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Estimation of turbulence dissipation rate by Large eddy PIV method in an agitated vessel
Kysela, Bohuš ; Jašíková, D. ; Konfršt, Jiří ; Šulc, R. ; Ditl, P.
The distribution of turbulent kinetic energy dissipation rate is important for design of mixing apparatuses in chemical industry. Generally used experimental methods of velocity measurements for measurement in complex geometries of an agitated vessel disallow measurement in resolution of small scales close to turbulence dissipation ones. Therefore, Particle image velocity (PIV) measurement method improved by large eddy PIV approach was used. Large eddy PIV method is based on modeling of smallest eddies by a sub grid scale (SGS) model. This method is similar to numerical calculations using Large Eddy Simulation (LES) and the same SGS models are used. In this work the basic Smagorinsky model was employed and compared with power law approximation. Time resolved PIV data were processed by Large Eddy PIV approach and the obtained results of turbulent kinetic dissipation rate were compared in selected points for several operating conditions (impeller speed, operating liquid viscosity).
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The impact of plasma induced flow on the boundary layer in a narrow channel
Procházka, Pavel P. ; Uruba, Václav
The induced flow generated by dielectric barrier discharge (DBD) actuator working in steady and unsteady regime will be used to modify properties of naturally developer boundary layer (BL) in short and length rectangular perspex channel which is connected to the blow-down wind tunnel. The actuator is placed in spanwise configuration and the inlet velocities will range between 5 and 20 m•s-1. Previously, mean flow field and statistical quantities were subjugated to investigation. In this paper, there will be presented dynamical features of the BL as well as tracking of coherent structures using phase-averaging. Proper Orthogonal Decomposition (POD) of influenced flow field and frequency analysis will be presented. These results should be taken into account regarding to use in the flow around a bluff body.
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Proposal for an electro/fluidic transducer based on wall-jet separation from a heated curved surface
Tesař, Václav
Transducers for converting electric input signals into a change of flow rate in a fluidic device are increasing in importance – and yet there is no fully satisfactory design solution. Standard two-stage approach is to perform in the first stage a conversion into a motion of a movable (or deformable) mechanical component. Then in the second stage the mechanical effect acts on the fluid flow. Mechanical components complicate manufacturing, have tendency to get stuck or worn – or, in the case of elastic support, to endanger reliability by breakages (e.g. due to material fatigue). Their inertia limits the frequency range. Author proposes an apparently not yet applied idea of transducer containing no moving or deformed parts. The electric input heats a surface to which an air jet is attached, causing its separation. Preliminary experiments show that relatively small heating suffices to change substantially the flowfield — and the reduction of the attachment wall to a thin metal foil can make the frequency range of responses quite acceptable.
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Induced fluid rotation and bistable fluidic turn-down valves (a survey)
Tesař, Václav
Paper discusses engineering applications of mutual interaction between fluid flow rotation in adjacent cavitiesthat communicate via a window through which is transferred moment momentum. The transfer may be merely byshear stress on the interface or enhanced by some flow crossing the boundary which is in the window. The first case of zero-time-mean mass transport through the window has led to methods of measuring the wall shear stress – a quantity of high importance in theoretical fluid mechanics – by measuring the speed of induced rotation in an adjacent vortex chamber. The case with momentum transfer by a non-zero flow became the starting point of development of fluidic valves combining two properties otherwise mutually incompatible: bistability on one side and turning-down the flow on the other side.
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Colliding wall-jets on a cylindrical surface
Tesař, Václav ; Peszyński, K.
Paper discusses aerodynamics and potential engineering applications of an unusual and in literature practically unknown fluid flow configuration, with two wall-jets attached to a cylindrical surface colliding head-on and generating a single jet directed away. Applications are seen in pneumatic sensors, particularly operating at low Reynolds numbers. Performed experimental investigation combined with numerical flowfield computations revealed several interesting aspects of this flowfield. The most interesting among them is the discovery of symmetry-breaking existence of three different stable flow regimes. This opens a possibility for fluidic tristable amplifiers and control systems operating with ternary logic.
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Prediction of surface tension of binary mixtures with the parachor method
Němec, Tomáš
The parachor method for the estimation of the surface tension of binary mixtures is modified by considering temperature-dependent values of the parachor parameters. The temperature dependence is calcu- lated by a least-squares fit of pure-solvent surface tension data to the binary parachor equation utilizing the Peng-Robinson equation of state for the calculation of equilibrium densities. A very good agreement between experimental binary surface tension data and the predictions of the modified parachor method are found for the case of the mixtures of carbon dioxide and butane, benzene, and cyclohexane, respectively. The surface tension is also predicted for three refrigerant mixtures, i.e. propane, isobutane, and chlorodifluoromethane, with carbon dioxide.
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Laser Doppler vibrometry experiment on a piezo-driven slot synthetic jet in water
Broučková, Zuzana ; Vít, T. ; Trávníček, Zdeněk
The present study deals with a slot synthetic jet (SJ) issuing from an actuator into quiescent surroundings and driven by a piezoceramic transducer. The actuator slot width was 0.36 mm. The working fluid was water. The present experiments used flow visualization (a laser-induced fluorescence technique - LIF) and laser Doppler vibrometry methods (LDV). Flow visualization was used to identify SJ formation, to demonstrate its function, and to estimate SJ velocity. LDV was used to quantify diaphragm displacement and refine operating parameters. Phase averaging yielded a diaphragm deflection during the actuation period. Taking incompressibility and continuity into consideration, the velocity in the actuator slot and the Reynolds number of the SJ were evaluated as 0.21 m/s and 157, respectively. The present results confirmed a SJ actuator function at the resonance frequency of approximately 46 Hz, which corresponds closely with the theoretical evaluation. The LDV results corresponded closely with an estimation of SJ velocity by the visualization.
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An apparatus with a horizontal capillary tube intended for measurement of the surface tension of supercooled liquids
Vinš, Václav ; Hošek, Jan ; Hykl, Jiří ; Hrubý, Jan
New experimental apparatus for measurement of the surface tension of liquids under the metastable supercooled state has been designed and assembled in the study. The measuring technique is similar to the method employed by P.T. Hacker [NACA TN 2510, 1951]. A short liquid thread of the liquid sample was located in a horizontal capillary tube partly placed in a temperature-controlled chamber. One end of the capillary tube was connected to a setup with inert gas which allowed for precise tuning of the gas overpressure. The open end of the capillary tube was precisely grinded and polished in order to assure its planarity and perpendicularity. The liquid meniscus at the open end was illuminated by a laser beam and observed by a digital camera. Application of an increasing overpressure of the inert gas at the inner meniscus of the liquid thread caused variation of the outer meniscus such that it gradually changed from concave to flat and subsequently convex shape. The surface tension at the temperature of the inner meniscus could be evaluated from the overpressure corresponding to exactly planar outer meniscus.
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