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Taxonomic trees of fluidic oscillators
Tesař, Václav
Fluidic no-moving-part oscillators generating fluid flow pulsation became recently very popular subject of investigations. The reason is their capability to increase efficiency of various chemical and physical/chemical processes. Also control of flows past bodies is more effective with pulsation. Advantages of fluidic oscillators are lowcost, robustness, long life, no maintenance, and similar factors associated with absence of mechanical moving components. New oscillator principles - as well as old nearly forgotten but now re-discovered ones - are currently developed. Sheer number of possible alternatives makes them difficult to survey. This paper attempts at clarifying the situation by proposing three different approaches.
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What can be done with microbubbles generated by a fluidic oscillator?
Tesař, Václav
Recently discovered possibility of generating sub-millimeter bubbles by means of pulsating the gas inlet by a fluidic oscillator has opened ways into many interesting and advantageous engineering applications. The paper, by presenting a survey of existing possibilities and their advantages, aims at providing to readers an inspiuration for their own developments.
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Invariants of simple nozzles
Tesař, Václav
Paper presents a new concept: invariance of second order. The idea bypasses the necessity geometric similarity (known as allowing only a scaling up or down) in derivation of dimensionless criterial parameters. The approach is demonstrated on the case of family of simple nozzles, the members of the family differing in their lengths of exit channel. While each nozzle is characterised by its primary invariant, characterisation of the whole family leads to two secondary invariants.
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Viscosity-dominated regime of flow in fluidic devices at very low Reynolds numbers
Tesař, Václav
Because of their small size – and typically due to high viscosity of handled fluids - some microfluidic devices have to operate at very low Reynolds number levels. The devices based on using inertia of a deflected jet under these conditions losetheir efficiency and finally exhibit a phenomenon so far not discussed in literature: they enter a regime in which their behaviour is no more specified by Reynolds number value. Paper presents a detailed case description of a valve and itstransition into the sub-dynamic regime. Pressure-assisted flow into the device terminal is characterised by newly introduced pressure parameter.
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Stochastic regimes in very-low-frequency fluidic oscillator
Tesař, Václav
Paper discusses interesting unexpected stochastic regimes discovered in a fluidic oscillator designed for operation at very low oscillation frequencies - without the inconvenience of the long feedback loops needed in standard low-frequency oscillator designs. The new oscillator contains a pair of bistable turn-down active valves operating in anti-parallel — essentially analogous to Abraham & Bloch electric “multibrateur” invented in 1919. Three different self-excited oscillation regimes were found. In the order of increasing supplied flow rate, these regimes are characterised by: (A) generation of stochastic-duration multi-pulse packs, (B) generation of individual pulses with a degree of periodicity, and (C) regime with randomly appearing flow pulses separated by intervals of the order of seconds.
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Fluid mechanics of additive manufacturing of metal objects by accretion of droplets – a survey
Tesař, Václav
Paper presents a survey of principles of additive manufacturing of metal objects by accretion of molten metal droplets, focusing on fluid-mechanical problems that deserve being investigated. The main problem is slowness of manufacturing due to necessarily small size of added droplets. Increase of droplet repetition rate calls for basic research of the phenomena that take place inside and around the droplets: ballistics of their flight, internal flowfield with heat and mass transfer, oscillation of surfaces, and the ways to elimination of satellite droplets.
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Fluidic low-frequency oscillator consisting of load-switched diverter and a pair of vortex chambers
Tesař, Václav ; Peszyński, K. ; Smyk, E.
Paper discusses a new configuration of fluidic oscillators, a subject of recent Patent application. There is some similarity with the standard Warren oscillator with its bistable jet-deflection diverter and two feedbacks – which is not suitable for situations demanding very low oscillation frequency. For these conditions the new design replaces jet-deflection switching in the diverter by load-switching effects, with the gradually increased loading by spin-up of fluid in the vortex chambers. The spin-up time also provides the needed time delays. Behaviour is characterised by the oscillation frequency increasing with increasing fluid flow rate – for which was derived a surprisingly simple theoretical solution.
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Plasma-discharge control in fluidics
Tesař, Václav ; Šonský, Jiří
A transducer controlling fluid flow by electric signals without the two-stage conversion involving moved or deformed components have been so far unrealisable. Authors solved the problem by means of “ionic wind” of non-temperature plasma discharge acting inside a fluidic diverter amplifier. Experiments demonstrated good performance at high frequenciesf > 100 Hz and small Reynolds numbers Re <1000.
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