Quiescent Air Research Articles

Characteristics of pulsed plasma synthetic jet and its control effect on supersonic flow

The plasma synthetic jet is a novel flow control approach which is currently being studied. In this paper its characteristic and control effect on supersonic flow is investigated both experimentally and numerically. In the experiment, the formation of plasma synthetic jet and its propagation velocity in quiescent air are recorded and calculated with time resolved schlieren method. The jet velocity is up to 100m/s and no remarkable difference has been found after changing discharge parameters. When applied in Mach 2 supersonic flow, an obvious shockwave can be observed. In the modeling of electrical heating, the arc domain is not defined as an initial condition with fixed temperature or pressure, but a source term with time-varying input power density, which is expected to better describe the influence of heating process. Velocity variation with different heating efficiencies is presented and discussed and a peak velocity of 850m/s is achieved in still air with heating power density of 5.0×1012W/m3. For more details on the interaction between plasma synthetic jet and supersonic flow, the plasma synthetic jet induced shockwave and the disturbances in the boundary layer are numerically researched. All the results have demonstrated the control authority of plasma synthetic jet onto supersonic flow.

Dynamics of energy coupling and thermalization in barrier discharges over dielectric and weakly conducting surfaces on µs to ms time scales

The paper presents experiments characterizing discharge development and energy coupling in a surface dielectric barrier discharge (SDBD), atmospheric air plasmas over dielectric and weakly conducting surfaces, over a wide range of time scales and electrical conductivities. The experiments are done using nanosecond pulse (NS) both single polarity and alternating polarity) and ac voltage waveforms. Discharge development and mechanisms of coupling with quiescent air are analysed using nanosecond gate camera imaging, schlieren imaging, and laser differential interferometry. It is shown that NS SDBD plasmas generate stochastic, localized, near-surface perturbations on a long time scale (>100 μs) after the discharge pulse. These perturbations, entirely different from compression waves generated on a short time scale (~1–10 μs), are caused by discharge contraction and originate from the ends of the filaments. Surface conductivity has almost no effect on discharge behaviour if RC time of the conducting surface layer is much longer compared to the characteristic time of NS or ac voltage waveforms. In the opposite limit of short RC time, the conducting layer acts as an extension of the high-voltage electrode. Discharge contraction significantly increases energy stored on the dielectric surface, which in this case exceeds energy dissipated as Joule heat. The stored energy is dissipated if the discharge pulse is followed by an opposite polarity pulse. In a single polarity discharge, on the other hand, surface charge accumulation limits energy coupled to the plasma by subsequent pulses. The results demonstrate that surface plasma actuator control authority may be significantly increased by using an alternating polarity pulse waveform, which is more effective than the removal of surface charge between the pulses using a weakly conducting surface.

Effect of Pulse Polarity on Nanosecond Surface Dielectric Barrier Discharge

This paper presents the optical emission characteristics of a surface dielectric barrier discharge sustained by repetitive nanosecond pulses with different polarities in the subsonic airflows. The plasma images presented here show that the positive polarity pulsed discharge is stable in the airflows, appearing as a brush-type discharge. While the negative polarity pulsed discharge plasma becomes a filamentary pattern that is evidently spatial inhomogeneous in quiescent air, it behaves unstable in airflows.

Mediatorless Carbohydrate/Oxygen Biofuel Cells with Improved Cellobiose Dehydrogenase Based Bioanode

AbstractDirect electron transfer (DET) between cellobiose dehydrogenase from Humicola insolens ascomycete (HiCDH) and gold nanoparticles (AuNPs) was achieved by modifying AuNPs with a novel, positively charged thiol N‐(6‐mercapto)hexylpyridinium (MHP). The DET enabled the use of the HiCDH enzyme as an anodic biocatalyst in the design of a mediatorless carbohydrate/oxygen enzymatic fuel cell (EFC). A biocathode of the EFC was based on bilirubin oxidase from Myrothecium verrucaria (MvBOx) directly immobilised on the surface of AuNPs. The following parameters of the EFC based on Au/AuNP/MHP/HiCDH bioanode and Au/AuNP/MvBOx biocathode were obtained in quiescent air saturated PBS, pH 7.4, containing: (i) 5 mM glucose‐open‐circuit voltage (OCV) of 0.65 ± 0.011 V and the maximal power density of 4.77 ± 1.34 μW cm−2 at operating voltage of 0.50 V; or (ii) 10 mM lactose‐OCV of 0.67 ± 0.006 V and the maximal power density of 8.64 ± 1.91 μW cm−2 at operating voltage of 0.50 V. The half‐life operation times of the EFC were estimated to be at least 13 and 44 h in air saturated PBS containing 5 mM glucose and 10 mM lactose, respectively. Among advantages of HiCDH/MvBOx FCs are (i) simplified construction, (ii) relatively high power output with glucose as biofuel, and (iii) the absence of the inhibition of the HiCDH based bioanode by lactose, when compared with the best previously reported CDH based bioanode.

Study of Shock and Induced Flow Dynamics by Nanosecond Dielectric-Barrier-Discharge Plasma Actuators

The shock wave behavior generated from a single shot of pulsed nanosecond dielectric-barrier-discharge plasma actuator with varying pulse voltages in quiescent air was studied by experiments and numerical simulations. The experiments included using the schlieren technique, a fast response pressure transducer, and a two-velocity-component particle image velocimetry system to measure the propagation of the shock wave, the shock overpressure, and the shock induced flow, respectively. For the numerical simulation, a simple “phenomenological approach” was employed by modeling the plasma region over the encapsulated electrode as a jump-heated and pressurized gas layer. The present investigation revealed that the behaviors of the shock wave generated by the nanosecond pulsed plasma were fundamentally a microblast wave, and their speed and strength were found to increase with higher input voltages. The blast wave occured about 1 to after the discharge of the nanosecond pulse, which was dependent on the input voltages, and then it decayed quickly from a speed of around () to a speed of around () within about (2–3 mm from the actuator surface). The shock-induced burst perturbations (overpressure and induced velocity) were found to be restricted to a very narrow region (about 1 mm) behind the shock front and lasted only for a few microseconds. In a relatively long time period after the discharge of the plasma, a fairly weak induced vortex structure was observed. These results indicated that the pulsed nanosecond dielectric-barrier-discharge plasma actuator had stronger local effects in time and spatial domain.

Acoustic forcing on swirling flow: experiments and simulation

We investigated the effect of sound irradiated from loudspeakers on the flow of preheated air in the combustion chamber of a swirl burner. The temporally periodic pattern of the flow generated by the sound was detected by fast particle image velocimetry (PIV), with a repetition rate that was adapted to the observation of 12 phase angles of the irradiated monochromatic sound. The strong observed movement of the air is related to the movement by the sound itself, as determined by the pressure measurements with microphones. The PIV measurements reveal also a nonlinear interaction between the irradiated sound and the precession of the vortex core. The accuracy of the sound measurements was tested by determining in quiescent air the acoustic velocity by microphones and as well by PIV; good agreement was obtained thereby. Numerical calculations, using large eddy simulation and accounting for the sound forcing by variation in the mass flow at the inlet of the computational domain, approximately reproduce some of the experimental results.

The characteristics of surface arc plasma and its control effect on supersonic flow

Abstract The characteristic of surface arc plasma included millisecond and microsecond actuation in supersonic flow is investigated both experimentally and numerically. In the experiment, the discharge characteristic of surface arc plasma in quiescent air and supersonic flow is recorded. The stable oblique shock could be observed with millisecond actuation. And the unstable compressive wave could be also observed with microsecond actuation. In the numerical investigation, plasma actuation is defined as a source term with input power density from discharge V–I characteristic, which is expected to better describe the influence of heating process. The numerical results are coincident with experimental results. In order to confirm the capability of surface arc plasma actuation to control supersonic flow, experimental investigations on control shock induced by ramp and separation of boundary layer induced by impinging shock are performed. All the results demonstrate the control effect of surface arc plasma actuation onto supersonic flow.

Experimental research of sliding surface distributed nanosecond discharge in supersonic air flow

Results are presented from the experimental study of the spatial structure and spectrum of the nanosecond distributed sliding surface discharge (plasma sheet) in quiescent air and in a supersonic and transonic gas flow behind a plane shock wave. Discharge investigations were carried out in air flow with velocities up to 1600 m/s within density range of 0.08-0.40 kg/m3. Different structures of plasma glow were observed for the laminar and turbulent flow regimes in the boundary layer. Analysis of the obtained images clearly shows that character of the glow of the discharge changes with change of the boundary layer structure. The analysis of the experimental data shows that the radiation spectrum of sliding surface discharge changes in gas flow after a shock wave

Turbulent Features in the Coherent Central Region of a Plane Water Jet Issuing Into Quiescent Air

A plane water jet issuing into quiescent air at a Reynolds number of 2.5 × 105 is experimentally studied using phase Doppler anemometry (PDA). The plane water jet contains a coherent central region, which is situated immediately downstream of the nozzle exit. Particular emphasis is placed upon the distinctive attributes of such a region. Both mean flow pattern and turbulent features are obtained statistically based upon instantaneous velocity data. The central region is overwhelmingly dominated by uniformly distributed velocity, and remarkably high velocity gradient is present near the boundary of this region. Evidence shows that self-preservation is not satisfied in the central region. Explicit energy dissipation mechanisms in the central region are appreciated from cross-sectional uniform distributions of Kolmogorov length scales. Turbulent kinetic energy increases as the plane jet progresses, which is opposite to the general tendency associated with self-preservation. Although the central region is filled with near-zero Reynolds shear stress, distributions of skewness and flatness in this region are non-Gaussian and the instability caused by small-scale flow structures is thereby substantiated.

Velocity field, surface profile and curvature resolution of steep and short free-surface waves

On steep, millimeter-scale, 2D water waves, surface profile, and subsurface velocity field are measured with high-spatio-temporal resolution. This allows resolving surface vorticity, which is captured in the surface boundary layer and compared with its direct computation from interface curvature and velocity. Data are obtained with a combination of high-magnification time-resolved particle image velocimetry (PIV) and planar laser-induced fluorescence. The latter is used to resolve the surface profile and serves as a processing mask for the former. PIV processing schemes are compared to optimize accuracy locally, and profilometry data are treated to obtain surface curvature. This diagnostic enables new insights into free-surface dynamic, in particular, wave growth and surface vorticity generation, for flow regimes not studied previously. The technique is demonstrated on a high-speed water jet discharging in quiescent air at a Reynolds number of 4.8 × 104. Shear-layer instability below the surface leads to streamwise traveling waves with wavelength λ ~ 2 mm and steepness \(2\pi a / \lambda \ge 2.0\), where a is the crest to trough amplitude. Flow structures are resolved at these scales by recording at 16 kHz with a magnification of 4.

Numerical simulation of nanosecond pulsed dielectric barrier discharge actuator in a quiescent flow

We present a numerical study of nanosecond pulsed dielectric barrier discharge (DBD) actuator operating in quiescent air at atmospheric condition. Our study concentrates on plasma discharge induced fluid dynamics and on exploration of parametric space of interest for voltage pulse in an attempt to shed some light into elucidation of the mechanisms whereby the generated shock wave propagates through and affects the external flow. Specifically, a one-dimensional, self-similar, local ionization kinetic model recently developed to predict key parameters of nanosecond pulsed plasma discharge is coupled with the compressible Navier-Stokes equations possibly for the first time. Within the considered range of parameters of the plasma model which is justified for the modeling of surface nanosecond pulsed discharge at atmospheric pressure, our coupled method is able to provide satisfactory prediction of the shock structure generated by the actuator for comparison with experiment, not only in the qualitative shock wave shape but also in quantitative shock front displacement. We provide a comprehensive analysis of the gas heating, shock wave initiation and evolution processes. For example, the characteristic time of the rapid localized heating responsible for shock wave generation, which is yet to be quantified experimentally, is found to be ∼350 ns. We conduct a parametric investigation by varying the peak voltage from 10 kV to 50 kV and rise time from 5 ns to 150 ns. The pressure wave whose behavior is found to be dominated by input voltage amplitude, introduces highly transient, localized disturbance to the quiescent air. In addition, the vortex induced by the shock passage is relatively weak. The interplay of the induced flows by a few successive plasma discharges operating at continuous mode does not appear to be significant, especially at low voltage amplitude.

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.

Nanosecond-pulsed plasma actuation in quiescent air and laminar boundary layer

An experimental investigation of the working principles of a nanosecond-pulsed dielectric barrier discharge (ns-DBD) plasma actuator has been conducted. Special emphasis is given on the thermal effects accompanying the rapid deposition of energy associated with this kind of actuation. A ns-DBD plasma actuator has been operated in quiescent air conditions as well as in a flat plate laminar boundary layer, with external flow velocity of 5 and 10 m s−1. Schlieren imaging and particle image velocimetry have been used to characterize the actuation. Additionally, the back-current shunt technique has been used for current measurements, from which energy input (per pulse) is calculated. Cases of 10-, 20- and 50-pulse bursts are tested. Schlieren imaging in still air conditions shows the formation of a high-temperature region in the vicinity of the discharge volume. The spatial extent of the visible ‘hot spot’ depends upon the number of pulses within the burst, following a power law. Schlieren imaging of the span-wise effect of the plasma actuator reveals weak compression waves originating from the loci of discharge filaments. The thermal ‘hot spots’ exhibit significant three-dimensionality. Particle image velocimetry is used to measure the velocity field resulting from the ns-DBDs acting on a laminar boundary layer. The disturbance leads to formation of a Tollmien–Schlichting wave train, with spectral content in good agreement with linear stability theory. It is observed that the group length of the wave train is proportional to the number of pulses within the burst.

Effects of the ambient temperature and initial diameter in droplet combustion

Results are presented from transient numerical simulations of combustion of an isolated heptane (n-C7H16) droplet in nearly quiescent ambient air. The focus is on the effects of environment temperature and droplet size on the combustion characteristics. Droplet sizes range from 10μm to 1mm, environment temperatures range from 800K to 1600K, and pressure is 1atm.The results show that ignition delay times have a very moderate dependence on droplet diameter. With an increase in droplet diameter, the ignition delay time first decreases near the ignitable diameter and then increases far from the ignitable diameter. The ignition stand-off ratio always decreases with an increase in droplet diameter. A strong but short-lived convection, which is generated following the ignition, is demonstrated for the first time. The convection is caused by gas expansion which ‘pushes’ the droplet toward the incoming flow. At lower temperatures, the transition from the kinetically controlled regime to the diffusion controlled regime is shifted to larger droplet diameters, and a larger range of droplet sizes is affected by the transition. Flame stand-off ratios are higher, and flame temperatures are lower, for small droplets. The transient evolution of heat release rate in droplet flames is presented for the first time. The heat release rate first exhibits thermal runaway (ignition), then decreases once the diffusion flame is established, and finally increases precipitously towards the end of the droplet lifetime due to an increase in strain rate. The heat release rates exhibit a strong dependence on droplet diameter.Finally, a comparison of the combustion characteristics of methanol and heptane droplets is presented, which establishes the differences in the phenomena reported in this paper and those reported for methanol. The increase in flame stand-off ratios and fuel vapor accumulation effects as droplet size is reduced, are qualitatively the same as those reported for methanol. The gasification rates and droplet surface temperatures for heptane exhibit less variation. Heptane flame locations lie in the fuel-rich zone whereas methanol flames are located at near stoichiometric mixture fractions.

Effects of pulse polarity on nanosecond pulse driven dielectric barrier discharge plasma actuators

Nanosecond pulse driven dielectric barrier discharge plasma actuators are studied in quiescent air using a power supply capable of producing negative and positive polarity waveforms. High voltage pulses are applied to the exposed electrode of typical asymmetric actuator geometry. In addition to polarity, the effects of pulse amplitude, actuator length, and dielectric thickness are also investigated. Schlieren images are used to estimate the relative near surface gas heating, while electrical measurements are acquired simultaneously. Negative polarity pulses develop slightly more energy per unit length for thin dielectrics, while positive polarity is slightly higher for thick dielectrics. In most cases, the difference in per unit length energy produced by positive and negative pulses on equivalent actuators is not outside the measurement uncertainty. Negative polarity pulses are found to produce a stronger pressure wave for a given peak voltage and pulse energy across the test matrix. Results indicate that the negative polarity pulse more efficiently couples electrical energy to the near surface gas as heat. This suggests negative polarity pulses may be preferred for aerodynamic flow control applications employing this actuator arrangement.

Investigation on the shockwave induced by surface arc plasma in quiescent air

The shockwave induced by surface direct-current (DC) arc discharge is investigated both experimentally and numerically. In the experiment, the shockwave generated by rapid gas heating is clearly observed from Schlieren images. The peak velocity of the shockwave is measured to be over 410 m/s; during its upright movement, it gradually falls to about 340 m/s; no remarkable difference is seen after changing the discharge voltage and the pulse frequency. In the modeling of the arc plasma, the arc domain is not simulated as a boundary condition with fixed temperature or pressure, but a source term with a time-varying input power density, which could better reflect the influence of the heating process. It is found that with a reference power density of 2.8×1012 W/m3, the calculated peak velocity is higher than the measured one, but they quickly (in 30 μs) become agreed with each other. The peak velocity also rises while increasing the power density, the maximum velocity acquired in the simulation is over 468 m/s, which is expected to be effective for high speed flow control.

S0550106 多電極プラズマアクチュエータ誘起噴流の推力向上([S055-01]プラズマアクチュエータ(1))

The flow control performance and jet vectoring characteristics of trielectrode (TED) plasma actuators with a sawtooth electrode on quiescent air were analyzed. To examine the body force generated from the actuator, the thrust of the induced jet was measured, as a reaction of the aerodynamic force from the actuator. The corresponding induced jet was visualized by means of high-speed Schrielen Visualization with the discharge conditions to understand physical phenomena on the actuator. The experiments show that the thrust generated by the TED actuator significantly enhanced by utilizing the sawtooth electode. When applying high negative DC voltage, sliding discharge channels between the two exposed electrodes were stabilized and enhanced. The induced flow at this condition forms strong inverted jet.

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.

Transport phenomena within the liquid phase of a laboratory-scale circular methanol pool fire

The effects of altering the lower thermal boundary condition of a methanol pool from −5°C to 50°C was investigated within a 90mm diameter and 12mm deep quartz burner under steady state burning condition in a quiescent air environment. Both the burning rate and the flame height were observed to increase by 15% with increasing bottom temperature over this range of bottom boundary conditions. The temperature and velocity within the liquid were measured by a single thermocouple traversed through the pool and PIV, respectively, in order to better understand the transport of mass and energy in the liquid. Temperature measurements revealed a distinct two-layer vertical thermal structure with the upper layer of the pool being almost uniform and near the boiling temperature of the fuel, while the lower layer experienced an increasing temperature gradient as the bottom boundary temperature was lowered. The thickness of the thermally uniform layer increased as the bottom temperature was increased. The measured fluid velocity showed a complementary two-layer structure with the upper layer being dominated by a pair of counter-rotating vortices that kept this portion of the liquid well mixed and transferred heat from the hot pool wall to the pool center, while the flow in the lower layer was uniformly low in value and vertical. A model was presented to aid in understanding the energy transfer within the liquid phase. In the lower layer, the Peclet Number was in the order of unity and required that the energy transfer throughout the liquid phase to be modeled as a combination of conduction and convection. Using this physical model, the change in burning rate over the full 55°C change in bottom temperature was predicted within 2%, thereby supporting the proposed mechanism for energy transfer into the pool’s depth.

Structural size and temperature dependence of solid to air heat transfer

Accurate prediction of heat transfer from micro and nanoscale devices is very important for their operation and reliability. While the general consensus is that the heat transfer coefficient increases at the smaller scales, the literature also indicates contradiction in the scaling across length scales. In this paper, we present an experimental technique to accurately characterize heat transfer from micro to nano scales using infrared microscopy on nanofabricated specimens that are integrated with micro heaters. The heat transfer coefficient for quiescent air is found to be about two orders of magnitude higher than its bulk counterpart. It also showed pronounced inversely proportional relationship with temperature and suggested that solid to air conduction is the dominant mode of heat transfer. It was found that pressure only indirectly influences heat transfer. Based on the experimental data, we present an empirical relationship for the heat transfer coefficient that depends on the ratio of the surface area to cross-sectional area, temperature of the heated solid and its proximity to other solid bodies.