Flow Control Actuators Research Articles

Dynamic characterization of piezoelectric micro-blowers for separation flow control

A micro-blower device, made by Murata™ Manufacturing Co. and originally designed for cooling electronic microprocessors, is used in the present study as an actuator for separation flow control. The dynamical performance of the device, working in pulsed mode, is experimentally investigated to deduce its abilities and limitations regarding active flow control. Both the device design and its dynamical characterization are addressed in the paper, paying particular attention to its ability to control flow. Both hot-wire anemometry and particle image velocimetry have been employed to investigate the dynamical evolution of the pulsed jet and allow the identification of the jet organized flow structures. The phase-averaging technique is applied to distinguish the large scale structure dynamics induced during the ejection phase. An array of active devices is also used to evaluate the flow control efficiency by forcing frequency strategy in order to reduce the separation area of a back facing step flow. Results show good performance in open loop flow control and applicability of such actuator for further closed loop flow control research.

Effects of Nanosecond Pulse Driven Plasma Actuators on Turbulent Shear Layers

The effects of nanosecond pulse driven dielectric barrier discharge plasma actuators on turbulent shear layers are examined experimentally on a mixing layer and backward facing step. The nanosecond pulse driven dielectric barrier discharge control mechanism is believed to be primarily thermal in contrast to most flow control actuators, including alternating current dielectric barrier discharges, which rely on periodic or pulsed momentum to excite shear-layer instabilities. Control authority using thermal perturbations has been demonstrated in various high-speed shear flows yet many questions on fundamental physics and scaling remain unanswered. This work aims to provide insight into both nanosecond pulse driven dielectric barrier discharges and thermal mechanisms in general for high-amplitude aerodynamic flow control. Previous studies suggest the efficacy of nanosecond pulse driven dielectric barrier discharges (and likely thermal perturbations in general) is strongly dependent on initial shear-layer conditions, namely some measure of the initial thickness. In an effort to support this hypothesis, boundary-layer suction is applied to a splitter plate upstream of a turbulent mixing-layer origin. This successfully reduces the initial mixing-layer momentum thickness, but does not result in substantial nanosecond pulse driven dielectric barrier discharge control authority. These results and the experimental conditions are documented in detail. Application of nanosecond pulse driven dielectric barrier discharge forcing to the turbulent shear layer downstream of a backward facing step having even smaller initial thickness produces the expected control authority for lower pulse amplitude than employed in the mixing-layer case. This supports the importance of initial shear-layer thickness (rather than state) for estimating potential control authority by nanosecond pulse driven dielectric barrier discharge and thermal perturbations in general.

Experimental investigations on cavity-actuated under-expanded supersonic oscillating jet

As one type of potential flow control actuators, cavity-actuated supersonic jet oscillators, which consist of a 2-D convergent nozzle and two face to face cavities, need to be investigated deeply to get the knowledge of their oscillating feature and underlying mechanism. Wind tunnel testing are conducted under different back pressures in a vacuum-type wind tunnel for two supersonic jet oscillators, to obtain their characteristics and the conditions for jet oscillating. The experimental results show that the continuous, nearly symmetric or asymmetric flipping between the two cavities appears over certain nozzle pressure ratio (NPR) range for both oscillators according to schlieren visualizations. Compared to the free jet, the oscillating jet with large exit achieves larger mixing; the oscillating jet with small exit has less mixing, based on the analysis of jet axial peak velocity and the entrainment. The cross-junction mode for estimating the resonance frequency in a pipe with two closed side branches is modified and obtained comparable estimations of the frequency of jet flipping with experimental data, but further investigations are needed to discover the underlying mechanism for the jet flipping.

Dissipated power and induced velocity fields data of a micro single dielectric barrier discharge plasma actuator for active flow control.

In recent years, single dielectric barrier discharge (SDBD) plasma actuators have gained great interest among all the active flow control devices typically employed in aerospace and turbomachinery applications [1,2]. Compared with the macro SDBDs, the micro single dielectric barrier discharge (MSDBD) actuators showed a higher efficiency in conversion of input electrical power to delivered mechanical power [3,4]. This article provides data regarding the performances of a MSDBD plasma actuator [5,6]. The power dissipation values [5] and the experimental and numerical induced velocity fields [6] are provided. The present data support and enrich the research article entitled “Optimization of micro single dielectric barrier discharge plasma actuator models based on experimental velocity and body force fields” by Pescini et al. [6].

Efficient needle plasma actuators for flow control and surface cooling

We introduce a milliwatt class needle actuator suitable for plasma channels, vortex generation, and surface cooling. Electrode configurations tested for a channel configuration show 1400% and 300% increase in energy conversion efficiency as compared to conventional surface and channel corona actuators, respectively, generating up to 3.4 m/s air jet across the channel outlet. The positive polarity of the needle is shown to have a beneficial effect on actuator efficiency. Needle-plate configuration is demonstrated for improving cooling of a flat surface with a 57% increase in convective heat transfer coefficient. Vortex generation by selective input signal manipulation is also demonstrated.

Experimental and numerical investigation on steady blowing flow control within a compact inlet duct

A combined experimental and numerical investigation of flow control actuation in a short, rectangular, diffusing S-shape inlet duct using a two-dimensional tangential control jet was conducted. Experimental and numerical techniques were used in conjunction as complementary techniques, which are utilized to better understand the complex flow field. The compact inlet had a length-to-hydraulic diameter ratio of 1.5 and was investigated at a free-stream Mach number of 0.44. In contrast to the baseline flow, where the flow field was fully separated, the two-dimensional control jet was able to eliminate flow separation at the mid-span portion of the duct and changed considerably the three-dimensional flow field, and ultimately, the inlet performance. A comparison between the baseline (no actuation) and forced flow fields showed that secondary flow structures dominated both flow fields, which is inevitably associated with total pressure loss. Contrary to the baseline case, the secondary flow structures in the forced case were established from the core flow stagnating on the lower surface of the duct close to the aerodynamic interface plane. High fidelity spectral analysis of the experimental results at the inlet’s exit plane showed that the baseline flow field was dominated by pressure fluctuations corresponding to a Strouhal number based on hydraulic diameter of 0.26. Not only did the two-dimensional tangential control jet improve the time-averaged pressure recovery at the inlet exit plane (13.3% at the lower half of the aerodynamic interface plane), it essentially eliminated the energy content of the distinct unsteady fluctuations which characterized the baseline flow field. This result has several implications for the design of a realistic engine inlet; furthermore, it depicts that a single non-intrusive static pressure measurement at the surface of the duct can detect flow separation.

Scanning localized magnetic fields in a microfluidic device with a single nitrogen vacancy center.

Nitrogen vacancy (NV) color centers in diamond enable local magnetic field sensing with high sensitivity by optical detection of electron spin resonance (ESR). The integration of this capability with microfluidic technology has a broad range of applications in chemical and biological sensing. We demonstrate a method to perform localized magnetometry in a microfluidic device with a 48 nm spatial precision. The device manipulates individual magnetic particles in three dimensions using a combination of flow control and magnetic actuation. We map out the local field distribution of the magnetic particle by manipulating it in the vicinity of a single NV center and optically detecting the induced Zeeman shift with a magnetic field sensitivity of 17.5 μT Hz(-1/2). Our results enable accurate nanoscale mapping of the magnetic field distribution of a broad range of target objects in a microfluidic device.

Multi-fluid modelling of pulsed discharges for flow control applications

Experimental evidence suggests that short-pulse dielectric barrier discharge actuators are effective for speeds corresponding to take-off and approach of large aircraft, and thus are a fruitful direction for flow control technology development. Large-eddy simulations have reproduced some of the main fluid dynamic effects. The plasma models used in such simulations are semi-empirical, however, and need to be tuned for each flowfield under consideration. In this paper, the discharge physics is examined in more detail with multi-fluid modelling, comparing a five-moment model (continuity, momentum, and energy equations) to a two-moment model (continuity and energy equations). A steady-state, one-dimensional discharge was considered first, and the five-moment model was found to predict significantly lower ionisation rates and number densities than the two-moment model. A two-dimensional, transient discharge problem with an elliptical cathode was studied next. Relative to the two-moment model, the five-moment model predicted a slower response to the activation of the cathode, and lower electron velocities and temperatures as the simulation approached steady-state. The primary reason for the differences in the predictions of the two models can be attributed to the effects of particle inertia, particularly electron inertia in the cathode layer. The computational cost of the five-moment model is only about twice that of the simpler variant, suggesting that it may be feasible to use the more sophisticated model in practical calculations for flow control actuator design.

Experiments on Active Drag Reduction on a Complex Outer Wing Model

The potential of active flow control by means of pulsed blowing to counter performance degradation at high angles of attack is demonstrated on a modern civil aircraft outer wing configuration provided by the European aeronautical industry. Flow control actuators are incorporated into the leading edge between the slat edge and the wingtip, where the wing’s slender shape and its high local curvature do not allow for the integration of mechanical leading-edge devices. Surface flow visualization identifies the highly three-dimensional topology of the uncontrolled flow, which is dominated by the slat edge vortex and leading-edge flow separation at high incidence angles. The influence of the varying-momentum coefficient on drag reduction, lift gain, and aerodynamic efficiency is studied, considering force and pressure measurements, as well as flow visualization. Results indicate that a momentum coefficient of suffices to reduce drag by up to 38%, compared to the uncontrolled baseline flow, and to offset aerodynamic performance degradation by 4 deg if the actuation effort is distributed efficiently along the span.

Plasma Virtual Actuators for Flow Control

Dielectric-barrier-discharge (DBD) plasma actuators are all-electric devices with no moving parts. They are made of a simple construction, consisting only of a pair of electrodes sandwiching a dielectric sheet. When AC voltage is applied, air surrounding the upper electrode is ionized, which is attracted towards the charged dielectric surface to form a wall jet. Control of flow over land and air vehicles as well as rotational machinery can be carried out using this jet flow on demand. Here we review recent developments in plasma virtual actuators for flow control that can replace conventional actuators for better aerodynamic performance.

Transient ejection phase modeling of a Plasma Synthetic Jet actuator

For several years, a promising Plasma Synthetic Jet actuator for high-speed flow control has been under development at ONERA. So far, its confined geometry and small space-time scales at play have prevented its full experimental characterization. Complementary accurate numerical simulations are then considered in this study in order to provide a complete aerothermodynamic description of the actuator. Two major obstacles have to be overcome with this approach: the modeling of the energy deposited by the electric arc and the accurate computation of the transient response of the cavity generating the pulsed jet. To solve the first problem, an Euler solver coupled with an electric circuit model was used to evaluate the energy deposition in the cavity. Such a coupling is performed by considering the electric field between the two electrodes. The second issue was then addressed by injecting these source terms in large Eddy simulations of the entire actuator. Aerodynamic results were finally compared with Schlieren visualizations. Using the proposed methodology, the temporal evolution of the jet front is remarkably well predicted.

SparkJet characterizations in quiescent and supersonic flowfields

The aerodynamic community has studied active flow control actuators for some time, and developments have led to a wide variety of devices with various features and operating mechanisms. The design requirements for a practical actuator used for active flow control include reliable operation, requisite frequency and amplitude modulation capabilities, and a reasonable lifespan while maintaining minimal cost and design complexity. An active flow control device called the SparkJet actuator has been developed for high-speed flight control and incorporates no mechanical/moving parts, zero net mass flux capabilities and the ability to tune the operating frequency and momentum throughput. This actuator utilizes electrical power to deliver high-momentum flow with a very fast response time. The SparkJet actuator was characterized on the benchtop using a laser-based microschlieren visualization technique and maximum blast wave and jet front velocities of ~400 and ~310 m/s were, respectively, measured in the flowfield. An increase in jet front velocity from 240 to 310 m/s during subatmospheric (60 kPa) testing reveals that the actuator may have greater control authority at lower ambient pressures, which correspond to high-altitude flight conditions for air vehicles. A SparkJet array was integrated into a flat plate and tested in a Mach 1.5 crossflow. Phase-conditioned shadowgraph results revealed a maximum flow deflection angle of 5° created by the SparkJet 275 µs after the actuator was triggered in single-shot mode. Burst mode operation of frequencies up to 700 Hz revealed similar results during wind tunnel testing. Following these tests, the actuator trigger mechanism was improved and the ability of the actuator to be discharged in burst mode at a frequency of 1 kHz was achieved.

Modeling and Parametric Study of a Plasma Synthetic Jet for Flow Control

In the past years, the plasma synthetic jet actuator has been proven as promising for aeronautical applications of aerodynamic flow control. In this paper, the plasma synthetic jet actuator’s operation is numerically modeled. Results of this simplified coupled numerical approach are presented, along with parametric studies that have been performed concerning geometrical and operational criteria, providing insights of its performance. The results of its pulsed mode have been coupled in a computational fluid dynamics solver via source terms, and basic simulations over a flat plate illustrate the main expected effect of the actuator for aerodynamic flow control: the vortices’ generation.

Flow Control Of Impingement JetsAnd Wall Jets

The purpose of this article is to provide useful information on the fl ow control technology of impingement jets in gas turbines and to discuss the recent trends in the passive and active fl ow control of impingement jets based on recent advancements. The temperature level in a gas turbine fi eld is so high that innovative developments in material and cooling technology are needed for usage in the hot sections. Therefore, innovative cooling systems will be employed in the future. Smart actuators for fl ow control of the jet impingement, which can be used under severe temperature and pressure conditions, will also be provided based on the progress in related technologies.

Pneumatic actuator for bulk material flow control and its diagnostics

A pneumatic actuator for bulk material flow control is presented. A system for diagnosing its possible malfunctions is offered.

Flow Response of Active Flow Control Actuators

Numerical and experimental analysis of a synthetic jet actuator in quiescent air is reported. The study focuses on the actuator itself and on the vorticity field as well as on structures that are generated by the actuator. Large eddies simulation was used for numerical analysis and phase-locked, two-dimensional microscopic-particle image velocimetry technique was used in the experiment. In the numerical simulation, the equations were integrated using a nondissipative scheme that enforces discrete conservation of kinetic energy. The internal cavity flow of the actuator was part of the simulation. The actuator under consideration is of a unique design, fabricated to accommodate practical issues associated with helicopter rotor application. However, it has the same operating principles as a conventional synthetic jet actuator. Harmonics of the forcing frequency were traced in the velocity field. Insight into the evolution of the vortical structures, jet flapping, and onset of turbulence in the jet was retrieved. This is an important building block for better understanding of the interaction of synthetic jets with boundary layers.

Measurement of Ozone Production in Non-thermal Plasma Actuator Using Surface Dielectric Barrier Discharge

Plasma actuators for flow control are intensively studied, but the production of ozone by the surface dielectric barrier discharge used in the actuators has never been quantified. Since ozone is harmful to human health, it is important to quantify its production for an application of this type of actuator on a land vehicle. This paper describes an experimental study to measure the concentration of ozone produced by an actuator with different parameters: amplitude and frequency of the applied high voltage, and the electrode configuration (shape, spacing and length). The results show that, under our experimental conditions, the production of ozone is directly proportional to the power dissipation. The production rate was measured at 21 g/kWh. Although the rate is much lower than that of an industrial ozonizer, it is still far from being negligible and should be taken into account for the future application of these actuators.

MODIFICATION OF AERODYNAMIC WING LOADS BY FLUIDIC DEVICES

Airplane wing load control systems are designed for modification/redistribution of aerodynamic loads in order to decrease risk of structural damage in conditions of excessive loads, to improve passenger comfort in turbulent atmosphere or to act as flight control systems. Classical examples include systems involving symmetric deflections of ailerons reducing wing root bending moments (Lockheed C-5 Galaxy) or deflections of spoilers stabilizing landing approach path (Lockheed TriStar). The fast development of Micro Electromechanical Systems and their application in Flow Control System opens the perspectives of designing practical wing load control systems based on fluidic actuators, modifying local aerodynamic loads by inducing changes to flow, for example, by inducing flow separation in the boundary layer or modifying Kutta condition on the trailing edge. This is the principle of operation of novel concepts of flow control actuators proposed by Institute of Aviation and discussed in the paper. The systems include actuators in the central part of the wing section, reducing local lift similarly to classical spoilers and actuators on the modified trailing edge, acting similarly to ailerons. The potential advantages in comparison to classical devices include potentially shorter reaction time because of avoiding the necessity of moving large surfaces against high dynamic pressure, which is important in conditions of fast-changing loads in turbulent atmosphere.

Propagating-arc magnetohydrodynamic plasma actuator for directional high-authority flow control in atmospheric air

A propagating-arc magnetohydrodynamic plasma actuator for aerodynamic flow control is reported. The actuator comprises two rail electrodes flush mounted on an aerodynamic surface. A pulsed arc is propelled down the length of the rails by Lorentz forces supported by a self-induced magnetic field. The arc induces a high velocity pulsed air wall jet due to the pushing and entrainment actions. Experiments in quiescent air demonstrate that the plasma arc achieves a peak velocity of around 100 m s−1 and requires a discharge energy on the order of 300 J per pulse. Wind tunnel tests on a 14.5 inch chord airfoil section, at a Reynolds number of 0.45 million show induced flow velocities on the order of 10's m s−1 with significant penetration of the flow actuation effect perpendicular to the wall surface.

Flow Physics of a Pulsed Microjet Actuator for High-Speed Flow Control

Flow control actuators based on a small-diameter source jet and a cylindrical cavity structure take advantage of the flow resonance within the cylindrical cavity to generate a variable-frequency, pulsed high-momentum microjet issuing through the cavity orifice. The flow-acoustic coupling, which leads to resonance within the cavity of the actuator, is the main driving mechanism behind the pulsed microjet. In the present study, a computational methodology based on high-order numerical techniques is used to simulate a highly unsteady and compressible pulsed actuator flowfield. Simulation generated flowfield results are analyzed to further understand the complex flow physics governing the pulsed actuator operation. The simulation provides significant details about the highly unsteady and complex microscale actuator flowfield, which are not observable from the experiments. Qualitative comparisons made between the simulated flowfield visualizations and the experimental microschlieren images show a reasonable level of agreement. We perform a dynamic mode decomposition to identify the dynamically important modes of the actuator flowfield. The additional insight gained into the flow physics through the simulation is useful in the design of more efficient and geometrically complex pulsed actuators for a range of high-speed flow and noise control applications.