Flow Separation Control by Plasma Actuator with Nanosecond Pulsed-Periodic Discharge

A review is presented of the studies in the former Soviet Union and in the USA of the mutual interactions of plasmas and high speed flows and shocks. There are reports from as early as the 1980s of large changes in the standoff distance ahead of a blunt body in ballistic tunnels, significantly reduced drag and modifications of travelling shocks in bounded weakly ionized gases. Energy addition to the flow results in an increase in the local sound speed that leads to expected modifications of the flow and changes to the pressure distribution around a vehicle due to the decrease in local Mach number. The critical question was, did a plasma provide a significant energy multiplier for the system? There have been a large number of experimental studies on the influence of a weakly ionized plasma on relatively low Mach number shocks and inherently also on the influence of the shock on the plasma. This literature is reviewed and illustrated with representative examples. The convergence through more controlled experiments and improved modelling to a physics understanding of the effects being essentially due to heating is outlined. It is demonstrated that the heating in many cases is global; however, tailored experiments with positive columns, dielectric barrier discharges and focused microwave plasmas can produce very localized heating. The latter appears more attractive for energy efficiency in flow control. Tailored localized ionization and thermal effects are also of interest for high speed inlet shock control and for producing reliable ignition for short residence time combustors, and work in these areas is also reviewed.

https://iopscience.iop.org/article/10.1088/0022-3727/38/4/R01/pdf

Plasmas in high speed aerodynamics

The kinetic modeling of low-pressure (p ∼ 1−10 torr) stationary nitrogen discharges and the corresponding afterglows is reviewed. It is shown that a good description of the overall behavior of nitrogen plasmas requires a deep understanding of the coupling between different kinetics. The central role is played by ground-state vibrationally excited molecules, N2(X 1 Σ + g ,v ), which have a strong influence on the shape of the electron energy distribution function, on the creation and destruction of electronically excited states, on the gas heating, dissociation and on afterglow emissions. N2(X 1 Σ + ,v ) molecules are actually the hinge ensuring a strong link between the various kinetics. The noticeable task done by electronically excited metastable molecules, in particular N2(A 3 Σ + u )a nd N2(a � 1 Σ − u ), is also pointed out. Besides contributing to the same phenomena as vibrationally excited molecules, these electronic metastable states play also a categorical role in ionization. Furthermore, vibrationally excited molecules in high v levels are in the origin of the peaks observed in the flowing afterglow for the concentrations of several species, such as N2(A 3 Σ + ), N2(B 3 Πg), N + 2 (B 2 Σ + u ) and electrons, which occur downstream from the discharge after a dark zone as a consequence of the V-V up-pumping mechanism.

Kinetic modeling of low-pressure nitrogen discharges and post-discharges

The paper discusses recent results on the development of localized arc filament plasma actuators and their use in controlling high-speed and high Reynolds number jet flows. Multiple plasma actuators (up to 8) are controlled using a custom-built 8-channel high-voltage pulsed plasma generator. The plasma generator independently controls pulse repetition rate (0–200 kHz), duty cycle and phase for each individual actuator. Current and voltage measurements demonstrated the power consumption of each actuator to be quite low (20 W at 20% duty cycle). Emission spectroscopy temperature measurements in the pulsed arc filament showed rapid temperature increase over the first 10–20 µs of arc operation, from below 1000 °C to up to about 2000 °C. At longer discharge pulse durations, 20–100 µs, the plasma temperature levels off at approximately 2000 °C.Modelling calculations using an unsteady, quasi-one-dimensional arc filament model showed that rapid localized heating in the arc filament on a microsecond time scale generates strong compression waves. The results of the calculations also suggest that flow forcing is most efficient at low actuator duty cycles, with short heating periods and sufficiently long delays between the pulses to allow for convective cooling of high-temperature filaments. The model predictions are consistent with laser sheet scattering flow visualization results and particle imaging velocimetry measurements. These measurements show large-scale coherent structure formation and considerable mixing enhancement in an ideally expanded Mach 1.3 jet forced by eight repetitively pulsed plasma actuators. The effects of forcing are most significant near the jet preferred mode frequency (ν = 5 kHz). The results also show a substantial reduction in the jet potential core length and a significant increase in the jet Mach number decay rate beyond the end of potential core, especially at low actuator duty cycles.

https://iopscience.iop.org/article/10.1088/0022-3727/40/3/S06/pdf

Development and use of localized arc filament plasma actuators for high-speed flow control

An essential role of CO2 and H2O during single-walled CNT synthesis from carbon monoxide

The paper discusses measurements of vibration-to-vibration (V–V) energy transfer rates for CO–CO using time-resolved step-scan Fourier transform infrared spectroscopy of optically pumped carbon monoxide. In the experiments, time evolution of all vibrational states of carbon monoxide excited by a CO laser and populated by V–V processes (up to v∼40) is monitored simultaneously. The V–V rates are inferred from these data using a kinetic model that incorporates spatial power distribution of the focused laser beam, transport processes, and multi-quantum V–V processes. Although the model predictions agree well with the time-dependent step-scan relaxation data, there is variance between the model predictions and the up-pumping data, however. Comparison of calculations using two different sets of V–V rates with experimental spectra showed that the use of the semi-empirical V–V rates of DeLeon and Rich provides better agreement with experiment. It is also shown that the multi-quantum V–V rates among high vibrational quantum numbers, calculated by Cacciatore and Billing, are substantially overpredicted. The results provide some new insight into nonequilibrium vibrational kinetics, and also demonstrate the capabilities of the step-scan Fourier transform spectroscopy for time-resolved studies of molecular energy transfer processes and validation of theoretical rate models.

Time-resolved Fourier transform infrared spectroscopy of optically pumped carbon monoxide

CO/N 2 , CO/Ar/O 2 , and CO/N 2 /O 2 gas mixtures are optically pumped using a continuous wave CO laser. Carbon monoxide molecules absorb the laser radiation and transfer energy to nitrogen and oxygen by vibration–vibration energy exchange. Infrared emission and spontaneous Raman spectroscopy are used for diagnostics of optically pumped gases. The experiments demonstrate that strong vibrational disequilibrium can be sustained in diatomic gas mixtures at pressures up to 1 atm, with only a few Watts laser power available. At these conditions, measured first level vibrational temperatures of diatomic species are in the range T V =1900–2300 K for N 2 , T V =2600–3800 K for CO, and T V =2200–2800 K for O 2 . The translational–rotational temperature of the gases does not exceed T =700 K. Line-of-sight averaged CO vibrational level populations up to v =40 are inferred from infrared emission spectra. Vibrational level populations of CO ( v =0–8), N 2 ( v =0–4), and O 2 ( v =0–8) near the axis of the focused CO laser beam are inferred from the Raman spectra of these species. The results demonstrate a possibility of sustaining stable nonequilibrium plasmas in atmospheric pressure air seeded with a few percent of carbon monoxide. The obtained experimental data are compared with modeling calculations that incorporate both major processes of molecular energy transfer and diffusion of vibrationally excited species across the spatially nonuniform excitation region, showing reasonably good agreement.

Vibrational energy storage in high pressure mixtures of diatomic molecules

An experimental study of shock modie cation in an M =2:5 supersonic e ow of nonequilibrium plasma over a cone is discussed. The experiments are conducted in a nonequilibrium plasma supersonic wind tunnel. Recent experiments at the Ohio State University using a supersonic plasma e ow over a quasi-two-dimensional wedge showed that an oblique shock can be considerably weakened by a transverse rf discharge plasma. The previously observed shock weakening, however, has been found consistent with a temperature rise in the boundary layers heated by the discharge. In thepresent study, theboundary-layereffects on theshock waveare reduced by placing an entire cone model into a supersonic inviscid core e ow. The electron density in the supersonic plasma e ow in the test section is measured using microwave attenuation. The ionization fraction in the discharge is in the same range as in the previous plasma shock experiments, up to ne/N =(1.2‐3.0)£10 i7 . The results do not show any detectable shock weakening by the plasma. This strongly suggests that the previously observed shock weakening and dispersion in nonequilibrium plasmas are entirely due to thermal effects.

Nonequilibrium Radio Frequency Discharge Plasma Effect on Conical Shock Wave: M = 2.5 Flow

Experiments conducted in a new, small-scale, nonequilibrium plasma wind tunnel recently developed at Ohio State University are discussed. The facility provides a steady-state supersonic e ow of cold nonequilibrium plasma with well-characterized, near uniform, properties. The plasma is produced in aerodynamically stabilized highpressureglowdischargethatformstheplenumofthesupersonicnozzle.Thepossiblemodie cation ofthesupersonic e ow due to ionization is studied by measuring the angle of oblique shocks attached to the wedge located in the nozzle test section. The results do not show any detectable shock weakening or attenuation in weakly ionized nitrogen plasma, compared to the measurements in a nonionized gas e ow. Experiments in supersonic e owing nitrogen and helium afterglow also demonstrate a novel technique for high-density supersonic e ow visualization. It allows identifying all key features of the supersonic e ow, including shocks, boundary layers, e ow separation regions, andwakesby recording intensevisibleradiation of theweakly ionizedplasmas.Interpretation ofradiation intensitydistributionsinnonequilibriumsupersonice owingafterglowmayprovideinformationonkeymechanisms of energy storage and ultraviolet radiation in high-altitude rocket plumes. In addition, these e ow visualization experiments can be used for validation of multidimensional computer e ow codes used for internal e ow simulation.

Supersonic Nonequilibrium Plasma Wind-Tunnel Measurements of Shock Modification and Flow Visualization

Electron production rate and electron density in cold optically pumped CO–Ar and CO–N2 plasmas in the presence of small amounts of O2 and NO have been measured using a Thomson discharge probe and microwave attenuation. Nonequilibrium ionization in the plasmas is produced by an associative ionization mechanism in collisions of highly vibrationally excited CO molecules. It is shown that adding small amounts of O2 or NO (50–100 mTorr) to the baseline gas mixtures at P=100 torr results in an increase of the electron density by up to a factor of 20–40 (from ne<1010 cm−3 to ne=(1.5–3.0)×1011 cm−3). This occurs while the electron production rate either decreases (as in the presence of O2) or remains nearly constant within a factor of 2 (as in the presence of NO). It is also shown that the electron–ion recombination rates inferred from these measurements decrease by two to three orders of magnitude compared to their baseline values (with no additives in the cell), down to β≅1.5×10−8 cm3/s with 50–100 mTorr of oxy...

Elke Plonjes

Papers

Time-resolved Fourier transform infrared spectroscopy of optically pumped carbon monoxide

Vibrational energy storage in high pressure mixtures of diatomic molecules

Nonequilibrium Radio Frequency Discharge Plasma Effect on Conical Shock Wave: M = 2.5 Flow

Supersonic Nonequilibrium Plasma Wind-Tunnel Measurements of Shock Modification and Flow Visualization

Electron density and recombination rate measurements in CO-seeded optically pumped plasmas