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HYBRID-SYNTHETIC IMPINGING JETS
Tesař, Václav
Impinging jets can achieve the highest convective heat or mass transfer rate. An increase beyond present limits is possible by pulsation of the flow. The extreme case of the synthetic jets could be particularly effective – were there not the re-ingestion of the fluid. A solution brings the new concept of non-zero time mean hybrid synthetic jet.
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Excitation of rotating disk by stationary permanent magnets
Půst, Ladislav ; Pešek, Luděk
Abstract: Flexural vibrations of rotating disk contain forward and backward traveling waves can be excited at certain speed of rotating disk also by a standing constant single point force or by multiple points standing forces, realized e.g. by permanent magnets fixed in non-rotating space. Presented paper is contribution to the theoretical background of a new experimental excitation technique elaborated in Institute of Thermomechanics for investigation of a bladed turbine disk model with imperfection by addition several damping heads on ends of blades. Mathematical model of imperfect rotating disk was derived by using data gained experimentally and by FE computed for three lowest split modes of vibrations. It was shown how either the whole spectrum of modes of vibrations can be recorded or some resonance of selected mode can be suppressed on emphasized by means of appropriate number and positions of permanent magnets.
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Microstructural models of trabecular bone - comparison of CT-based FE models
Jiroušek, Ondřej ; Zlámal, Petr
In the article a detailed comparison of techniques used to developed detailed FE models of trabeculra bone microstructure is presented. The FE models are built using a stack of microtomographic images of trabecular bone. Compression test of a cylindrical sample taken from human proximal femur is virtually performed using FE models developed using different procedures. Effect of improper segmentation, mesh smoothing, mesh density, use of hexahedral/ tetrahedral elements, linear/ quadratic shape function is evaluated in parametric study. Material model is based on nanoindentation and numerical elastic properties of sample are compared to experimental results.
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Pore Size Distribution of Human Trabecular Bone - Comparison of Intrusion Measurements with Image Analysis
Doktor, Tomáš ; Valach, Jaroslav ; Kytýř, Daniel ; Jiroušek, Ondřej
This paper deals with pore size distribution assessment of trabecular structure of human proximal femur. Two distinct methods for estimation of the pore size distribution histograms are tested, mercury intrusion porosimetry and image analysis. For the image analysis two-dimensional images of crosssections of the specimens were used and the pore size distribution was estimated using a stereological calculation method based on the geometrical properties of an idealized pore. Comparison of the results of both methods is presented. Suitability of both methods for biological materials is discussed regarding to mechanical properties of these materials, characteristics of pores and range of pore sizes present in the structure.
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Assessment of C/PPS composites degradation indicators using acoustic measurement
Kytýř, Daniel ; Valach, Jaroslav ; Doktor, Tomáš ; Jiroušek, Ondřej
Changes of mechanical properties, namely of modulus of elasticity, often play a role of damage accumulation indicators in the framework of fatigue behaviour of composite materials. In order to improve the reliability of these indicators for identification of material degradation process it is necessary to eliminate the influence of external loading and volume forces associated with thermal expansion of heterogeneous material at various temperatures. Therefore the paper deals with description of experimental assessment of relation between external load and material stiffness and also studies the dependency of material stiffness on temperature. For the monitoring of degradation process ultrasonic measurement was used. Obtained results are discussed with respect to fatigue testing of material and also interpreted from the micromechanical point of view.
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Coarse-particles conveying in pipes
Vlasák, Pavel ; Kysela, Bohuš ; Chára, Zdeněk
Flow behavior of coarse-grained slurry depends generally on particle size, shape, density and concentration. The present paper describes the results of experimental investigation of the model coarse-grained slurries on the pipe loop of inner diameter 36 mm with smooth pipes. Graded pebble gravel and glass balls conveyed in water were studied. Research was focused on evaluation of the effect of slurry velocity and concentration on hydraulic gradient versus slurry velocity relationship and the slurry flow behavior. Particle distribution in the pipe vertical cross section and motion of particles along the pipe bottom were studied in a transparent pipe section.
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Influence of the spheroid prolongation on the drag force
Lukerchenko, Nikolay ; Keita, Ibrahima ; Kvurt, Y. ; Miles, Jiří
The drag force acting on a spheroid moving perpendicularly to its axis of rotation in water was studied experimentally. Along the spheroid axis, which is normal to its axis of rotation, a round narrow hole was bored. The spheroid moved along a thin vertical thread stretched in water. A video system recorded the spheroid motion and the spheroid velocity was determined from the record. The drag force coefficient was calculated from the balance of forces acting on the spheroid. Two oblate, two prolate spheroids and one sphere with ratio of the axes 0.67; 0.81; 1.33; 2 and I (sphere), respectively, with approximately the same volumes, were used. The friction coefficient between the thread and spheroid was determined from the comparison of the experimental and calculated motions of the sphere, for which the drag force coefficient is known. The dependence of the drag force coefficient of the spheroid on the ratio of its semi-axes was obtained.
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Modelling of bed in process of particle saltation in channel
Kharlamova, Irina ; Kharlamov, Alexander A. ; Chára, Zdeněk ; Vlasák, Pavel
For numerical modelling particle saltation in channel with rough bed is important to define a bed configuration. The paper deals with the bed consisting of spherical particles of the different size then the saltating particle. In horizontal x-z plane the particles are arranged hexagonally. In vertical direction the particles are distributed according to Gaussian distribution. Before each collision of the saltating particle with bed the bed is shifted on a random distance and it is rotated by a random angle, so there is uniform probability to find a bed particle in any point of the x-z plane. The bed structure is chosen with aim to represent the natural bed as much as possible, thus the known information about distribution of the bed particles along y-direction is used and the location of bed particles in the x-z plane is controlled by principles of equal probability and minimal dense packing.
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