Flow Control Actuators Research Articles

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.

Effect of wind turbine size on load reduction with active flow control

Decades of wind turbine research, development and installation have demonstrated reductions in levelized cost of energy (LCOE) resulting from turbines with larger rotor diameters and increased hub heights. Further reductions in LCOE by up-scaling turbine size can be challenged by practical limitations due to a mass increase trend. On-blade, active flow control devices have the potential to disrupt this trend by allowing longer blades with less mass through an active load control system. Typically, these load control systems are developed for specific wind turbines, making it difficult to study load reduction trends with turbine size to gain further insights into the benefits and risks of this control technology. This paper quantifies the variation in load reduction, complexity, and robustness of load control systems with flow control actuators for three turbines of increasing size. It is shown that, under limited control authority, load reduction can increase with turbine size provided more elaborate control algorithms are used to preserve the bandwidth and robustness of the control system.

Discharge Stability Enhancement in Surface Corona Actuators

A surface corona actuator for flow control, already tested in wind tunnel experiments, is here characterized at the bench in still air. The anodic electrode is provided with periodic triangular tips and is operated with a pulsed waveform. The operating stability and voltage range turn out to be wider than for dc operation. The actuator is characterized by means of several measurements as a function of the frequency, including power absorption and ionic wind measurements in different locations. The ionic wind is measured first by a micropitot probe obtaining time-averaged values, then by a hot wire anemometer obtaining instantaneous velocities and turbulence spectra. The behavior of the actuator is interpreted accounting for the creation and removal of charges in the gap subject to a periodic excitation.

Improved performance of polyimide Cirlex‐based dielectric barrier discharge plasma actuators for flow control

AbstractDielectric barrier discharge (DBD) plasma actuators are simple electrohydrodynamic devices, which are able to provide effective aerodynamic control. One of the main components of these devices is the thin dielectric layer, which allows to separate and prevent the arc between the high‐voltage electrodes. Different materials can be used as dielectric layer to reduce the power consumption or boost the flow controlling effect of the actuators. In this context, this report presents a comparative study of two commercial polyimides, Kapton and Cirlex, used as dielectric layer of surface DBD plasma actuators. The electrical, dielectric, mechanical, electromechanical, and thermal properties were obtained to evaluate overall performance. It was verified that Cirlex (8.3 W) consumes less power than Kapton (21.3 W) to generate higher induced flow velocity of ≈3.4 m/s for an input voltage of 11 kVpp and 24 kHz. During one AC cycle at 11 kVpp the charge transferred for Cirlex (70 nC) is lower than for Kapton (100 nC), as well as the dielectric breakdown voltage to ignite the plasma discharge, 1.5 and 2.2 kVpp, respectively. The Cirlex DBD presents a higher voltage operation limit (at least 14 kVpp) and a more regular plasma discharge, which results in a more homogenous thermal profile and temperature distribution during its operation. The Cirlex actuator delivered higher mechanical power (6.2 mW) and achieved higher electromechanical efficiency (0.004%). The polyimide Cirlex proved to be a suitable alternative for Kapton to fabricate DBD plasma actuators for flow control with improved performance.

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.

Experimental study on the plasma actuators for the tip leakage flow control in a turbine cascade

In this paper, the effects of dielectric barrier discharge (DBD) plasma actuators (PAs) on the leakage flow structures and loss in a turbine cascade have been experimental studied. Two group cases have been conducted to evaluate the performance of the DBD plasma actuators and its effect varied with the attack angle (α). By measuring the static pressure on the casing and the flow field at the exit section, the effects of the DBD PAs have been illustrated. Results show that the DBD plasma actuators on the tip of turbine cascade could make effect on the tip leakage flow by weaken the strength of the TLV near the casing. Rising the actuation strength, this effect would be enhanced. Besides, this effect on the flow structure mainly focuses on the TLV near the casing, changes in the blade passage could be ignored with the action of the DBD PAs. In the study on the attack angle, the largest loss reduction of 7.8% is noticed with the inflow Re=0.64×105 and α=5°.

Parametric study of a frequency-modulated pulse jet by measurements of flow characteristics

Recently, pulsed jet actuators have been discovered as an efficient and promising technique for flow separation control. In this paper, a systematic experimental investigation is carried out using a flow control approach, in order to study the flow field characteristics of a baseline single pulsed jet actuator, under different excitation conditions. The findings are used to provide a better understanding of pulsed blowing actuator operation and the associated flow physics. In these experiments, two types of signal waveforms have been implemented to produce the pulsed excitation of jet; a simple square-wave excitation signal for the first case and a burst modulated excitation signal for the second case. All measurements were conducted using a single hot-wire system to an axial distance x/D ≤ 30 along the centerline. The effects of electrical parameters on properties of a single pulsed jet, including the excitation frequency from 10 up to 220 Hz and the duty cycle from 15% up to 80% for the first case, and different combinations of the carrier frequency and the modulating frequency for the second case have been studied. These measurements revealed a significant dependence of mean centerline velocity and turbulence intensity to excitation signal, frequency, and duty cycle. The statistical flow properties, such as mean centerline velocity decay and turbulence intensity distribution of pulsed jets are quantified and compared together over a wide range of pulsing parameters. This investigation aids in designing and optimizing flow control actuators in aerodynamics and combustion chambers.

Fiber optic pressure measurement on a complex outer winglet model with active flow control actuators

The next generation of aircraft requires novel approaches on lift increase and drag reduction technologies as active flow control to counter local flow separations. Testing and monitoring its effectiveness in flight require precise pressure evaluation as a possible control input. Pressure-based measurements of local flow separation are challenging utilizing conventional electrical pressure sensors. These sensors might show degradation of their long-term performance in harsh environments, e. g., in real aircraft applications, due to their low overload protection, susceptibility to electromagnetic influences, and corrosion. Fiber optic measurements overcome those challenges and promise to be a robust sensing solution. We investigate the potential application of a novel fiber optic measurement system that monitors the active flow control system's effectiveness. Furthermore, the results are compared to conventional static pressure measurements. Due to the high-frequency capabilities of the fiber optic sensors, the dynamic behavior of the boundary layer separation and vortex generation was captured in the frequency domain. An industry-relevant outer wing was equipped with 25 fiber optic pressure sensors positioned at one wingspan section for this experiment. The experiments were conducted at a Reynolds number Re=1.1⋅106and a Mach number Ma=0.12. Wind tunnel testing results show that fiber optic pressure sensors can measure aerodynamic characteristics and, furthermore, even allow a detailed look at the dynamic behavior.

Background-oriented schlieren measurement of near-surface density field in surface dielectric-barrier-discharge

Surface dielectric-barrier-discharge (DBD) is widely utilized for flow control actuators called DBD plasma actuator (DBDPA). With the aim of ensuring an accurate background-oriented schlieren (BOS) measurement of the near-surface density field in surface DBD, we investigate the effects of the depth of field (DoF), the wall surface and the background image deformation on measurement results. Experiments using a glass plate as the measurement target reveal that there is no appreciable effect of whether the DoF includes the measurement target. Additionally, the DoF should be shallow from the viewpoint of the error introduced by the wall surface. Moreover, it is found that the error introduced by the wall surface and the dot deformation can be characterized by specific dimensionless parameters. Finally, we conduct the BOS measurement of the DBDPA. We confirm that the density field is qualitatively valid from a physical respect, and we present the density field while discussing specific measurement errors.

Vibration Reduction on Helicopter Rotors Using Open-Loop Flow Control

The effectiveness of active flow control for reducing vibrations in helicopter rotors is examined using numerical simulations. The flow control actuators employ pulsed fluidic jets on the pressure side (PS) and suction side (SS) of the airfoil and are installed near the trailing edge of the blades. The flow control region extends over 12% of the blade span and is centered at the 75% span location. A reduced-order model (ROM) of the unsteady aerodynamic effects introduced by the actuators is developed. The ROM is trained using data obtained from unsteady Reynolds-averaged Navier–Stokes simulations performed across a broad range of flow conditions representing the rotor environment. The ROM is incorporated in a comprehensive aeroelastic rotor simulation code. Open-loop control, consisting of alternating PS and SS actuation at various frequencies, is applied on a four-blade hingeless rotor resembling the Messerschmitt–Bölkow–Blohm Bo-105 rotor in level flight with an advance ratio of 0.30. The results indicate that the flow control actuators have significant control authority to modify the vibratory loads, with up to 85% reduction in the four-per-revolution vertical shear force at the rotor hub. The tradeoff between vibratory load reduction and rotor performance is also examined. A 5.3% increase in rotor power accompanies the vibratory load reduction.

The ACONIT project: an innovative design approach of active flow control for surge prevention in gas turbines

The objective of the ACONIT project is to design, manufacture and test actuators for flow control for an implantation in an aircraft engine. The actuators will fulfil aeronautics requirement in order to increase the Technology Readiness Level (TRL) in this domain. In particular, for the present proposal, one plans to focus on the extension of the stable operating range of axial compressor, allowing thus a reduction of the surge margin through postponing the stall onset. To do so, the first objective of the work is to improve the knowledge of the flow physics of an efficient flow control system by joint numerical and experimental analyses performed in a low speed, single stage axial compressor. The results of this analysis will be used to derive the fluidic specifications for high-TRL actuators and control systems. These specifications will be the base for the design and manufacturing of amplified piezo-electric actuator prototypes whose fluidic performance and operational performance in an environment with vibration and controlled level of temperature will be precisely evaluated before manufacturing final actuators that will be integrated in a full-scale engine test facility. Their performance will be evaluated in terms of Surge Margin Improvement (SMI) as well as in terms of energy balance between the induced consumption and the machine performance improvements. The consortium grouped for carrying out this project is composed of a SME (CTEC), two academic institutions (Bundeswehr University Munich and ENSAM) and a Research Centre (ONERA). It groups skills ranging from internal flow analysis in turbomachinery, to flow control or actuators design, manufacturing and characterisations.

Performance enhancement of single dielectric barrier discharge flow control actuators by means of rear linking tunnels on a reference bluff body using CFD

This paper studies drag reduction on a reference bluff body (Ahmed body) with single dielectric barrier discharge (SDBD) actuators, also called plasma actuators, located at different positions on the model’s rear part with a spanwise arrangement. This active actuator modifies the laminar-to-turbulent transition leading to reattachments of separated flows on the body. A reduction of actuator efficiency when increasing velocity is the main limitation of this method. The main aim of this work is to find a solution for this problem by employing passive flow control to improve the SDBD actuators’ efficiency. For this purpose, rear linking tunnels are added to the model as a novel passive flow control. Numerical simulations were performed to determine the best position of the active actuators on the model. The next step is to study the effect of the passive and active flow control on drag reduction and investigating the combination of these methods for improving the active flow control performance at four different freestream velocities (Re ​= ​0.289 ​× ​106–0.722 ​× ​106). The positive effect of this combination is noticeable (a 59% increase in the SDBD actuator’s efficiency for Re ​= ​0.722 ​× ​106), and the non-linear behaviour of this combination leads to high drag reduction of between 13.29% and 17.96% for different Reynolds numbers.

Multi-Sectional Lift Actuation for Wind Turbine Load Alleviation

The development of large rotors can be limited by the ability to manage dynamic loads. Active Flow Control (AFC) actuators have been proposed to reduce these dynamic loads. This paper aims to study and quantify the advantages of using multiple AFC actuators compared to using a single AFC actuator for each blade. For this study, AFC actuators are idealized and modeled as instantaneous changes to the local lift coefficient at the corresponding blade sections. We refer to this idealized actuators as sectional lift actuators (SLA). A comparison between a single SLA and two SLAs per blade is made using feedforward and feedback control. Blade loads in above-rated wind conditions are used for the comparative analysis, which is conducted using the DTU-10MW reference turbine model.

Effects of plasma kinetic modeling on performance characterization of plasma actuators for active flow control

This work focuses on the development of a multiscale computational fluid dynamics (CFD) simulation framework for the investigation of the effects of plasma kinetics on the performance of a microscale dielectric barrier discharge plasma actuator (DBD-PA). To this purpose, DBD-PA multi-scale dual-step modelling approach has been implemented, by considering plasma chemistry and flow dynamic. At first, a microscopic plasma model based on the air plasma kinetics has been defined and plasma reactions have been simulated in zero-dimensional computations in order to evaluate the charge density. At this aim computations have been performed using the toolbox ZDPlasKin, which solves plasma reactions by means of Bolsig+ solver. An alternate current (AC) electrical feeding has been assumed: in particular, the sinusoidal voltage amplitude and the frequency have been fixed at 5 kV and 1 kHz at atmospheric pressure and 300 K temperature in quiescent environment. The predictal charge density has been in a macroscopic plasma-fluid model based on Suzen Dual Potential Model (DPM), which has implemented in the computation fluid dynamic CFD code OpenFoam. Hence, as second step, 2D-CFD simulations of the electro-hydrodynamic body forces induced by the microscale DBDPA have been performed, based on the previously predicted charge densities at the operating conditions. Quiescent flow over a dielectric barrier discharge actuator has been simulated using the plasma-fluid model. The novel modelling framework has been validated with experimental data.

Ultrasound generated by alternating current dielectric barrier discharge plasma in quiescent air

Dielectric barrier discharge (DBD) driven by an alternating current (AC) high-voltage power supply, also called silent discharge, has been known for more than a century since Siemens first proposed the term DBD in 1857, and penetrates into our daily lives. In our traditional view, AC-DBD is considered as an ozone generator for disinfection, an electronic emitter for depollution of gas streams, a jet actuator for flow control, and so on. However, the acoustic effect of AC-DBD is always neglected. Here, we demonstrate that an asymmetrical DBD plasma actuator which was powered by sinusoidal AC waveforms with an AC voltage of 16 kVp–p at a high voltage frequency of a few kHz can generate ultrasound in quiescent air by using a pressure-field microphone and a high accuracy phase-lock image Schlieren technique. The frequency of this induced ultrasound is approximately 100 kHz which is much higher than the sin high voltage frequency of a few kHz. It is the first time that ultrasound created by the AC-DBD is observed. This finding provides a comprehensive understanding of AC-DBD and might open up possibilities for novel industrial applications of AC-DBD by using the induced ultrasonic energy. In addition, according to the experimental results, a hypothesis is proposed on the formation mechanism of ultrasound generated by the AC-DBD plasma actuator.

Development of an ultrasound acoustic streaming actuator for flow control

Development of an ultrasound acoustic streaming actuator for flow control

A Numerical RANS‐Based Model of a Nanosecond Pulsed Plasma Actuator for Flow Control over an Airfoil

AbstractAirfoil stall influences the performance of flight vehicles and remains a challenge for the design of modern aircraft. A Dielectric Barrier Discharge (DBD) device seems to be a promising tool to control the flow over various parts of an aircraft and to suppress separation. A phenomenological model based on dynamic similarity is developed to simulate the control effect of a Nanosecond Dielectric Barrier Discharge (NS‐DBD) actuator. A two‐dimensional numerical simulation considers the response of the flow past a NACA 0015 airfoil at 14° post stall angle of attack and a Reynolds number of 250,000 to pulsed surface heating at the leading edge. The RANS‐based numerical results have been obtained for a baseline simulation (no actuation) and an open‐loop control simulation of the airfoil. A one‐equation local correlation‐based transition model is implemented to capture laminar‐turbulent transition. The numerical results of both the baseline and the actuated case are in good agreement with experiments performed by other authors.

Helicopter Fuselage Model Drag Reduction by Active Flow Control Systems

A comprehensive experimental investigation of a helicopter blunt fuselage model was carried out to evaluate the effectiveness of active flow control (AFC) systems in reducing parasite fuselage drag. The main objective was to demonstrate the capability of different active technologies to decrease fuselage drag by alleviating the flow separation in the loading ramp region of large transport helicopters. The work was performed on a simplified blunt fuselage at model scale. Two different flow control actuators were considered for evaluation: steady blowing and unsteady blowing (i. e., pulsed jets). Laboratory tests of each individual actuator were performed to assess their performance and properties. The fuselage model was investigated with and without the AFC systems located along the loading ramp edges. Significant drag reductions were achieved for a wide range of fuselage angles of attack and sideslip angles without negatively affecting other aerodynamic characteristics.

High frequency characterization of a sweeping jet actuator

Sweeping jets are an emerging type of actuators that have gained interest due to their potential use in flow control applications. The working principle of these devices is based on the bi-stable attachment of a jet to adjacent walls. They are able to produce unsteady blowing within a wide range of operating frequencies. Nevertheless, the state of art shows a lack of space-time characterization of these actuators for high sweeping frequencies. This paper presents a conditional approach that reconstructs the spatial dynamic response of sweeping jets for sweeping frequencies above 500 Hz. The time-dependent velocity is measured with two single-hot-wire sensors: a reference one placed at the edge of the exit nozzle, and a flying one. The method is then tested to characterize the flow at the exit nozzle of an in-house sweeping jet actuator with 1 mm space resolution, and 50 μs time resolution. These measurements are performed with a sweeping frequency of 639 Hz. Overall this paper demonstrates that the conditional approach is very useful for understanding the physics of flow control actuators.

Post-stall flow control using nanosecond pulse driven dielectric barrier discharge plasma actuators

The efficacy of thermal perturbations generated by nanosecond pulse driven dielectric barrier discharge (ns-DBD) plasma actuators for post-stall flow control is explored on a NACA 0012 airfoil. Baseline and controlled flow fields are studied using static pressure measurements, particle image velocimetry and constant temperature anemometry. Experiments are primarily performed at Re = 0.74 × 106 (U∞ = 40 m s−1) and α = 18°. Three regimes of forcing are identified corresponding to separation control (0.92 < F+ < 1.52), bluff body shedding (0.23 < F+ < 0.92) and an impulse-like response (F+ < 0.23). The response of the flow to a single high voltage pulse is also examined and compared to other studies of transient separation control. Results show that the global structure of the controlled flow is not specific to ns-DBD plasma actuator forcing. Rather, it is the local behavior of the ns-DBD that results in control authority at conditions that are challenging for the majority of active flow control actuators that rely on zero net mass flux momentum addition. The implications of these findings as well as open questions and suggestions for future work are discussed.