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Wall Shear Stress Induced by Taylor Bubbles in Inclined Flow Channels
Tihon, Jaroslav ; Pěnkavová, Věra ; Vejražka, Jiří
The motion of single air bubbles in flat channels is experimentally investigated. The electrodiffusion technique of near-wall flow diagnostics is applied to measure the wall shear stress distribution under large rising bubbles. The measurements are synchronized with the visual observation of bubble movement by a high-speed camera. The analysis of video records provides information on the bubble shape and terminal velocity. The experiments are carried out for three different channel configuration (with heights of 1.5, 4, and 8 mm), cover a wide range of channel inclination angles (from horizontal to vertical position), and dealing with both the bubbles in stagnant and in co-flowing water.
Plný tet: SKMBT_C22012112614030 -  PDF
Plný text: content.csg - PDF
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Oscillations of Bubbles Attached to a Capillary: Case of Pure Liquid
Vejražka, Jiří ; Vobecká, Lucie ; Tihon, Jaroslav
In this contribution, we therefore present results of a linear inviscid theory for shape oscillations of a spherical bubble, which is in contact with a solid support. The theory allows determining eigenmode, but also response of the bubble shape to a motion of its support or to volume variations. Present theory covers also the cases previously analyzed by Strani and Sabettaand Bostwick and Steen, and it can be applied to both bubbles and drops. The theory has been compared to experiments. Good agreement is found for the case of small bubbles, which have spherical static shape. Experimental results for larger bubbles and drops deviate from the theory, if a neck is formed. It is shown that this deviation correlates well with a ratio of bubble volume to the maximum volume, when a detachment occurs.
Plný tet: SKMBT_C22012112614031 - PDF
Plný text: content.csg - PDF
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The Transitional Regime of Backward-Facing Step Flows
Tihon, Jaroslav ; Pěnkavová, Věra ; Havlica, Jaromír ; Šimčík, Miroslav
The backward-facing step flow is investigated experimentally (electrodiffusion technique) and numerically (CFD software Fluent) at moderate Reynolds numbers achieved in a water channel. The direction sensitive sensors are used to measure the wall shear rate behind the step and detect the near-wall extent of different flow-recirculation regions. 2D numerical simulations provide additional information on the global flow rearrangement caused by the change of operational parameters (channel expansion ratios, inlet flow conditions). At the transitional flow regime, the near-wall flow exhibits an unsteady character with a high sensitivity to external low-frequency perturbations (intrinsic flow instability). The inlet pulsatile forcing is found to affect strongly the overall flow structure behind the step, resulting in the reattachment length reduction and the reverse flow intensification.
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Dynamics of Shape Oscillations of a Bubble Attached to a Capillary Tip
Vobecká, Lucie ; Vejražka, Jiří ; Orvalho, Sandra ; Zedníková, Mária ; Tihon, Jaroslav
The shape oscillations of a bubble, which is attached to a tip of a capillary tube, are studied experimentally. It is observed that the attachment leads to an appearance of an additional low-frequency mode (denoted as mode 1), when compared to standard theories of oscillations of free bubbles or drops. It is observed that the frequency of mode 1 scales with the bubble size as f ~ D^(-2). The oscillation dynamics of a bubble is strongly influenced if some surface active agent is present in the liquid. Due to interfacial elasticity, the surfactants presence in low concentration increases the oscillation frequency. The decay of oscillations is also faster, probably due to an increase of energy dissipation by Marangoni stresses.
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Oscillation of a Bubble in Surfactant Solution
Vobecká, Lucie ; Vejražka, Jiří ; Zedníková, Mária ; Orvalho, Sandra ; Tihon, Jaroslav
The decay of shape oscillations of a bubble in solutions of a soluble non-ionic surfactant are studied by means of high-speed imaging. The oscillations are induced by a sudden motion of a capillary, to which the bubble is attached. The bubble then either detaches or remains attached at the capillary. In both cases, the observed bubble shape is decomposed into eigenmodes (Legendre polynomials). The frequency f and the decay time τ of the lowest oscillation mode are evaluated. Their dependences on the surfactant concentration c, on the bubble size and on the bubble age are studied.
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Experimentální studium pohybu bublin v nakloněném plochém kanálu
Tihon, Jaroslav ; Pěnkavová, Věra ; Vejražka, Jiří
Elektrodifúzní diagnostika přístěnného toku je použita pro měření rozložení smykové rychlosti pod velkou bublinou stoupající podél stěny nakloněného kanálu. Směrově citlivá čidla umožňují též detekci reverze toku v tenkém kapalném filmu oddělujícím pohybující se bublinu od stěny kanálu. Elektrodifúzní měření jsou synchronizována s vizuálním pozorováním pohybu bubliny pomocí rychloběžné kamery. Analýza těchto videozáznamů poskytuje informace o tvaru a rychlosti bublin. Experimenty jsou provedeny pro tři různé výšky tokového kanálu a pokrývají široký rozsah úhlů jeho naklonění. Získané výsledky charakterizující pohyb bublin jak v nehybné kapalině, tak i v kapalině proudící vzhůru kanálem, jsou konfrontovány s předpověďmi teoretických modelů.
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Tlumení tvarových oscilací bublin surfaktanty
Vejražka, Jiří ; Orvalho, Sandra ; Vobecká, Lucie ; Zedníková, Mária ; Tihon, Jaroslav
Prostřednictvím rychloběžné kamery studujeme tvarové oscilace bublin, které jsou buď přichyceny na hrotu kapiláry, nebo volně stoupají v kapalině. Přitom vyhodnocujeme, jak přítomnost surfaktantu (alfa-terpineolu) v kapalině (vodě) ovlivňuje dobu, za kterou se tvarové oscilace utlumí. Je pozorováno, že přítomnost surfaktantů vede k významnému zkrácení doby útlumu.
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