Synthetic Jet Actuator Research Articles

Large Eddy Simulation of optimal Synthetic Jet Actuation on a SD7003 airfoil in post-stall conditions

Aerodynamic performances may be optimised by the appropriate tuning of Active Flow Control (AFC) parameters. For the first time, we couple Genetic Algorithms (GA) with an unsteady Reynolds-Averaged Navier-Stokes (RANS) solver using the Spalart-Allmaras (SA) turbulence model to maximise lift and aerodynamic efficiency of an airfoil in stall conditions [1], and then validate the resulting set of optimal Synthetic Jet Actuator (SJA) parameters against well-resolved three-dimensional Large Eddy Simulation (LES). The airfoil considered is the SD7003, at the Reynolds number Re=6×104 and the post-stall angle of attack α=14∘. We find that, although SA-RANS is not quite as accurate as LES, it can still predict macroscopic aggregates such as lift and drag coefficients, provided the free-stream turbulence is prescribed to reasonable values. The sensitivity to free-stream turbulence is found to be particularly critical for SJA cases. Baseline LES simulation agrees well with literature results, while RANS-SA would seem to remain a valid model to a certain degree. For optimally actuated cases, our LES simulation predicts far better performances than obtained by suboptimal SJA LES computations as reported by other authors [2] for the same airfoil, Re and α, which illustrates the applicability and effectiveness of the SJA optimisation technique applied, despite using the less accurate yet computationally faster SA-RANS. The flow topology and wake dynamics of baseline and SJA cases are thoroughly compared to elucidate the mechanism whereby aerodynamic performances are enhanced.

Lift enhancement based on virtual aerodynamic shape using a dual synthetic jet actuator

The Dual Synthetic Jet Actuator (DSJA) is used to develop a new type of lift enhancement device based on circulation control, and to control the flow over the two-dimensional (2D) NACA0015 airfoil. The lift enhancement device is composed of a DSJA and a rounded trailing edge (Coanda surface). The two outlets of the DSJA eject two jets (Jet 1 and Jet 2). Jet 1 ejects from the upper trailing edge, which increases the circulation of airfoil with the help of the Coanda surface. Jet 2 ejects from the lower trailing edge, which acts as a virtual flap. The Reynolds number based on the airfoil chord length and free flow velocity is 250000. The results indicate that the circulation control method based on Dual Synthetic Jet (DSJ) has good performance in lift enhancement, whose control effect is closely related to momentum coefficient and reduced frequency. With the increase of the reduced frequency, the control effect of the lift enhancement is slightly reduced. As the momentum coefficient increases, the control effect becomes better. When the angle of attack is greater than 4°, the increments of lift coefficients under the control of DSJ are larger than those under the control of the steady blowing at a same momentum coefficient. The maximum lift augmentation efficiency can reach 47 when the momentum coefficient is 0.02, which is higher than the value in the case with steady blowing jet circulation control.

Influence of Strouhal number and phase difference on the flow behavior of a synthetic jet array

A computational study is presented to characterize the flow behavior of independently controlled multiple synthetic jet actuators (SJA). For fixed geometric configuration and Reynolds number (Re) 300, the influence of Strouhal number (St=0.028–0.172) and phase difference (∅=0°–180°) between the actuators on the evolution, and interaction of the vortices are highlighted. Directivity plots are employed to illustrate the effect of ∅ on the vectoring behavior of a synthetic jet array (SJ array). It has been observed that a high jet vectoring angle (β) is achieved while operating the SJ array at low St. The vectoring angle seems to be independent of ∅ for the low and high St. However, for an intermediate St=0.086, the vectoring angle varies with the ∅. The phase averaged vorticity contour for St=0.028 reveals that the evolution of the anti-clockwise vortex from the leading actuator (SJA1) decides the vectoring of the jet. By contrast, for higher Sts(0.115 and 0.13), the remnant vortices play a significant role in vectoring the jets toward the leading actuator. Based on the cross-stream distribution of time-averaged streamwise velocity, three distinct flow regimes are characterized: near-field, intermediate-field, and far-field. The strength of the jet is quantified by the downstream distribution of the jet momentum flux. Proper evolution of the vortices results in the enhancement of the jet momentum flux. It is recommended to operate the SJ array at an intermediate St (0.086) to vary the jet vectoring angle with phase differences as well as to achieve maximum jet momentum flux.

Study on propagation mechanisms of the actuations generated by plasma synthetic jet actuator in a supersonic flow

The plasma synthetic jet actuator (PSJA) has a potential application in the flow control, especially for the supersonic and hypersonic flow. In order to make a deeper understanding on the propagation processes of the actuation generated by the PSJA in a supersonic flow, both the schlieren experiment and the numerical simulation are adopted for the analyses. Based on the analytic geometry method, the propagation of the diffracted wave, the synthetic jet and the dis-charged heat are discussed in detail. The effects of the mainstream Mach number and the discharge strength on the propagation processes are also deeply analyzed. Besides, an analytic model and relevant motion equations are established for the forecast of the actuations. The results suggest that the 1st diffracted wave expands around and propagates along the mainstream direction. The synthetic jet will change the start point of the oblique wave. Further, the propagation of the dis-charged heat is also related to the mainstream.

Control of flow separation over a wing model with plasma synthetic jets

An array of 30 plasma synthetic jet actuators (PSJAs) is deployed using a modified multichannel discharge circuit to suppress the flow separation over a straight-wing model. The lift and drag of the wing model are measured by a force balance, and the velocity fields over the suction surface are captured by a particle imaging velocimetry system. Results show that the flow separation of the straight wing originates from the middle of the model and expands towards the wingtips as the angle of attack increases. The flow separation can be suppressed effectively by the PSJAs array. The best flow control effect is achieved at a dimensionless discharge frequency of F + = 1, with the peak lift coefficient increased by 10.5% and the stall angle postponed by 2°. To further optimize the power consumption of the PSJAs, the influence of the density of PSJAs on the flow control effect is investigated. A threshold of the density exits (with the spanwise spacing of PSJAs being 0.2 times of the chord length in the current research), below which the flow control effect starts to deteriorate remarkably. In addition, for comparison purposes, a dielectric barrier discharge (DBD) plasma actuator is installed at the same location of the PSJAs. At the same power consumption, 4.9% increase of the peak lift coefficient is achieved by DBD, while that achieved by PSJAs reaches 5.6%.

Experimental and Numerical Investigations of Modified Synthetic Jet Actuator for Active Flow Control

This paper presents the results of experimental and numerical investigations on synthetic jet actuators with various arrangements. First, an actuator with one membrane in the cavity is a standard, reference arrangement; and second, one novel modified arrangement has two membranes in the cavity. Authors use constant temperature-anemometry measurements to identify the characteristic frequencies in which the output jet velocity reaches a maximal value, namely, the resonant frequency of the actuator (Helmholtz frequency) and of the membrane (structural resonance). A numerical model of a synthetic jet actuator has been validated using experimental data for the case with one membrane in a cavity. The simulation results confirm the existence of a toroidal vortex at the actuator exit for the frequencies determined in the experiment. Numerical results of a modified actuator with two membranes in the cavity give promising results in the context of separation control by synthetic jet devices, i.e., increase of output velocity and vorticity from the same occupied space is obtained. Numerical simulations show that in the modified actuator with two membranes in the cavity, the jet output velocity and vorticity of a created toroidal vortex at the actuator outlet can be increased by 80%, compared to the case with one membrane. The new geometry of a synthetic jet actuator with perpendicular membranes is to be implemented into the wind tunnel measurements to prove the use of a modified synthetic jet actuator as a device applicable for active flow control.

Active flow control using deep reinforcement learning with time delays in Markov decision process and autoregressive policy

Classical active flow control (AFC) methods based on solving the Navier–Stokes equations are laborious and computationally intensive even with the use of reduced-order models. Data-driven methods offer a promising alternative for AFC, and they have been applied successfully to reduce the drag of two-dimensional bluff bodies, such as a circular cylinder, using deep reinforcement-learning (DRL) paradigms. However, due to the onset of weak turbulence in the wake, the standard DRL method tends to result in large fluctuations in the unsteady forces acting on the cylinder as the Reynolds number increases. In this study, a Markov decision process (MDP) with time delays is introduced to model and quantify the action delays in the environment in a DRL process due to the time difference between control actuation and flow response along with the use of a first-order autoregressive policy (ARP). This hybrid DRL method is applied to control the vortex-shedding process from a two-dimensional circular cylinder using four synthetic jet actuators at a freestream Reynolds number of 400. This method has yielded a stable and coherent control, which results in a steadier and more elongated vortex formation zone behind the cylinder, hence, a much weaker vortex-shedding process and less fluctuating lift and drag forces. Compared to the standard DRL method, this method utilizes the historical samples without additional sampling in training, and it is capable of reducing the magnitude of drag and lift fluctuations by approximately 90% while achieving a similar level of drag reduction in the deterministic control at the same actuation frequency. This study demonstrates the necessity of including a physics-informed delay and regressive nature in the MDP and the benefits of introducing ARPs to achieve a robust and temporal-coherent control of unsteady forces in active flow control.

Experimental and numerical investigation of compressibility effects on velocity derivative flatness in turbulence

Compressibility effects on the velocity derivative flatness F∂u′/∂x are investigated by experiments with opposing arrays of piston-driven synthetic jet actuators (PSJAs) and direct numerical simulations (DNS) of statistically steady compressible isotropic turbulence and temporally evolving turbulent planar jets with subsonic or supersonic jet velocities. Experiments using particle image velocimetry show that nearly homogeneous isotropic turbulence is generated at the center of a closed box from interactions between supersonic synthetic jets. The dependencies of F∂u′/∂x on the turbulent Reynolds number Reλ and the turbulent Mach number MT are examined both experimentally and using DNS. Previous studies of incompressible turbulence indicate a universal relationship between F∂u′/∂x and Reλ. However, both experiments and DNS confirm that F∂u′/∂x increases relative to the incompressible turbulence via compressibility effects. Although F∂u′/∂x tends to be larger with MT in each flow, the F∂u′/∂x in the turbulent jets and the turbulence generated from PSJAs deviate from those in incompressible turbulence at lower MT compared with isotropic turbulence sustained by a solenoidal forcing. The PSJAs and supersonic planar jets generate strong pressure waves, and the wave propagation can cause an increased F∂u′/∂x, even at low MT. These results suggest that the compressibility effects on F∂u′/∂x are not solely determined from a local value of MT and depend on the turbulence generation process.

Comparison of the Axial Fan and Synthetic Jet Cooling Systems

Choosing the right cooling device is crucial for the proper operation of electronic equipment. A comparison of the two different cooling devices is presented in this paper: one with a standard axial fan and the other with a synthetic jet actuator. Two distinct sets of operating conditions of the fan and two different loudspeakers for the synthetic jet actuator were used. The experimental setup consisted of a radial heat sink mounted onto a round electric heater and two cooling systems: one with the axial fan and the other with a synthetic jet actuator. From the thermal balance in the specified control volume, the heat sink’s thermal resistance. as well as the coefficient of performance, were determined. The highest difference between the thermal resistance of both cooling systems occurred at a low input power of P = 0.5 W. The heat sink cooled with a synthetic jet had the thermal resistance of R = 0.39 K/W, while the same heat sink cooled with a fan achieved R = 0.23 K/W. Thus, the fan cooling exhibited almost 70% better performance than synthetic jet cooling. For a higher input power of P = 7.0 W, the relative difference in the thermal resistance decreased to the value of 42%. For the input power of P = 7.0 W, the fan-cooled heat sink dissipated the thermal power of Q˙HS=487 W under the temperature difference between the heat sink base and ambient air equal to 60 K. For the same input power and temperature difference, the synthetic jet cooling of the same heat sink dissipated a thermal power of Q˙HS=339 W. Under natural convection, the heat sink dissipated the thermal power of Q˙HS=57 W. Thus, the heat transfer enhancement with fan cooling relative to natural convection was equal to 8.5, while the enhancement with synthetic jet cooling relative to natural convection was equal to 6.0. The modified coefficient of performance and the heat transfer rate of the heat sink per unit temperature difference and unit volume of the cooling device ε are presented. The axial fan performed better in terms of both parameters under consideration. The ε of the investigated device with a fan was around four times higher than in the case of the synthetic jet actuator and eight times higher than in the case of natural convection.

Aerodynamic Efficiency Improvement on a NACA-8412 Airfoil via Active Flow Control Implementation

The present paper introduces a parametric optimization of several Active Flow Control (AFC) parameters applied to a NACA-8412 airfoil at a single post-stall Angle of Attack (AoA) of 15∘ and Reynolds number Re = 68.5×103. The aim is to enhance the airfoil efficiency and to maximize its lift. The boundary layer separation point was modified using Synthetic Jet Actuators (SJA), and the airfoil optimization was carried on by systematically changing the pulsating frequency, momentum coefficient and jet inclination angle. Each case has been evaluated using Computational Fluid Dynamic (CFD) simulations, being the Reynolds Averaged Navier–Stokes equations (RANS) turbulence model employed the Spalart Allmaras (SA) one. The results clarify which are the optimum AFC parameters to maximize the airfoil efficiency. It also clarifies which improvement in efficiency is to be expected under the operating working conditions. An energy balance is presented at the end of the paper, showing that for the optimum conditions studied the energy saved is higher than the one needed for the actuation. The paper clarifies how a parametric analysis has to be performed and which AFC parameters can be initially set as constant providing sufficient previous knowledge of the flow field is already known. A maximum efficiency increase versus the baseline case of around 275% is obtained from the present simulations.

A novel three-dimensional center energy deposition model for the hybrid synthetic jet actuator

The low energy conversion efficiency of the plasma synthetic jet actuator (PSJA) and the misfiring problem caused by the low gas backfill rate have hindered the application of the PSJA in supersonic aircraft for fast response aerodynamic control. Although the hybrid synthetic jet actuator (HSJA) based on the PSJA has the potential to solve these problems, the heat and mass transfer mechanism of hybrid synthetic jets is still unclear. According to the capacitive discharge process, a novel three-dimensional center energy deposition model was proposed in this paper. The accuracy of the three-dimensional center energy deposition model was verified by comparing it with the experimental results. Considering the damped oscillation of the discharge intensity, the effect of nonconstant center energy deposition on the heat and mass transfer inside the cavity was further discussed. The simulation results show that the electrodes have an impact on the creation and evolution of the vortices inside the cavity. And the three-dimensional nonconstant center energy deposition model reveals the real heat and mass transfer mechanism of the HSJA, which provides the theoretical foundation for improving energy conversion efficiency and gas backfill rate of the HSJA.

Interaction of synthetic jets with a massively separated three-dimensional flow field

The interaction of an array of finite span synthetic jet actuators with a massively separated flow was explored experimentally over a cantilevered, swept, and tapered model having a deflected control surface. The interaction of one synthetic jet with the local flow led to a flow field associated with formation and advection of vortex rings that bent into the flow, resulting in flow reattachment in their trajectory. Under specific conditions, when multiple jets were actuated, their interaction was either constructive or distractive, depending on the separation severity and spanwise flow magnitude. This work improves control of three-dimensional flow separation using distributed actuators.

Role of phase-difference between two adjacent rectangular synthetic jet actuators in active control of flow over a rounded ramp

In the current study, the role of phase-difference between signals of two adjacent synthetic jet actuators (SJAs) in active control of flow over a rounded ramp geometry has been investigated. In order to accurately predict the separation and reattachment locations, wall-resolved large eddy simulation has been utilized to capture the locations of separation and reattachment. The two adjacent SJAs were placed upstream of the separation point. Six phase-differences between the two SJAs were considered, and two momentum coefficients were applied. First, the role of phase-difference in active flow control of a separation bubble behind a ramp-down region was investigated. Furthermore, the impact of an increased momentum ratio on the size and length of the separation zone was investigated to assess the effectiveness of phase-difference with respect to a higher velocity ratio. The effect of increased momentum ratio on the wall pressure fluctuations was also explored. As the second objective of this study, the flow and turbulent features were discussed to unveil the SJA actuation impact on the downstream flow. The time-averaged velocity and turbulent kinetic energy profiles and the turbulent production were examined and compared to the uncontrolled baseline case. It was found that a higher velocity ratio tremendously increased the turbulent energy before the separation point, while further downstream, the level of turbulent energy was uncoupled from the SJA momentum coefficient. Our study showed that by increasing the momentum ratio, the role of phase-difference in reducing the separation thickness lessened. Nevertheless, applying either a positive or a negative phase-difference of pi/2 still postponed the separation point.

Effect of varying frequency of a synthetic jet on flow separation over an airfoil

An experimental investigation on the effects of the synthetic jet actuator (SJA) was conducted on a National Advisory Committee for Aeronautics 0025 airfoil in a low-speed recirculating wind tunnel at a chord Reynolds number of 100 000 and at an angle of attack 12°. Particle image velocimetry was used to visualize the flow separation for the uncontrolled baseline flow, and the flow attachment for the SJA controlled flows. The location of the SJA was at −1.3% from the separation point, and a blowing ratio of 0.8 was chosen for this study. The blowing ratio proved to be effective in suppressing the separation of the flow. The reduced frequency (Ste) was varied between 1, 2, 14, and 58. The momentum bursts from the SJA based on the reduced frequency determined the effectiveness of the control method. The Reynolds stresses and turbulence production decreased dramatically with increasing frequency up to the shear layer frequency (Ste= 14), but further excitation (Ste= 58) resulted in a regain of turbulence levels. Proper orthogonal decomposition was performed which showed that the low frequency operations globally affect the modes in the shear layer while the high frequency operations are confined to the airfoil surface.

Synthetic Jet Actuator Techniques in Quiescent Flow Condition: A Review

Synthetic Jet Actuator Techniques in Quiescent Flow Condition: A Review

Turbulence generated by an array of opposed piston-driven synthetic jet actuators

Turbulence generated by an array of opposed piston-driven synthetic jet actuators

Experimental and CFD Characterization of a Double-Orifice Synthetic Jet Actuator for Flow Control

The paper presents a joint experimental and numerical characterization of double-orifice synthetic jet actuators for flow control. Hot-wire measurements of the flow field generated by the device into a quiescent air environment were collected. The actuation frequency was systematically varied to obtain the frequency response of the actuator; its coupled resonance frequencies were detected and the velocity amplitude was measured. Direct numerical simulations (DNS) of the flow field generated by the device were subsequently carried out at the actuation frequency maximizing the jet output. The results of a fine-meshed parametric analysis are outlined to discuss the effect of the distance between the orifices: time-averaged flow fields show that an intense jet interaction occurs for small values of the orifice spacing-to-diameter ratio; phase-averaged velocity and turbulent kinetic energy distributions allow to describe the vortex motion and merging. A novel classification of the main regions of dual synthetic jets is proposed, based on the time- and phase-averaged flow behaviour both in the near field, where two distinct jets converge, and in the far field, where an unique jet is detected. The use of three-dimensional DNS also allows to investigate the vortex merging for low values of the jet spacing. The work is intended to provide guidelines for the design of synthetic jet arrays for separation control and impinging configurations.

Implementation of Continuous Blowing and Synthetic Jet Actuators to Control the Flow Separation over a Fully Stalled Airfoil

Continuous blowing and synthetic jet actuators were implemented to investigate their effects on a fully stalled airfoil. An opening tangential to the boundary layer configuration was installed over the suction surface of the Selig-Donovan airfoil at the angle of attack of 16° and Reynolds number of 60,000. An optimization analysis was carried out to look for the optimum operational design point. Genetic algorithm, artificial neural network, and computational fluid dynamic simulations were combined to perform the optimization. Inserting location, opening diameter, velocity amplitude, and synthetic jet frequency were considered as design variables. Results indicated a significant improvement in aerodynamic characteristics, performance, and lift and drag coefficients. Using unsteady actuation caused a better improvement in aerodynamic characteristics compared to the steady case and also led to a remarkable reduction in the applied momentum coefficient. Contours of different flow field parameters were depicted for both cases and their similarities and dissimilarities were identified. Moreover, the synthetic jet actuator displayed a lower increase in the friction coefficient than the continuous blowing actuator. Therefore, it showed a higher performance improvement in comparison with the continuous blowing jet.

Parametric study of high-frequency characteristics of plasma synthetic jet actuator

A major issue of plasma synthetic jet actuator (PSJA) is the severe performance deterioration at high working frequency. In this study, experiments and numerical simulation are combined together to investigate the influence of thermal conductivity, throat length (L th) and discharge duration (T d) on the high-frequency characteristics of PSJA. Results show that the variation of the actuator thermal conductivity and discharge duration will not alter the saturation frequency of the actuator, whereas decreasing the throat length results in an increase of the saturation frequency. For a short-duration capacitive discharge of 1.7 μs, a clear shock wave is issued from the orifice, followed by a weak jet. As a comparison, when the discharge duration is increased up to 202.6 μs, a strong jet column is formed and no obvious shock wave can be visualized. Based on numerical simulation results, it becomes clear that the long-duration pulse-DC discharge is able to heat the cavity gas to a much higher temperature (3141 K) than capacitive discharge, greatly improving the conversion efficiency of the arc discharge energy to the internal energy of the cavity gas. In addition, high-speed Schlieren imaging is deployed to study the performance degradation mechanism of PSJA at high working frequency. Monitor of the exit jet grayscale indicates that as long as the saturation frequency is exceeded, the actuator becomes unstable due to insufficient refresh time. The higher the discharge frequency, the more frequently the phenomenon of ‘misfires’ will occur, which explains well the decaying jet total pressure at above saturation frequency.

Simplified Steady-State Representation of Slot Synthetic Jet Actuators to Enable Numerical Optimization With Steady RANS Simulations

To address the growing energy use of data centers, waste heat recuperation offers a solution to better integrate these facilities into the broader energy system, thus facilitating a transition to the decarbonization of the energy system. The use of liquid coolants for full immersion cooling or local heat extraction from high power density components is considered for this purpose. However, heat can also be extracted by novel air cooling approaches, perhaps in combination with localized liquid cooling. To optimize heat extraction from air-cooled systems and maximize the heat grade, synthetic jets (SJs) can be used for targeted adaptive cooling in conjunction with air ducting to facilitate maximum heat recuperation potential in rack or server-mounted air-to-liquid heat exchangers. Internal server layouts can be optimized numerically, e.g., using a multiple objective genetic algorithm approach based on minimization of entropy generation rates. However, since SJs are inherently transient flow phenomena, this would require transient flow simulations, which forms a bottleneck in a numerical optimization loop. This research aims to develop a simplified steady-state representation of a synthetic jet actuator (SJA) with slot orifice using a localized body force to generate a similar time-averaged flow field to a real SJA, suitable for steady Reynolds-averaged Navier–Stokes simulations within a numerical optimization loop. Both flow fields are compared in terms of the mean flow field, jet spreading rate, and turbulence intensity distributions.