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Synthetic Jet Characteristic Velocity Estimated nn The Basis of Momentum Flux Measurement
Kordík, Jozef ; Trávníček, Zdeněk
The paper compares two methods of evaluation of synthetic jet characteristic velocity. The first method of the characteristic velocity estimation is based on a simple measurement of the synthetic jet momentum flux (the jet thrust) by means of precision scales. The second method is a standard one, it is based on spatial-temporal integration of the velocity profile at the synthetic jet actuator nozzle exit. The velocity profile was obtained here using hotwire measurements in air. Comparison of the methods is based on dimensionless stroke length of the synthetic jet. It is confirmed that the results of both methods are nearly the same for higher dimensionless stroke length. On the other hand, if the jet regime approaches to the synthetic jet criterion threshold (i.e. to the dimensionless stroke length of 0.5) the results of both methods diverge. Using the current results a correction is suggested for the momentum-flux-based method to match the standard one.
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Generation of Intermittent Jet by Pulse-Modulated Synthetic Jets
Broučková, Zuzana
An axisymmetric air jet was excited using a pair of counter synthetic jets running in opposed direction. First, the control synthetic jets were measured alone. The driving signal was pulse modulated sinusoidal signal. After an adjustment, the primary axisymmetric jet was excited to the bifurcating mode, and its behavior was studied experimentally. For comparison purposes, a reference steady (unforced) jet from the same nozzle was also measured (Reynolds number was approximately 1300). The flow visualization and hot-wire anemometry techniques were used.
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Analysis of non-harmonic excitation of synthetic jet actuators
Kordík, Jozef ; Trávníček, Zdeněk
Axisymmetric synthetic jet actuator based on a loudspeaker was tested experimentally. The actuator was driven by a voltage waveform, whose shape was derived from a pulse-width modulated signal. While the duty cycle and frequency of the excitation signal were varied the outlet velocities were measured by a hot-wire probe. Integral quantities such as characteristic velocity, momentum flux, and kinetic energy flux were evaluated from the outlet velocities. Their maxima were located as functions of frequency and the duty cycle. The found maximal values were compared with results obtained for harmonic excitation. The highest improvement (about 18%) was achieved for kinetic energy flux.
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Comparison of synthetic jet actuators based on sharp-edged and round-edged nozzles
Kordík, Jozef ; Trávníček, Zdeněk
Axisymmetric synthetic jet actuators based on a loudspeaker and on two types of flanged nozzles were tested and compared experimentally. The first type of the nozzle was a sharp-edged circular hole. The second one had a special design with fillets at inner and outer nozzle exit and with a small step in the middle of the nozzle. The function of the step was to prevent the flow reattachment during the extrusion stroke. The actuators with the two types of nozzles were operated at resonance and were compared first qualitatively using a simple phase locked flow visualization. Then the hot-wire anemometer was used to measure velocity \ndistributions along nozzle axis and velocity profiles at the nozzle exit. Comparison of the nozzles was based on evaluation of the characteristic velocity and integral quantities (volumetric, momentum, and kinetic energy fluxes). It was found out that these quantities, which were evaluated at the nozzle exit, differ substantially for both nozzles. On the other hand the velocity flow field in farther distances from the nozzle exit area did not exhibit such prominent differences.
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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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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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Laser Doppler vibrometry experiment on a piezo-driven slot synthetic jet in water
Broučková, Zuzana ; Vít, T. ; Trávníček, Zdeněk
The present study deals with a slot synthetic jet (SJ) issuing from an actuator into quiescent surroundings and driven by a piezoceramic transducer. The actuator slot width was 0.36 mm. The working fluid was water. The present experiments used flow visualization (a laser-induced fluorescence technique - LIF) and laser Doppler vibrometry methods (LDV). Flow visualization was used to identify SJ formation, to demonstrate its function, and to estimate SJ velocity. LDV was used to quantify diaphragm displacement and refine operating parameters. Phase averaging yielded a diaphragm deflection during the actuation period. Taking incompressibility and continuity into consideration, the velocity in the actuator slot and the Reynolds number of the SJ were evaluated as 0.21 m/s and 157, respectively. The present results confirmed a SJ actuator function at the resonance frequency of approximately 46 Hz, which corresponds closely with the theoretical evaluation. The LDV results corresponded closely with an estimation of SJ velocity by the visualization.
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