M. N. Shneider

Effect of continuous current during pauses between successive strokes on the decay of the lightning channel

A one-dimensional model is used to study the dynamics of the hydrodynamic parameters of the lightning channel in the return stroke and after the pulse current is damped. The effect of the continuous residual electric current during pauses between the successive strokes on the plasma cooling in the channel is analyzed. It is shown that a continuous electric current, which is several orders of magnitude lower than the peak current in the return stroke, is capable of maintaining the channel conductivity. This effect cannot be explained merely by Joule heating but is largely governed by the fact that the turbulent heat transport is substantially suppressed. In this case, even a continuous current as low as 50–100 A is capable of maintaining the conductivity of the lightning channel at a level at which only M-components can develop in the channel rather than the dart leader of the subsequent stroke.

Near-electrode sheath dynamics, current anharmonicity and battery effect in asymmetrical, low-pressure radio frequency discharges

The asymmetrical RF discharge is considered on the basis of a simple model. Simple and clear formulae are given to describe the motion of near-electrode sheath-plasma interfaces and evolution of anharmonic current, and to describe self-bias when there is a blocking capacitor in an external circuit. It is shown that in the case of a rather high blocking capacitor in an asymmetric discharge, positive charge flows into the large electrode, not electrons (as usual). The battery effect is discussed, i.e. the arising of constant EFM and direct current flow in an asymmetric discharge in the case when only conductors are in the external circuit. The model and simple computational formulae are convenient to interpret experiments and to make it possible to estimate discharge and plasma parameters from electric measurements of current and potentials.

Experimental and Computational Investigation of Drag Reduction by Electric‐Arc Airspikes at Mach 10

Aerodynamic drag reduction effects of an on‐axis electric‐arc airspike have been investigated in Mach 10 flow using RPI’s 24‐inch Hypersonic Shock Tunnel (HST). The long‐range purpose of this study is to examine the feasibility of airspike‐augmented flight for future transatmospheric vehicles. The research objective was to characterize airspike phenomena as a function of input arc power (0 to 50‐kW) for a 6‐inch diameter blunt‐body with a fixed arc‐to‐forebody separation of 6.38 inches. An array of lead‐acid batteries supplied power for the self‐sustaining discharge. The joint experimental/ numerical investigation began with the acquisition of a substantial experimental database — against which CFD simulations could be calibrated. The results correlated very well. The HST tests were conducted under low enthalpy, ‘ideal gas’ conditions with a stagnation pressure of 260‐psia and total temperature of 560‐K. A fast response PCB accelerometer measured drag forces on the blunt‐body model during 1–5 ms of ‘unres...

Experimental and Computational Investigation of Hypersonic Electric‐Arc Airspikes

Drag reduction effects of an electric arc airspike in a hypersonic flow are currently being studied in the Rensselaer Polytechnic Institute 24‐inch Hypersonic Shock Tunnel (RPI HST). In tandem these results are being modeled computationally, and compared to existing theory. The arc is driven by a high current lead‐acid battery array, producing a maximum of 75‐kilowatts into the self‐sustaining electrical discharge. The test conditions were for Mach 10, 260 psia stagnation pressure, and 560 K stagnation temperature flow — a low enthalpy, “ideal gas” condition. Schlieren photographs are taken of the arc apparatus and downstream blunt body, with a variety of arc powers and source/body distances. Fast‐response accelerometers are used to measure drag on the hanging blunt body. These tests are conducted with and without the arc to establish the most efficient placement and power of the airspike. The computational effort employs the Euler gasdynamic equations to represent a heat source in flow conditions and geo...

Drag and Total Power Reduction for Artificial Heat Input in Front of Hypersonic Blunt Bodies

The effect of an air‐spike in hypersonic flow is considered in the paper. The similarity laws of shape dependence and shock wave parameters, as a function of the strength of the heat source and characteristics of incident flow, are given. The numerical modeling is performed, based on the Euler gasdynamic equations for conditions identical to those tested in the RPI Hypersonic Shock Tunnel (M=10.1), where heat deposition took place with and without a blunt body in the stream. Good agreement between the individual shock wave shapes given by asymptotic theory, numerical modeling, and experiment is demonstrated. Results of numerical modeling show significant drag reduction and confirm the energy efficiency of the air‐spike concept.

Combined Experimental and Numerical Investigation of Electric‐Arc Airspikes For Blunt Body at Mach 3

Electric‐arc airspike experiments were performed with a 1.25‐inch diameter blunt body in the vacuum‐driven Mach 3 wind tunnel at Rensselaer Polytechnic Institute. Schlieren movies at 30‐Hz frame rate were recorded of the airspike flowfields, revealing substantial evolution over the 6‐second run durations. Arc powers up to 2‐kW were delivered into the airspike by an arc‐welding power supply, using zirconiated tungsten electrodes. Aerodynamic drag was measured with a piezo‐electric load cell, revealing reductions up to 70% when the airspike was energized. The test article was a small‐scale model of the Mercury lightcraft, a laser‐propelled transatmospheric vehicle designed to transport one‐person into orbit. Numerical modeling of this airspike is based on the Euler gasdynamic equations for conditions identical to those tested in the RPI supersonic tunnel. Excellent agreement between the shock wave shapes given by first‐order asymptotic theory, numerical modeling, and experiment is demonstrated. Results of t...

Longitudinal structure of capacitative radiofrequency gamma discharge and its analogy with a direct current glow discharge

The longitudinal structure of a gamma -type RF discharge is computed and shown to have much in common with the structure of a direct current glow discharge. In a sufficiently long gap or at rather high pressures, there are regions of cathode fall, negative glow and Faraday dark space near each of the electrodes. A uniform positive column exists between the electrodes. This pattern is obtained when taking into account non-local effects of the electron spectrum and abandoning Townsends coefficient to describe the ionization, electron diffusion and electron losses as functions of a local field. The reverse field regions obtained numerically agree with experimental values. It is shown that very low magnitude of the field amplitude and mean electron energy in the plasma are not intrinsic properties of the gamma discharge. The low magnitudes are typical only for sufficiently short gaps and low pressures when the plasma is concentrated in the negative glow region and the Faraday dark space and there is no room for the positive column to form. In long gaps and for high pressure the positive column is formed where the field and electron temperature are as high as in an alpha discharge or in the positive column of a glow discharge.

Dispersion of sound induced by a non-resonant interaction of an optical lattice with collisional gases

Dependence of the speed of sound in carbon dioxide as a function of the optical lattice wavelength, varied by changing the angle between two intersecting laser beams, is shown numerically with the direct simulation Monte Carlo method. The change in the speed of sound is related the excitation of vibrational degrees of freedom when the optical lattice - gas interaction regime changes from high to low frequency.

1 Energy Addition in a Hypersonic Flow: Analysis of the Interaction Region

An upstream structure of a parabolic shock wave induced in a hypersonic flow by a steady-state high-intensity heat source is examined. A similarity analysis is used to derive a simple analytic expression that allows one to predict the shock wave upstream stand-off distance. The solution of Navier-Stokes is obtained to provide basis for the validation of the analytic expression; a good agreement is obtained between the analytical and numerical results for a number of power source intensities.