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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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EXTRACTING INFORMATION ABOUT COHERENCE IN JET FLOWS
Tesař, Václav ; Něnička, Václav ; Šonský, Jiří
Several alternative approaches to processing jet flow visualization images were recently introduced by the authors. Common feature is processing a sequence of 2D images to detect presence and evaluate development dynamics of 3D vortical structures, which are recognised as regions that differ from the uncorrelated turbulent background in the processed pictures by their high negative values of correlation coefficient.
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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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Mechano/fluidická zařízení využívající „aerodynamický paradox“
Tesař, Václav
A mechanism of aerodynamically excited vibration of a movable or elastic solid body was investigated and suggested for use in a number of fluidic (or mechano/fluidic) devices. The vibration is based on a phenomenon little known and so far practically not used, despite of its very old history. The effect leads to an alternatingdirection of a fluid force acting a movable object exposed to fluid flow. One of the directions is interesting: it is the very opposite to what an uninitiated observer may expect. Early investigations of this "paradox" by the present author in 1971 became recently of renewed interest because of the possible topical use in biosensors.
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Vlastnosti trysek – experimentální ověření na zažízení stavěném pro výzkum šroubovicových nestabilit
Tesař, Václav ; Kellnerová, Radka ; Kukačka, Libor ; Kordík, Jozef
Nozzle characterisations in current use may be described as zero-order (neglected loss) and first-order (constant loss coefficient). The first author (V. Tesar) introduced in 1985 a second-order theory, fully predicting all the variations of nozzle properties with Reynolds number Re. The basic idea is an assumption of the loss being entirely specified by displacement effect of wall boundary layer in the exit. The other assumption is the layer thickness being governed by laminar boundary layer growth law. All evidence for this theory so far was only indirect. This paper presents recent direct experimental verification by measurements of boundary layer profiles on a large-scale nozzle model across a wide range of Re. The predictions of the 1985 model are fully supported.
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