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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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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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Zpracování informací získaných z termochromních kapalných krystalů
Tesař, Václav ; Randa, Zdeněk
Thermochromic liquid crystals respond to local changes of temperature by varying the colour of reflected light. Their use in experiments with anisothermal fluid flows seems to be simple - just taking pictures of the colours on the walls exposed to the fluid flow. However, practical extraction of quantitative informationfrom the pictures is not straightforward and brings a number of pitfalls. Unfortunately, these details are generally not found in available literature since most authors mention these practical procedure aspects very briefly, if at all. The present paper concentrates on these practical steps following them on a practicalexample of an investigation of an impinging jet, presenting the experience acquired in such investigations.
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Frekvenční charakteristika generátoru anulárního syntetizovaného proudu s velkým vnitřním průměrem
Krejčí, A. Petr ; Trávníček, Zdeněk ; Tesař, Václav ; Kordík, Jozef ; Pavelka, Miroslav
An annular synthetic jet actuator of large inner/outer diameter ratio of the nozzle has been developed, and four variants were designed, manufactured, and tested. The resonance frequency was estimated theoretically, based on the mechanical energy balance, and then measured by the pneumometric method. The resultant value was found to be 40 Hz. The nozzle geometry effect on the resonance frequency and on the velocity was quantified; both effects were concluded to be surprisingly small, less than 5%.
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Mikrofluidika v řízení živých tvorů
Tesař, Václav
Microfluidics had recently interesting uses in biomedical sciences. Its development towards smaller scales has reached the stage making possible an interfacing with the brain cortex at the cellular level: reading and inputting the neural signals. The signals and their processing are mostly of electronic character, but fluidics plays in this application one of the key supporting roles. The contribution on the one hand lists therapeutical applications, but on the other hand notes current trend of military uses of insects or small rodents who can intrude into military establishments and gather there pictorial information.
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