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Počáteční chování bubliny páry v přehřáté kapalině
Vejražka, Jiří
The model is based on a simplified law for heat transfer of a growing and moving bubble and on solving the forces acting on this bubble. The model is represented by a system of ordinary differential equations and can be used until the time when bubbles start to wobble. An partially successful (but not convincing) attempt to validate this model by comparison with available experimental data was done. In most cases, the bubble behavior is controlled by a balance of the buoyancy with the added-mass force. In this aspect, the bubble behavior differs considerably from the case of constant-volume bubbles, for which the added-mass force is usually negligible. For the case of spherical bubbles, a simple analytical solutions are provided.
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Optické sondy a jejich přesnost měření
Vejražka, Jiří ; Večeř, Marek ; Orvalho, Sandra ; Sechet, P. ; Cartellier, A.
The optical probes are commonly used for experimental measurements in gas-liquid multiphase systems. During the measurement, it is evaluated the intensity of the light, which reflects from the tip of the optical fiber placed in the measuring point. From the obtained signals, it is possible to evaluate the bubble residence time, local gas hold-up and it is possible to estimate the size and the velocity of the bubbles. In this study, the signal of the optical probe is compared with the data obtained by direct observation by high-speed camera. It was found that optical probe works well in the case of large and fast moving bubbles. In the case of small and slow bubbles the data measured by optical probe are not usable. The dimensionless criterion for minimal Weber number characterizing the reliability of the probe was derived on the base of motion equation for the bubbles.
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Magnus and Drag Forces Acting on Golf Ball
Kharlamov, Alexander ; Chára, Zdeněk ; Vlasák, Pavel
The paper describes the results of experiments with a rotating golf ball moving quasi-steadily in calm water. The motion of the ball was recorded on a digital video camera. The Cartesian coordinates and the angle of rotation of the ball were determined from the records of motion. The dimensionless drag force coefficient, Magnus force coefficient and translational and rotational Reynolds numbers were calculated from the time series of the ball coordinates and the angle of rotation for each recorded frame. The calculated data were averaged over rectangular cells on experimental domain on the plane of translational and rotational Reynolds numbers, i.e. 1.2 104 < Re < 1.6 104 and 3.8 103 < Reω < 2.7 104. The coefficients were presented in tabulated form.
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The calculation of the forces and moments acting on the fluttering profile using interferometric measurements
Vlček, Václav ; Kozánek, Jan ; Zolotarev, Igor
Experimental investigation of the profile NACA0015 fixed as a dynamic system with two degree of freedom vibrating in self-excitation modes were measured in the wind tunnel of the Institute of Thermomechanics AS CR by interoferometric methods. The profile moved in the vertical direction and rotated around the elastic axis in 1/3 of the profile chord. Presented results of the aeroelastic measurement correspond to the flow velocity M = 0.28 and Reynolds number 0.34·106. Time depends of the airflow forces and moments acting on the vibrating profile were obtained newly in this contribution. Applied method of the evaluation is now in the process of its evolvement, therefore presented results should be regards as preliminary conclusions.
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Study of velocity field in crossroads areas
Kellnerová, Radka ; Jaňour, Zbyněk
The aim of this project is to study the influence of boundary conditions on contaminant spreading in a urban area model. The urban area model was composed of simple elements shapes of crossroads: X-shaped, T-shaped and skew intersection. The project at the beginning - by now only results for T-shaped crossroads are available. The experiment took place in an environmental aerodynamics tunnel IT AS CR in Nový Knín. First results show accumulation of high values of Reynolds stress above the area of crossroads center, whereas the velocity field is weaker when compared with more remote area. In specific distance from the intersection the recirculating zone appears. With increasing distance from intersection the center of zone moves counter main flow and up.
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Flow of coarse-grained particles-water mixture in pipes
Vlasák, Pavel ; Chára, Zdeněk ; Konfršt, Jiří ; Krupička, Jan
The advanced knowledge of coarse-grained slurry flow behavior is important for safe, reliable, and economical design and operation of the pipeline transport. The effect of the mixture velocity, solid concentration, and pipe inclination on the coarse-grained particle – water mixtures flow behavior and pressure drop in the turbulent flow regime was experimentally studied in horizontal, vertical, and inclined pipe of inner diameter D = 100 mm. The respective experimental methods, including radiometric methods for particle concentration distribution in pipe cross-section, were used. Graded basalt gravel was used as a model of solid particles. Concentration distribution in the pipe cross-section and motion of particles along the horizontal pipe invert were also studied. The study revealed that the coarse-grained particle-water mixtures in the horizontal and inclined pipes were significantly stratified. The particles moved principally in a layer close to the pipe invert. However, for higher and moderate flow velocities particle saltation becomes dominant mode of particle conveying and particles moved also in the central and upper part of the pipe cross-section.
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Flow in the nozzle with moving wall
Foldyna, Josef ; Říha, Zdeněk ; Sitek, Libor
The paper presents simulation of 3D, unsteady, turbulent flow of water in the high-pressure nozzle device with moving wall. The wall performs rectilinear reciprocating motion with high frequency and very small amplitude that results in generation of very high velocity pulsations at the nozzle outlet. The pulsations lead to the break-up of continuous jet into regular slugs of water. The objective of the simulation was to verify whether dynamic meshes can be used to simulate above mentioned problem and satisfactory agreement of the simulation and reality can be obtained.
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