothesis and under assumption of nonzero lateral stress component enables moderately large deflections but with small strains. This is here extended by the unilateralWinkler foundation. The attribution unilateral means that the foundation is not connected with the beam. For this problem we demonstrate a mathematical formulation resulting from its natural decomposition which leads to a saddle-point problem with a proper Lagrangian. Next we are concerned with methods of solution for our problem by means of the finite element method as the paper (Gao, D. Y., Nonlinear elastic beam theory with application in contact problems and variational approaches, Mech. Research Communication, 23 (1) 1996) has no mention of it. The main alternatives are here the solution of a system of nonlinear nondifferentiable equations or finding of a saddle point through the use of the augmented Lagrangian method. This is illustrated by an example in the final part of the article.Item Numerical analysis of bypass model geometrical parameters influence on pulsatile blood flow(University of West Bohemia, 2011) Jonášová, Alena; Vimmr, Jan; Bublík, OndřejThe present study is focused on the analysis of pulsatile blood flow in complete idealized 3D bypass models in dependence on three main geometrical parameters (stenosis degree, junction angle and diameter ratio). Assuming the blood to be an incompressible Newtonian fluid, the non-linear system of Navier-Stokes equations is integrated in time by a fully implicit second-order accurate fractional-step method. The space discretization is performed with the help of the cell-centred finite volume method formulated for unstructured tetrahedral grids. In order to model a realistic coronary blood flow, a time-dependent flow rate taken from corresponding literature is considered. For the analysis of obtained numerical results, special emphasis is placed on their comparison in the form of velocity isolines at several selected cross-sections during systolic and diastolic phases. The remainder of this paper is devoted to discussion of walls shear stress distribution and its oscillatory character described by the oscillatory shear index with regard to areas prone to development of intimal hyperplasia or to thrombus formation.Item Airflow visualization in a model of human glottis near the self-oscillating vocal folds model(University of West Bohemia, 2011) Horáček, Jaromír; Uruba, Václav; Radolf, Vojtěch; Veselý, Jan; Bula, VítězslavThe contribution describes PIV (Particle Image Velocimetry) measurement of airflow in the glottal region of complex physical models of the voice production that consist of 1:1 scaled models of the trachea, the self-oscillating vocal folds and the human vocal tract with acoustical spaces that correspond to the vowels /a:/, /u:/ and /i:/. The time-resolved PIV method was used for visualization of the airflow simultaneously with measurements of subglottal pressure, radiated acoustic pressure and vocal fold vibrations. The measurements were performed within a physiologically real range of mean airflow rate and fundamental phonation frequency. The images of the vibrating vocal folds during one oscillation period were recorded by the high-speed camera at the same time instants as the velocity fields measured by the PIV method. In the region above the model of the ventricular folds and epilarynx tube it is possible to detect large vortices with dimensions comparable with the channel cross-section and moving relatively slowly downstream. The vortices disappear in the narrower pharyngeal part of the vocal tract model where the flow is getting more uniform. The basic features of the coherent structures identified in the laryngeal cavity models in the interval of the measured airflow rates were found qualitatively similar for all three vowels investigated.Item New implicit method for analysis of problems in nonlinear structural dynamics(University of West Bohemia, 2011) Gholampour, Ali Akbar; Ghassemieh, MehdiIn this paper a new method is proposed for direct time integration of nonlinear structural dynamics problems. In the proposed method the order of time integration scheme is higher than the conventional Newmark’s family of methods. This method assumes second order variation of the acceleration at each time step. Two variable parameters are used to increase the stability and accuracy of the method. The result obtained from this new higher order method is compared with two implicit methods; namely the Wilson- and the Newmark’s average acceleration methods.Item Determination of principal residual stresses’ directions by incremental strain method(University of West Bohemia, 2011) Civín, Adam; Vlk, MilošThe ring-core method is the semi-destructive experimental method used for evaluation of the homogeneous and non-homogeneous residual stresses, acting over depth of drilled core. By using incremental strain method (ISM) for the residual state of stress determination, this article describes procedure how unknown directions and magnitudes of principal residual stresses can be determined. Finite element method (FEM) is used for the numerical simulation of homogenous residual state of stress and for subsequent strain determination. Relieved strains on the top of the model’s core are measured by simulated three-element strain gauge, turned by the axis of strain gauge “a” from the direction of the principal stress σ1 about unknown angle α. Depth dependent magnitudes of relieved strains, their differences and set of known values of calibration coefficients K1 and K2 or relaxation coefficients A and B are used together for determination of the angle α and for re-calculation of principal stresses.Item Using a tensor model for analyzing some aspects of mode-II loading(University of West Bohemia, 2011) Seitl, Stanislav; Fernández-Zúñiga, Daniel; Fernández-Canteli, AlfonsoWhen analyzing the scatter and discrepancies arising among the fracture toughness resulting for differentmaterials and given mixity ratio KIIC/KIC three factors seems to be influential in contributing to the still unsatisfactory state of affairs in this field: a) the lack of established requirements as regards geometry and minimal in- and out-ofplane dimensions of specimens regulating the test for determining mode-II fracture toughnessKIIC or, in the more general case, its equivalent in mixed mode cases, b) the role played by the micro-cracking present in the process zone, acknowledged as a microstructural phenomenon already pointed out by Kalthoff and co-workers, needs to be experimentally investigated, and is not considered in the mainly analytical and numerical focussing pursued here, and c) the insufficient attention paid to the particularity of the stress fields around the crack front before and after the daughter crack is formed. In this work, the last question is addressed with the intention of contributing to the clarification of some points with regard to crack instability under mode-II and mixed-mode loading, in particular, why it is difficult to formulate a sufficiently simple failure model for mechanical components or real structures for which the type of load or the geometry results in stress states from which the potential of mixed mode failure arises.Item Finite element modelling of vocal tract changes after voice therapy(University of West Bohemia, 2011) Vampola, Tomáš; Laukkanen, Anna Maria; Horáček, Jaromír; Švec, Jan G.Two 3D finite element (FE) models were constructed, based on CT measurements of a subject phonating on [a:] before and after phonation into a tube. Acoustic analysis was performed by exciting the models with acoustic flow velocity at the vocal folds. The generated acoustic pressure of the response was computed in front of the mouth and inside the vocal tract for both FE models. Average amplitudes of the pressure oscillations inside the vocal tract and in front of the mouth were compared to display the cost-efficiency of sound energy transfer at different formant frequencies. The formants F1–F3 correspond to classical vibration modes also solvable by 1D vocal tract model. However, for higher formants, there occur more complicated transversal modes which require 3D modelling. A special attention is given to the higher frequency range (above 3.5 Hz) where transversal modes exist between piriform sinuses and valleculae. Comparison of the pressure oscillation inside and outside the vocal tract showed that formants differ in their efficiency, F4 (at about 3.5 kHz, i.e. at the speaker’s or singer’s formant region) being the most effective. The higher formants created a clear formant cluster around 4 kHz after the vocal exercise with the tube. Since the human ear is most sensitive to frequencies between 2 and 4 kHz concentration of sound energy in this frequency region (F4–F5) is effective for communication. The results suggest that exercising using phonation into tubes help in improving the vocal economy.Item Finite element for non-stationary problems of viscoelastic orthotropic beams(University of West Bohemia, 2011) Zajíček, Martin; Adámek, Vítězslav; Dupal, JanThe main aim of this work is to derive a finite beam element especially for solving of non-stationary problems of thin viscoelastic orthotropic beams. Presented approach combines the Timoshenko beam theory with the consideration of nonzero axial strain. Furthermore, the discrete Kelvin-Voight material model was employed for the description of beam viscoelastic material behaviour. The presented finite beam element was derived by means of the principle of virtual work. The beam deflection and the slope of the beam have been determined by the analytical and numerical (FEM) approach. These studies were made in detail on the simple supported beam subjected to the non-stationary transverse continuous loading described by the cosine function in space and by the Heaviside function in time domain. The study shows that beam deformations obtained by using derived finite element give a very good agreement with the analytical results.Item A computational method for determination of a frequency response characteristic of flexibly supported rigid rotors attenuated by short magnetorheological squeeze film dampers(University of West Bohemia, 2011) Zapomněl, Jaroslav; Ferfecki, Petr; Čermák, LiborLateral vibration of rotors can be significantly reduced by inserting the damping elements between the shaft and the casing. The theoretical analysis, confirmed by computational simulations, shows that to achieve the optimum compromise between attenuation of the oscillation amplitude and magnitude of the forces transmitted through the coupling elements between the rotor and the stationary part, the damping effect must be controllable. For this purpose, the squeeze film dampers lubricated by magnetorheological fluid can be applied. The damping effect is controlled by the change of intensity of the magnetic field in the lubricating film. This article presents a procedure developed for investigation of the steady state response of rigid rotors coupled with the casing by flexible elements and short magnetorheological dampers. Their lateral vibration is governed by nonlinear (due to the damping forces) equations of motion. The steady state solution is obtained by application of a collocation method, which arrives at solving a set of nonlinear algebraic equations. The pressure distribution in the oil film is described by a Reynolds equation modified for the case of short dampers and Bingham fluid. Components of the damping force are calculated by integration of the pressure distribution around the circumference and along the length of the damper. The developed procedure makes possible to determine the steady state response of rotors excited by their unbalance, to determine magnitude of the forces transmitted through the coupling elements in the supports into the stationary part and is intended for proposing the control of the damping effect to achieve optimum performance of the dampers.Item Modal properties of the flexural vibrating package of rods linked by spacer grids(University of West Bohemia, 2011) Zeman, Vladimír; Hlaváč, ZdeněkThe paper deals with the modelling and modal analysis of the large package of identical parallel rods linked by transverse springs (spacer grids) placed on several level spacings. The rod discretization by finite element method is based on Rayleigh beam theory. For the cyclic and central symmetric package of rods (such as fuel rods in nuclear fuel assembly) the system decomposition on the identical revolved rod segments was applied. A modal synthesis method with condensation is used for modelling of the whole system. The presented method is the first step for modelling the nuclear fuel assembly vibration caused by excitation determined by the support plate motion of the reactor core.
