Formation Of Synthetic Jet Research Articles

Effects of piezoelectric synthetic jet actuator parameters on synthetic jet formation process and actuator performance

Aiming at the problem that the non-linearity between the piezoelectric vibrator deflection and the excitation amplitude of the piezoelectric synthetic jet actuator (SJA) causes it difficult to obtain the variation laws of the synthetic jet (SJ) velocity accurately, a theoretical model of the whole flow field of the piezoelectric SJA is established and four important parameters (half period jet mean velocity, Reynolds number, effective jet length and energy) for evaluating the performance of the SJ are proposed. Then a two-dimensional axisymmetric numerical simulation is carried out with k-ω turbulence model to study the SJ formation process and the velocity characteristics of the SJ at the orifice. The results show that the fluid pressure in the cavity is the main reason causing the non-linearity between the piezoelectric vibrator deflection and the excitation amplitude, the influence of which decreases with the increase of the orifice diameter. The SJ velocity increases first and then decreases slowly with the increase of excitation frequency. The extreme point occurs at the first resonant frequency of the piezoelectric vibrator and the position of the maximum velocity occurs at 0.5 mm outside the orifice. Other conditions being constant, both the piezoelectric vibrator deflection and jet velocity have a maximum value when the radius ratio of the piezoelectric layer to the elastic layer is 0.75. Smaller piezoelectric layer thickness leads to larger lateral displacement of the piezoelectric vibrator, and the optimum thickness ratio is 0.25. The results of this study have important guiding significance for accurately obtaining the variation laws of the SJ velocity as well as rationally designing the piezoelectric SJA.

Synthetic jet flow driven by a standing-wave thermoacoustic heat engine

This paper presents an experimental investigation of the formation of synthetic jets into external quiescent environment for cooling purposes using a thermoacoustic engine which converts heat into acoustic oscillations. Thermoacoustic technology is proposed in this study as an option for enhancing the cooling effect in equipment normally relying on natural convection. For this purpose, a standing-wave thermoacoustic heat engine was modelled and built and its ability to generate synthetic jet flow structures as well as their propagation were examined. It was found that the engine is able to drive synthetic jets through 10 orifices at a temperature difference less than 150 °C. The working medium is CO2 at ambient pressure. In experiments, the performance of thermoacoustic engine was evaluated in terms of jet velocity and the required temperature difference across the stack by varying the heating power.

Acoustics and Heat Transfer Characteristics of Piezoelectric Driven Central Orifice Synthetic Jet Actuators

ABSTRACT Growth in computational capacity combined with a decrease in the size of digital devices has led to increasing demand for more active and efficient cooling of electronics. In this study, an experimental investigation into two different sizes of central orifice Synthetic Jet Actuators (SJAs) is conducted to evaluate their heat transfer as well as noise generation characteristics. Two SJAs (40 mm and 20 mm) are examined, covering a distinct span of frequencies ranging from low to medium (<5500 Hz) in regards to the effect of SJA size over performance. The SJAs’ disk deflection, structural frequency, and jet exit velocity were measured to fully characterize the jet performance. The maximum Nusselt number for the largest SJA was 3 times more than the smallest SJA, where the evaluation of stroke length suggests no effective synthetic jet formation for the smallest SJA. The noise from the SJAs was measured in an anechoic chamber using three microphones, Fast Fourier Transform (FFT) of the sound pressure levels provide contributions to different tones in the resulting noise. 1/3 Octave Constant Percentage Bandwidth (CPB) analysis was performed to identify the frequency bands making the largest contribution to the noise. The largest SJA showed the highest heat transfer at acceptable noise levels during the operation below resonance frequency.

Design approach to predict synthetic jet formation and resonance amplifications

A predictive approach for the Synthetic Jet (SJ) formation, which couples formation criteria already available from the literature, with outcome to Lumped Element Modeling (LEM), which is able to predict the resonance velocity amplifications, is presented. In this respect, the formation criteria yield the minimum jet velocity to obtain an experimentally observable jet, whereas the LEM approach predicts the jet velocity, to be compared with the minimum one of the formation criterion. A deep experimental investigation is carried out to detect jet formation and to measure resonance velocity amplifications on various types of actuators manufactured in house, including, among others, the influence of single and double orifices. Jet velocity is recorded by means of hot-wire anemometry, at both the device orifice exit and one orifice diameter downstream, i.e. downstream of the expected stagnation (saddle) point. Jet formation and resonance amplification thresholds are identified in terms of the dimensionless Stokes and Strouhal parameters. Based on the experimental finding, a description of the relevant modeling is reported, devoted to the evaluation of the velocity magnification factor with respect to the so-called incompressible (static) solution. It takes advantage from the physical consideration that the actuator behaves as a driven system of two-coupled mechanical oscillators, the Helmholtz’s one and the structural one, for which resonance frequencies are predicted by means of simple relationships, together with appropriate damping factors. Jet formation thresholds closely agree with the present experimental findings. The predicted overdamped conditions are compared with classic analytic boundary correlations of the literature and previous experimental results as well. Practical and simple relationships, that can help the designer to manufacture a device having desired performances, are presented.

Water Synthetic Jet Driven by a Piezoelectric Actuator – LIF and PIV Experiments

A synthetic jet (SJ) is a fluid jet flow generated from fluid oscillations during a periodical fluid exchange between an actuator cavity and surrounding fluid. A water synthetic jet was generated from submerged piezoelectric-driven SJ actuator. The actuator slot width was 0.36 mm. The experiments were performed using laser induced fluorescence (LIF) flow visualization and particle image velocimetry (PIV) techniques, both in a phase locked setup. The LIF visualization was used to demonstrate three-dimensional nature of the SJ formation process and to estimate SJ velocity. The PIV experiment quantified SJ velocity cycles in chosen plans. The driven frequency was adjusted near the resonance at approximately 46 Hz. It was evaluated theoretically and confirmed experimentally by means of LIF visualization. The time-mean orifice velocity and the Reynolds number were estimated asU0= 0.07–0.10 m/s andRe= 100–150, respectively.

Visualization of synthetic jet formation in air

This experimental study deals with a round synthetic jet (SJ) issuing into quiescent surroundings. Flow visualization in air is used to identify different flow field regimes. Hot-wire anemometry and theoretical evaluations are used to quantify parameters in terms of the Reynolds (Re) and Stokes (S) numbers. To verify the theoretical evaluation, additional experiments were performed using the laser Doppler vibrometry. Four regimes of oscillatory suction and extrusion are distinguished and presented by means of a Re–S parameter map: (a) creeping flow without SJ formation, (b) SJ formation and propagation without vortex rollup, (c) SJ with vortex rollup, and (d) vortex structure breakdown, instability and transition to turbulence. Differences in the SJ regimes at low, moderate and high Stokes numbers are found. While all four (a–d) regimes are identified for lower (S < 10) and moderate (S = 10–30) Stokes numbers, the SJ formation process for higher Re and S is coupled with the laminar–turbulent transition. The results are reasonably consistent with those in the available literature.

Effect of orifice inner lip radius on synthetic jet efficiency

A synthetic jet is formed by periodic oscillation of a fluid through an orifice. The oscillatory motion is driven by a diaphragm or other driver. Previous studies have demonstrated that synthetic jet formation and time-averaged cavity pressure are a function of the orifice shape. Traditionally, the performance of the jet is evaluated with varying configurations of fixed driver input voltage or fixed driver displacement. Neither of these measures accurately reflect the efficiency of the actuator. Defining efficiency as “desired output divided by required input,” these traditional measures may not account for increase in required driving current or force. A sharp inside edge of a thin synthetic jet orifice can result in separated flow and increased momentum flux (due to the decreased flow area) for a fixed driver displacement. This can lead one to believe that efficiency has been improved, when, in reality, much more power was required for the driver. Acoustic power, which is the time-average of volume flow rate through the orifice multiplied by the driving pressure, accurately accounts for the power required to drive the actuator. For any synthetic jet actuator, the power to the driver is the power to the fluid (acoustic power) divided by the driver efficiency. If we assume that the driver efficiency is not a strong function of the load, any change to the acoustic power will result in the same proportional change in the driver input power. This study investigates the efficiency of a round (axisymmetric) synthetic jet actuator as a function of the radius of curvature of the interior edge of the orifice. Simultaneous particle image velocimetry measurements at the jet exit and cavity pressure measurements are used to measure the acoustic power required to generate the jet. The resultant momentum flux of the jet is used as a measure of output of the jet. Results are obtained for a range of displacement amplitudes (or stroke lengths) and radii of curvature, while Reynolds number is held fixed.

Flow structure generated by two synthetic jets in a channel: Effect of phase and frequency

A three-dimensional computational model was developed to investigate the interaction of two synthetic jets with cross flow in micro-channel. An experimental validation of the formation of synthetic jet was presented at the same scale as those used in this numerical work. Good agreement was obtained between the experimental results and the numerical data. The numerical approach was then used to perform studies with the two jets being in-phase and 180° out-of-phase at various operating frequencies and diaphragm amplitudes with fixed jet Reynolds number. The addition of one synthetic jet was shown to achieve greater mixing of flow in the micro-channel than those with single synthetic jet. Velocity field obtained from 180° out-of-phase configurations show a distinct pattern of vortex formation which results in continuous mixing of the flow in the channel throughout the cycle. When the synthetic jet actuators were driven at different membrane frequencies and amplitudes, the main difference was the variation of the size and number of the vortices downstream and upstream of the office. The structure of the flow in the micro-channel is strongly dependent on the period of the oscillation of the membrane of the synthetic jet actuators.

An experimental study on the behaviours of circular synthetic jets at low Reynolds numbers

In the work presented in this article, the behaviour of circular synthetic jets issuing into quiescent surrounding fluid at low Reynolds numbers is experimentally studied for potential mixing applications of synthetic jets at micro-scales or in highly viscous fluids. Sugar solutions and silicone oil are used as the flow media in order to achieve the required low Reynolds numbers. The conditions for jet instability, vortex rollup and synthetic jet formation are investigated using both flow visualisation techniques and particle image velocimetry, and the typical behaviour of synthetic jets at a Reynolds number around unity is also illustrated. The roles of Reynolds number, dimensionless stroke length and Stokes number in determining the characteristics of synthetic jets are examined and found to be largely consistent with the finding obtained at higher Reynolds numbers. Finally, a parameter map of synthetic jet flow patterns is produced based on the results from this study, which can be used to aid the choice of synthetic jet operating conditions for specific applications or anticipate if a desired vortex structure can be obtained at a given synthetic jet operating condition.

Vortex Roll-Up Criterion for Synthetic Jets

In this paper, both theoretical analysis and numerical simulations are undertaken to study the parameters that affect the strength of vortex roll-up of synthetic jets. A dimensional analysis reveals that the dimensionless vorticity of vortex roll-up produced by an orifice flow depends on the dimensionless stroke length, Stokes number, and the ratio between the orifice diameter and the thickness of the Stokes layer. Based on the results from a fully developed oscillating laminar pipe flow, the Stokes number is found to play an important role in determining the thickness ofthe Stokes layer inside the orifice and hence the shape of the velocity profile. Results from the numerical simulations confirm that the Stokes number also determines the strength of vortex roll-up of a synthetic jet issued from an orifice of a finite depth, for the same reason. Finally, a parameter map, which marks the three different regimes of synthetic jets (classified as no jet, jet formation without vortex roll-up, and jet formation with vortex roll-up) is produced based on the numerical simulation results. It is shown that for the synthetic jet actuator used in the present study, a minimum Stokes number of about 8.5 is required to ensure the occurrence of an appreciable vortex roll-up at a dimensionless stroke length greater than 4. In addition, a very low Stokes number can also suppress the formation of synthetic jets. This study provides a further understanding of the behavior of synthetic jets in quiescent conditions, which will be useful for designing more effective synthetic jet actuators in which vortex roll-up is desired.

Formation Criterion for Synthetic Jets

Af ormation criterion for synthetic jets is proposed and validated. A synthetic jet actuator is a zero-net mass-flux device that imparts momentum to its surroundings. Je tf ormation is defined as the appearance of a time-averaged outward velocity along the jet axis and corresponds to the generation and subsequent convection or escape of a vortex ring. It is shown that over a wide range of operating conditions synthetic jet formation is governed by the jet Strouhal number Sr (or Reynolds number Re and Stokes number S). Both numerical simulations and experiments are performed to supplement available two-dimensional and axisymmetric synthetic jet formation data in the literature. The data support the jet formation criterion 1/Sr = Re/S 2 > K, where the constant K is approximately 1 and 0.16 for two-dimensional and axisymmetric synthetic jets, respectively. In addition, the dependence of the constant K on the normalized radius of curvature of a rounded orifice or slot is addressed. The criterion is expected to serve as a useful design guide for synthetic jet formation in flow control, heat transfer, and acoustic liner applications, in which a stronger jet is synonymous with increased momentum transfer, vorticity generation, and acoustic nonlinearities.

Synthetic Jets in Cross-Flow

Results of an experimental investigation on synthetic jets from round orifices with and without cross-flow are presented. Jet Reynolds number up to 46,000 with a fully turbulent approach boundary layer, and Stokes number up to 400. are covered. The threshold of stroke length for synthetic jet formation. in the absence of the cross-flow, is found to be Lo /D approximately 0.5. Above Lo /D is approximately 10, the profiles of normalized centerline mean velocity appear to become invariant. It is reasoned that the latter threshold may be related to the phenomenon of saturation of impulsively generated vortices. In the presence of the cross-flow, the penetration height of a synthetic jet is found to depend on the momentum- flux ratio . When this ratio is defined in terms of the maximum jet velocity and the cross-flow velocity. not only all data collapse but also the jet trajectory is predicted well by correlation equation available for steady jets-in-cross-flow. Distributions of mean velocity, streamwise vorticity as well as turbulence intensity for a synthetic jet in cross-flow are found to be similar to those of a steady jet-in-cross-flow. A pair of counter-rotating streamwise vortices, corresponding to the bound vortex pair of the steady case, is clearly observed. Mean velocity distribution exhibits a dome of low momentum fluid pulled up from the boundary layer, and the entire domain is characterized by high turbulence.