Sergey Macheret

Magnetohydrodynamic Control of Hypersonic Flows and Scramjet Inlets Using Electron Beam Ionization

The possibility of controlling scramjet inlets in off-design conditions by operating a near-surface magnetohydrodynamic (MHD) system upstream of the inlet is examined. The required electrical conductivity in air is supposed to be created by electron beams injected into the air from the vehicle along magnetic field lines. A simple model of a beam-generated ionization profile is developed and coupled with plasma kinetics, MHD equations, and two-dimensional inviscid flow equations. Calculations show that an MHD system with reasonable parameters could bring shocks back to the cowl lip when flying at Mach numbers higher than those for which the inlet was optimized. The MHD effect is not reduced to heating only because the work by j X B forces is a substantial part of the overall effect. Power requirements for ionizing electron beams could be lower than the electrical power extracted with MHD, so that a net power would be generated onboard. Problems associated with high Hall fields are discussed

Modeling of dielectric barrier discharge plasma actuator in air

A detailed physical model for asymmetric dielectric barrier discharge (DBD) in air at low voltages (1.5–2 kV) is developed. Modeling of DBD with an applied sinusoidal voltage is carried out in two dimensions. The leading role of charging the dielectric surface by electrons in the cathode phase is shown to be critical, acting as a harpoon that pulls positive ions forward and accelerates the gas in the anode phase. The positive ion motion back toward the exposed electrode is shown to be a major source of inefficiency in the sinusoidal or near-sinusoidal voltage cases. Based on understanding of the DBD physics, an optimal voltage waveform is proposed, consisting of high repetition rate, short (a few nanoseconds in duration), negative pulses combined with a positive dc bias applied to the exposed electrode.

Modeling of dielectric barrier discharge plasma actuators driven by repetitive nanosecond pulses

A detailed physical model for an asymmetric dielectric barrier discharge (DBD) in air driven by repetitive nanosecond voltage pulses is developed. In particular, modeling of DBD with high voltage repetitive negative and positive nanosecond pulses combined with positive dc bias is carried out. Operation at high voltage is compared with operation at low voltage, highlighting the advantage of high voltages, however the effect of backward-directed breakdown in the case of negative pulses results in a decrease of the integral momentum transferred to the gas. The use of positive repetitive pulses with dc bias is demonstrated to be promising for DBD performance improvement. The effects of the voltage waveform not only on force magnitude, but also on the spatial profile of the force, are shown. The crucial role of background photoionization in numerical modeling of ionization waves (streamers) in DBD plasmas is demonstrated.

Vibrational energy transfer rates using a forced harmonic oscillator model

This paper addresses the analysis, validation, and application of analytic, nonperturbative, semiclassical vibration-translation (V-T) and vibration-vibration-translation (V-V-T) rate models for atom-diatom and diatom-diatom vibrational molecular energy transfer collisions. These forced harmonic oscillator (FHO) rate models are corrected and validated by comparison with recent experiments, and with three-dimensional semiclassical trajectory calculations for N 2 -N 2 , O 2 -O 2 , and N 2 -O 2 , which are considered to be the most reliable theoretical data available. A remarkably good overall agreement is shown for both the temperature and quantum number dependence of single-quantum and double-quantum V-V-T transitions in the temperature range 200 < T < 8000 K and for vibrational quantum numbers 0 < ν < 40. It is demonstrated that the multiquantum vibrational energy transfer processes occur via a sequential FHO mechanism, as a series of virtual single-quantum steps during one collision. An important exception, asymmetric multiquantum V-V exchange at low temperatures, that occurs via a direct first-order mechanism, is discussed. Analytic thermally averaged FHO V-T and V-V rates are suggested. The FHO model gives new insight into vibrational kinetics and may be easily incorporated into kinetic modeline calculations under conditions when first-order theories are not applicable.

Experimental investigation of dielectric barrier discharge plasma actuators driven by repetitive high-voltage nanosecond pulses with dc or low frequency sinusoidal bias

Experimental studies were conducted of a flow induced in an initially quiescent room air by a single asymmetric dielectric barrier discharge driven by voltage waveforms consisting of repetitive nanosecond high-voltage pulses superimposed on dc or alternating sinusoidal or square-wave bias voltage. To characterize the pulses and to optimize their matching to the plasma, a numerical code for short pulse calculations with an arbitrary impedance load was developed. A new approach for nonintrusive diagnostics of plasma actuator induced flows in quiescent gas was proposed, consisting of three elements coupled together: the schlieren technique, burst mode of plasma actuator operation, and two-dimensional numerical fluid modeling. The force and heating rate calculated by a plasma model was used as an input to two-dimensional viscous flow solver to predict the time-dependent dielectric barrier discharge induced flow field. This approach allowed us to restore the entire two-dimensional unsteady plasma induced flow ...

Electron-Beam-Generated Plasmas in Hypersonic Magnetohydrodynamic Channels

A novel concept is analyzed of hypersonic cold-air magnetohydrodynamic (MHD) power generators and accelerators with ionization by electron beams. Ionization processes are considered in detail. Strong coupling between hypersonic boundary layers and electrode sheaths is demonstrated, and anode voltage fall in hypersonic MHD channels is shown to be very high. A potential anode sheath instability and ways to suppress it are discussed. Electron beams are shown to be capable of generating an adequate conductivity in cold air, while allowing full control and stable operation of MHD channels. Example calculations of hypersonic accelerator and power generator performance appear to be promising.

Surface charge in dielectric barrier discharge plasma actuators

Direct measurements of the dielectric surface potential and its dynamics in asymmetric dielectric barrier discharge (DBD) plasma actuators show that the charge builds up at the dielectric surface and extends far downstream of the plasma. The surface charge persists for a long time (tens of minutes) after the driving voltage has been turned off. For a sinusoidal voltage waveform, the dielectric surface charges positively. With the voltage waveform consisting of nanosecond pulses superimposed on a dc bias, the sign of the dielectric surface charge is the same as the sign (polarity) of the bias voltage. The surface charging significantly affects DBD plasma actuator performance.

Semiclassical modeling of state-specific dissociation rates in diatomic gases

A nonempirical, containing no adjustable parameters, theoretical model is suggested for calculations of state-specific dissociation rates in diatomic gases. Effects of molecular rotation and three dimensionality of collisions are consistently accounted for. The model is based upon a modified forced harmonic oscillator (FHO) scaling, with anharmonic frequency correction and energy symmetrization. The FHO scaling allows close-coupled calculations of multiquantum transitions between vibrational states, and it requires evaluation of collisional energy transfer to classical oscillator. Three-dimensional classical energy transfer models in both free-rotation and impulsive (sudden) approximations were used in conjunction with the FHO quantum scaling. The new theory describes the role of various degrees of freedom in dissociation both qualitatively and quantitatively. One of the predictions is that at low and moderate temperatures, dissociation is strongly preferential, with state-specific rates sharply increasin...

Nonequilibrium dissociation rates behind strong shock waves : classical model

Abstract A model is suggested for the analytical calculation of dissociation rates behind shock waves where the vibrational temperature T v is less than the gas temperature T . The model is based on an analysis of the threshold translational energy for collision-induced dissociation as a function of initial vibrational and rotational energies. The threshold function method combined with a classical impulsive model for energy exchange yields explicit formulae for the rate coefficient k ( T v , T ) and the mean vibrational energy removed in dissociation. The mechanism of nonequilibrium dissociation is predicted to change during vibrational relaxation: dissociation from low vibrational levels dominates at low T v / T , while dissociation from all levels contributes almost equally as T v / T approaches unity. The formulae obtained exhibit an explicit dependence on the mass ratio of the dissociating molecule and its collision partner, the lighter mass of the partner making dissociation from high levels more favorable. Dissociation in a molecular gas at T > T v is demonstrated to occur predominantly via noncollinear collisions with simultaneous transfer of rotational and translational energy to the vibrational mode of the dissociating molecule.

Phenomenological Analysis of Shock-Wave Propagation in Weakly Ionized Plasmas

Shock propagation into weakly ionized gases shows several features differing markedly from conventional, nonionized-gas shock structure. Phenomenological analysis of general macroscopic features of the previously observed plasma shock effects allows only two possible interpretations: existence of an energy (momentum) flux toward the wave precursor or volumetric energy release (exothermic phase transition) in the upstream portion of the wave (precursor) followed by reverse transition in the downstream portion of the wave. It is shown that known microscopic mechanisms are not capable of producing such a flux or energy release: Typical processes involving electrons, ions, and excited species do not couple strongly to neutral atoms and molecules, and there is not enough energy stored in these species because of the low ionization fraction. The theoretical basis for phase transitions in low-density, weakly ionized plasmas also is unknown. Analysis of the steady two-wave system created by either of the two effects raises a question as to whether the observed plasma shocks are stable objects. Another question is whether there exists phase transition within the plasma shock. It also remains unclear to what extent twodimensional thermal inhomogeneity effects contribute to the observed phenomena. Answering these fundamental questions requires additional experimental studies of the problem. p D d E E/N F h j k