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Control of flow separation by vestigial "synthetic jet"
Tesař, Václav ; Pavelka, Miroslav ; Smyk, E. ; Peszyński, K.
A useful aerodynamic control action may be causing a separation of flow from the wall past which the fluid flows. Several current research activities aim at doing so by oscillating zero-time-mean flow generated in a nozzle located in the wall. The effect is often described as “flow control by synthetic jet”, although it is obvious that the vortex rings generated in the outflow phase of the oscillation are deformed by the controlled flow and also are carried too far away so that they cannot form the classical synthetic jet configuration of vortex rings. Authors describe experiments with such a control applied successfully in an axisymmetric fluidic valve. Presented is a hypothesis explaining the separation mechanism.
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Identification of coherence in turbulent flows by local correlations
Tesař, Václav ; Něnička, Václav
Coherent structures are not included in practically useful models of turbulence due to scarcity of data upon which the modelling could be based. The problem is indistinct, vague, and continuously varying boundary of each structure, making difficult even identification of its very existence. The introduced identification method operates with a pair of flow visualisation images, computing local correlation coefficients in sequentially interrogated small image parts. The coherence is revealed by high local values of the coefficient. There are two variants, a scalar and vector approaches. The method is shown applied to an experiment in which the structures play an important role.
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Atomiser with excitation by a fluidic oscillator
Tesař, Václav ; Hykl, Jiří
Paper summarises results obtained in preliminary feasibility study of a novel pneumatic atomiser for generation o fine droplet spray. The new feature is pulsating the air supply by means of a no-moving-part fluidic oscillator. Tests involved recording by high-speed camera and optical measurements of the droplet size by laser light scattering and suggested a promising future potential necessitating, however, further development and optimisation.
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Preventing microbubble conjunctions by gas flow excited by a fluidic oscillator
Tesař, Václav
Recently demonstrated was efficient generation of desirable small-size microbubbles in liquids by applying oscillation to the air flow supplied into the aerator - preferably by means of a simple and inexpensive fluidic oscillator. It remained unexplained what the oscillation actually does with the bubbles. In this paper was established that microbubbles are produced in a process consisting of several discrete steps, in each growing in size. Oscillation of the gas flow can eliminate this growth and keep the resultant bubbles small. Two alternative mechanisms were identified than can suppress the growth.
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Mechanism of Multiple Microbubble Conjunctions
Tesař, Václav ; Šonský, Jiří
In typical generation of microbubbles by blowing the gas through tiny holes in an aerator, the size of bubbles is much larger than hole exit diameter. Explanation for this discrepancy was found by analysing high-speed camera images of bubble generation. Primary microbubbles, of initial size corresponding to the aerator exits, tend to remain near the location of their formation at the aerator exit - and coalesce there with subsequently formed other primary microbubbles. Typically, this coalescence is repeated several times before the increased final size and position of the resultant larger bubble reaches the limit for separation and ascent.
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Water oxygenation by fluidic microbubble generator
Tesař, Václav ; Peszyński, K.
Oxygenation of water by standard means is rather ineffective. The classicalapproach to improvements – decreasing the size of the aerator exits - have already reached their limits. Recent idea is to decrease the size of the generated air bubbles by oscillating the supplied air flow using fluidic oscillators. Authors made extensive performance measurements with an unusual high-frequency fluidic oscillator, designed to operate within the submersed aerator body. The performance was evaluated by the dynamic method of recording the oxygen concentration increase to saturation in the aerated water. Experiments proved the fluidic generator can demonstrably increase the aeration efficiency 4.22-times compared with the aeration from a plain end of a submerged air supply tube.
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Integral fluidic generator of microbubbles
Tesař, Václav ; Jílek, Miroslav
Very small gas bubbles dispersed in liquid are desirable in many applications. If smaller than 1 mm in diameter, they are called microbubbles. Until recently, the advantages they offer have been little usedbecause of generally low efficiency of microbubble generation methods. A solution was found in pulsating the gas flow into the submerged aerator by a fluidic oscillator. At present, the oscillator and aerator are separate devices. The flow pulsation is strongly damped in the connecting tubes. New solution presented in this paper relies on oscillator robustness and no need for maintenance, which allows integrating it with the aerator into a single body submerged in the liquid. Small bubble size requires driving frequency much higher than has been so far applied. The present paper shows some of the ways leading to high frequencies – in particular the use of dominant third harmonic of the basic switching frequency.
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Role of microfluidics in discovering new marketable substances – a survey
Tesař, Václav
Paper surveys developments in combinatorial chemistry and related search for new medicaments and biomedical procedures, characterised by simultaneous testing of many substances (and their mixtures). Attention is focused on advantages offered in this field by small-scale no-moving-part microfluidics. Typical problems and their solution are then demonstrated on author’s development of a microfluidic unit for parallel tests of catalyst having potential to improve the effectiveness of Fischer-Tropsch synthesis of liquid fuels from biomass.
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Fluidics: what it is, where it is heading - and how it will change the world we line in
Tesař, Václav
Fluidics, technique of handling fluids, is usually meant to imply absence of moving components. It is less known and used than it deserves considering the advantages it offers. Recently its applications became more widespread thanks to two development directions. On one hand, it is the power fluidics, handling larger flow rates. Newly developed are no-moving-part fluidic pumps for dangerous liquids (in particular radioactive ones) — and fluidic oscillators for generation of microbubbles used in waste water processing and growth of unicellular plants. The other rapidly growing development direction is microfluidics, often integral with electronic circuitry on the same substrate. It has immense possibilities in sensors and also in handling small amounts of fluids e.g. in microchemistry.
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Interesting properties of microbubbles
Tesař, Václav
Paper investigates properties of very small, sub-millimetre sized gas bubbles in liquids,which are generated by oscillation produced by a fluiidic no-moving-part oscillato in the gas supply into the aerator. Of particular interest is the tendency of microbubbles to coalesce already while in immediate neighbourhood of the aerator exit. Another investigated property is oscillation driven by released surface energy. Autorr derived an equation describing the natural frequencty of the oscillation and with the help of this equation found interesting parameters of the processes, such as the amount of outside water that takes part in the oscillation and has a decisive role in the inertial properties of the variations of the microbubble shape.
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