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Přechod z laminárního do turbulentního proudění pro zahuštěné nenewtonovské suspenze
Vlasák, Pavel ; Chára, Zdeněk
The paper deals with flow behaviour of fine-grained highly concentrated non-Newtonian slurries and their laminar/turbulent transition. The non-Newtonian slurry behaviour changes significantly if the laminar flow changes to the turbulent one. Results of experimental investigation of water mixtures of kaolin or fluidic ash in horizontal straight pipes are presented. A pipe loop with hydraulically smooth stainless steel pipes of inner diameter D of 17.5, 26.8 and 36 mm was used to study the slurry flow parameters.
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Zahájení pohybu dnových sedimentů v režimu saltace
Kharlamov, Alexander ; Chára, Zdeněk ; Vlasák, Pavel
The paper deals with dependence of threshold shear stress on Reynolds number for saltation mode of bed load transport. The incipient of particle saltation in turbulent flow was determined and presented in variables introduced by Shields. For the condition of the threshold there was chosen the equality of the submerged gravitational force and the lift force on a bed particle. An additional dimensionless parameter ξ characterizing the bed surface formation was introduced. The critical shear stress as function of bed particle Reynolds number was determined analytically and reported in graphical form for different values of the bed parameter. The calculated values of an introduced parameter ξ characterising bed formation differ from results of Ling (ξ=3) and approximately agree with experimental results of Ancey et al. (ξ=2/3).The obtained results agree with available experiments on saltation start.
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Computational modelling of fatigue crack propagation path in gears
Ševčík, Martin ; Vrbka, M. ; Hutař, Pavel ; Náhlík, Luboš
The paper deals with estimation of fatigue crack behavior for crack propagation from the tooth root. The conditions of linear elastic fracture mechanics are considered. The extension of LEFM – two-parameter LEFM is used for crack path estimation. The effect of the rim thickness on the path of fatigue crack was studied and procedure for more accurate description of the fatigue crack propagation at the gear rim is proposed in the work. Modified maximal tangential stress (MTS) criterion was used for an estimation of the fatigue crack path in the gear rim. Good agreement with experimental results was obtained. More accurate two-parameter description of the stress field around the crack tip can explain the discrepancies between experimental results and previous numerical calculations based on classical one parameter LEFM.
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Judgement of Possibilities of Decrease of Vibration Magnitude of Forced Response by Actuating Kinematic Excitation Effect
Markopoulos, A. ; Ondrouch, Jan ; Ferfecki, Petr
Summary: The task is to determine time history of a kinematic excitation so that after the initial transient component dies out, the magnitude of displacement a kinematicaly excited system has been lower than the magnitude of the system without kinematic excitation. The best solution of the investigated problem can be calculated by means of a procedure using minimization of the functional. However, numerical simulations shows that due to complexity of analysis proposed approach is not useful for solving systems with more than one of degree of freedom. They also tell that the best approaches to decrease a vibration magnitude of a rotor system supported by hydrodynamic bearings are methods using linearization equation of motion.
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Postup pro zkoumání stability rovnovážné polohy a nelineární odezvy rotoru uloženého v kapalinných ložiskách a majícím kotouč ponořený v neviskózní, nesmáčivé kapalině
Zapoměl, Jaroslav
Lateral vibration of rotors is significantly influenced by their supports and by their interaction with the medium in the ambient space. If the disc of the rotor is submerged in a liquid and if it vibrates, the pressure field is induced and the liquid acts by a force on the wall of the disc. It is assumed that the disc performs oscillations only with small amplitudes and it enables to describe the produced pressure field by a Laplace's equation and by the relation for the boundary conditions. The liquid is inwettable and it means that it does not lean to the disc surface and therefore no tangential forces acting on the disc are produced. The resulting force is obtained by integration of the pressure distribution around the circumference and along the height of the submerged part of the disc. Its components are proportional to the disc accelerations and it implies that the negatively taken coefficients of proportionality can be considered as additional masses.
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Vliv šířky zářezu na lomovou odezvu ohýbaných betonových vzorků
Veselý, V. ; Keršner, Z. ; Seitl, Stanislav ; Klusák, Jan
An influence of the test specimen’s preparation – shape and width of a stress concentrator – on the values of parameters of nonlinear fracture models of quasi-brittle cement-based composites is analyses in the paper presented. The parameters – e.g. effective crack extension, effective fracture toughness, and specific fracture energy – can be calculated from results of loading tests performed on notched three point bend specimens. The effects are investigated by means of a comparison of numerically simulated fracture process in the cracked specimen with the specimens with the double V-notch. The numerical simulations of the fracture according to the linear elastic fracture mechanics were performed by using finite element method programs (ANSYS, and FRANC2D). The influence on the load-carrying capacity and the initial compliance of the cracked specimen are introduced and discussed.
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