Active Flow Control Applications Research Articles

Novel Fluidic Oscillator Evaluation Considering Dimensional Modifications

Although flow mixing and cooling can be greatly enhanced when considering the use of fluidic oscillators (FOs), they are more commonly employed in active flow control (AFC) applications where the injected pulsating flow interacts with the boundary layer, usually in order to delay its separation. In fact, prior to any FO implementation in a given application, it is essential to study the range of frequencies and amplitudes it can generate as a function of the incoming mass flow and its dimensions. This is what is being performed in the present manuscript for a rather novel FO configuration. A numerical study of a standard three-dimensional (3D) FO configuration, and also using a two-dimensional (2D) approach, is initially presented. After comparing the 3D and the 2D results and analyzing the main differences, we modified some of the internal dimensions of the FO in order to evaluate the variation in its dynamic performance. The present results clarify which internal dimensional modifications are more effective in generating larger output frequencies and velocity field variations. Care is taken to analyze the origin of self-sustained oscillations. This paper links, for the first time, the origin of the pressure force oscillations at the feedback channel’s outlet, with the interaction of the mixing chamber central jet and the reverse feedback channel flow at the mixing chamber’s converging walls. A novel equation relating the FO outlet mass flow frequency with the time-averaged FC reverse flow is presented and discussed. In fact, the present study needs to be seen as the continuation of a former one, recently published by authors, where the effects of several Reynolds numbers as well as some different internal dimensions were considered.

Data-Driven Control-Oriented Modeling for Response of Fluidic Thrust Vectoring

Response to control input is of significance to the application of real-time active flow control (AFC). In this paper, a novel data-driven framework is used to discover the underlying physics of the dynamic response process of fluidic thrust vectoring (FTV), a typical application of AFC. In the proposed framework, sparse identification of a nonlinear dynamics (SINDy) algorithm is used to identify the governing equations of the flow control responses of sets of noisy measurement data. The clustering algorithm is then used to seek the generalized coefficients of basis functions for different sets of data, which improve the robustness of the model to noisy measurement data. First, a simulated mechanical system is used to validate the effect of the framework. To simplify the modeling, control performance and characteristics are investigated in a detailed manner. Then a dimensionless parameter ΔCp based on the pressure coefficient is found to exhibit a linear relationship with the vector angle under different working conditions. This parameter is introduced in the proposed framework to model the dynamic process of response to control input. The obtained governing equations can describe the dynamic process accurately based on the validation of testing data. The form of the governing equation is rewritten and analyzed based on the control theory, revealing the physics of this process, which is significant to practical AFC implementation.

Experimental investigation of suction and oscillatory blowing as an alternative for a leading edge slat

High lift devices such as flaps and slats have conventionally served in multi-element wings to enhance aerodynamic performances in general, and lift in particular. However, more innovative solutions are required to address the evolving challenges the aviation industry faces to reach higher lift capabilities. Active flow control (AFC) has been studied for many years but applications have remained strictly experimental and did not reach any widespread industrial use to enhance aerodynamic performance. This experimental study investigates applying a novel AFC concept to a high-lift wing configuration to replace the slat with AFC actuators that incorporate suction and oscillatory blowing (SaOB) into the main element of a thick airfoil. The multi-element wing is equipped with an AFC system to investigate the possibility of replacing the leading edge slat with an AFC actuator. The benefits of having an AFC system acting as a virtual slat are discussed quantitatively, with experimental results, and practically as an efficient solution for designing a lighter and more capable high-lift device. This research aims to characterize the application of AFC to a wing in terms of aerodynamics and energy efficiency.

Analysis of a Novel Fluidic Oscillator under Several Dimensional Modifications

To activate the boundary layer in Active Flow Control (AFC) applications, the use of pulsating flow has notable energy advantages over constant blowing/suction jet injections. For a given AFC application, five parameters, jet location and width, inclination angle, frequency of injection, and the momentum coefficient, need to be tuned. Presently, two main devices are capable of injecting pulsating flow with a momentum coefficient sufficient to delay the boundary layer separation: these are zero-net-mass-flow Actuators (ZNMFAs) and fluidic oscillators (FOs). In the present study, a novel FO configuration is analyzed for the first time at relatively high Reynolds numbers, and fluid is considered to be incompressible. After obtaining the typical linear correlation between the incoming Reynolds number and the outlet flow oscillating frequency, the effects of dimensional modifications on outlet width and mixing chamber wedge inclination angle are addressed. Modifications of the outlet width were observed to create large variations in FO performance. The origin of self-sustained oscillations is also analyzed in the present manuscript and greatly helps in clarifying the forces acting on the jet inside the mixing chamber. In fact, we can conclude by saying that the current FO configuration is pressure-driven, although the mass flow forces appear to be much more relevant than in previously studied FO configurations.

Plasma Actuators Based on Alumina Ceramics for Active Flow Control Applications

Plasma actuators have demonstrated great potential for active flow control applications, including boundary layer control, flow separation delay, turbulence control, and aircraft noise reduction. In particular, the material used as a dielectric barrier is crucial for the proper operation of the device. Currently, the variety of dielectrics reported in the literature is still quite restricted to polymers including Kapton, Teflon, poly(methyl methacrylate) (PMMA), Cirlex, polyisobutylene (PIB) rubber, or polystyrene. Nevertheless, several studies have highlighted the fragilities of polymeric dielectric layers when actuators operate at significantly high-voltage and -frequency levels or for long periods. In the current study, we propose the use of alumina-based ceramic composites as alternative materials for plasma actuator dielectric layers. The alumina composite samples were fabricated and characterized in terms of microstructure, electrical parameters, and plasma-induced flow velocity and compared with a conventional Kapton-based actuator. It was concluded that alumina-based dielectrics are suitable materials for plasma actuator applications, being able to generate plasma-induced flow velocities of approximately 4.5 m/s. In addition, it was verified that alumina-based ceramic actuators can provide similar fluid mechanical efficiencies to Kapton actuators. Furthermore, the ceramic dielectrics present additional characteristics, such as high-temperature resistance, which are not encompassed by conventional Kapton actuators, which makes them suitable for high-temperature applications such as turbine blade film cooling enhancement and plasma-assisted combustion. The high porosity of the ceramic results in lower plasma-induced flow velocity and lower fluid mechanical efficiency, but by minimizing the porosity, the fluid mechanical efficiency is increased.

Aerodynamic force reduction of rectangular cylinder using deep reinforcement learning-controlled multiple jets

The drag and lift forces of rectangular cylinders with four aspect ratios (AR) are mitigated at a Reynolds number of 1000 using deep reinforcement learning (DRL) controlled multiple jet actuators at four corners of the cylinders in this study. Surface pressure probes are set to monitor the flow state, featuring a practical engineering solution. For each jet actuator, the control law is optimized using the powerful capabilities of DRL with the surface pressure signals as the control input and the jet velocities as the control output. Different jet configurations are tested on the rectangular cylinder with AR = 1, 2, 3, and 5 to choose the best multiple jet control configurations. The results have shown that under eight independent jets at the rectangular cylinder corners, the mean drag coefficient is reduced by 77.1%, 39.9%, 52.4%, and 2.8% for the rectangular cylinder with AR = 1, 2, 3, and 5, respectively. Furthermore, the lift fluctuation is reduced by 94.2%, 66.6%, 77.1%, and 59.8%, indicating a significant stabilization of the wake. This configuration with eight independent jets shows the top performance among these jet configurations. This study highlights the significance of the multiple jet actuation and can guide the practical application of DRL-based active flow control of rectangular cylinders.

Stabilizing the square cylinder wake using deep reinforcement learning for different jet locations

We mitigate the drag and lift forces of a square cylinder at three Reynolds numbers of 500, 1000, and 2000 using deep reinforcement learning (DRL) and two different positions of a zero flux jet actuator couple based on computational fluid dynamics simulations. The jet actuators are symmetrically deployed at the leading and trailing corners of the square cylinder and tested at those three Reynolds numbers. Surface pressure probes monitor the flow state featuring a practical engineering solution as compared to velocity sensors in the wake. For each jet actuator position and Reynolds number, the feedback law is optimized using the powerful capabilities of DRL with the pressure signals as control input and the jet velocities as control output. Under leading jet control, the mean drag coefficient is reduced by 44.4%, 60.6%, and 57.8% for increasing Reynolds numbers. In addition, the lift fluctuation is reduced by 85.9%, 82%, and 86.2%, demonstrating a substantial stabilization of the wake. In contrast, the optimized trailing jet control performs much worse. This study highlights the significance of the location of zero-net-mass-flux actuation and may guide the practical application of DRL-based active flow control of square cylinders.

Suction and Pulsed Blowing Applied to Local Separation Downstream of Slat Cutout

An experimental study on the application of active flow control (AFC) to a .4 scale model of a swept wing in a landing configuration was conducted. The wing is fitted with an ultrahigh bypass ratio (UHBR) engine nacelle. The highly efficient UHBR engines are characterized by a large diameter that interferes with the flow around the wing, especially with slats deflected, degrading its performance. An innovative AFC device, creating steady suction and pulsed blowing (PB), was installed in the leading-edge region of the wing, above the nacelle, and its performance was experimentally evaluated. The effects of the suction and PB mechanisms were examined individually and simultaneously using relevant normalized parameters to pave the way to a full-scale wind-tunnel test. It was shown that the AFC actuators increased lift by up to 3%, redirected the flow to the desired downstream direction, and reduced the size of the separation zone created due to the implementation of the UHBR nacelle. The next step is validating the small-scale results of this study in a full-scale wind-tunnel tests that would be the last step before the technology is flight test ready.

Binary Repetitive Model Predictive Active Flow Control Applied to an Annular Compressor Stator Cascade With Periodic Disturbances

Abstract Novel pressure gain combustion concepts invoke periodic flow disturbances in a gas turbine's last compressor stator row. This contribution presents studies of mitigation efforts on the effects of these periodic disturbances on an annular compressor stator rig. The passages were equipped with pneumatic active flow control (AFC) influencing the stator blade's suction side, and a rotating throttling disk downstream of the passages inducing periodic disturbances. For steady blowing, it is shown that with increasing actuation amplitudes cμ, the extension of a hub corner vortex deteriorating the suction side flow can be reduced, resulting in an increased static pressure rise coefficient Cp of a passage. The effects of the induced periodic disturbances could not be addressed intrinsically, by using steady blowing actuation, Considering a corrected total pressure loss coefficient ζ*, which includes the actuation effort, the stator row's efficiency decreases with higher cμ due to the increasing costs of the actuation mass flow. Therefore, a closed-loop approach is presented to address the effects of the disturbances more specifically, thus lowering the actuation cost, i.e., mass flow. For this, a repetitive model predictive control (RMPC) was applied, taking advantage of the periodic nature of the induced disturbances. The presented RMPC formulation is restricted to a binary control domain to account for the used solenoid valves' switching character. An efficient implementation of the optimization within the RMPC is presented, which ensures real-time capability. As a result, Cp increases in a similar magnitude but with a lower actuation mass flow of up to 66%, resulting in a much lower ζ* for similar values of cμ.

Applying deep reinforcement learning to active flow control in weakly turbulent conditions

Machine learning has recently become a promising technique in fluid mechanics, especially for active flow control (AFC) applications. A recent work [Rabault et al., J. Fluid Mech. 865, 281–302 (2019)] has demonstrated the feasibility and effectiveness of deep reinforcement learning (DRL) in performing AFC over a circular cylinder at Re = 100, i.e., in the laminar flow regime. As a follow-up study, we investigate the same AFC problem at an intermediate Reynolds number, i.e., Re = 1000, where the weak turbulence in the flow poses great challenges to the control. The results show that the DRL agent can still find effective control strategies, but requires much more episodes in the learning. A remarkable drag reduction of around 30% is achieved, which is accompanied by elongation of the recirculation bubble and reduction of turbulent fluctuations in the cylinder wake. Furthermore, we also perform a sensitivity analysis on the learnt control strategies to explore the optimal layout of sensor network. To our best knowledge, this study is the first successful application of DRL to AFC in weakly turbulent conditions. It therefore sets a new milestone in progressing toward AFC in strong turbulent flows.

Design of a synthetic jet actuator for flow separation control

This paper describes the development of a piezo-electric synthetic jet actuator (SJA) of the AFLoNext project. Active flow—loads and noise control on next generation wing (AFLoNext) is a project within European Union’s 7th Framework Program. One of the main project goals is the application of active flow control (AFC) techniques, such as SJAs and pulsed jet actuators (PJAs) in two different application scenarios to evaluate the potential benefit for retrofit of current aircraft and also for future aircraft designs. This paper is focusing on the SJAs. For large-scale wind tunnel testing, an actuator panel with 84 SJAs including the drive electronics system was designed and pre-tested in a laboratory environment. The performance exceeds 100 m/s with outlet nozzles of 2.5 mm diameter and a span wise clearance of 10 mm. A second actuator design was prepared for the application on the outer wing region and was investigated in a harsh environmental test campaign. Two span wise rows of five actuators were integrated in a panel with 10 × 0.5 mm2 slotted outlet nozzles. With this design also velocities exceeding 100 m/s were measured. The actuators withstand different harsh environmental conditions including extreme temperature, rain, mechanical vibration and shock. With the results of the project, a technology readiness level (TRL) evaluation will conclude the maturity of the technology. Depending on the final test and evaluation results, achievement of TRL4 is expected.

Magneto-mechanical micro-valve for active flow control

This paper presents and discusses the development of a normally-off magneto-mechanical airflow microvalve designed for active aerodynamic flow control application. The architecture of the microvalve is composed of a highly flexible mechanical resonator, a miniature coil enabling the magnetic resonator actuation and a packaging with oval shape. The resonator system consists in an elastomer membrane and miniature magnets. The device fabrication process is based on micro-molding, micromachining and rapid prototyping. The pulsed micro-jet delivered by this microvalve can be used at different duty cycles and frequencies going up to 750 Hz. The micro-jet velocity reached 100 m/s for a corresponding airflow rate of 3 L/min. An array of twenty microvalves was implemented on an Ahmed body model and separation flow control experiments were successfully performed. The actuation provided by the micro-jets decreased by 4.3 % the drag on the vehicle-like shape.

Vortex rings produced by non-parallel planar starting jets

Abstract

Numerical Analysis of Sweeping Jets for Active Flow Control Application

Abstract It has become apparent recently that the fluidic oscillators, also known as sweeping jets, can be used to create a combination of steady (streamwise vortices) and unsteady (spanwise vortices) forcing mechanisms which have the potential to fulfill many of the promises of active separation control. The fluidic oscillators contain no moving parts, but produce an unsteady component via a natural feedback loop inherent to their geometry. The oscillations are entirely self-induced and self-sustaining. Their simple and robust design and their effectiveness over a wide range of flow conditions make them more attractive than other flow control devices, such as synthetic jets and plasma actuators. Figure 1 shows the instantaneous jet generated in quiescent environment using the Improved Delayed Detached Eddy Simulation (IDDES) model, where the Large Eddy Simulation (LES) branch of the IDDES model is able to capture the turbulence structures properly. An instantaneous iso-surface of vorticity magnitude, colored by streamwise velocity for flow over a wall-mounted hump is depicted in Figure 2. As expected, a massive flow separation occurs behind the hump in the uncontrolled condition (Figure 2 (a)), with a nearly two-dimensional free shear layer at the edge of the separation line. Breakdown of the shear layer by an array of sweeping jets located slightly downstream of the separation line is seen in Figure 2 (b), which is followed by the elimination of the separation region behind hump. The three-dimensional structures generated by the sweeping jets are smaller and closer to the hump wall than those produced by the steady jets shown in Figure 2 (c). Presence of a large region of reversed flow near the hump wall in its aft section is also seen in the case of the steady jet. This study indicates a superior effectiveness of sweeping jets on separated flows.

Aerodynamic and energy‐efficiency‐based assessment of plasma actuator's position on a NACA0015 airfoil

AbstractIn recent years, there has been an increase in the number of research papers on plasma and its use in active flow control applications. The main objective of this study is to assess the plasma actuator's position on a NACA0015 airfoil in terms of aerodynamic forces. In addition, optimization of the plasma actuator's position and its configuration are studied in order to identify the optimum configuration for improvement in lift coefficient. The experiments are conducted in an open‐suction‐type wind tunnel at Reynolds numbers of 48,000, 75,000, and 100,000. The plasma actuators are mounted on various positions (x/C) starting from the leading edge to trailing edge of the airfoil. The experimental results on aerodynamic force measurement are presented to illustrate the increasing lift effect of the generated plasma. It is also shown that the plasma actuators used as an active flow control device appears to shift the stall angle of the airfoil. The results of the experimental study suggest that the energy efficiency of airborne systems can be improved with the use of plasma actuators due to its increasing lift coefficient effect. This result becomes a vital finding considering that the same flight can be achieved with less fuel and less amount of environmental pollution for the same distance of journey. It is also worth mentioning that increasing lift effect would mean taking off from a shorter runway or allowing the airborne vehicle with the ability to fly with additional payload.

Influence of exit configuration on the flow fields and oscillation characteristics inside and outside the fluidic oscillator

The fluidic oscillator shows very promising outcomes as an actuator in active flow control applications as it features several characteristics including no-moving parts, self-excitation, rigidity and reliability. The fluidic oscillator can be utilized in diverse applications including active-control of combustion and flow separation. The present work investigates the exit configuration influence on the flow dynamics of a small-scale double-feedback fluidic oscillator. Three configurations were modelled to examine the effects of the inclusion of a splitter inside the exit channel as well as the whole elimination of both the exit channel and splitter. Experimental results were obtained using particle image velocimetry (PIV) laser system with water as a working fluid and time-resolved pressure measurements using a hydrophone. Two-dimensional computational models based on Unsteady Reynolds-averaged Navier-Stokes (URANS) equations and shear stress transport (SST) were utilized considering the flow to be turbulent, incompressible, and isothermal. The results indicate that the exit configuration has no significant effects on the frequencies, while the jet deflection angle of the outflowing jet increases and the jet tends to be continuous when the splitter is removed. The splitter increases the outflow fluctuation amplitudes which would be suitable for diverse applications.

Reinventing the wheel: excitation of flow instabilities for active flow control using plasma actuators

Flow control is typically used to manipulate a flow’s natural behavior to alter its effects on a vehicle or system. When active flow control (AFC) is used in aerospace applications, the actuation can be turned on/off or adapted to changing flight conditions. AFC techniques in which significant gains can be achieved with relatively small cost are most beneficial and are the subject of this paper. Flows with the Kelvin–Helmholtz (K–H) instability, which are present in many aerospace applications, are amenable to AFC and have been the subject of research for over five decades. The K–H instability can amplify any natural or artificially-seeded thermal, acoustic, or hydrodynamic perturbations over a wide range of frequencies. These perturbations can grow and eventually roll up into large-scale flow structures, which, in turn, can dominate important processes such as entrainment, mixing, momentum and heat trasport, and aeroacoustic noise. However, the application of active flow control was limited to low-speed flows in the earlier decades of study due to the shortcomings of the available actuators. The recent development of plasma actuators, capable of producing simultaneously high-amplitude and high-bandwidth thermal perturbations, has extended the AFC applications to high-speed and high-Reynolds number flows of interest in aerospace, hence the title: reinventing the wheel. In this paper, two classes of such plasma actuators, namely localized arc filament plasma actuators and nanosecond dielectric-barrier discharge plasma actuators, are briefly discussed and their applications in two very different flows are presented to highlight the advances made in the community in the use of plasma actuators for aerospace applications. In addition, there is a discussion of further advances that must be made in the development of these actuators to move the techniques from laboratory to in-flight use.

Atmospheric-pressure pulsed plasma actuators for flow control: shock wave and vortex characteristics

Pulsed plasma actuators are used for an active flow control application since the 2000s. In this paper, we discuss shock wave and vortex characteristics in pulsed plasma actuators after an introduction of research progress on atmospheric-pressure discharge plasma actuators. First, the shock wave characteristics in surface dielectric barrier discharge (SDBD) actuator operating in diffuse-like and multi-streamer modes are discussed. In the most general case, a shock wave in a diffuse-like SDBD actuator is stronger and faster than in a multi-streamer SDBD actuator. Improved plasma actuators, such as the three-electrode SDBD actuator and the plasma synthetic jet actuator have the enhanced shock wave characteristics. Second, in order to investigate the effects of pulse parameters on the shock wave characteristics in nanosecond-pulse SDBD actuator, a particle image velocimetry system is used to capture the formation of starting vortexes at different pulse rise times, pulse durations, and pulse repetition frequencies (PRFs). It is shown that the velocity of a starting vortex significantly increases when the pulse rise time decreases from 400 ns to 50 ns due to a more significant hydrodynamic effect generated during a shorter rise time. This phenomenon is confirmed by calculating a reduced electric field E/N at a short rise time, which turns out to be higher at a shorter rise time than that at a relatively longer rise time. It is also shown that the velocity of a starting vortex increases and its active area enlarges when the PRF increases.

Full-Scale Testing of Active Flow Control Applied to a Vertical Tail

Full-scale wind tunnel tests were carried out on a Boeing 757 vertical tail equipped with 37 sweeping jet actuators on the starboard side along the rudder hinge line. The tests were performed at the National Full-Scale Aerodynamics Complex (NFAC) 40- by 80-Foot Wind Tunnel () at NASA Ames Research Center. The model was tested at a nominal airspeed of 100 knots across rudder deflections and sideslip angles that covered the airplane’s emergency single engine climb. Active flow control (AFC) was optimized at the maximum rudder deflection of 30° and sideslip angles of 0° and . A threshold success criterion of 20% increase in the maximum side force was exceeded at largest rudder deflection using 12 sweeping jet actuators in the absence of sideslip but barely reached it with 18 actuators at the maximum sideslip at a momentum coefficient input of approximately 0.5%; consequently, 31 actuators were selected for most tests. AFC caused significant increases in suction pressure and associated side force on the actuated side. The successful demonstration of this application cleared the way for a subsequent flight demonstration on the Boeing 757 ecoDemonstrator in 2015.

Analysis of Active Flow Control Application in Subsonic Intake Integrated with Aircraft Glider

Analysis of Active Flow Control Application in Subsonic Intake Integrated with Aircraft Glider