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Microbubble generation by fluidics. Part I: Development of the oscillator
Tesař, Václav
Author investigates generation of sub-millimetre air bubbles in water. An economical alternative is pulsating the supplied air flow, using a no-moving-part fluidic oscillator. The paper summarises the existing oscillator principles and discusses the author's development of the chosen oscillator – which resulted in a device with behaviour different from what has been so far described in literature.
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Trapped Vortex Ring
Tesař, Václav
Paper discusses an almost unknown and yet interesting operating principle of fluidic nomoving-part devices for flow control. The principle is based on the properties of vortex rings. A standing vortex ring is kept in a semi-toroidal recession positioned opposite to an annular nozzle from whichissues an annular fluid jet. The ring can exist in the recession with two alternative senses of rotation so that the annular jet is led to either the central exit through the centre of the vortex, or to the outer spacepast the outer vortex circumference.
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Enigma of submerged fence skin friction sensor
Tesař, Václav
The problem with the known “sublayer fence” skin friction sensor is the disturbance caused by the fence protruding above the surface. Author comes with the solution preventing the premature transition into turbulence or even flow separation. In his original version of thesensor the fence is submerged slightly below the surface. Detailed investigations revealed the pressure difference output to be dependent on creation of two recirculation regions. An attempt tofacilitate the recirculation, quite surprisingly, has led to the merging of the vortices and total disappearance of a useful output signal.
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Annular Jets and Annular Impinging Jets
Turek, Tomáš ; Trávníček, Zdeněk ; Šafařík, Pavel ; Tesař, Václav
A near field of annular jets and annular impinging jets is experimentally investigated at three inner/outer diameter ratios 0.84, 0.90 and 0.95. Time-mean velocity profiles are evaluated, and three main flow field zones are distinguished – in agreement with the literature, namely the initial merging zone, the intermediate one, and the fully merged one. A distribution of the pressure on the impingement wall is measured for various nozzle-to-wall distances. Two basic annular impinging jet regimes are clearly distinguished, with either small or large recirculation bubbles. Moreover, a hysteretic behavior has been identified under specific parameters.
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High-Frequency, Small-Scale Fluidic Oscillators for Boundary Layer Control
Tesař, Václav
No-moving-part fluidic oscillators are promising candidates for use in jet-type actuators for suppressing the separation in adverse pressure gradient flows and for elmination of the effect of hairpin vortices that absorb the turbulent fluid flow energy. The requests for very high oscillation frequency has led to several new ideas applied to the oscillator configurations.
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“Master and Slave" Fluidic Amplifier Cascade
Tesař, Václav
Paper díscusses mutual matching of two no-moving-part fluidic unvented amplifiers in a two-device module, where small MASTER device, with feedback loop converting it into a small high-frequency oscillator dictates the flow switching in a large powerful SLAVE device. Interest in this configuration was caused by it providing a solution in generation of gas microbubbles. The oscillation frequency has to be high and this calls for small amplifier size - while a the total gas flow has to be large, calling for a large size. The conditions in the circuit is complicated by the need to considerfive terminals in each amplifier.
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Visualization of synthetic jets at higher stokes numbers
Broučková, Zuzana ; Trávníček, Zdeněk ; Šafařík, Pavel
Visualization of synthetic jets at higher Stokes numbers (S = 90 and 127) by the phase-locked smoke-wire technique is presented and discussed. The working fluid is air. The Reynolds numbers are quantified using hot-wire anemometry. Although our method of visualization essentially provides only qualitative results, the present study also demonstrates some quantitative results, namely the behavior of the zero-net-mass-flux jet near its critical stage. Visualization of the sub-critical stage is also shown.
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