PIV and LIF study of flow and thermal fields of twine plumes in water

Flow and thermal fields of a pair of plane plumes in water are investigated by means of PIV and LIF experiments. The plumes are generated from thermal line sources, which are made out of electrically heated cylinders with a diameter of D = 1.21 mm. A cylinder-to-cylinder distance was 17.9 D. Either continuous or pulsating heating were used with the same heating input power. Because the cylinder-to-cylinder distance is moderately small, deflections of plumes from a vertical direction occur and the plumes are inclined together. This behavior is caused by a confined entrainment from a space between the both plumes. For a continuous heating, low frequency oscillations were identified and the natural frequency was evaluated as 0.5 Hz. Based on this finding, pulsating heating was used at the subharmonic frequency of 0.25 Hz. The maximum time-mean velocity magnitude at the continuous and pulsating heating were commensurable, approximately 0.007 m/s. On the other hand, pulsating heating achieves by 36 % higher velocity peaks. A very strong velocity oscillations were generated by pulsating heating at the distance approximately 8.3 D above the cylinders, where the velocity maxima oscillate along the time-mean value of 0.0057 m/s from −30% to +70 %. Temperature fields reasonably agree with this findings, despite a relatively fast equalization of the temperature field was concluded. The results demonstrate enhancement effects of pulsations in flow/thermal fields

Active control of the wake behind the cylinder

Active control of the flow around a circular cylinder using a synthetic jet (SJ) array was experimentally investigated in a small wind tunnel. 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). It was found that the control moves a separation point more downstream, i.e. the width of the wake can be reduced. Moreover, increasing frequency ratio Cf causes refining of the vortex structure. When the SJs are placed at the front or rear stagnation points, the wake tends towards the symmetrical pattern

Laser Doppler vibrometry experiment on a piezo-driven slot synthetic jet in water

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

Active control of an annular jet by means of the radial synthetic jet

This experimental study deals with an annular jet, which is actively controlled. Firstly, the following partial tasks were measured: axisymmetric continual jet, axisymmetric synthetic jet, radial synthetic jet, and annular jet without control. Measurement was performed in air by means of the Pitot probe and hot-wire anemometer. The results of partial steady tasks confirm knowledge, available from literature. The expected resonant behavior of the synthetic jet actuators was confirmed too. A possibility of flow control of the mean annular jet was proved. A suitable frequency and amplitude of the control synthetic jet was 600Hz and 23m/s, respectively

The interaction of continual and synthetic jets

This work deals with an interaction of continual and cross synthetic jets. Both jets are axisymmetric. All measurements have been performed in air by means of the Pitot tube and hot-wire anemometry. Flow visualization has been also performed. The possibility of flow control is shown and quantified. For the moderate momentum ratio, the mean jet deflection reasonably agrees with the momentum conservation

Generation of Intermittent Jet by Pulse-Modulated Synthetic Jets

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

Visualization of synthetic jets at higher stokes numbers

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

Bubble dynamics in drinks

This study introduces two physical effects known from beverages: the effect of sinking bubbles and the hot chocolate sound effect. The paper presents two simple „kitchen” experiments. The first and second effects are indicated by means of a flow visualization and microphone measurement, respectively. To quantify the second (acoustic) effect, sound records are analyzed using time-frequency signal processing, and the obtained power spectra and spectrograms are discussed

Active control of an annular jet by means of the radial synthetic jet

This experimental study deals with an annular jet, which is actively controlled. Measurement was performed in air by means of the Pitot tube and hot-wire anemometer. The expected resonant behavior of the synthetic jet actuator was confirmed. A possibility of flow control of the mean annular jet was proved. A suitable frequency and velocity amplitude of the control synthetic jet was 600Hz and 23m/s, respectively

Visualization of the annular synthetic jet

Visualization of the annular synthetic jet at three Stokes numbers (S = 16, 24 and 27) is performed by the phase-locked smoke-wire technique. Moreover, measurement of the velocity fields is also presented. These measurements are performed by means of the Pitot tube for several configuration of the annular jet. The working fluid is air