Synthetic Jet Actuator Research Articles

Impingement heat transfer to the synthetic jet issuing from a nozzle with an oscillating cross section

This experimental study is focused on a rectangular synthetic (zero-net-mass-flux) jet impinging on a wall. The working fluid is air. A novel variant of a synthetic jet actuator (SJA) is proposed. Following biomimetic principles, the actuator incorporates a nozzle in which the cross-sectional area periodically oscillates during the driven cycle. Two independent actuating systems are used to drive the main SJA diaphragm (as seen in common SJAs with rigid nozzles) and the oscillating nozzle walls (unique to this study). Namely, the diaphragm is driven electrodynamically, and the nozzle walls are driven using a pair of piezoelectric transducers. The operating frequency of 62 Hz is selected by tuning both systems at their resonances. The nozzle slot width oscillates around an average value of approximately 6 mm. Four experimental methods were used: phase-locked visualization of the oscillating nozzle lips, direct measurement of the jet momentum flux using a precision scale, hot-wire anemometry, and an evaluation of the heat transfer coefficients on the exposed wall using heat flux sensors. The local distribution of the heat transfer coefficient on the wall (Nusselt number in a dimensionless form) indicated the three-dimensional character of the flow field with an axis-switching effect. It was concluded that the momentum flux and the heat transfer rate could be enhanced by the phase delay between the diaphragm and nozzle cycles. The reason is the fact that the nozzle cross-sectional area during the extrusion stroke is smaller than the area during the suction, thus the SJ velocity and momentum flux are promoted during extrusion while the losses are reduced during suction. The maximal effect was found at the phase shift of 270° for the ratio of the cross-section areas during the extrusion and suction of 0.74.

Effect of geometric parameters on synthetic jet: A review

A synthetic jet actuator is a fluidic device that produces a jet flow by the periodic ingestion of fluid into and expulsion of fluid out of a cavity across an orifice. Since such a mechanism transfers linear momentum to the fluid without introducing a net mass into the system over an actuation cycle, the synthesized jet is also termed a zero-net-mass-flux jet. Over the last two decades, synthetic jets have been the subject of intense research. It has been shown that the geometric parameters of a synthetic jet actuator can strongly influence the flow characteristics and performance of synthetic jets. The aim of this paper is to provide a comprehensive review of the influence of the geometric parameters of a synthetic jet actuator on the characteristics and performance of synthetic jets. These parameters include the height and diameter of the cavity and the orifice and the shape and edge configuration of the orifice.

Experimental analyses of synthetic jet control effects on aerodynamic characteristics of helicopter rotor

ABSTRACTExperimental analyses of synthetic jet control (SJC) effects on aerodynamic characteristics of rotor in steady state and in hover were conducted. To ensure the structural strength of rotor and enough interior space for holding the synthetic jet actuators (SJAs), a particular blade with a frame-covering structure was designed and processed, and the experiment was conducted with low free stream velocities and rotor rotation speeds. There were three test conditions. In steady state, there were three free stream velocities (10m/s, 15m/s and 20m/s). In hover state, the rotor was worked with two rotation speeds of 180RPM and 240RPM. In forward flight, the rotor was worked with a rotation speed of 180RPM and a free stream velocity of 7.5m/s. To measure the synthetic jet control effect on rotor in stall, the range of collective pitch was set from 10° to 28° in steady state. The aerodynamic forces and sectional velocity field were measured by using the six-component balance and the Particle Image Velocimetry (PIV) system in the wind tunnel. Flow control effects on the blade based on the synthetic jets (SJ) were experimentally investigated with different jet parameters, such as jet locations, jet angles, and jet velocities. In steady state, the jet closer to the leading edge, and the jet angle of 90° had more advantages in improving the aerodynamic characteristics. Furthermore, the aerodynamic forces and sectional velocity field measurement of rotor in hover were conducted, it showed that SJAs could increase flow velocity at the upper surface, which led to lower upper surface pressure. As a result, the normal forces of rotor with two rotation speeds were increased significantly. These results indicated that the synthetic jet has a capability of increasing the normal force and delaying or preventing the stall of rotor.

Aspekty akustyczne strugi syntetyzowanej – przegląd

Synthetic jet finds more and more applications in technology and science. It is generated most often by loudspeakers or piezoelectric elements. The loudspeakers provide greater velocity momentum, but also generates more noise, which can be problematic in commercial applications of the synthetic jet actuator. The paper reviews articles on the aspect of the sound level generated by the synthetic jet actuators and show the direction of future development in this field.

Cephalopod‐Inspired Swimming Robot Using Dielectric Elastomer Synthetic Jet Actuator

Jet propulsion is the main method of locomotion developed by cephalopods to swim through water, either for hunting or escaping from predators. Under this inspiration, diverse underwater robots utilizing jet propulsion‐based locomotion are studied. This article presents a cephalopod‐inspired robot based on a dielectric elastomer actuator, utilizing jet‐propulsion actuation. The actuator is designed and optimized under the guidance of a corresponding electromechanical model. Then, the flow field characteristics of the synthetic jet actuator are simulated and analyzed. Equipped with the actuator, the bioinspired robot can locomote either with a speed of 0.66 body length per second on the surface of the water by jetting air or with a speed of 0.43 body length per second while almost completely submerged underwater by jetting water. The jet actuator presents even more environmental adaptability, which powers dual swimming locomotion by jetting two flow media, and can potentially be applied to the design of underwater robots.

Active flow control for supersonic aircraft: A novel hybrid synthetic jet actuator

In order to improve the performance of traditional synthetic jet actuators in active flow control for supersonic aircraft, a novel millimeter-scale hybrid synthetic jet actuator (HSJA) is proposed in this paper. It provides additional power for the jet spout stage and gas refresh stage by adding a piezo-driven diaphragm at the bottom of the plasma synthetic jet actuator (PSJA). On the one hand, the problem of the low jet velocity of the piezo-driven synthetic jet actuator (PDSJA) could be avoided due to the heating and pressurization of gas by arc discharge. On the other hand, more external gas would be refilled during the gas refresh stage due to the restoration of the piezo-driven diaphragm compared with the plasma synthetic jet actuator. In an attempt to optimize the structure of the novel hybrid synthetic jet actuator, the flow field of the actuator is numerically simulated. The results show that the peak velocity of synthetic jet and the gas refilled rate are both increased compared with the plasma synthetic jet actuator. The effectiveness of the proposed hybrid synthetic jet actuator is verified.

A novel spray cooling device based on a dual synthetic jet actuator integrated with a piezoelectric atomizer

To solve the heat dissipation problem of electronic equipment, a novel spray cooling device based on a dual synthetic jet actuator integrated with a piezoelectric atomizer (DSJAPA) is proposed and designed. The flow field, spray characteristics and cooling performance of DSJAPA are investigated experimentally. The experimental results show that the spray angle increases with the increase of driving voltage, and it can achieve the maximum of 74°. Under a certain driving voltage, there exists an optimum driving frequency making the spray angle reach to the maximum. The spray flow field is investigated by droplet image velocimetry (PIV) system. The results show that the spray droplet velocity can be accelerated to about 22.5 m/s by the high-speed ejection of the jet. With the increase of dual synthetic jet velocity, the dense spray with low velocity changes into the dilute spray with high velocity. A dual synthetic jet actuator can improve the cooling capability of spray, reduce the temperature non-uniformity of surface (from 32 °C in spray cooling to 18 °C in DSJAPA cooling) and enlarges the direct impingement range (from (−30 mm, 25 mm) in spray cooling to (−60 mm, 57 mm) in DSJAPA cooling).

New combinational active control strategy for improving aerodynamic characteristics of airfoil and rotor

In order to improve the aerodynamic characteristics of rotor, a new active flow control strategy by combining a synthetic jet actuator and a variable droop leading-edge or a trailing-edge flap has been proposed. Their control effects are numerically investigated by computational fluid dynamics (CFD) method. The validated results indicate that variable droop leading-edge and synthetic jet can suppress the formation of dynamic stall vortex and delay flow separation over rotor airfoil. Compared with the baseline state, Cdmax and Cmmax are significantly reduced. Furthermore, parametric analyses on dynamic stall control of airfoil by the combinational method are conducted, and it indicates that the aerodynamic characteristics of the oscillating rotor airfoil can be significantly improved when the non-dimensional frequency ( k*) of variable droop leading-edge is about 1.0. At last, simulations are conducted for the flow control of rotor by the combinational method. The numerical results indicate that large droop angle of variable droop leading-edge can better reduce the torque coefficient of rotor and the trailing-edge flap has the capability of increasing the thrust of rotor. Also, the synthetic jet could further improve the aerodynamic characteristics of rotor.

Consensus-Based Decentralized Aerodynamic Moment Allocation Among Synthetic Jets and Control Surfaces

In this brief, a decentralized allocator is presented for synthetic jet actuators (SJAs) and control surfaces onboard a small unmanned aerial vehicle to cooperatively generate desired aerodynamic moments. In this brief, the first-order dynamics are assumed for both the SJAs and control surfaces. A weighted consensus algorithm with limited feedback is used for the aerodynamic moment contribution allocator considering constraints. The proposed allocator is validated via simulations.

Optimization of Flow Control Parameters Over SD7003 Airfoil With Synthetic Jet Actuator

Abstract An optimization study was conducted to find the optimum operational characteristics of a synthetic jet actuator (SJA) to postpone the static stall separation over an SD7003 airfoil at Reynolds number of 60,000. A genetic algorithm (GA) coupled with an artificial neural network (ANN) was employed. Aerodynamic performance (L/D) was chosen as the objective function. Both tangent to the boundary layer (TBL) and the cross to the boundary layer (CBL) configurations of SJA were used and their effectiveness in separation control were compared. The following design variables of the SJA were allowed to change within a predetermined range: location, the opening length, the injection velocity amplitude, the injection angle, and the nondimensional frequency. It was found that for location, opening length, and velocity amplitude ratio, a narrow range near the peak optimum values achieved the best performance. However, for the nondimensional frequency and jet injection angle, the optimum values providing highest performance were in a wider range of values. Activation of SJ actuator improved the aerodynamic performance of the airfoil significantly. However, TBL configuration of SJA produced superior improvement in aerodynamic performance. The optimum aerodynamic performance achieved by TBL-SJA was 34.4, in comparison to 25.3 for CBL-SJA and 5 for the uncontrolled stalled airfoil at 13 deg angle of attack.

VOC emissions from nail salon products and their effective removal using affordable adsorbents and synthetic jets

Volatile organic compounds (VOCs) like acetone, ethyl and n-butyl acetate, toluene, and formaldehyde have been measured inside nail salons, where they are emitted from nail care products. Exposure to these compounds can have negative health impacts on both workers and costumers, as well as the environment. The objectives of this research were to characterize VOC emissions from typical nail care products and to investigate VOC removal by 3 low-cost adsorbent materials (coco coir, biochar, and activated carbon). The removal studies were performed using acetone as the model VOC in both chamber experiments and mathematical models. The chamber experiments were conducted under both passive and active flow conditions using synthetic jet actuators to determine the effect of local ventilation on the rate of removal. Mass-normalized adsorption rates were determined for acetone using nth-order rate equations. Adsorption rate models were then developed for all adsorbent materials under both flow conditions and used to estimate the effect of low-cost adsorbents in an actual nail salon. Activated carbon provided the best VOC removal compared to the other 2 adsorbent materials tested. The use of active flow with synthetic jets was also shown to enhance adsorption processes for both biochar and activated carbon, but not for coco coir. Model results predicted activated carbon with active flow enhancement could achieve an effective ventilation rate of nearly 300 m3 h−1 if 250 kg were placed in a 1400 m3 salon.

Heat transfer coefficients during the impingement cooling with the use of synthetic jet

The paper presents experimental studies on heat transfer phenomena which occurs during the impingement cooling with the use of synthetic jet. The synthetic jet was created by a special actuator consisting in a plexiglass cavity and a loudspeaker. An air was a working fluid. The effect of synthetic jet actuator geometry and supply parameters on heat transfer enhancement has been investigated. The experiment was performed with the use of classical thermal balance method. Heat transfer coefficients at a stagnation point have been obtained. Measurements were carried out in the range of excitation frequency f = 5 ÷ 600 Hz and of root mean square voltage E = 1, 2, 3, 4, 6 V. Changes of actuator geometry included: orifice diameters d = 9, 15, 24 mm, orifice thickness t = 3, 5, 20 mm and cavity depth H = 20, 40, 60 mm. On the basis of measurements results the correlations for the stagnation point Nusselt number of an axisymmetric impinging synthetic jet characterized by large Reynolds numbers have been obtained. Nusselt number has been determined in the function on scaling parameters: Reynolds number, jet-to-surface spacing x/d and the dimensionless stroke length L0. Reynolds numbers were ranging from 3600 to 22950, x/d = 1 ÷ 20 and L0 = 0.84 ÷ 170.5. The scaling parameters have significantly extended the range of research so far. The obtained Nusselt number correlations provide heat transfer calculations at large Reynolds numbers and are expected to have practical applications.

Numerical investigation of synthetic jets driven by thermoacoustic standing waves

We have carried out a preliminary study of the physical processes leading to the formation of a jet into external quiescent surroundings driven by thermoacoustic standing waves. The standing waves are initiated in a thermoacoustic engine (TAE) utilizing the thermoacoustic effect, and the synthetic jet is produced via a jet ejector where a sudden change in the cross-section is employed. We investigate the characteristics of the proposed synthetic jet actuator using both reduced-order network model and computational fluid dynamics (CFD) simulations. The network technique, which is based on linear thermoacoustic theory, can predict the onset of thermoacoustic instability in the frequency domain. The CFD code solves the fully coupled nonlinear compressible flow equations and enables the time-domain analysis of complex flow patterns, which facilitates comprehension of the jet formation process. Both theoretical analysis and numerical simulations reveal that spontaneous, self-excited oscillations inside the TAE will happen when the temperature ratio is greater than the onset temperature ratio for thermoacoustic instability. CFD simulations further identified the transition from no jet to a clear synthetic jet, which determines the onset temperature ratio for jet formation and the threshold value of a non-dimensional parameter. Finally, we carried out a parametric study to investigate the influence of resonator length and orifice diameter on the onset characteristics as well as other performance parameters including the acoustic intensity, space-averaged mean momentum flux, space-averaged velocity and the jet effectiveness. The proposed thermoacoustically-driven synthetic jet actuator may outperform conventional actuators driven by a piston, loudspeaker or piezoelectric transducers on occasions where the surface temperature is high, and therefore has the potential to be utilized for self-cooling purposes.

Flow past a hollow cylinder with two spanwise rows of holes

The flow over fixed, hollow, rigid circular cylinders with and without two series of equally spaced holes of equal diameter (0.2 cylinder diameter) along the surface of the cylinder has been studied by means of dye injection flow visualization, particle image velocimetry (PIV), and force measurements. Holes are connecting the front and rear stagnation lines or the upper and lower sides (i.e., they are positioned at ± 90° from the stagnation line) of the cylinder. The center-to-center distance between holes was 7-hole diameter. The experimental data were obtained from water channel and wind tunnel measurements performed at Reynolds numbers of 6900 and 20000, respectively. It was found that cylinder with holes located on the surface at an angle ± 90° from the front stagnation line behaves like a double-chamber synthetic jet actuator when subjected to cross-flow. The interaction of synthetic jet arrays with external flow over the side surfaces instigates the three-dimensional instabilities in the near wake and produces a 14.4% drag reduction compared to the drag of a plain cylinder. It was shown that cylinder with holes located on the surface at angles of 0° and 180° from the front stagnation line also instigates the three-dimensional instabilities in the near wake, but the primary mechanism responsible for drag reduction is self-bleed through downstream holes. This system producing drag reduction was found to have a drag coefficient equal to about 78.9% of that for a plain cylinder. Important flow structures inside and behind the cylinders with holes were also identified.

Active Aerodynamic Control of a Separated Flow Using Streamwise Synthetic Jets

LES simulations at Re = 1 × 105 and wind tunnel experiments at Re = 5 × 105 were conducted to investigate the beneficial effect of an active flow control (AFC) technique on the aerodynamic performance of a simplified truck geometry. The paper involves the investigation of a synthetic jet actuator characterized by periodic blowing and suction that defines a zero net mass flux flow control mechanism. The actuation aims to suppress the flow separation occurring at the A-pillar (front rounded corner) of a truck cabin. The work flow is defined as it follows. First, LES at low Reynolds number are conducted for different disposition of the actuation slots. The results show a beneficial effect when the actuation slots are positioned in streamwise direction compared to spanwise (vertical) direction. Second, based on the previous considerations, wind tunnel experiments are conducted to verify and support the numerical findings. Both numerical solutions and experimental data show the same trend and the superiority of the streamwise slots actuation when compared to traditional vertical slot actuation. In particular, this work shows the weakness of a vertical slot actuation, when its location is not optimized. A small change in its positioning greatly worsen the efficacy of the separation control in terms of drag reduction and separation bubble length. The slot location directly affects the length of the separated flow region which its reduction can vary between 40–70% based on the positioning. Conversely, a streamwise actuation, spanning a larger portion of the curvature of a rounded A-pillar, is not affected by this behaviour and contributes up to 80% of the recirculation bubble reduction measured in the unactuated case. The effect of the location change and the orientation of a zero net mass flux jet slot is therefore investigated and discussed in this work.

Experimental investigation on the performance of a novel dual synthetic jet actuator-based atomization device

A synthetic jet actuator is a zero net mass flux device. In order to improve the compactness of spray cooling system, a novel dual synthetic jet actuator-based atomization device (DSJAA) is proposed and designed. In this atomization system, water is atomized by the oscillating dual synthetic jets instead of high-pressure air source. Moreover, no extra power for supplying water is needed. The effects of driving voltage, driving frequency, fluid volumetric flow rate and spray height are investigated on atomization rate, water droplet size, surface temperature and heat flux. The maximum atomization rate is 3.4 L/h at driving frequency of 700 Hz and driving voltage of 210 V. The dimensionless correlations between the physical properties of dual synthetic jet and the size of water droplet are presented, which provides guideline for the design of a dual synthetic jet actuator-based fluid atomizer. The experimental results show that when the resonance frequency of piezoelectric diaphragm is equal to half the Helmholtz frequency of DSJAA, the atomization performance is the best by setting driving frequency to resonance frequency. The best spray height is equal to the fluid stroke length of DSJAA. The cooling capability is mainly influenced by the Reynolds number of dual synthetic jet and volumetric flow rate of water. A correlation for DSJAA cooling is established. A maximum heat flux of the novel spray cooling method reaches as high as 59 W/cm2 in the experiments.

Experimental investigation on the flow regime and impingement heat transfer of dual synthetic jet

An experimental investigation is carried out to analyze the flow regime and impingement heat transfer of a dual synthetic jet (DSJ) actuator for the compact thermal management. The time-averaged flow field of DSJ is observed to consist of four regions, namely, the formation region, merging region, development region and decay region other than three regions of SJ without the merging region. The downstream vorticity-concentrated partitions of the SJ and the DSJ spread outwards at about 22.5° and 26.6°, respectively. The heat transfer characteristic of an impinging DSJ is analyzed by comparing with a synthetic jet (SJ). It is found that the local Nusselt number reaches a maximum at a same dimensionless impingement spacing of H/dh = 5.5 both for the SJ and the DSJ, and the maximum local Nusselt number of the impinging DSJ is 11.4% higher. The maximum area-averaged Nusselt number of the impinging DSJ is 8.6% higher at only half of H/dh than the impinging SJ, which is more preferable for the space-restricted electronic cooling. In addition, the effects of slot dimensions on the heat transfer of the impinging DSJ are studied. The peak Nusselt number is directly proportional to the slot neck length and inversely proportional to the slot aspect ratio at a same H/dh. The area-averaged Nusselt number reduces with the increase of the slot neck length at a smaller H/dh, and varies contrarily at a larger H/dh. The best heat transfer performance of the impinging DSJ is achieved at a midlevel aspect ratio. The results provide a design guideline for the cooling packages of space-restricted electronics.

Influence of the Excitation Frequency and Orifice Geometry on the Fluid Flow and Heat Transfer Characteristics of Synthetic Jet Actuators

Synthetic jet is a novel fluctuating time dependent flow technique that transfers linear momentum to the surroundings by ingesting and expelling fluid from a cavity containing an oscillating diaphragm and is conceivably useful for electronic cooling. In this paper numerical analysis is performed to address the effects of the variation in geometric parameters of orifice and excitation frequency on the synthetic jet fluidic. The moving piezoelectric diaphragm is modeled with constant voltage amplitude boundary condition. Computations are carried out by using COMSOL 5.3a Multiphysics software. The present study focuses on the synthetic jets which are formed from a single cylindrical cavity but with different orifices, such as single-hole, three-holes, single-rectangular slots and three-rectangular slots. The exit areas for single-hole and single rectangular-slot has been chosen as 7.0 mm2, while exit areas for multiple holes and slots has been chosen as 21.0 mm2. The velocity of the synthetic jet reaches a maximum value when the diaphragm is excited at an optimum frequency. In case of single-hole synthetic jet, the optimum frequency is nearly the same as that of single rectangular- slot type orifice; however the optimum frequency for the multi-orifices is lower than that of singleorifice synthetic jet. The quality of the simulation results is verified by grid, time and domain independence studies, and validated with the existing experimental data. The simulation results obtained in this study are remarkable as they provide primary design guidepost for the excitation frequency and orifice shape. The present investigation also indicates the maximum value of average heat transfer coefficient is 86.5 W/m2 K with single rectangular-slot orifice, which is 21% higher as compared with a single-hole orifice. For single rectangular-slot orifice when distance ratio of orifice-to-heater is (Z/b) = 80 the value of heat transfer coefficient is 112.5 W/m2 K which is about 20.3% higher as compared to that of single-hole at (Z/d) = 14 thereby leading to better performance.

Effect of Synthetic Jets Spacing on Flow Separation over Swept, Flapped Airfoils

An experimental study was performed on flow field interactions and aerodynamic performance enhancement due to the activation of an array of synthetic jet actuators on a swept, flapped, modified NACA 0012 airfoil model at two angles of attack (0° and 5°) and two control surface deflection angles (20° and 30°) at a chord-based Reynolds number of 720,000 and sweep angle of 20°. The focus was on understanding the mechanisms for separation reduction and lift enhancement due to the application of the flow control. Activating all the jets resulted in up to increase in the sectional lift, which was measured via an array of surface-mounted pressure ports. Stereoscopic particle image velocimetry revealed the presence of trains of pinched, skewed vortical rings that were created by the synthetic jets and that caused flow reattachment over the control surface. The characteristics of the train of these vortical rings varied with the spacing between active jets, where smaller spacing resulted in the largest modification to the flow field. The proposed mechanisms for separation reduction in a swept cross flow via pinched vortex rings are discussed, revealing further potential for how to leverage synthetic jets for separation postponement and lift enhancement over control surfaces.

Synthetic jet actuator efficiency based on the reaction force measurement

The efficiency of the devices is often a determinant of the technological advancement. The aspects of increasing the efficiency of the devices has been studied and improved over the years. The energetic efficiency of the synthetic jet actuators is often defined as the ratio of the kinetic power to the electric power feeding the actuators. In this paper, a new practical experimental procedure for evaluating a kinetic power based on the measurement of the time-mean reaction force was presented. The new procedure is independent from the actuator type, the number of actuators and orifices, as well as the shape of the orifices. The paper presents a comparison of the SJAs efficiency based on a spatially and temporally averaged synthetic jet momentum velocity and a time averaged reaction force measurements. The results of the efficiency in both methods are convergent. The paper presents the experimental investigation of the efficiency for single and double acting, single and multiple axisymmetric orifices synthetic jet actuators. It was noticed that the double acting synthetic jet actuator achieved 12% higher efficiency than the single acting SJA.