Volume 13, number 1 (2019)
Permanent URI for this collection
Browse
Recent Submissions
Item Wang tiling for particle heterogeneous materials: algorithms for generation of tiles/cubes via molecular dynamics(University of West Bohemia, 2019) Šedlbauer, David; Lepš, MatějThis paper aims at a reduction of periodicity artefacts during a generation of random heterogeneous material models. The traditional concept of the Periodic Unit Cell is compared with a novel approach of the stochastic Wang tiling. Since modelled structures consist of hard circular/spherical particles in a matrix, the algorithm for placement of inclusions is based on themodifiedmolecular dynamics.We introduce two types ofWang tile boundary conditions to decrease periodicity artefacts. Tested samples for 2D applications form sets of both monodisperse and polydisperse microstructures. The overall volume fractions of these samples are approximately 0.2, 0.4, and 0.6, respectively. The generated sets are analysed both visually and statistically via a two-point probability function. An extension of the stochastic Wang tiling enables to create 3D structures, as well. Therefore, artificial periodicity is also investigated on a 3D sample consisting of spherical particles of identical radii distributed in a continuous phase.
Item Response probability density of a system with cross-correlated parametric and additive input noises(University of West Bohemia, 2019) Náprstek, Jiří; Fisher, CyrilData processing and subsequent mining is a widely followed task. Employment of suitable evaluation and in- terpretation procedures can significantly improve the effective resolution of measuring facility using an identical hardware equipment. Recording of time variable processes is accompanied by various internal disturbing effects as a rule. They influence parameters of the measuring facility, transducer-device transmitting, etc. These parasitic processes are usually of the random character and, consequently, they exercise as parametric noises. Moreover, the input signal mostly consists of a useful signal, which can be taken for deterministic, and of a random additive part. Due to interaction of additive noises with the device itself, the cross-correlation of both additive and multiplicative noises cannot be neglected as a rule. Various combinations of noises are the origin of random and also systematic measuring errors which can have under certain circumstances a cumulative character. Their influence deteriorates the output signal quality and can lead finally to the stochastic stability loss. These effects can be theoretically described using differential systems with stochastic coefficients and a stochastic right hand side considering all input and output processes to be of the Markov type. A direct investigation of the relevant Fokker-Planck equation is employed as the main tool. Two first stochastic moments (mathematical mean value and variance) as evolutionary processes are investigated for a general deterministic useful signal and subsequently for two special cases of this one. Both types of input random noises are considered. Conditions of stochastic stability with respect to intensities of input random processes are formulated. The probability density function is deduced as well, in order to illustrate the probabilistic character of the system response as a whole. The stochastic asymmetry of the output signal is identified. Limitation procedures show a smooth transition from a general stochastic problem to deterministic noise free input signal and its processingItem Computational study of a novel valveless design of a macroscale piston pump(University of West Bohemia, 2019) Machů, Tomáš; Pochylý, František; Šulc, JanThis paper deals with a novel valveless piston pump, which is using modified Venturi diodes as a replacement for common check valves. The pump characteristic was obtained from CFD simulations in ANSYS Fluent software. The simulations were carried out on a simplified 2D geometry model of the pump. The dependencies of the volumetric efficiency of the pump on parameters of piston motion (frequency and amplitude) and backpressure are the results of the simulations. The principle of the pump is based on different rates of dissipation in the discharge and the suction diode. The paper also presents a qualitative analysis of dissipation function for turbulent flow, which provides further insight into the principle of valveless pumping.Item Helmet to head coupling by multi-body system(University of West Bohemia, 2019) Hynčík, Luděk; Bońkowski, TomaszThe number of fatalities proves that traffic accidents and their consequences are still a serious problem to be solved. Almost half of the people dying on the world’s roads are pedestrians, cyclists, and two-wheeler riders, which are together called vulnerable road users. Future challenges such as multi-modal transport including autonomous cars and other modes of transport bring additional safety issues to be investigated. New impact scenarios, covering multi-directional impacts, can be hardly assessed using hardware dummies. Hence, the numerical simulation using human body models is the technology to be addressed for future safety assessment. Especially for vulnerable road users, the complex dynamical loading must be taken into account. Long impact scenarios with complex loading usually cost a lot of calculation time. The presented work contributes to the numerical assessment of two-wheeler riders’ safety with a further view of the optimization of personal protective equipment. For running plenty of impact scenarios for optimization of the personal protective equipment, the short calculation time is necessary. The paper presents a multibody model of a helmeted two-wheeler rider, which results in a short calculation time for assessing complex impact scenarios. The previously developed and validated human body model is addressed and it is coupled to a helmet model using the multibody system approach. The helmeted human body model is validated in oblique impact scenarios by comparing the results to an existing finite element human body model.Item Buckling analysis of sandwich orthotropic cylindrical shells by considering the geometrical imperfection in face sheets(University of West Bohemia, 2019) Ahmadi, Seyed Ali; Pashaei, Mohammad Hadi; Jafari-Talookolaei, Ramezan-AliA three dimensional analysis for the buckling behavior of a sandwich orthotropic thick cylindrical shell under uniform lateral pressure has been presented. It is assumed that both ends of the shell have the simply supported conditions and the face sheets have an axisymmetric initial geometrical imperfection in the axial direction. The governing differential equations are derived based on the second Piola–Kirchhoff stress tensor and are reduced to a homogenous linear system of equations using differential quadrature method (DQM). The buckling pressures have been calculated for the shell with isotropic core and orthotropic face sheets with 0-degree orientation with respect to the hoop direction. Moreover, buckling pressure reduction parameter has been defined and computed for different imperfection parameters of face sheets and geometrical properties of sandwich shells. The results obtained in the present work are compared with finite element solutions and results reported in the literature and very good agreements have been observed. It is shown that the imperfections have higher effects on the buckling load of thick shells than thin ones. Likewise, it is found that the present method can capture the various geometrical imperfections observed during the manufacturing process or service life.
