Plasma actuators are a type of actuator currently being developed for active aerodynamic flow control. Plasma actuators impart force in a similar way to ionocraft. Plasma flow control has drawn considerable attention and been used in boundary layer acceleration, airfoil separation control, forebody separation control, turbine blade separation control, axial compressor stability extension, heat transfer and high-speed jet control.
Dielectric Barrier Discharge (DBD) plasma actuators are widely utilized in airflow control applications. [2] DBD is a type of electrical discharge commonly used in various Electrohydrodynamic (EHD) applications. [1]
In DBDs, the emitter electrode is connected to a high-voltage source and exposed to the surrounding air, while the collector electrode is grounded and encapsulated within the dielectric material (see figure). When activated, they form a low-temperature plasma between the electrodes by application of a high-voltage AC signal across the electrodes. Consequently, air molecules from the air surrounding the emitter electrode are ionized, and are accelerated towards the counter electrode through the electric field. [1]
Plasma actuators operating at the atmospheric conditions are promising for flow control, mainly for their physical properties, such as the induced body force by a strong electric field and the generation of heat during an electric arc, and the simplicity of their constructions and placements. [3] In particular, the recent invention of glow discharge plasma actuators by Roth (2003) [4] that can produce sufficient quantities of glow discharge plasma in the atmosphere pressure air helps to yield an increase in flow control performance.
Either a direct current (DC), an alternating current (AC) power supply, or a microwave microdischarge can be used for different configurations of plasma actuators. [5] [6] One schematic of an AC power supply design for a dielectric barrier discharge plasma actuator is given here as an example. The performance of plasma actuators is determined by dielectric materials and power inputs, later is limited by the qualities of MOSFET or IGBT.
The driving waveforms can be optimized to achieve a better actuation (induced flow speed). However, a sinusoidal waveform may be preferable for the simplicity in power supply construction. The additional benefit is the relatively less electromagnetic interference. Pulse-width modulation can be adopted to instantaneously adjust the strength of actuation. [7]
Manipulation of the encapsulated electrode and distributing the encapsulated electrode throughout the dielectric layer has been shown to alter the performance of the dielectric barrier discharge (DBD) plasma actuator. Locating the initial encapsulated electrode closer to the dielectric surface results in induced velocities higher than the baseline case for a given voltage. [8] In addition, Actuators with a shallow initial electrode are able to more efficiently impart momentum and mechanical power into the flow. [9] [10]
No matter how much funding has been invested and the number of various private claims of a high induced speed, the maximum, average speed induced by plasma actuators on an atmospheric pressure conviction, without any assistant of mechanical amplifier (chamber, cavity etc.), is still less than 10 m/s. [11]
The surface temperature plays an important role in limiting the usefulness of a dielectric barrier discharge plasma actuator. The thrust produced by an actuator in quiescent air increases with a power law of the applied voltage. For voltages greater than a threshold, the exponent of the power-law reduces limiting the thrust increase, and the actuator is said to have “saturated,” limiting the actuator’s performance. The onset of saturation can visually be correlated by the inception of filamentary discharge events. The saturation effect can be manipulated by changing the local surface temperature of the dielectric. [12] Also, when dealing with real-life aircraft equipped with plasma actuators, it is important to consider the effect of temperature. The temperature variations encountered during a flight envelope may have adverse effects in actuator performance. It is found that for a constant peak-to-peak voltage the maximum velocity produced by the actuator depends directly on the dielectric surface temperature. The findings suggest that by changing the actuator temperature the performance can be maintained or even altered at different environmental conditions. Increasing dielectric surface temperature can increase the plasma actuator performance by increasing the momentum flux whilst consuming slightly higher energy. [13] [14]
Although plasma actuators have been extensively characterized for their performance as flow control devices, the notion that they might fail under adverse conditions such as dew, drizzle or dust makes them less popular in practical applications. Earlier publications have shown the effect of moisture, [15] [16] water adhesion, [17] and even icing. [18] [19] A recent publication has simulated light rain by directly spraying water droplets on to a working plasma actuator and showed its effect on thrust recovery as the performance metric. [20] It was shown that wet actuators quickly recover plasma glow, and gradually regain thrust comparable to the dry actuator.
Some recent applications of plasma actuation include high-speed flow control using localized arc filament plasma actuators, [21] and low-speed flow control using dielectric barrier discharges for flow separation, replacing mechanical high-lift devices, [22] 3D wake control, [23] sound control, [24] andsliding discharges. [25] The present research of plasma actuators is mainly focused on three directions: (1) various designs of plasma actuators; (2) flow control applications; and (3) control-oriented modeling of flow applications under plasma actuation. In addition, new experimental and numerical methods [26] are being developed to provide physical insights.
A plasma actuator induces a local flow speed perturbation, which will be developed downstream to a vortex sheet. As a result, plasma actuators can behave as vortex generators. The difference between this and traditional vortex generation is that there are no mechanical moving parts or any drilling holes on aerodynamic surfaces, demonstrating an important benefit of plasma actuators. Three dimensional actuators such as Serpentine geometry plasma actuator generate streamwise oriented vortices, [22] [27] which are useful to control the flow. [28] Recent work showed significant turbulent drag reduction by modifying energetic modes of transitional flow using these actuators. [29]
Active noise control normally denotes noise cancellation, that is, a noise-cancellation speaker emits a sound wave with the same amplitude but with inverted phase (also known as antiphase) to the original sound. However, active noise control with plasma adopts different strategies. The first one uses the discovery that sound pressure could be attenuated when it passes through a plasma sheet. The second one, and being more widely used, is to actively suppress the flow-field that is responsible to flow-induced noise (also known as aeroacoustics), using plasma actuators. It has been demonstrated that both tonal noise [11] and broadband noise [24] (difference can refer to tonal versus broadband) can be actively attenuated by a carefully designed plasma actuator.
Plasma has been introduced to hypersonic flow control. [30] [31] Firstly, plasma could be much easier generated for hypersonic vehicle at high altitude with quite low atmospheric pressure and high surface temperature. Secondly, the classical aerodynamic surface has little actuation for the case.
Interest in plasma actuators as active flow control devices is growing rapidly due to their lack of mechanical parts, light weight and high response frequency. The characteristics of a dielectric barrier discharge (DBD) plasma actuator when exposed to an unsteady flow generated by a shock tube is examined. A Study shows that not only is the shear layer outside of the shock tube affected by the plasma but the passage of the shock front and high-speed flow behind it also greatly influences the properties of the plasma [32]
Plasma actuators could be mounted on the airfoil to control flight attitude and thereafter flight trajectory. The cumbersome design and maintenance efforts of mechanical and hydraulic transmission systems in a classical rudder can thus be saved. The price to pay is that one should design a suitable high voltage/power electric system satisfying EMC rule. Hence, in addition to flow control, plasma actuators hold potential in top-level flight control, in particular for UAV and extraterrestrial planet (with suitable atmospheric conditions) investigations.
On the other hand, the whole flight control strategy should be reconsidered taking account of characteristics of plasma actuators. One preliminary roll control system with DBD plasma actuators is shown in the figure. [33]
It can be seen that plasma actuators deployed on the both sides of an airfoil. The roll control can be controlled by activating plasma actuators according to the roll angle feedback. After studying various feedback control methodologies, the bang–bang control method was chosen to design the roll control system based on plasma actuators. The reason is that bang-bang control is time optimal and insensitive to plasma actuations, which quickly vary in difference atmospheric and electric conditions.
Another study for rolling moment control using three-dimensional actuation has also been reported for an aircraft wing where actuators were employed as the leading-edge slat, spoiler, flap, and leading-edge aileron. [22] Results show that the serpentine plasma actuators may be employed as high-lift devices (as DBD slat and DBD spoiler) working at low Reynolds numbers and they can have the same effect of a conventional aileron for normal flight maneuvering, with low power consumption.
Plasma-actuated heat transfer (or plasma-assisted heat transfer) is a method of cooling hot surfaces assisted by an electrostatic fluid accelerator (EFA) such as a dielectric barrier discharge (DBD) plasma actuator or corona discharge plasma actuator. Plasma-actuated heat transfer is one of the proposed applications of DBD plasma actuators, [34] and needle plasma actuator. [35]
All electronic devices generate excess heat which must be removed to prevent premature failure of the device. Since heating occurs at the device, a common method of thermal management for electronics is to generate a bulk flow (for example by external fans) which brings the cooler, ambient air into contact with the hot device. A net heat transfer occurs between the hotter electronics and cooler air, lowering the mean temperature of the electronics. In plasma-actuated heat transfer, EFA plasma actuators generate a secondary flow to the bulk flow, cause local fluid acceleration near the plasma actuator, and ultimately may thin the thermal and velocity boundary layer near the electronics. [36] [37] The result is that the cooler air is brought closer to the hot electronics, improving the forced air cooling. Plasma-actuated heat transfer may be used as a thermal management solution for mobile devices, notebooks, ultra-mobile computers, and other electronics or in other applications which use similar forced air cooling configurations. [38] [39]
In engineering applications which experience significantly high temperature environments such as those encountered in gas turbine blades, hot structures must be cooled to mitigate thermal stresses and structural failure. In those applications, one of the most common techniques used is film cooling where a secondary fluid such as air or another coolant is introduced to a surface in a high temperature environment. The secondary fluid provides a cooler, insulating layer (or film) along the surface that acts as a heat sink, lowering the mean temperature in the boundary layer. [40] Since the secondary fluid is injected onto the surface at discrete holes on the surface, a portion of the secondary fluid is blown off the surface (especially at high momentum ratios of injected air to cross flow), decreasing the effectiveness of the film cooling process. [40] In plasma-actuated heat transfer, EFA plasma actuators are used to control the secondary fluid via a dynamic force which promotes attachment of the secondary fluid to the hot surface and improves the effectiveness of the film cooling. [34] [41] [42] [43]
Various numerical models have been proposed to simulate plasma actuations in flow control. They are listed below according to the computational cost, from the most expensive to the cheapest.
The most important potential of plasma actuators is its ability to bridge fluids and electricity. A modern closed-loop control system and the following information theoretical methods can be applied to the relatively classical aerodynamic sciences. A control-oriented model for plasma actuation in flow control has been proposed for a cavity flow control case. [48]
Electrical discharge machining (EDM), also known as spark machining, spark eroding, die sinking, wire burning or wire erosion, is a metal fabrication process whereby a desired shape is obtained by using electrical discharges (sparks). Material is removed from the work piece by a series of rapidly recurring current discharges between two electrodes, separated by a dielectric liquid and subject to an electric voltage. One of the electrodes is called the tool, or simply the tool or electrode, while the other is called the workpiece-electrode, or work piece. The process depends upon the tool and work piece not making physical contact. Extremely hard materials like carbides, ceramics, titanium alloys and heat treated tool steels that are very difficult to machine using conventional machining can be precisely machined by EDM.
A magnetohydrodynamic drive or MHD accelerator is a method for propelling vehicles using only electric and magnetic fields with no moving parts, accelerating an electrically conductive propellant with magnetohydrodynamics. The fluid is directed to the rear and as a reaction, the vehicle accelerates forward.
A corona discharge is an electrical discharge caused by the ionization of a fluid such as air surrounding a conductor carrying a high voltage. It represents a local region where the air has undergone electrical breakdown and become conductive, allowing charge to continuously leak off the conductor into the air. A corona discharge occurs at locations where the strength of the electric field around a conductor exceeds the dielectric strength of the air. It is often seen as a bluish glow in the air adjacent to pointed metal conductors carrying high voltages, and emits light by the same mechanism as a gas discharge lamp (Chemiluminescence). Corona discharges can also happen in weather, such as thunderstorms, where objects like ship masts or airplane wings have a charge significantly different from the air around them.
Electrowetting is the modification of the wetting properties of a surface with an applied electric field.
A thermionic converter consists of a hot electrode which thermionically emits electrons over a potential energy barrier to a cooler electrode, producing a useful electric power output. Caesium vapor is used to optimize the electrode work functions and provide an ion supply to neutralize the electron space charge.
An ion-propelled aircraft or ionocraft is an aircraft that uses electrohydrodynamics (EHD) to provide lift or thrust in the air without requiring combustion or moving parts. Current designs do not produce sufficient thrust for manned flight or useful loads.
An electroactive polymer (EAP) is a polymer that exhibits a change in size or shape when stimulated by an electric field. The most common applications of this type of material are in actuators and sensors. A typical characteristic property of an EAP is that they will undergo a large amount of deformation while sustaining large forces.
A nonthermal plasma, cold plasma or non-equilibrium plasma is a plasma which is not in thermodynamic equilibrium, because the electron temperature is much hotter than the temperature of heavy species. As only electrons are thermalized, their Maxwell-Boltzmann velocity distribution is very different from the ion velocity distribution. When one of the velocities of a species does not follow a Maxwell-Boltzmann distribution, the plasma is said to be non-Maxwellian.
Plasma activation is a method of surface modification employing plasma processing, which improves surface adhesion properties of many materials including metals, glass, ceramics, a broad range of polymers and textiles and even natural materials such as wood and seeds. Plasma functionalization also refers to the introduction of functional groups on the surface of exposed materials. It is widely used in industrial processes to prepare surfaces for bonding, gluing, coating and painting. Plasma processing achieves this effect through a combination of reduction of metal oxides, ultra-fine surface cleaning from organic contaminants, modification of the surface topography and deposition of functional chemical groups. Importantly, the plasma activation can be performed at atmospheric pressure using air or typical industrial gases including hydrogen, nitrogen and oxygen. Thus, the surface functionalization is achieved without expensive vacuum equipment or wet chemistry, which positively affects its costs, safety and environmental impact. Fast processing speeds further facilitate numerous industrial applications.
Dielectric-barrier discharge (DBD) is the electrical discharge between two electrodes separated by an insulating dielectric barrier. Originally called silent (inaudible) discharge and also known as ozone production discharge or partial discharge, it was first reported by Ernst Werner von Siemens in 1857.
The plasma pencil is a dielectric tube where two disk-shaped electrodes of about the same diameter as the tube are inserted, and are separated by a small gap. Each of the two electrodes is made of a thin copper ring attached to the surface of a centrally perforated dielectric disk. The plasma is ignited when nanoseconds-wide high voltage pulses at kHz repetition rate are applied between the two electrodes and a gas mixture is flown through the holes of the electrodes. When a plasma is ignited in the gap between the electrodes, a plasma plume reaching lengths up to 12 cm is launched through the aperture of the outer electrode and into the surrounding room air. The cold plasma plume emitted by the plasma pencil can be used to kill bacteria without harming skin tissue.
A microplasma is a plasma of small dimensions, ranging from tens to thousands of micrometers. Microplasmas can be generated at a variety of temperatures and pressures, existing as either thermal or non-thermal plasmas. Non-thermal microplasmas that can maintain their state at standard temperatures and pressures are readily available and accessible to scientists as they can be easily sustained and manipulated under standard conditions. Therefore, they can be employed for commercial, industrial, and medical applications, giving rise to the evolving field of microplasmas.
A turbine blade is a radial aerofoil mounted in the rim of a turbine disc and which produces a tangential force which rotates a turbine rotor. Each turbine disc has many blades. As such they are used in gas turbine engines and steam turbines. The blades are responsible for extracting energy from the high temperature, high pressure gas produced by the combustor. The turbine blades are often the limiting component of gas turbines. To survive in this difficult environment, turbine blades often use exotic materials like superalloys and many different methods of cooling that can be categorized as internal and external cooling, and thermal barrier coatings. Blade fatigue is a major source of failure in steam turbines and gas turbines. Fatigue is caused by the stress induced by vibration and resonance within the operating range of machinery. To protect blades from these high dynamic stresses, friction dampers are used.
The Wingless Electromagnetic Air Vehicle (WEAV) is a heavier than air flight system developed at the University of Florida, funded by the Air Force Office of Scientific Research. The WEAV was invented in 2006 by Dr. Subrata Roy, plasma physicist, aerospace engineering professor at the University of Florida, and has been a subject of several patents. The WEAV employs no moving parts, and combines the aircraft structure, propulsion, energy production and storage, and control subsystems into one integrated system.
Atmospheric-pressure plasma is a plasma in which the pressure approximately matches that of the surrounding atmosphere – the so-called normal pressure.
Plasma is one of four fundamental states of matter characterized by the presence of a significant portion of charged particles in any combination of ions or electrons. It is the most abundant form of ordinary matter in the universe, mostly in stars, but also dominating the rarefied intracluster medium and intergalactic medium. Plasma can be artificially generated, for example, by heating a neutral gas or subjecting it to a strong electromagnetic field.
Plasma medicine is an emerging field that combines plasma physics, life sciences and clinical medicine. It is being studied in disinfection, healing, and cancer. Most of the research is in vitro and in animal models.
The serpentine plasma actuator represents a broad class of plasma actuator. The actuators vary from the standard type in that their electrode geometry has been modified in to be periodic across its span.
The MARHy Hypersonic low density Wind Tunnel, located at the ICARE Laboratory in Orléans, France, is a research facility used extensively for fundamental and applied research of fluid dynamic phenomena in rarefied compressible flows. Its name is an acronym for Mach Adaptable Rarefied Hypersonic, and the wind tunnel is recorded under this name in the European portal MERIL.
Subrata Roy is an Indian-born American inventor, educator, and scientist known for his work in plasma-based flow control and plasma-based self-sterilizing technology. He is a professor of Mechanical and Aerospace Engineering at the University of Florida and the founding director of the Applied Physics Research Group at the University of Florida.
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