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PIV and LIF study of flow and thermal fields of twine plumes in water
Broučková, Zuzana ; Trávníček, Zdeněk
Flow and thermal fields of a pair of plane plumes in water are investigated using PIV and LIF experiments. The plumes are generated from thermal line sources (electrically heated cylinders, diameter D = 1.21 mm). Either continuous or pulsating heating were used with the same heating input power. For a continuous heating, low frequency oscillations were identified and the natural frequency was evaluated as 0.5 Hz. Pulsating heating was used at frequency of 0.25 Hz. The maximum time-mean velocity magnitude at the continuous and pulsating heating were commensurable, approximately 0.007 m/s. Temperature fields reasonably agree with these findings.
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Active control of the wake behind the cylinder
Broučková, Zuzana ; Flídr, Erik ; Šafařík, P. ; Trávníček, Zdeněk
Active control of the flow around a cylinder using synthetic jet (SJ) array was experimentally investigated. The prevailing method was flow visualization. Modification of the wake was investigated at two values of a dimensionless momentum coefficient (Cμ = 0.21 and 1.02). Various α and Cf parameters were tested (where α is the angle between SJs and the main flow direction and Cf is the frequency ratio). The control moves separation point more downstream. Increasing frequency ratio Cf causes refining of the vortex structure. When SJs are placed at the front or rear stagnation points, wake tends towards the symmetrical pattern.
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Jet flow issuing from an axisymmetric pipe-cavity-orifice nozzle
Broučková, Zuzana ; Pušková, P. ; Trávníček, Zdeněk ; Šafařík, P.
Axisymmetric air jet flow is experimentally investigated under passive flow control. Jet issues from a pipe (inner diameter 10mm, length 150 mm) equipped with an axisymmetric cavity at the end. Cavity operates as resonator creating self-sustained acoustic excitations. Mechanism of excitations is complex. Experimental methods were: flow visualization, microphone measurements and velocity measurements by the Pitot probe. Power spectral density (PSD) and sound pressure level (SPL) were evaluated from microphone measurements. Jet Reynolds number ranged Re = 1600–18 000. Peaks in PSD indicated a function of the resonator. The most effective acoustic response was found at higher Re. Results demonstrate effects of the control: mixing and velocity decay along the axis are intensified. It causes shortening of the transition region. However, an inverse proportionality of the velocity decay in the fully developed region is not changed. Momentum and kinetic energy fluxes decrease more intensively.
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PIV and LIF study of slot continuous jet at low Reynolds number
Broučková, Zuzana ; Hsu, S.-S. ; Wang, A. B. ; Trávníček, Zdeněk
This study deals with a continuous jet issuing from a small narrow slot with a width of 0.36 mm. The experimental arrangement is based on the piezoelectric synthetic jet actuator studied previously for easy comparisons. The working fluid is water at room temperature. The experiments were performed using methods of particle image velocimetry (PIV) and flow visualization (laser induced fluorescence, LIF). The time-mean volume flux through the exit nozzle was quantified using precise scales. The mean velocity and the Reynolds number were evaluated as Um = 0.12 m/s and Re = 90, respectively. The results of LIF and PIV techniques revealed the three-dimensional character of the flow field, namely the saddle-shape velocity profiles. This behavior is typical for steady jets from a rectangular nozzle. The obtained results were compared with previous measurements of the synthetic jet issuing from the same cavity and the slot nozzle.
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Maximization of integral outlet quantities of an axisymmetric synthetic jet actuator based on a loudspeaker
Kordík, Jozef ; Trávníček, Zdeněk
The goal of this paper is to find an optimal nozzle size of an axisymmetric synthetic jet actuator based on a loudspeaker. The desirable maximized output quantities are: volumetric flow, momentum flux, and kinetic energy flux. To evaluate these quantities velocity profiles were measured using a hot-wire probe at the actuator nozzle exit. Six different nozzle diameters and three supplied real power levels were tested to find the maxima of the quantities. The actuator operated always at resonance during experiments. It was found out that the momentum flux and the kinetic energy flux reach distinguishable local maxima at particular diameters of the nozzle. Besides, the maxima of the particular quantities do not coincide and the best nozzle size slightly increases with the supplied real power to the actuator.
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Malý aerodynamický tunel
Flídr, E. ; Šafařík, P. ; Trávníček, Zdeněk ; Broučková, Z.
Tato experimentální práce popisuje malý aerodynamický tunel vhodný pro vizualizaci proudového pole. Byla vyhodnocena rychlost proudění uvnitř měřicí oblasti několika metodami: anemometr se žhaveným drátkem, Prandtlova sonda, Pitotova sonda a videozáznam zviditelněného proudění. Kromě toho je ukázáno použití tunelu pro vizualizaci příčně obtékaného válce v režimu Kármánovy vírové řady.
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Pasivní řízení symetrického proudu vzduchu
Pušková, P. ; Šafařík, P. ; Trávníček, Zdeněk ; Broučková, Z.
Práce se zabývá pasivním řízením osově symetrického proudu vzduchu pomocí rezonátoru. Cílem je zjistit, jak ovlivňuje rezonátor chování proudu. Pro ověření známých teoretických poznatků je experimentálně naměřena frekvenční charakteristika Helmholtzova rezonátoru. Dále je zjištěna spektrální výkonová hustota signálu z mikrofonu pro jednotlivé režimy proudění (Re = 1600, 5000, 10 000 a 30 000) a různé objemy rezonátoru. U vybraných režimů jsou naměřeny vlastnosti proudu (poklesy rychlosti v ose proudu a rychlostní profily) pomocí Pitotovy sondy. Následně jsou porovnány vlastnosti proudu bez rezonátoru a s rezonátorem.
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