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Struktury vyvolané nestabilitou v impaktním proudění
Tesař, Václav ; Něnička, Václav
Authors investigated by flow visualisation an air jet impinging on a perpendicular flat plate. The jet was supplied with tiny water droplets, produced by condensation, which caused scattering of laser light illuminating the section of the jet in meridian plane. A high-speed camera, operating in an adjustable phase relationship with acoustic signals triggering the formation of the instabilities was used to record the visualisation images. These were then processed using advanced procedures aimed at identifying the structures.
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Dynamika impaktních proudů zkoumaná v příčných řezech proudem
Tesař, Václav ; Něnička, Václav
Authors investigated experimentally air jets impinging on a flat plate. The object of interest were vortical instability structures in the mixing layer of the jet. Their formation was triggered by azimuthal waves generated in the nozzle exit. The structures were visualised by planar laser-light sheet illuminating a jet cross section where it scattered on by water mist particles added into air. The cross section was, photographed by a high-frequency camera located behind transparent impingement plate.
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Systematické vyšetřování vzdáleného rychlostního pole syntetizovaných proudů
Tesař, Václav ; Kordík, Jozef
Paper presents results evaluated from anemometric data on a large number of synthetic jets. The results are based on a one-equation turbulence model solution of time-mean synthetic jets. This solution follows the classical concept of similarity transformation, but depends on a parameter the value of which varies along the jet axis. This is idea has been there before, but the scarcity of so far available experimental data made the previous attempt unreliable. This is now improved by the large number of new data that correct some previous coclusions.
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Výzkum fázově spouštěných vírů v impaktním proudění
Tesař, Václav ; Něnička, Václav
Impinging jets can transfer the highest achievable thermal flux into (or out from) solid bodies. The transport is significantly influenced by complex and so far not fully understood vortical structures that develop due to hydrodynamic instability. This contribution describes experiments with visualisation of impinging air jets in which periodic formation of vortices was triggered by azimuthal travelling waves generated in nozzle exit - and was phase-synchronised with the camera. Visualisation was by laser light scattered from water mist particles added to the supplied air. The instability structures were identified in the images by using their coherence.
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Jet Bouncing from Weaving Reed
Tesař, Václav
Subject of this contribution is a little known and seemingly impossible mechanism of the jet “reflecting” or“bouncing back” from an impact on a solid wall. This, of course, cannot occur if the wall were impermeable — the trick is to provide the wall with perforations that allow a part of the jetto pass through. On doing so this part has to change its direction and this has to be compensated by the remaining partchanging its direction too, to the opposite side from the jet axis. This part forms the “reflected” jet. Thephenomenon, which has applications in shuttle-less looms, was investigated by flow visualisation and a detailed flowfield computations.
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Fluidická oscilátorová ústrojí založená na „Aerodynamickém Paradoxu“
Tesař, Václav
Discussed unusual flow-induced oscillation mechanism is based on an old but nearly forgotten phenomenon contradicting simple logic (motion opposing the direction of incoming flow). The author proposed using this mechanism in a number of interesting fluidic devices. The paper concentrates on observed existence of two different vibration regimes and the fact that the transition between these regimes exhibits a local maximum of Strouhal number Sh.
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Parametry modelu turbulence aplikovaného na syntetizované proudy
Tesař, Václav ; Kordík, Jozef
Anemometric data obtained on two synthetic jets issuing from 4 mm dia nozzle, differing in their frequency (25 Hz and 50 Hz), were applied to one-equation turbulence model solution of jets. It made possible evaluating spatial distributions of unsteady motion in the jets. The classical concept of similarity, however, ceased to be applicable and a concept of quasi-similarity was introduced. An important finding was the discontinuity found in the structure of the unsteady motions – the intial region dominated by large vortices is changed into a region of initially smaller/scale turbulent eddies.
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Tlakem poháněná mikrofluidika
Tesař, Václav
Microfluidics is a new field of fluid flow generation and control at small scales, making possible fundamental changes in some engineering and biomedical fields. Particular advantages bring devices operating without moving parts – but at the usually small Reynolds number it is difficult to use jet inertia,upon which no-moving-part fluidics is based. The paper presents, on an example, a solution: flow assisted - or downright driven - by applied pressure difference.
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