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Complex Effects of Free Stream Turbulence and Surface Roughness on the Transitional Intermittency

Author(s): Ondřej Hladík,Pavel Jonas,Oton Mazur,Vaclav Uruba / Language(s): English Issue: 4A/2012

The comparison of transitional intermittency distributions in boundary layers on smooth and rough surface is presented. The conditional analysis of the instantaneous wall friction is applied on time records made in transitional boundary layers originating on smooth surface or surface covered by sand paper. Special procedures of measurement and evaluation are applied that allowed evaluations of turbulent and nonturbulent zone averages.

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Investigation of High Speed Cavitation Flows

Author(s): Robert Olsiak,Branislav Knizat,Marek Mlkvik / Language(s): English Issue: 4A/2012

The paper presents an investigation of cavitating flow behind a micro-orifice. Experiments were performed with a device making visualization of a cavitation cloud possible. Two micro-orifices of the sizes 0.58 mm and 0.35 mm were tested at different cavitation numbers and at different flow rates. Parameters of the cavitation cloud collapse on a smooth surface were found. The experimental device, experiment methodology and some results are presented. Results of visualization are compared with CFD simulations.

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Effects of Leading Edge Protuberances on the Aerodynamic Performance of 2D Cambered Airfoil

Author(s): Abeer Mukhtar,Humera Golandaz,Young Hwan Kim / Language(s): English Issue: 4A/2012

The pressure and lift coefficients were measured for airfoils with leading edge protuberances in a subsonic wind tunnel and compared with the performance of a baseline NACA 4415 airfoil with a straight leading edge. These protuberances of 2D airfoil mimic the leading edge configuration of the flippers of a humpback whale. Wavelengths were varied from 25% to 50% of the chord length whilst amplitude was varied from 5% to 12% of the chord length of the baseline. Surface flow visualisation using kaolin was also performed to further analyze the flow patterns and surface swirls and vortices were observed. The presence of protuberances enhanced the stall characteristics of the baseline and low amplitude protuberances exhibited the maximum lift coefficient. However, at low angles of attack, the performance of the modified airfoils is similar to that of the baseline.

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Fatigue Performance of 6063-T66 Aluminum Alloy under Combined Cyclic Loading of Parent and Welded Specimens

Author(s): Peter Kopas,Milan Saga,Milan Uhricik / Language(s): English Issue: 4A/2012

The article deals with the determining of fatigue lifetime of structural materials during multiaxial cyclic loading. The aim of this investigation is to evaluate the influence on fatigue behavior of the welded aluminum alloy EN AW 6063-T66. Experimental device was designed for combined bending - torque loading. The experimental part was focused on the modeling of combined biaxial loading and determining the number of cycles to fracture in the region of low-cycle fatigue. In-phase loading can be treated fairly well using the conventional hypotheses (von Mises or Tresca) on basis of nominal, structural or local strains or stresses. Based on the experimental results the fatigue design curves are compared to fatigue data from base metal and weldments.

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A New Filling Material for Cold Sleeve

Author(s): Pavol Novak,Milan Zmindak / Language(s): English Issue: 4A/2012

The aim of this paper is to review material properties of a new filling material for cold sleeve. The first experimental measurements were performed on samples to determine required material properties. Then the structural analysis was performed for a pressurized pipe with insufficiently welded root and installed cold sleeve. The case of depressurized pipes that could cause a breach of cohesion between the filling material and surface of pipe or sleeve with the usage of cohesive finite elements was simulated.

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Changes of Material Properties of Silumin During Age-Hardening

Author(s): Jan Kohout / Language(s): English Issue: 4/2012

Simple phenomenological description of age-hardening kinetics of supersaturated solid solution is presented using the sum of exponential functions with negative arguments containing rate constants whose temperature dependence is described by the Arrhenius equation. To determine the number of exponentials in the sum is very important: it should be sufficiently high to obtain accurate description of kinetics and sufficiently low to keep the simplicity and practical applicability of this description. The HB hardness and growth changes (i.e. relative length increase) of silumin during age-hardening are studied separately and then commonly as various manifestations of the same hardening.

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Mechanical Properties of Heat Treated Secondary AlSi12Cu1Fe Cast Alloy at Room Temperature

Author(s): Lenka Hurtalova,Eva Tillova,Maria Chalupova / Language(s): English Issue: 4/2012

The contribution describes changes of mechanical properties (strength tensile, absorbed energy and Brinell hardness) in secondary (recycled) eutectic aluminium-silicon cast alloy - AlSi12Cu1Fe during solution treatment. This work presents the influences of the solution treatment by 525 °C, 545 °C and 565 °C with the holding time 2, 4, 8, 16 and 32 hours, water quenching at 40 °C and natural aging with holding time 24 hours. Mechanical properties were measured in line with STN EN ISO at room temperature. Solution treatment has led to changes in mechanical properties, caused by the changes in microstructure including the spheroidization and coarsening of eutectic silicon, gradual disintegration, shortening and thinning of intermetallic phases.

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Structural (Super)Plasticity of Magnesium Materials

Author(s): Zuzanka Trojanova,Zoltan Szaraz,Oksana Padalka,Talant Ryspaev,Pavel Lukáč / Language(s): English Issue: 4/2012

Superplastic behaviour of two magnesium alloys QE22 and EZ33 having a grain size of 1μm was studied at a temperature of 420 °C and in the strain rate interval from 1 104 to 1 103 s1. The substructure of samples was analysed by the transmission electron microscope before and after deformation. The substructure of samples exhibits an existence of particles in the grain boundaries. These precipitates are very stable even at high deformation temperature and they are reason for the relatively high ductility of alloys. Observed ductility of the WE54/SiCp composite is much lower due to the existence of ceramic particles in the grain boundaries. They inhibit the grain boundary sliding and are the reason for the cavitations and early fracture of the composite.

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T- Joints - Experimental and Numerical Analysis

Author(s): Vincent Kvocak,Pavol Beke,Renata Vargova / Language(s): English Issue: 4/2012

The article presented focuses on the comparison of the measurement results obtained in laboratory experiments on joints of a lattice girder. Because of the geometric variety of joints, the article presented concentrated selectively on hinged T-joints only. The geometry of this type of joint consists of two fundamental members: the chord and the brace member. The T-joints examined were always composed of the rectangular chord member in combination with a variety of types (rectangular, circular and open) and dimensions of brace members. The scientific research results and evaluations presented characterize the correlations regarding the global resistance of joints in lattice structures in the light of the latest scientific knowledge that should be responsibly taken into consideration in their reliable and cost-effective design.

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Analysis of the Initial Imperfections Effect on the Thin-Walled Cold-Formed Compressed Steel Members

Author(s): Mohamad Al Ali,Michal Tomko,Ivo Demjan,Vincent Kvocak / Language(s): English Issue: 4/2012

The paper deals with the influence of initial imperfections and presents fundamental information about experimental and theoretical-numerical research to determinate the load-carrying capacity of thin-walled cold-formed compressed steel members. The investigated members had closed cross-sections made of homogeneous material. The theoretical-numerical analysis in this paper is oriented on the investigation and modeling of initial imperfections effects, caused by production process, on the load-carrying capacities of mentioned members, while the experimental investigation is to verify the theoretical results and to investigate the behavior of mentioned members during the loading process.

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The Least Square and the Weighted Least Square Methods for Estimating the Weibull Distribution Parameters - A Comparative Study

Author(s): Ivana Pobocikova,Zuzana Sedliackova / Language(s): English Issue: 4/2012

In this paper we study the performance of the least square method and the weighted least square method for estimating the Weibull distribution parameters. In engineering practice these methods are commonly used due to their simplicity. The estimates of the parameters can be calculated easily by the closed-form formula. We consider three estimators of the cumulative distribution function and the weight factor proposed by Bergman (1986). The methods are compared in terms of the root mean square error and sample size. The comparison is based on the Monte Carlo simulation. The comparison shows that the weight factor improves the accuracy of the estimation the Weibull distribution parameters.

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The Role of Shot Peening in Increasing X70 Steel Fatigue Properties

Author(s): Katarina Mikova,Mario Guagliano,Otakar Bokuvka,Libor Trsko,Frantisek Novy / Language(s): English Issue: 4/2012

The paper presents the authors' results of experimental examination of pipeline steel X70 fatigue properties before and after shot peening application, including relaxation of residual stresses during rotating bending fatigue loading (f 30 Hz, T 20 5 °C, R 1). The obtained results confirmed the positive effect of surface hardening realized by shot peening. The fatigue properties incl. fatigue limit were higher after shot peening and this effect was visibly higher for notched specimens than for smooth specimens. During rotating bending fatigue loading the residual stresses in subsurface layers relaxes which decreases the strengthening effect after a certain number of cycles.

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Physical Modeling of Flow in a Tundish for Continuous Casting Machine

Author(s): Maria Carnogurska,Miroslav Prihoda / Language(s): English Issue: 3/2012

The paper describes a procedure for obtaining similarity criteria for a real object - a tundish for continuous casting machine (CCM) and its reduced physical model. The procedure is demonstrated on the basis of Navier-Stockes differential equations and with the help of dimensional analysis. To obtain similarity criteria dimensional analysis makes use of dimensions of those physical quantities which significantly influence the mentioned phenomenon. From the above it results that when modeling a concrete phenomenon with the help of this method it is inevitable to thoroughly know the physical substance of the phenomenon to expressly ensure the choice of all relevant quantities. On the basis of elaborated model laws the basic characteristics of flow (rate, flow volume, time intervals) were designed for the reduced model of the real CCM. The obtained dependences of characteristics feature the universal validity for all the models of tundishes.

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Meshless Modelling of Laminate Mindlin Plates under Dynamic Loads

Author(s): Milan Zmindak,Daniel Riecky / Language(s): English Issue: 3/2012

Collocation method and Galerkin method have been dominant in the existing meshless methods. A meshless local Petrov-Galerkin (MLPG) method is applied to solve laminate plate problems described by the Reissner-Mindlin theory for transient dynamic loads. The Reissner-Mindlin theory reduces the original three-dimensional (3-D) thick plate problem to a two-dimensional (2-D) problem. The bending moment and the shear force expressions are obtained by integration through the laminated plate for the considered constitutive equations in each lamina. The weak-form on small subdomains with a Heaviside step function as the test functions is applied to derive local integral equations. After performing the spatial MLS approximation, a system of ordinary differential equations of the second order for certain nodal unknowns is obtained. The derived ordinary differential equations are solved by the Houbolt finite-difference scheme as a time-stepping method.

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Optimization of Thin Shell Structures Using FSD Algorithms

Author(s): Milan Saga,Martin Dudinsky,Peter Pechac / Language(s): English Issue: 3/2012

The paper presents a theoretical and numerical study of the efficiency of the fully stress design (FSD) algorithm in the case of thin shell finite elements. Relation between membrane forces, bending moments and the element thickness is analysed by means of numerical tests. Subsequent numerical testing and a new iterative algoritm to providing the rapid convergence of the optimizing process is proposed.

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The Effect of Metric Space on the Results of Graph Based Colour Image Segmentation

Author(s): Martina Zachariasova,Robert Hudec,Miroslav Benco,Patrik Kamencay,Peter Lukac,Slavomir Matuska / Language(s): English Issue: 3/2012

This paper deals with the impact of the metric space on the results of colour image segmentation algorithm. Distance and similarity measures are important tasks for quality of colour image segmentation. Main idea of this research is to make a comparison of algorithm results with using different metrics. Euclidean distance is the most used metric in many colour image segmentation algorithms. This paper shows comparison of this metric with many other metrics. Nine different metrics are gradually used in efficient graph based colour image segmentation algorithm created in the C++ language. The efficiency of precision and recall is one of the investigation tasks of colour image segmentation.

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Applications of Cutting Stock Problem

Author(s): Vladimir Medvid / Language(s): English Issue: 3/2012

We present one heuristic solution for the well-known cutting stock problem which was formulated by Kantorovich in 1939. It is the problem of filling an order at minimum cost for specified numbers of lengths of material to be cut to given stock lengths of given cost. When expressed as an integer programming problem the large number of variables involved generally makes computation infeasible. The same difficulty persists when only an approximate solution is being sought by linear programming.

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Dynamics of Haematopoietic Stem Cells Model

Author(s): Julia Stoberova / Language(s): English Issue: 3/2012

The article deals with the haematopoietic stem cells model with delay introduced by M. C. Mackey [1]. Our aim is to establish the new conditions under which the equilibria of the model are stable or unstable. The model without delay is also investigated.

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Study of the Chalcogenide Nonlinear Fiber Bragg Gratings for All-Optical Switching

Author(s): Eliska Jurisova,Jarmila Mullerova / Language(s): English Issue: 2/2012

In this paper uniform and apodized nonlinear chalcogenide glass fiber Bragg gratings (FBG) as novel and promising devices for all-optical switching are investigated. The transmission response of these devices is discussed theoretically. The continuous wave in the investigated FBGs induces the nonlinear variation of the refractive index that causes changes in the FBG transmission characteristics. This effect can be modified depending on incident light power what is assumed to be due to the optical Kerr effect responsible for optical switching. The incident intensity thresholds of the bistable regimes were analyzed numerically to deduce possible switching properties of FBGs based on various chalcogenide glass samples. The influence of grating parameters on the optical bistability behaviour is researched. The nonlinear coupled mode theory was used for numerical investigation.

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Effectivity Analysis of Chosen Numerical Methods for Solution of Mechanical Systems with Uncertain Parameters

Author(s): Robert Bednar,Milan Saga,Milan Vasko / Language(s): English Issue: 4/2011

The paper deals with usability and efficiency problem for the chosen solution methods for mechanical systems with structural uncertainties, which are significantly influencing the analysis results and the analysis itself. In the centre of interest will be the chosen approaches and methods. An application of the chosen approaches is presented – the first one, a simple combination of only inf-values or only sup-values; the second one presents full combination of all inf-sup values; the third one uses the optimizing process as a tool for finding out an inf-sup solution and last one is Monte Carlo technique as a comparison tool.

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