# Volume 11, number 1 (2017)

## Permanent URI for this collection

## Browse

### Recent Submissions

Item Whirl flutter optimisation-based solution of twin turboprop aircraft using a full-span model(University of West Bohemia, 2017) Čečrdle, JiříShow more parts as a propeller or a gas-turbine engine rotor. The proposed optimisation-based analytical procedure is used to determine the critical values of the engine attachment stiffness parameters for the preselected flutter speed. For the half-span model, two design variables are used. The objective function is defined as the minimization of the engine vibration mode frequency sum. Design constraints keep the engine frequency ratio and the flutter stability at the selected velocity. However, application of a full-span model is necessary in some cases. In this case, special models of both symmetric and antisymmetric engine vibrations and four design variables must be used. Design constraints maintain the pitch mode frequency ratio, the yaw mode frequency ratio and the critical mode frequency ratio. Critical modes are dependent on the relation between the rotational direction of both propellers (identical or inverse). A flutter design constraint is applied as well. The described methodology is demonstrated on the application example of a twin-engine commuter aircraft. Demonstrated cases include symmetrical revolutions of propellers for both identical and inverse directions of rotation, cases of single engine failure and single propeller feathering, and finally, cases of unsymmetrical revolutions including the reduced and increased revolutions of a single propeller, for both identical and inverse directions of rotation.Show more Item Determination of group velocity of propagation of Lamb waves in aluminium plate using piezoelectric transducers(University of West Bohemia, 2017) Lašová, Zuzana; Zemčík, RobertShow more A prior knowledge of group velocities of Lamb wave modes is a key for analysis of time signals in guidedwave based structural health monitoring. The identification of multiple wave modes may be complicated due to dependency of group velocity on frequency (dispersion). These dependencies for infinite plate of constant thickness can be calculated by a numerical solution of analytic equation. Two alternative approaches to determine group velocities of zero-order Lamb wave modes in aluminum plate were used in this work: Two-dimensional Fast Fourier Transform (2D-FFT) and methods of time-frequency processing. 2D-FFT requires a high number of time signals in equidistant points, therefore it was applied on data from finite element analysis of wave propagation in the plate. Group velocities for chosen frequencies were also determined using wavelet transform (WT) of signals as differencies of times of arrival measured by a pair of piezoelectric transducers. The results from 2D-FFT and wavelet transform were compared to the analytic solution.Show more Item Rotary oscillations of a micropolar fluid sphere in a bounded medium(University of West Bohemia, 2017) Madasu, Krishna Prasad; Gurdatta, Manpreet KaurShow more The present study examines the axisymmetric rotary oscillation of a micropolar fluid sphere in concentric spherical cavity filled with Newtonian viscous fluid. A continuity of velocity components and stress together with the spin vorticity relation are used at the interface between fluid-fluid regions. The torque exerted on the micropolar fluid sphere is obtained analytically and the real and imaginary torque coefficients are presented graphically. The effect of the micropolarity parameter, viscosity ratio and spin parameter on the torque are studied numerically. In the limiting cases, the torque acting on the rotating micropolar fluid sphere and solid sphere in concentric spherical cavity are obtained from the present analysis.Show more Item Propagation of plane waves in a rotating magneto-thermoelastic fiber-reinforced medium under G-N theory(University of West Bohemia, 2017) Maity, Narottam; Sakti, Barik Pada; Chaudhuri, Pranay KumarShow more The article is concernedwith the possibility of plane wave propagation in a rotating elastic medium under the action of magnetic and thermal fields. The material is assumed to be fibre-reinforced with increased stiffness, strength and load bearing capacity. Green and Nagdhi’s concepts of generalized thermoelastic models II and III have been followed in the governing equations expressed in tensor notation. The effects of various parameters of the applied fields on the plane wave velocity have been shown graphically.Show more Item Multiscale probabilistic modeling of a crack bridge in glass fiber reinforced concrete(University of West Bohemia, 2017) Rypl, Rostislav; Vořechovský, MiroslavShow more The present paper introduces a probabilistic approach to simulating the crack bridging effects of chopped glass strands in cement-based matrices and compares it to a discrete rigid body spring network model with semi-discrete representation of the chopped strands. The glass strands exhibit random features at various scales, which are taken into account by both models. Fiber strength and interface stress are considered as random variables at the scale of a single fiber bundle while the orientation and position of individual bundles with respect to a crack plane are considered as random variables at the crack bridge scale. At the scale of the whole composite domain, the distribution of fibers and the resulting number of crack-bridging fibers is considered. All the above random effects contribute to the variability of the crack bridge performance and result in size-dependent behavior of a multiply cracked composite.Show more Item Calculation of locomotive traction force in transient rolling contact(University of West Bohemia, 2017) Voltr, PetrShow more To represent thewheel-rail contact in numerical simulations of rail vehicles, simplified models (Fastsim, Pola´ch etc.) are usually employed. These models are designed for steady rolling only, which is perfectly suitable in many cases. However, it is shown to be limiting for simulations at very low vehicle speeds, and therefore it does not actually allow simulation of vehicle running at arbitrarily variable speed. The simplified model of transient rolling, which involves calculation of the stress distribution in the discretised contact area, overcomes this disadvantage but might be unnecessarily complex for more simple simulations. In this paper, an approximative creep force computation method for transient rolling is presented. Its purpose is not to study the transient phenomena themselves but provide a simple and readily available way to prevent incorrect results of the numerical simulation when the vehicle speed approaches zero. The proper function of the proposed method is demonstrated by a simulation of start-up and interrupted sliding of a four-axle locomotive.Show more Item Periodic solutions of a graphene based model in micro-electro-mechanical pull-in device(University of West Bohemia, 2017) Wei, Dongming; Kadyrov, Shirali; Kazbek, ZhassulanShow more Phase plane analysis of the nonlinear spring-mass equation arising in modeling vibrations of a lumped mass attached to a graphene sheet with a fixed end is presented. The nonlinear lumped-mass model takes into account the nonlinear behavior of the graphene by including the third-order elastic stiffness constant and the nonlinear electrostatic force. Standard pull-in voltages are computed. Graphic phase diagrams are used to demonstrate the conclusions. The nonlinear wave forms and the associated resonance frequencies are computed and presented graphically to demonstrate the effects of the nonlinear stiffness constant comparing with the corresponding linear model. The existence of periodic solutions of the model is proved analytically for physically admissible periodic solutions, and conditions for bifurcation points on a parameter associated with the third-order elastic stiffness constant are determined.Show more Item Numerical procedure for fluid-structure interaction with structure displacements limited by a rigid obstacle(University of West Bohemia, 2017) Yakhlef, Othman; Murea, Cornel MariusShow more A fixed point algorithmis proposed to solve a fluid-structure interaction problem with the supplementary constraint that the structure displacements are limited by a rigid obstacle. Fictitious domain approach with penalization is used for the fluid equations. The surface forces from the fluid acting on the structure are computed using the fluid solution in the structure domain. The continuity of the fluid and structure velocities is imposed through the penalization parameter. The constraint of non-penetration of the elastic structure into the rigid obstacle is treated weakly. A convex constrained optimization problem is solved in order to get the structure displacements. Numerical results are presented.Show more