Synthetic Jet Actuator Research Articles

Active flow control of bi-stable asymmetric vortices on a slender body by alternating synthetic jets

A synthetic jet actuator with double nozzles is developed to control the leeward vortices behind the slender body at a high angle of attack. Periodic alternating disturbance at the nose tip of the slender body is continuously provided by the alternating synthetic jets. A series of experiments is conducted in a low speed and low turbulence wind tunnel. The results show that the alternating synthetic jets at the nose tip can achieve a suitable control effect in which the lateral force is eliminated. For a very high angle of attack, the leeward vortices behind the slender body are in the inherently bi-stable state; in this state, the symmetric distribution formation by the leeward vortices is nearly impossible. However, the alternating disturbance of the nose tip can break the bi-stable state and transform it into other two states, which are the unsteady switched state of the leeward vortices and the quasi-steady symmetric state. Varying the control frequency significantly affects the state of the leeward vortex. In the range of the lower control frequency, the leeward vortices switch back and forth and follow the cycle of the alternating disturbance at the nose tip. In the range of the higher control frequency, the leeward vortices are in a stable symmetric distribution state. As the alternating disturbance frequency increases from lower to higher, the switched amplitude of the leeward vortices decreases and the unsteady switched state of the leeward vortices gradually changes to the quasi-steady symmetric state. This amplitude reduction occurs because the position of the leeward vortices is very sensitive to the alternating disturbance of the nose tip and is related to the inertia effect of the fluid.

The predominant effect of stroke length on velocity profiles at the exit of axisymmetric synthetic jet actuators

This paper addresses the effect of stroke length on velocity profiles at the exit of axisymmetric synthetic jet actuators. An actuator with a “short-orifice” nozzle geometry was studied (the length of the sharp-edged orifice nozzle being always shorter than the stroke length). The investigation was carried out using hot-wire experiments and numerical approaches in the range of the Reynolds number from 400 to 5600 and a dimensionless stroke length from 1.5 to 86 (i.e. the Stokes number was from 16 to 102). To quantify the velocity profiles at the actuator exit, the ratio of the two characteristic velocities was used. The former one was evaluated as the time- and spatially-averaged orifice velocity and the latter as the time-mean centerline velocity. It was found that this average-to-centerline velocity ratio depends mainly on the dimensionless stroke length, with a minor dependence on the Reynolds number. An approximation function of this predominant relationship has been proposed. The results of both experimental and numerical approaches are in reasonable agreement showing variation of average-to-centerline velocity ratio in the range of 0.7–1.3.

Response of a Streamwise Vortex–Wall Interaction to Unsteady Forcing

Streamwise vortices exist in a number of engineering flows in both external and internal aerodynamics, often with undesirable effects. An experimental investigation of a streamwise vortex–wall interaction is performed through the use of a vortex-generating wing section and a suspended splitter plate. A synthetic jet actuator is implemented below the vortex, and the response of the vortex to unsteady forcing is evaluated with a focus on spatial amplification through convective instabilities. At low forcing frequencies, a large displacement of the vortex is recorded near the actuator, but with almost no spatial amplification. This behavior agrees qualitatively with transient growth studies of columnar vortices. Within a specific frequency range, it is shown that spatial amplification of the vortex displacement occurs. The range of amplified forcing frequency agrees well with predictions for the Crow instability. The vortex–wall interaction generates a secondary vortex structure that rolls up from the boundary layer. When the actuator displaces the primary vortex structure, this causes a strong dynamic reaction in the secondary structure. This occurs due to a mutual induction mechanism between the two vortices as well as a spatial oscillation of the separation point from which the secondary structure originates.

Investigation of efficient air pollutant removal using active flow control

Efficiency in maintaining indoor air quality is central to the operation of high-performance buildings. The purpose of this work was to investigate the effect of airflow velocities generated by a low-power annular, multi-orifice synthetic jet actuator (SJA) on the degradation rate of a model air pollutant, nitrogen dioxide (NO2), by a photocatalytic surface. In this study, the active flow fields generated by the SJA were first characterized, and the effect of SJA-to-wall distance was analyzed as the airflow impinged on a wall of varying surface textures. Second, the impact of flow characteristics, namely surface velocity and velocity distribution, on the removal rates of NO2 by the photocatalytic surface was investigated in a closed chamber. Results showed that a SJA-to-wall (L) distance of 315 mm had the greatest reduction (dampening) on peak airflow velocities. Also, the surface with the highest roughness used in this study resulted in increased turbulence at the wall surface. The use of the SJA enhanced the removal rate of NO2 compared to passive (control) conditions. Increases in air velocity, however, did not monotonically enhance the removal rate of NO2. The highest removal rate (k = 0.0013 min−1) was measured at L = 315 mm, where the highest velocity dampening was observed. It also corresponded to an average surface velocity of approximately 0.1 m/s across the photocatalytic surface.

LEM Characterization of Synthetic Jet Actuators Driven by Piezoelectric Element: A Review.

In the last decades, Synthetic jet actuators have gained much interest among the flow control techniques due to their short response time, high jet velocity and absence of traditional piping, which matches the requirements of reduced size and low weight. A synthetic jet is generated by the diaphragm oscillation (generally driven by a piezoelectric element) in a relatively small cavity, producing periodic cavity pressure variations associated with cavity volume changes. The pressured air exhausts through an orifice, converting diaphragm electrodynamic energy into jet kinetic energy. This review paper considers the development of various Lumped-Element Models (LEMs) as practical tools to design and manufacture the actuators. LEMs can quickly predict device performances such as the frequency response in terms of diaphragm displacement, cavity pressure and jet velocity, as well as the efficiency of energy conversion of input Joule power into useful kinetic power of air jet. The actuator performance is also analyzed by varying typical geometric parameters such as cavity height and orifice diameter and length, through a suited dimensionless form of the governing equations. A comprehensive and detailed physical modeling aimed to evaluate the device efficiency is introduced, shedding light on the different stages involved in the process. Overall, the influence of the coupling degree of the two oscillators, the diaphragm and the Helmholtz frequency, on the device performance is discussed throughout the paper.

Effect of ambient temperature on piston-type synthetic jet actuator

The effect of the ambient temperature on the piston-type synthetic jet actuator performance is investigated numerically. And the effects of the actuation frequency and orifice diameter on mass flow, cylinder pressure and jet momentum are also analysed under high and normal temperatures as well as the effect of the auxiliary air inlet on synthetic jet actuator performance under high temperature. The results indicated that ambient temperature has a significant effect on synthetic jet actuator performance. As the ambient temperature rises, the mass flow, cylinder pressure and jet maximum momentum decrease significantly. Compared with that at normal temperature, the synthetic jet actuator performance with high actuation frequency or large orifice diameter has large degradation under high ambient temperature. In addition, auxiliary air inlet can significantly improve the suction capability and performance of the synthetic jet actuator.

MHD-RLC discharge model and the efficiency characteristics of plasma synthetic jet actuator

Major factors affecting efficiency of plasma synthetic jet actuator (PSJA) are analyzed based on a new discharge model in the present paper. The model couples the magnetohydrodynamics (MHD) equations with the resistor–inductor–capacitor (RLC) equations, and is able to resolve the time-dependent voltage fall on the sheath region and arc region, which is critical in analyzing energy loss in the heating process. This model is integrated into the commercial CFD software by a two-equation method. Results show that in a typical capacitive discharge at microsecond scale, the maximum energy loss is the sheath energy loss, which accounts for nearly half of the discharge energy, while the radiation loss is less than 5%. The discharge time is an important parameter for the PSJA efficiency. A short discharge time less than 1μs will effectively reduce the sheath energy loss, while a longer discharge time will decrease the thermodynamic efficiency.

Characterization of a Commercial Synthetic Jet Actuator for Air Quality Applications

In this study, a commercial electromagnetic synthetic jet actuator (SJA) was characterized to determine its flow patterns and sphere of influence within a room. Its axisymmetry was also evaluated. The objective of this research is to explore the potential application of this commercial SJA for indoor air quality (IAQ) control and its limitations for such application. A small fan of similar size was also partially characterized. Results showed that the SJA is axisymmetric, and it uses a small power input to impact the airflow over 1 m from the jet exit. Further, while the SJA showed less flow output than the fan in this study, it had significantly greater dynamic pressure per unit power. This feature could be potentially useful for indoor air quality control applications.

Optimal diameter of nozzles of synthetic jet actuators based on electrodynamic transducers

The formulation of the optimization problem was based on the maximization of an objective function, which was alternatively selected from the four integral quantities: volumetric flux, momentum flux, kinetic energy flux, and energetic efficiency. With two potential applications intended (active flow control and heat transfer enhancement), the most appropriate objective function is suggested to be the momentum flux.In the experimental part of this study, an axisymmetric actuator was investigated in several variants. Two transducers (loudspeakers) and nine nozzle diameters were tested for ten supplied power levels. The working fluid was air, and the driving frequency was adjusted near the first resonance of the actuator. The velocity profiles at the actuator nozzle exit were measured by a hot-wire anemometer.In the theoretical part of this study, a lumped element model was used with an assumption of an incompressible working fluid. The optimal diameter of the nozzle was found and its dependence on the chosen objective function was revealed. Moreover, the theoretical results were generalized into explicit relationships between the dimensionless optimal nozzle diameter and the loudspeaker parameters. A maximum overall energetic efficiency of 15% was achieved. Outstanding agreement between the theoretical and experimental results was concluded.

Interaction between plasma synthetic jet and subsonic turbulent boundary layer

This paper experimentally investigates the interaction between a plasma synthetic jet (PSJ) and a subsonic turbulent boundary layer (TBL) using a hotwire anemometer and phase-locked particle imaging velocimetry. The PSJ is interacting with a fully developed turbulent boundary layer developing on the flat wall of a square wind tunnel section of 1.7 m length. The Reynolds number based on the freestream velocity (U∞ = 20 m/s) and the boundary layer thickness (δ99 = 34.5 mm) at the location of interaction is 44 400. A large-volume (1696 mm3) three-electrode plasma synthetic jet actuator (PSJA) with a round exit orifice (D = 2 mm) is adopted to produce high-speed (92 m/s) and short-duration (Tjet = 1 ms) pulsed jets. The exit velocity variation of the adopted PSJA in a crossflow is shown to remain almost identical to that in quiescent conditions. However, the flow structures emanating from the interaction between the PSJ and the TBL are significantly different from what were observed in quiescent conditions. In the midspan xy plane (z = 0 mm), the erupted jet body initially follows a wall-normal trajectory accompanied by the formation of a distinctive front vortex ring. After three convective time scales the jet bends to the crossflow, thus limiting the peak penetration depth to approximately 0.58δ99. Comparison of the normalized jet trajectories indicates that the penetration ability of the PSJ is less than steady jets with the same momentum flow velocity. Prior to the jet diminishing, a recirculation region is observed in the leeward side of the jet body, experiencing first an expansion and then a contraction in the area. In the cross-stream yz plane, the signature structure of jets in a crossflow, the counter-rotating vortex pair (CVP), transports high-momentum flow from the outer layer to the near-wall region, leading to a fuller velocity profile and a drop in the boundary layer shape factor (1.3 to 1.2). In contrast to steady jets, the CVP produced by the PSJ exhibits a prominent spatiotemporal behaviour. The residence time of the CVP is estimated as the jet duration time, while the maximum extent of the affected flow in the three coordinate directions (x, y, and z) is approximately 32D, 8.5D, and 10D, respectively. An extremely high level of turbulent kinetic energy production is shown in the jet shear-layer, front vortex ring, and CVP, of which the contribution of the streamwise Reynolds normal stress is dominant. Finally, a conceptual model of the interaction between the PSJ and the TBL is proposed.

Experimental research on multichannel discharge circuit and multi-electrode plasma synthetic jet actuator

To solve the low efficiency of plasma synthetic jet actuator (PSJA), a multichannel discharge technique based on the concept of voltage relay is put forward and a new multi-electrode plasma synthetic jet actuator (ME-PSJA) is designed. The experiment shows the multichannel discharge technique can enlarge the discharge channel distance by multiplying the discharge channel number without increasing the input voltage. With a 1 nF discharge capacitor, the discharge efficiency of three channels discharge increases by 135% compared with the standard one channel discharge. When the discharge capacitor increases to 0.3 μF, four discharge channels still improve the discharge efficiency by 119% as well. Schlieren flow visualization confirms that ME-PSJA also outperforms the 2-electrode PSJA in terms of jet velocity and duration time, and both are increased by about 50%.

Numerical study on the effects of a synthetic jet actuator on S809 airfoil aerodynamics at different flow regimes and jet flow angles

Due to importance of active flow control methods for future Horizontal axis wind turbines (HAWT), we numerically investigated the effects of Synthetic jet actuators (SJA) on the separation and aerodynamic lift enhancement of a finite span wing with the S809 airfoil. The investigation was performed at a realistic AOA range (0°~25°), which is associated to a wide range of AOA, from an attached flow to a highly separated condition. Also, effects of SJA injection angle on flow characteristics at a highly separated condition were studied. The unsteady simulations were performed using DES turbulence model. The results show that for an attached flow, the SJA triggers an early separation and reduces the lift coefficients. However, for a separated flow, the SJA considerably enhances lift performance. In addition, at small AOAs the lift values fluctuate harmonically over time; however, as the AOA increases and the separation becomes severe, the lift becomes more chaotic. Finally, a study of the present SJA configuration reveals that the SJA can enhance the lift values at a wide range of injected flow angle. However, lift values do not show distinguished pattern for different jet angles at a high AOA.

A 2D Simulation of the Flow Separation Control over a NACA0015 Airfoil Using a Synthetic Jet Actuator

The present study aims to investigate numerically the flow control possibility using a synthetic jet actuation over a bi-dimensional NACA0015 airfoil manoeuvring at a highly turbulent flow (8.9e105 Reynolds to chord number). The 2-D flow behaviour was computed using the ANSYS Fluent commercial code. The so-called Reynolds Averaged Navier-Stocks (RANS) approach has been tested for one (Spalat-Allmaras S-A) and two (K-ε) transport equations for the turbulence modelling. Both present a weakness to predict the stall angle effectively. The S-A lift coefficient slope seems to be the closest to the experimental data. The synthetic jet control exhibits an extraordinary lift coefficient enhancement at high Angles Of Attack (AOA) but seems to be less obvious at low AOA, where the flow is still attached. A synthetic jet of a Strouhal (St = 2) and momentum (Cμ of 0.56%), delays the stall onset from 15 to 19deg with enhancing the lift coefficient by 40%. The actual work has been enriched by studying the effect of the jet’s frequency and momentum on the lift temporal signal. Also, the interaction between the mean flow and the synthetic jet structures topology was undertaken.

Vortex Ring Formation Characteristics in Synthetic Jet due to Changes of Excitation Frequency in the ½-Ball Cavity Actuator

A jet flow that contains vortex ring has a large energy compared to a regular jet. As one of the causes of the aerodynamic drag to the vehicle, the flow separation that occurs behind the bluff body must be controlled, so that aerodynamic drag can be significantly reduced. This study is a basic work on the development of turbulent flow separation control for aerodynamic purpose, especially in the design of the vehicle body. The main objective of this study is to analyze the performance of the synthetic jet (SJA) as one of flow control tool to reduce separation area. To get the maximum performance of the synthetic jet actuator, the research starts by characterizing the actuator. Characterization of ½ ball-shaped cavity is done with excitation frequency changes and orifice diameter of 3, 5 and 8 mm. The study was conducted using computational and experimental methods. The experimental data was obtained by testing synthetic jet actuator with providing sinusoidal signal to drive the membrane and at the orifice end a hotwire probe that is set and plugged into a CTA (Constant Temperature Anemometry) to obtain the speed velocity of the exhaust jet. Computational methods used a commercial CFD software (FLUENT 6.3) with a Reynolds Stress Model as a model of turbulence. Each of these calculations or measurements was conducted under the same conditions. The research result is displayed in frequency testing curve to get the maximum velocity of the jet stream. The results are further indicative of the synthetic jet actuator capability to generate vortex rings. In the experimental results, the determination of ring vortex formation taken from the calculation of the flow velocity, while the CFD simulations, the formation of vortex rings can be seen from the visualization of the flow contour. Vortex ring formed from this ½ -ball cavity, occurred at 3 mm and 5 mm orifice diameter, while the 8 mm orifice diameter cavity cannot form a ring vortex.

Numerical Study of a Novel Piston-type Synthetic Jet Actuator with a Quick-return Characteristic

A novel designed piston-type synthetic jet actuator has been developed, which has a quick-return characteristic. When moving to the Top Dead Centre, the piston in high speed route moves faster than the classical one. Compared with classical piston-type SJA, the pressure ratio of the cavity, jet velocity and the jet momentum of the novel SJA increase significantly, which greatly enhances the performance of the actuator. A parametric study has been carried out focusing on the affection in different actuation frequencies and duty cycle. Results show that the performance of the quick-return piston-type SJA is significantly improved.

Effect of slotted exit orifice on performance of plasma synthetic jet actuator

This study experimentally investigates the influence of exit orifice shape on the performance characteristics of a three-electrode plasma synthetic jet actuator. High-speed Schlieren imaging system and phase-locked two-component PIV measurements are used for flowfield characterisation in quiescent conditions. Two actuator configurations with the same exit area but different exit orifice shape (round orifice and slot orifice) are studied. Results indicate a close correspondence between the shapes of the starting vortex ring with the shapes of the respective exit orifices. For the slot orifice, the elongated starting vortex ring gradually expands during propagation, while its ends become warped. A distinct K–H instability structure is observed, inducing continuous oscillation of the high-speed jet. Compared with the jet from the round orifice, the slot jet has a higher entrainment rate of surrounding air, thus resulting in a lower propagation velocity of the jet front. The exit velocity of PSJA within one period initially shows a rapid increase, then persists at a relatively high level (100–130 m/s), and finally drops with some small-scale oscillations. The oscillation amplitude is less than 10 m/s, and the oscillation period is approximately 600 µs. Under conditions of same exit area, orifice shape has little influence on the variation of the exit velocity.

Design and analysis of a synthetic jet actuator-based fluid atomization device

High-pressure nozzles and ultrasonic atomizers are the two most common devices used to generate sprays. Each of these has some disadvantages, such as controllability in high-pressure nozzles and fluid management challenges in ultrasonic devices. To overcome these limitations, a new atomization technology using a synthetic jet actuator was developed and is presented here. The work includes design and experimental analysis of both the stand-alone synthetic jet actuator and the synthetic jet-based atomization device. The synthetic jet actuator is designed using a model-based approach and characterized by measuring dynamic orifice pressure, diaphragm peak-to-peak displacement, flow rate, and power consumption. Orifice pressure reaches 296 Pa at a flow rate of 16 mL/s and 186 Pa at a flow rate of 37 mL/s for two possible synthetic jet actuator geometries, respectively. Piezoelectric diaphragm displacement reaches 50 µm with a brass substrate thickness of 0.20 mm. The synthetic jet-based atomization device is characterized with high-speed imaging and measurement of water atomization rate. It produces droplets with average sizes of 92–116 µm at maximum rates of 19–28 µL/s, depending on the geometry of the synthetic jet actuator. The outcomes of this work are principles for designing effective synthetic jet-based atomization devices, as well as system-level implementation concepts and control schemes.

Reducing the pressure drag of a D-shaped bluff body using linear feedback control

The pressure drag of blunt bluff bodies is highly relevant in many practical applications, including to the aerodynamic drag of road vehicles. This paper presents theory revealing that a mean drag reduction can be achieved by manipulating wake flow fluctuations. A linear feedback control strategy then exploits this idea, targeting attenuation of the spatially integrated base (back face) pressure fluctuations. Large-eddy simulations of the flow over a D-shaped blunt bluff body are used as a test-bed for this control strategy. The flow response to synthetic jet actuation is characterised using system identification, and controller design is via shaping of the frequency response to achieve fluctuation attenuation. The designed controller successfully attenuates integrated base pressure fluctuations, increasing the time-averaged pressure on the body base by 38%. The effect on the flow field is to push the roll-up of vortices further downstream and increase the extent of the recirculation bubble. This control approach uses only body-mounted sensing/actuation and input–output model identification, meaning that it could be applied experimentally.

A novel ram-air plasma synthetic jet actuator for near space high-speed flow control

As a promising high-speed flow control technique, plasma synthetic jet actuator (PSJA) has the superiorities of requiring no moving parts or flow supplies, extremely fast response, wide frequency band and high efflux speed. However, it has limitations for application: in near space, the air in the cavity which is used to generate the puled plasma jet becomes rare, and the low refill rate often leads to insufficient recovery which limits the working frequency. In order to overcome these limitations, a novel actuator called ram-air plasma synthetic jet actuator (RPSJA) is proposed. Inspired by the ramjet, the principle of this actuator is to take advantage of the tremendous dynamic pressure of the high-speed inflow using an added ram-air inlet. Numerical investigations were conducted to demonstrate the feasibility of such an actuator. The results show that, compared with PSJA, the air in the chamber becomes denser and the refill rate is notably increased owing to the “ram-air effect” of RPSJA. Based on the flow characteristic analysis, a revised actuator with a stepped ram-air inlet is proposed and investigated as well, and the results show that the performance is improved as the stepped height rises.

Analytical Model: Characteristics of Nanosecond Pulsed Plasma Synthetic Jet Actuator in Multiple-Pulsed Mode

AbstractMulti-field coupling simulation method based on the physical principles is used to simulate the discharge characteristics of nanosecond pulsed plasma synthetic jet actuator. Considering the effect of the energy transferring for air, the flow characteristics of nanosecond pulsed plasma synthetic jet actuator are simulated. The elastic heating sources and ion joule heating sources are the two main sources of energy. Through the collisions, the energy of ions is transferred to the neutral gas quickly. The flow characteristics of a series of blast waves and the synthetic jet which erupt from the plasma synthetic jet (PSJ) actuator are simulated. The blast wave not only promotes outward, but also accelerates the gas mixing the inhaled gas from the outside cavity with the residual gas inside the cavity. The performances of PSJ actuator fluctuate in the first three incentive cycles and become stable after that.