Active flow control is a topic in full expansion due to associated industrial applications of huge importance, particularly for aeronautics. Among all flow control methods, such as the use of mechanical flaps, wall synthetic jets or MEMS, plasma-based devices are very promising. The main advantages of such systems are their robustness, simplicity, low power consumption and ability for real-time control at high frequency. This paper is a review of the worldwide works on this topic, from its origin to the present. It is divided into two main parts. The first one is dedicated to the recent knowledge concerning the electric wind induced by surface non-thermal plasma actuators, acting in air at atmospheric pressure. Typically, it can reach 8 m s−1 at a distance of 0.5 mm from the wall. In the second part, works concerning active airflow control by these plasma actuators are presented. Very efficient results have been obtained for low-velocity subsonic airflows (typically U∞ ≤ 30 m s−1 and Reynolds number of a few 105), and promising results at higher velocities indicate that plasma actuators could be used in aeronautics.

https://iopscience.iop.org/article/10.1088/0022-3727/40/3/S01/pdf

Airflow control by non-thermal plasma actuators

This paper describes the status circa 2001, of the HITRAN compilation that comprises the public edition available through 2001. The HITRAN compilation consists of several components useful for radiative transfer calculation codes: high-resolution spectroscopic parameters of molecules in the gas phase, absorption cross-sections for molecules with very dense spectral features, aerosol refractive indices, ultraviolet line-by-line parameters and absorption cross-sections, and associated database management software. The line-by-line portion of the database contains spectroscopic parameters for 38 molecules and their isotopologues and isotopomers suitable for calculating atmospheric transmission and radiance properties. Many more molecular species are presented in the infrared cross-section data than in the previous edition, especially the chlorofluorocarbons and their replacement gases. There is now sufficient representation so that quasi-quantitative simulations can be obtained with the standard radiance codes. In addition to the description and justification of new or modified data that have been incorporated since the last edition of HITRAN (1996), future modifications are indicated for cases considered to have a significant impact on remote-sensing experiments. (C) 2003 Elsevier Ltd. All rights reserved.

The HITRAN molecular spectroscopic database: edition of 2000 including updates through 2001

Actuators are transducers that convert an electrical signal to a desired physical quantity. Active flow control actuators modify a flow by providing an electronically controllable disturbance. The field of active flow control has witnessed explosive growth in the variety of actuators, which is a testament to both the importance and challenges associated with actuator design. This review provides a framework for the discussion of actuator specifications, characteristics, selection, design, and classification for aeronautical applications. Actuator fundamentals are discussed, and various popular actuator types used in low-to-moderate speed flows are then described, including fluidic, moving object/surface, and plasma actuators. We attempt to highlight the strengths and inevitable drawbacks of each and highlight potential future research directions.

Actuators for Active Flow Control

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

Localized arc filament plasma actuators are used to control an axisymmetric Mach 1.3 ideally expanded jet of 2.54 cm exit diameter and a Reynolds number based on the nozzle exit diameter of about 1.1×106. Measurements of growth and decay of perturbations seeded in the flow by the actuators, laser-based planar flow visualizations, and particle imaging velocimetry measurements are used to evaluate the effects of control. Eight actuators distributed azimuthally inside the nozzle, approximately 1 mm upstream of the nozzle exit, are used to force various azimuthal modes over a large frequency range (StDF of 0.13 to 1.3). The jet responded to the forcing over the entire range of frequencies, but the response was optimum (in terms of the development of large coherent structures and mixing enhancement) around the jet preferred Strouhal number of 0.33 (f = 5 kHz), in good agreement with the results in the literature for low-speed and low-Reynolds-number jets. The jet (with a thin boundary layer, D/θ ∼ 250) also responded to forcing with various azimuthal modes (m = 0 to 3 and m = ±1, ±2, ±4), again in agreement with instability analysis and experimental results in the literature for low-speed and low-Reynolds-number jets. Forcing the jet with the azimuthal mode m = ±1 at the jet preferred-mode frequency provided the maximum mixing enhancement, with a significant reduction in the jet potential core length and a significant increase in the jet centreline velocity decay rate beyond the end of the potential core.

Active control of high-speed and high-Reynolds-number jets using plasma actuators

Active control of high Reynolds number and high-speed jets has been hampered due to the lack of suitable actuators. Some of the attributes that would make an actuator suitable for such flows are: high amplitude and bandwidth; small size for distribution around the jet; phase-locking ability for jet azimuthal mode forcing; and sufficient ruggedness for hot jets. We have been developing a class of actuators termed ‘localized arc filament plasma actuators,’ which possess such characteristics. In this paper, we present the development and characterization of these actuators as well as preliminary results on their applications in high Reynolds number Mach 0.9 and ideally expanded Mach 1.3 jets.

Development and characterization of plasma actuators for high-speed jet control

Experimental studies of shock modification in weakly ionized supersonic gas flows are discussed. In these experiments, a supersonic nonequilibrium plasma wind tunnel, which produces a highly nonequilibrium plasma flow with the low gas kinetic temperature at M = 2, is used. Supersonic flow is maintained at complete steady state. The flow is ionized by a high-pressure aerodynamically stabilized dc discharge in the tunnel plenum and by a transverse rf discharge in the supersonic test section. The dc discharge is primarily used for the supersonic flow visualization, whereas the rf discharge provides high electron density in the supersonic test section. High-pressure flow visualization produced by the plasma makes all features of the supersonic flow, including shocks, boundary layers, expansion waves, and wakes, clearly visible. Attached oblique shock structure on the nose of a 35-deg wedge with and without rf ionization in a M = 2 flow is studied in various nitrogen-helium mixtures. It is found that the use of the rf discharge increases the shock angle by 14 deg, from 99 to 113 deg, which corresponds to a Mach number reduction from M = 2.0 to 1.8. Time-dependent measurements of the oblique shock angle show that the time for the shock weakening by the rf plasma, as well as the shock recovery time after the plasma is turned off, is of the order of seconds. Because the flow residence time in the test section is of the order of 10 μs, this result suggests a purely thermal mechanism of shock weakening due to heating of the boundary layers and the nozzle walls by the rf discharge. Gas flow temperature measurements in the test section using infrared emission spectroscopy, with carbon monoxide as a thermometric element, are consistent with the observed shock angle change. This shows that shock weakening by the plasma is a purely thermal effect. The results demonstrate the feasibility of both sustaining uniform ionization in cold supersonic nitrogen and airflows and the use of nonequilibrium plasmas for supersonic flow control. This opens a possibility for the use of transverse stable rf discharges for magnetohydrodynamic energy extraction and/or acceleration of supersonic airflows.

Shock Wave Control by Nonequilibrium Plasmas in Cold Supersonic Gas Flows

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.

Peter Palm

Papers

Development and characterization of plasma actuators for high-speed jet control

Shock Wave Control by Nonequilibrium Plasmas in Cold Supersonic Gas Flows

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