James Menart

Hypersonic Flow Control Using Surface Plasma Actuator

Plasma-fluid-dynamic interaction has been shown to be a viable mechanism for hypersonic flow control. An effective and verified flow control process using direct current surface discharge is summarized. The operating principle is based on a small electromagnetic perturbation to the growth rate of the displacement thickness of a shear layer that is strongly amplified by a subsequent pressure interaction. The aerodynamic control is delivered in less than a millisecond time frame and produces no parasitic effect when deactivated. The magnitude of the resultant aerodynamic force and moment can be significant and does not require a large amount of power for plasma generation to overcome the inefficient ionizing process, thus reducing the weight of a high-speed vehicle. The electromagnetic perturbation is derived from a surface gas discharge with or without an externally applied magnetic field. An embedded plasma actuator near the leading edge of a flat plate has produced high surface pressure equivalent to more than a 5 deg flow deflection at Mach 5, and the flow control effectiveness will increase with an increasing oncoming Mach number. The detailed flow structure of weakly ionized airstreams has been investigated by a combination of experimental effort and computational simulation solving the magneto-fluid-dynamic equations in the low magnetic Reynolds number limit with a drift-diffusion plasma model. The identical plasma actuator is investigated as a variable geometry cowl of a hypersonic inlet. All phenomena are replicated by computational results and are fully validated by experimental observations.

Rotational and Vibrational Temperature Distributions for a Dielectric Barrier Discharge in Air

Spatially resolved rotational and vibrational temperatures for N 2 and rotational temperatures for N + 2 , as a function of voltage, have been obtained for an asymmetric surface mode dielectric barrier discharge using emission spectroscopy. The rotational temperatures were obtained from a nonlinear least-squares fit of a two-temperature theoretical spectrum with the measured spectra of the N 2 (C 3 Π u ― B 3 Π g ) and N + 2 (B 2 Σ + u ― X 2 Σ + g ) electronic band systems. The vibrational temperatures were obtained by applying the Boltzmann plot method to the Δv = ―2 sequence of the N 2 (C 3 Π u ― B 3 Π g ) electronic band system. It was observed that the rotational temperatures for N 2 ? and N + 2 decreased in the induced flow direction and increased with increasing voltage. Values started at 390 ± 10 K and decreased to 340 ± 10 K for N 2 and started at 500 ± 15 K and decreased to 450 ± 15 K for N + 2 . The vibrational temperatures also decreased in the induced flow direction from 3250 to 2850 ± 300 K. A difference in rotational temperatures between N 2 and N + 2 was observed for all voltages studied, and these differences increased with increasing voltage. The rotational temperatures of both species fluctuated in the spanwise direction. These fluctuations damped out in the streamwise direction and were weakly correlated with the attachment points of the microdischarges on the edge of the exposed electrode.

Study of Plasma Electrode Arrangements for Optimum Lift in a Mach 5 Flow

Abstract : This work is an experimental effort to study the power efficiency of using a plasma discharge to alter the lift on a body or surface. In this paper several electrode geometries are considered in an effort to reduce the plasma power required for a given change in lift. The cathode electrode position and electrode size are studied. For all cases studied the anode electrode is kept the same. Results are presented for four different size cathodes and four different cathode positions. The primary result presented is the lift change produced by the discharge per unit power input. The lift is determined by measuring the deflection of the model under the applied plasma. This type of a measurement system has some advantages and disadvantages compared to a load cell lift measurement system used by the authors in past work. Results from each of these lift measurement tools compare well. Results for 9 and 24 mA DC discharges are shown in this paper. For the conditions utilized in this work the results indicate that both cathode position and cathode size affect the lift change caused by a plasma discharge per unit of power input.

Grid Study on Blunt Bodies with the Carbuncle Phenomenon

the capturing of shock waves when low-dissipative, upwind schemes are used to numerically analyze high speed flows. The carbuncle phenomenon is best illustrated by the distorted bow shock predicted upstream of a blunt body in a supersonic flow. Various cures have been proposed for the carbuncle problem. These cures generally involve adding dissipation to the numerical routine in order to eliminate the carbuncle. This work will show results of a detailed study of how the structured grid aects the carbuncle phenomenon and how well it captures a strong shock. The CFD (computational fluid dynamics) code used to perform this study is AVUS (Air Vehicles Unstructured Solver). During this work it was found that heat transfer profiles in the stagnation region of a hypersonic blunt body are sensitive to perturbations upstream in the flow field. It is believed that the upstream perturbations are errors generated in the shock which are convected downstream to the surface of the blunt body. These upstream perturbations are amplified in the calculated heat flux profiles on the body surface. These errors arise from the Riemann solver which depends on grid quality in the region of the shock. It appears that the grid quality in the region of the shock is a major factor contributing to the inability of some Riemann solvers to accurately predict the flow field in this region. The grid study proposed here will provide recommendations on what types of structured grids should be used to accurately capture strong shocks and accurately predict the heat transfer profile on a body surface.

PRIMARY ELECTRON MODELING IN THE DISCHARGE CHAMBER OF AN ION ENGINE

In this work a computer code called P RIMA was further developed and utilized to study the motion of primary electrons in a two -dimensional, axisymmetric discharge chamber of an ion engine. Primary electrons are key to ion engine performance in that they produce the ions that produce the thrus t. Maxwell equations are solved to determine the magnetic field and the equations of motion are solved to determine the trajectory of the primary electrons. A study was conducted to gain insights as to what constitutes a good magnetic field. This was done by looking at three types of ideal magnetic fields and several, ring -cusp configurations. The actual discharge chamber design parameters studied are the discharge chamber shape and the number of magnetic rings.

Numerical Study of High-Intensity Free-Burning Arc

The behavior of a free-burning argon arc including the cathode region is investigated from a theoretical perspective. Two-dimensional differential equations describing the conservation of mass, momentum, energy, and electrical current density are solved together with Ohms law and Maxwells equation for the magnetic field in a cylindrical coordinate system using an iterative finite volume method. Recent data of the radiative losses from an argon plasma in the form of net emission coefficients are used. Simulations are made at various electrical currents for different electrode gaps. Predicted isotherms of the arc are in fair agreement with existing experimental results. Particularly, reasonable profiles of current density at a plane beneath the cathode tip are predicted, even though nonequilibrium behavior in the cathode sheath is neglected. Comparisons of the present current density profile with reported results are made. In addition, the effect of the length scale required in the use of net emission coefficients to model the radiative characteristics of the arc is studied.

Dynamic Electric Field Calculations Using a Fully Kinetic Ion Thruster Discharge Chamber Model

In this work, we present a fully kinetic particle-in-cell - Monte Carlo Collision (PIC-MCC) computer model developed to calculate the dynamic electric field inside an ion engine discharge chamber. This model self- consistently tracks primary electrons, secondary electrons, singly charged and doubly charged xenon ions, and xenon neutrals inside the discharge chamber. Both electric and magnetic field effects are included in the particle tracking. The plasma potential results are evaluated at every time step based on the charged particle distribution. This model avoids using an artificial inflated permittivity assumption and also avoids the use of the experimentally measured plasma potential data in the electric field calculations. Instead, a self-similar scaling system is considered. The computer model has been applied to study NASAs Next Generation Xenon Thruster (NEXT) operating at 5.3 kW, 3.1 A beam current, and 1567 V beam voltage. We discuss the recent algorithm development and present preliminary results such as the electric potential maps, the particle number density distributions, and the particle energy distributions from the NEXT ion thruster discharge chamber simulations.

Application of Plasma Discharge Arrays to High-Speed Flow Control

Surface DC plasma discharges were created in the boundary layer of a plate in a Mach 5 flow. The electrodes consisted of three circular cathodes and three pin anodes arranged to create transverse discharges. The cathodes were arranged linearly along the centerline of the plate, with the anodes displaced laterally. The cathodes and anodes were each held at common voltages. The discharge was run in two configurations, one with a single cathodeanode pair lit, and one in which all three cathode-anode pairs were lit. The discharge from these electrode configurations and its effect on the flow were strikingly different from the streamwise discharge created by linear electrodes. Flow modification from the circular electrode discharges was confined primarily to the boundary layer. The circular electrode discharges act similarly to crossflow jets or bumps , creating a weak wave structure with a pronounced vortex in the boundary layer downstream of the cathode. In contrast to the linear electrode discharges, surface static pressures and the inviscid flow above the boundary layer were largely unaffected by the circular electrode discharge. Substantial boundary layer distortion from the circular electrodes occurred at powers as low as 15 Watts. The circular cathode arrays have potential applications as high -bandwidth vortex generators.

Fully Coupled Electric Field/PIC-MCC Simulation Results of the Plasma in the Discharge Chamber of an Ion Engine

In this paper simulation results for the plasma in the NASAs Evolutionary Xenon Thruster (NEXT) ion engine discharge chamber are presented. The unique aspect of these results is that the Poisson equation solution of the electric field is fully coupled with the particle tracking portion of the particle- in-cell Monte Carlo collision (PIC-MCC) model. This means the effects of charged particles on the electric field are accounted for in a precise, detailed manner. This fidelity of a simulation has never been performed for the plasma in the discharge chamber of an ion engine, until now. In the past, the present authors have presented results where the particle tracking portion of the PIC-MCC solution was weakly coupled to the electric field solution. This approximation was made to reduce the computational time from years to weeks. In this work, full coupling is simulated. The reason this full coupling can be performed in weeks of computational time, instead of years, is the self-similar scaling routine used and the convergence routines used. Many results for the plasma in the discharge chamber are presented in this paper including neutral, first ion, second ion, primary electron, and secondary electron number density distributions; as well as electron energy distributions and, of course, the electric potential distribution. These results are presented for the NEXT throttling level TL35. Our simulation results have been validated against experimental plasma measurements made on the laboratory model NEXT ion thruster at the University of Michigan. In addition, self-consistent ion bombardment sputter yield calculations were performed with our PIC-MCC model to compute the erosion profile of the cathode keeper face plate.

Development of a Langmuir Probe for Plasma Diagnostic Work in High Speed Flow

At the present time there is an interest in controlling high-speed flow over a body utilizing a plasma and an applied magnetic field. In order to critically understand what is happening in this process it is necessary to characterize the plasma. This paper outlines the development of a Langmuir probe for this task and presents some data taken in a quiescent plasma. A Langmuir probe can be used to determine the electron number density, the electron temperature, and the electric field as a function of position in the plasma. From this information the electrical conductivity can be determined utilizing drift velocity data available in the literature. Both a single and a double Langmuir probe were developed and tested.