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Střihové napětí na stěně pod velkou bublinou stoupající v nakloněném plochém kanále
Tihon, Jaroslav ; Pěnkavová, Věra ; Vejražka, Jiří
The motion of single air bubbles in an inclined flat water channel is investigated experimentally. The electrodiffusion technique of near-wall flow diagnostics is applied to measure the wall shear stress distribution under large bubbles rising along the upper channel wall. The electrodiffusion measurements are synchronized with the visual observation of bubble movement done 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 heights and cover a wide range of channel inclination angles (from horizontal to vertical position). The experimental results on bubble motion characteristics obtained in stagnant and upward-flowing liquid arrangements are confronted with the recent predictions and scaling provided by theoretical models.
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Vliv viskoelastických vlastností mezifázového rozhraní na oscilace bubliny
Vejražka, Jiří ; Vobecká, Lucie ; Zedníková, Mária ; Orvalho, Sandra ; Tihon, Jaroslav
Shape oscillations of bubbles in surfactant solutions are investigated by means of high-speed imaging. Two cases are studied: bubbles attached to a capillary tip, and bubbles freely rising after detachment. The frequency f and the decay time τ of shape oscillations are evaluated, and their dependence on the surfactant concentration and on the bubble size is investigated. If the surfactant is present, τ shortens and a minimum is found for a certain concentration. This observation demonstrates that in the surfactant solutions, the energy dissipation linked to interfacial effects can be much more important than the dissipation due to liquid viscosity. The frequency f increases with increasing concentration of the surfactant first, and then it drops rapidly at concentration, at which τ is minimal. The initial increase of the oscillation frequency suggests that the interfacial elasticity can play an important role in surfactant solutions.
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Fyzikální modelování změn difúze znečišťujících látek v oblasti křižovatky v závislosti na směru nabíhajícího proudění
Kukačka, Libor ; Jurčáková, Klára ; Kellnerová, Radka ; Jaňour, Zbyněk
The objective of this study is physical modelling of emissions from vehicle exhausts in street canyons within an urban area and the influence of the approach flow direction on contaminant spreading. The requirements of the similarity of the atmospheric boundary layer and a boundary layer in the wind tunnel are satisfied. For several flow directions mean velocity and concentration fields are measured within street canyon intersection inside symmetrical urban built-up area. Scalar fluxes are computed from measured data to quantify pollutant spreading. Results of the experiment show a significant sensitivity of the mean velocity field on the approach flow direction. The approach flow extensively influence contaminant spreading within the built-up area and therefore intensity of street canyons ventilation vary with the approach flow direction. The approximate flow direction with the most favourable dispersion conditions in the studied area is found out.
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Turbulent Boundary Layer generated Over Two Urban Roughnesses
Kellnerová, Radka ; Jaňour, Zbyněk
As a basis for PIV measurement, turbulent characteristics of a flow were investigated by LDA over two different types of urban roughness in wind-tunnel. The roughness consists from long series of street canyons with unit aspect ratio. Different shapes of roof - flat and pitched - provide a different internal boundary layer above canyons. The latter one, generated by pitched roofs, is significantly more turbulent and thus slower then above flat roofs. Momentum flux and corresponding transport exhibits the largest deviation specially at the roof level. Ventilation in the streets is therefore distinct from each other. Quadrant analysis was applied for vertical profiles and cross-sectional areas in canyons in order to detect a domination of sweep or ejection events and to find a linkage to the third moment of velocity for both cases. The overall goal of the project is to find any of coherent structure produced by the roof edge as a low momentum region or an ejection formed by hairpin.
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Řízení mezní vrstvy na profilu s vychýlenou klapkou pomocí vírových generátorů
Součková, Natálie ; Šimurda, David ; Popelka, Lukáš
The paper focuses on reducing eventually removing flow separation, which is occurred on deflected flap, with vortex generators. Airfoil NACA 63A421 with simple flap and low profile vane-type vortex generators were used. The investigation was carried out using experimental and numerical method. The flow over the airfoil was visualized by tuft filaments technique in closed circuit wind tunnel for two flap deflections and four angles of attack. Two size and two different shapes of vortex generators were tested. The data from numerical simulation of flapped airfoil without VG's control were used for vortex generator designing.
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