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A new method for fluid input into a hybrid synthetic jet actuator
Kordík, Jozef ; Trávníček, Zdeněk ; Tesař, Václav
A new principle of flow rectification for hybrid synthetic jet actuators is introduced in this paper. As is well known, the flow rectification can be best accomplished by means of fluidic diodes. Novelty of the present study are fluidic diodes with two mutually opposed nozzles. Interaction between the periodic jet flows from the nozzles causes a difference between the blowing and suction strokes, resulting in a particularly efficient rectification effect. The distance between the nozzle exits as well as the oscillation frequency were the parameters, which were varied during hot-wire measurements. The combination of those parameters achieving the highest volumetric efficiency was identified.
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Bubble dynamics in drinks
Broučková, Zuzana ; Trávníček, Zdeněk ; Šafařík, P.
This study introduces two physical effects known from beverages: the effect of sinking bubbles and the hot chocolate sound effect. The paper presents two simple „kitchen” experiments. The first and second effects are indicated by means of a flow visualization and microphone measurement, respectively. To quantify the second (acoustic) effect, sound records are analyzed using time-frequency signal processing, and the obtained power spectra and spectrograms are discussed.
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Near-exit coalescence – a so far unknown limit to microbubble smallness
Tesař, Václav ; Šonský, Jiří
Gas bubbles in a liquid are required in applications to be of the smallest possible bubble size. Authors studied microbubble formation using a high-speed camera and discovered that bubbles increase their volume near the aerator exits by mutual coalescence. This effect presents a limit to how small microbubbles can be produced. The discovered conjunction effects are very fast and this is probably the reason why they have so far escaped attention.
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Microfluidics in search for catalysts for biomass conversion into liquid fuels
Tesař, Václav
Paper surveys recent developments of combinatorial chemistry characterised by testing many substances and their mixtures simultaneously. Attention focuses on the progress made possible by applying small-scale no-moving-parts fluidics. Encountered problems like small Re and their solutions are demonstrated on author's development of a microfluidic unit for parallel testing of catalysts improving effectiveness of Fischer-Tropsch synthesis of liquid fuels from biomass.
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Holographic interferometry as a tool for visualization of temperature fields in fluids
Dančová, Petra ; Vít, Tomáš ; Lédl, V. ; Trávníček, Zdeněk ; Doleček, R.
Measurement of temperature field in moving fluids is connected with many difficulties. Usage of point temperature measurement methods, such as Constant Current Anemometry (CCA), is limited to frequencies up to 3kHz. This frequency should be the limiting factor for using the CCA in fluid when fast change of temperature occurs. This shortcoming of CCA measurements should be overcome by using of optical methods such as digital holographic interferometry. It is necessary to employ a special holographic setup with double sensitivity instead of the commonly used Mach-Zehnder type of holographic interferometer in order to attain the parameters sufficient for the studied case. This setup is not light efficient like the Mach-Zehnder type but has double sensitivity. The special technique of acquiring and phase averaging of results from holographic interferometry is presented.
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Large-amplitude pulsatile nozzle flow
Tesař, Václav ; Kordík, Jozef
Flowfield inside a simply-shaped nozzle with periodic unsteadiness of so large amplitude that a flow reversal occurs during a part of the cycle was investigated. The range of frequencies was from 20 Hz to 320 Hz, and ratios of the pulsation amplitude to the steady flow were from 1.0 to 4.0. Anemometric velocity measurements were combined with numerical flowfield computations.
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Visualization of the flow around rotating cylinder
Broučková, Z. ; Trávníček, Zdeněk ; Šafařík, P.
The visualization of the flow around a rotating and standing cylinder has been performed in air. This visualization has been performed for two velocities of a freestream for Reynolds numbers Re = 920 a 990 and for four frequencies of the rotation (0 s-1, 5 s-1, 12.5 s-1 and 20 s-1). The results have been compared with the theoretical model – potential flow. The velocity of the freestream has been evaluated by measurement of the frequency of the shedding vortices behind the standing cylinder in the regime of the Kármán vortex street. In the regime of the Kármán vortex street the Reynolds numbers were Re = 110 a 120. The known relations between Stouhal and Reynolds numbers were used.
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Microbubble Generation by Fluidics. Part II: Bubble Formation Mechanism
Tesař, Václav
Author develops a facility for generation of sub-millimetre sized air bubbles in water. A promising and economically viable method, based on author's earlier experience, is fragmentation of bubbles by oscillation while they are still at the aerator exit locations. Understanding the mechanism by which the oscillation affects bubble formation is a pre-requisite to new aerator design. Results obtained so far reveal a strange bubble size independence on the applied acoustic power, frequency of oscillation and the character of aeratoir body.
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