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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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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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Small wind tunnel for flow field visualization
Flídr, Erik ; Broučková, Zuzana ; Pavelka, Miroslav ; Trávníček, Zdeněk
This experimental work describes a small wind tunnel for flow field visualization and quantifies its main parameters. Velocity of fluid flow was evaluated by a few experimental methods: - Hot wire anemometry - Videorecord of flow - Velocity measurement at the outlet of the tunnel by hot wire anemometry - Velocity measurement at the outlet of the tunnel by Pitot tube - Continuous velocity measurement at the outlet of the tunnel using the reference Pitot tube Visualization of homogeneous flow field around a circular cylinder was shown.
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Small wind tunnel
Flídr, E. ; Šafařík, P. ; Trávníček, Zdeněk ; Broučková, Z.
The aim of this paper was to describe a small wind tunnel for flow field visualization. Flow velocity was investigated by four methods: hot wire anemometry, Prandtl tube, Pitot tube and flow field visualization were used. Flow pas circular cylinder in Von Kármán regime was shown.
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Passive control of axisymmetric air flow
Pušková, P. ; Šafařík, P. ; Trávníček, Zdeněk ; Broučková, Z.
The work deals with passive management axially symmetrical air flow through the resonator. The aim is to see how it affects the behavior of the resonator current. To verify the theoretical knowledge is known experimentally measured frequency response of the Helmholtz resonator. It is also detected spectral power density signal from the microphone for various flow regimes (Re = 1600, 5000, 10000 and 30000) and various volumes of the resonator. For selected modes stream characteristics (speed decreases in the axis of flow and velocity profiles) are measured by Pitot probes. Then the flow characteristics are compared without a resonator and with resonator.
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