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Control of flow separation by vestigial "synthetic jet"
Tesař, Václav ; Pavelka, Miroslav ; Smyk, E. ; Peszyński, K.
A useful aerodynamic control action may be causing a separation of flow from the wall past which the fluid flows. Several current research activities aim at doing so by oscillating zero-time-mean flow generated in a nozzle located in the wall. The effect is often described as “flow control by synthetic jet”, although it is obvious that the vortex rings generated in the outflow phase of the oscillation are deformed by the controlled flow and also are carried too far away so that they cannot form the classical synthetic jet configuration of vortex rings. Authors describe experiments with such a control applied successfully in an axisymmetric fluidic valve. Presented is a hypothesis explaining the separation mechanism.
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Measurement of velocity and frequency in synthetic jet
Flídr, E. ; Šafařík, P. ; Trávníček, Zdeněk ; Broučková, Z.
The aim of this paper is to evaluate resonance frequency of pulsation in synthetic jets of air. Resonance frequency was determined by theoretical calculation. This frequency was compared with experimental results. To obtain frequency characteristics three experimental methods were used. Velocity was measured by Pitot tube and hot wire anemometry in CTA regime. Last method was based on calculation of reaction force from relative weight changes.
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Influence of Carrying Frequency Phase Shift of Synthetic Jet Generator on The Noise
Pick, P. ; Matějka, Milan ; Skála, V.
The main topic of this article is the influence of phase shift of neighbouring synthetic jet generators to the flow field past a hump and noise reduction. Syntheetic jet generators are placed in the hump in front of point of boundary layer separation. The carrying frequency of actuating signal, wich is driving the synthetic jet generator, is shifted for about 180°. The modulated frequency, which corresponds to the shading frequency of the flow, is superposed on the carrying signal. Finally, the benefits of phase shift signal of neighbouring synthetic jet generators are described, as far as the produced noise and the total loss coefficient decrease are concerned.
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A new method for fluid input into a hybrid synthetic jet actuator
Kordík, Jozef ; Trávníček, Zdeněk ; Tesař, Václav
A new principle of flow rectification for hybrid synthetic jet actuators is introduced in this paper. As is well known, the flow rectification can be best accomplished by means of fluidic diodes. Novelty of the present study are fluidic diodes with two mutually opposed nozzles. Interaction between the periodic jet flows from the nozzles causes a difference between the blowing and suction strokes, resulting in a particularly efficient rectification effect. The distance between the nozzle exits as well as the oscillation frequency were the parameters, which were varied during hot-wire measurements. The combination of those parameters achieving the highest volumetric efficiency was identified.
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Spectral analysis of continual and synthetic jet
Broučková, Z. ; Trávníček, Zdeněk ; Šafařík, P.
This experimental work deals with a characterization of three types of air jet flows (continual jet, synthetic jet and continual jet which is controlled by a synthetic jet). The characterization is performed using the power spectral density (PSD). All the characteristics were obtained from single-point measurements of flow velocity. The velocity was measured by a hot-wire anemometer (CTA). Power spectral density was acquired directly from measured signal (or from velocity data) by means of MATLAB software. Obtained graphs showed the control effect on the main flow behaviour. The significant qualitative change was revealed near a boundary of synthetic jet formation. Moreover it was concluded that this significant change can be also well quantified.
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Region of parameters of synthetic jets
Broučková, Z. ; Šafařík, P. ; Trávníček, Zdeněk
This work presents a summary of parameters which are needed for the descriptionof synthetic jets. Examples from available literature are discussed. A reliability of severalformation criteria of these jets is also verified. Most of the criteria have similar character andgive reliable results - all chosen examples satisfy the criteria. It is shown that one of theinvestigated criteria is not very trustful.Furthermore, the measurements of velocity of the axisymmetric synthetic jet were performedby means of the Pitot tube and the Hot-Wire Anemometry. The results are in very goodaccordance with the ones from literature. Moreover, it was revealed that the decreasing of theReynolds number and the working point approaching towards the critical value on theboundary of formation lead to the gradual shortening of the reach of the jet.
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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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Experimental investigation of a control synthetic jet
Němcová, L. ; Kordík, Jozef ; Trávníček, Zdeněk ; Kopecký, V.
Synthetic jets were used in previous studies to control a continuous round air jet. This study focuses on the single synthetic jet, which is measured using Constant Temperature Anemometry (CTA) and Particle Image Velocimetry (PIV). Velocity profiles and frequency characteristics of the single synthetic jet are discussed and the velocity maps at frequencies of 30 Hz, 81 Hz and 190 Hz are presented. Finally, velocity, flow rate and momentum coefficients featured the present flow control are evaluated, and examples of actively controlled round air jet are put on. The results confirm efficiency of the present flow control method.
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