AIAA 92-3975

NASA Ames's Electric Arc-Driven Shock Tube Facility and Research on Nonequilibrium Phenomena in Low Density Hypersonic Flows

S. P. Sharma NASA Ames Research Center Moffett Field, CA

AIAA 17th Aerospace

Ground Testing Conference July 6-8, 1992 / Nashville, TN

For permission to copy or republlsh, contact the Amerlcan lnstltute of Aeronautlcs and Astronautics 370 L'Enfant Promenade, S.W., Washington, D.C. 20024

A I A A 92-3975 1

NASA AMES'S ELECTRIC ARC-DRIVEN SHOCK TUBE FACILITY AND RESEARCH ON NONEQUILIBRIUM PHENOMENA

I N LOW DENSITY HYPERSONIC FLOWS

by

Surendra P . Sharma. NASA Ames Research Center, Moffett Field, CA

Abstract Basic requirements for a ground test facility simu-

lating low density hypersonic flows are discussed. Such facilities should he able to produce shock velocities in the range of 10-17 km/sec in an initial pressure of 0.010 to 0.050 Torr. The facility should he equipped with di- agnostics systems to be able to measure the emitted radiation, characteristic temperatures and populations in various energy levels. In the light of these require- ments, NASA Ames's electric arc-driven low density shock tube facility is described and available exper- imental diagnostics systems and computational tools are discussed.

I. Introduction

Future space vehicles intended for use in Mars sprint missions' and aerobraking maneuversZ in upper at- mosphere, a t very high velocities, will face very hos- tile aerothermodynamic environment. The free stream densities may be as low as 0.01 Torr and the re-entry velocities as high as 17.0 km/sec. The flow field around such vehicles will be highly dissociated with a high de- gree of ionization. Our present day capability in being able to compute such a flow is rather limited3 and the experimental data for this velocity and density range are scarce.

\ ' v

W

e

In the low density and high Knudsen number envi- ronment the flow field becomes rarefied and the con- tinuum description of the fluid breaks down. The most useful solution technique for the rarefied flow fields is the Direct Simulation Monte Carlo (DSMC) method. The method simulates the fluid flow as a collection of particles that move through the physical space un- dergoing interparticle collisions and boundary interac- tions appropriate to the local flow conditions. DSMC models have been successfully validated against ex- perimental data for the simulation of low tempera- ture r ~ t a t i o n a l , ~ vibrational,' and coupled vihration- dissociation6 phenomena in diatomic molecules. How- ever, research simulation of ionization and radiative heat loss is still in its infancy.

The radiation emission in this regime is dominated by the free-free, hound-free transitions and strong atomic line radiation. Depending upon the optical

'Research Scientist. Associate Fellow AIAA.

path the atomic lines tend to be self-ahsorbed in the media. Thus, in a moderately optically thick medium, which is a more realistic scenario for this regime, com- plex line by line spatial integration is required. In an optically thick approximation, of course, only free- free and bound-free radiation need to he considered. Therefore, in this regime, the emphasis of the model is more on the radiative transport computations than on the accuracy of the chemical and/or thermodynamic nonequilibrium model, whereas in a moderate ioniza- tion regime, the reverse is true.

Electrons play a very dominant role in the excita- tion and emission process. Electrons are about 5 x lo6 times more efficient than neutral particles and about 230 times more efficient than the ions in causing elec- tronic excitations of atoms and molecules by collisions. Furthermore, experiments show7 tha t high electronic states, which primarily contribute to the emission, re- main in equilibrium with the electrons described by a local Saha equation. Thus, for singly ionized species ( N e - Ni,,,,), the population of excited electronic states is maintained proportional to the square of the electron number density. In which case

Spontaneous radiative Power = (1) hvAi,jN;gi - hvAijN,' 0: N,'

where Ai,j is the Einstein transition probability for spontaneous emission for the i -+ j transition, N; is the number density of the ith electronic level, g; is the degeneracy of the ith level and Ne is the electron num- ber density. The electrons not only affect the line ra- diation hut also the continuum radiation as well. The continuum radiation, either due to free-free transition ( a N:) or due to bound-free recombination process (a N , ) or due to densely packed bound-bound line ra- diations ( a N:), can add up t o a significant portion of the total radiative power, especially in an optically thick environment.

Since, chemical and thermodynamic processes in the relaxation zone are primarily binary, i.e. they re- sult from two body collisions, the intensity of radia- tion from such reactions is proportional to the density and the thickness of the relaxation zone is inversely proportional to the density. Consequently, as long as the hinarv mechanism dominates. the inteerated radi-

This paper is declared a work of US. Government and is not subject to copyright protection in the United Stales.

ation flux emitted from such a nonequilibrium region remains independent of density a t a given velocity"'

2 A I A A 92-3975

( “binary scaling”). In the low density, however, it is very likely that the “binary scaling” law will be violated. This may be caused by three factors: 1) collisional limiting,” which reduces the radiation in- tensity at low densities when the collisions to rnain- tain the population of excited states against the deple- tion by emission are not sufficient, 2) radiation cool- ing, which is a major phenomena in this regime, and 3) reabsorption of line radiation in an optically thick medium.

At low density, with decreased collision rate, the time lapsed between collisions is longer. At the same time, due to elevated translational temperature, the average number of collisions (e.g. Z,, ZR and Z”) required to achieve equilibrium in various degrees of freedom increase. For example, DSMC computat.ions4 show that for nitrogen the number of collisions to achieve rotational equilibrium likely to increase from 10 at T=1000 K to 15 at T=1000 K. In other words, whereas, the rotational relaxation time behind a 10.0 km/sec in 0.1 Torr Nz is about 3.0 x iisec, it will increase to 2.0 x 10V3 psec behind a 12.0 kin/sec shock in 0.01 Torr (Fig. 1 ) . Shock tube data”-” indicate that , even at moderate densitics and toinper- atures (for example, p l = l Torr and T ~ 7 2 0 0 I<) , the rotational temperature lags behind the translational temperature and the assumption T = Tn is violated. In the low density regimes the inequality between the rotational and translational will be enhanced.

At early stages of nonequilibrium behind a normal shock, when the electron number density is still low, the electron temperature, T, may be slightly ahead of the vibrational temperature, Tu, in this regime. Ilow- ever, after the onset of the electron avalanche, due to the large electron population, the vibrational terripera- ture most likely will be in equilibrium with the clcctron temperature. In expanding flows, on the other hand, three body and dissociative recombination processes are dominant and about 80% of the recombination en- ergy is transferred to the electrons, thereby raising the electron temperature well above the vibrational tem- perature. Therefore, in expanding flow the inequality between the vibrational and electron temperatures is prevalent throughout the flow.

From the discussion it evident that the nathire of relaxation phenomena in low density (< 0.050 Torr) is slightly different as compared to that in moderate density ( > 0.100 Torr) flows. In order to acquire the much needed data for this regime, one has to pay spe- cial attention to these features. At NASA Ames we are in a process of resurrecting the GO cm low density Electric Arc-driven Shock Tube (E.A.S.T.) facility in order to collect data on rotational, vibrational and ra- diative nonequilibrium. In this paper we present a brief description of the facility and its capability with an emphasis on the following points:

At first we will discuss A . Facility Requirements:

the essential features, including the simulation capabilities, of a low density hypersonic facility. In the light of these basic features we describe the Ames’s E.A.S.T. facility. A brief historical account of the past research activities in the low density environment is also included.

B. Diagnostics:: In the light of parameters relevant to this regime, the diagnostics systems available at the facility are reviewed.

C . DSMC and Fluid Codes: Status of DSMC codes available at NASA Ames, which will be exten- sively used to analyze the experimental data, is also examined. Future plans for model develop- ments are discussed.

11. Requirements for a Low Density Facility

A low density shock tube must have the following

(i) The driven tube must be operational down to a minimum initial pressure of 0.010 Torr.

(i i) Shock velocities up to 17 km/sec should be pos- sible to achieve.

( i i i ) In order to study the chemical kinetics at low densities by spectroscopic techniques, one must fullfil a primary prerequisite of producing the test gas sample spectroscopically clean. Metallic im- purities are specially of concern in steel driven tubes.I3 The tubes must be made of a material, such as aluminum, less susceptible to inducing impurities into the test gas. The vacuum system must maintain a pressure of about 10Wa Torr dur- ing pump down and the leak rates must be such that less than 1 ppm level of impurities may be assured.

(iv) On average one requires a test time of at least 3.0-3.5 psec to be able to capture the entire range of the relaxation process. Thus for a maximum shock velocity of 17 km/sec, one must be able to produce a test gas slug of 3.0 t 17.0 % 50 mm. For a given shock velocity, the tcst time in low density shock tubed4 is approximately propor- tional to - D z p l , where D is shock tube inter- nal diameter. Experiments”-I3 with the 10 cin driven tube indicate that for a shock velocity of 13.0 km/sec in an initial pressure of 0.1 Torr the test time of 0.5-1.0 psec can be achieved. Thus in order to achieve the same shock velocity in an initial pressure of 0.01 Torr with a test time of 3.5 psec, the driven tube diameter should be - 10.0~(0.01/0.1)(3.5/1.0) = 59 cm. Depen- dence of test time on the shock velocity is gov- erned by many factor^'^ and difficult to model for high velocities.

(v) At these low densities the boundary layer thick- ness is bound to be large. This thick bound- ary layer will tend to absorb a bulk of the ul- t ra violet radiation from the gas, which will be

operational capabilities:

A I A A 92-3975 3 t

the primary source of radiation a t these shock velocities.’ In order t o minimise this affect, the

the range of 3.0 - 13.0 km/sec have been obtained Following is a list of operating conditions: 2

driven tube should he larger in diameter. Also use of boundary layer stripper plate may he con- sidered.

111. NASA Ames’s E.A.S.T. Facility

d

The electric arc-driven shock-tube facility a t NASA Ames Research Center13 consists of one driver system and two parallel driven tubes. One is a 10 cm i.d. tube 12 m in length, and the other is a G O cm i.d. tube 21 m in length (Fig. 2). The 10 cm driven tube is used for experiments at moderate density (initial pressure, PI > 0.1 Torr) and the G O cm driven tube for low density experiments (PI < 0.050 Torr). The driver can be operated in two configurations: 1) a 17.7 cm conical configuration with a 10.16 cm exit diameter (driver volume = 632 cm3), and 2) a variable length (34 - 137 cm) IO-cm i.d. cylindrical configwadion (driver volume = 2,669 to 10,752 cm3). The length of the cylindrical drivers can be varied by using a Lexan filler plug. Schematic of the cylindrical drivcr are shown in Figure 3. The current collector ring, which acts as the electrode assembly for the driver, consists of t.wo coaxial copper cylinders. The outer cylinder flanged to the driver tube, is electrically grounded, while the inner cylinder is connected by a copper spring contact plate t o the main electrode. The high voltage electrode has a hollow core through which a rod extends back t o the piston of a pneumatic solenoid. A trigger wire, which is actuated by the solenoid, is used to activate the high voltage circuit. Several different materials have been used for the trigger wire, but most of the tests have been made with tungsten wire. The trigger wire is extended along the length of the arc chamber to the ground plate (Fig. 3). When the slack wire is drawn to the high-voltage electrode, the current flow is initiated. The thermionic emission from the trigger wire helps to ignite the d i~charge’~ .

Energy to the driver is supplied by a 1.24 mega Joule 40 kV capacitor energy storage system. The 6- tier capacitor bank has 220 capacitors. Ry using dif- ferent combinations of series-parallel connections the capacitance of the hank can be varied from 149 p F to its maximum value of 6,126 p F (1,530 p F for 40 kV operation). Nominal total system inductance exclu- sive of the load (arc) is 0.26 pH and the resistance is 1.6 mQ.

The electrical characteristics of the driver, such as the load current, are recorded during each run. The shock velocity is comput,ed by recording the time of shock arrival a t various locations along the length of the tube, using conductivity probes and digital coun-

Using the 60 cm driven tube, by varying the driverfdriven gas combination, driver charge pressure and preset capacitor hank voltages, shock vclocities in

\ ’ v

s.,

., ters.

a. Driver charge pressure: 8.0 - 27.2 a tm. b. Driven tube initial pressure: 0.010 - 3.0 Torr. c. Driver gas: Hz, He, Nz, Hz/ Ne. d. Driven gas: Air, H z , AI, Ne, Kr. e. Capacitor Bank:

(i) Voltage: 14.0 - 38.0 kV. (i i) Capacitance: 861.3 - 1530 pF.

In tests conducted before 1988 mylar diaphragms, 0.35 to 0.50 mm in thickness, have been used to sepa- rate the driver and driven tube sections. However, ex- periments have shown13 that mylar is a source of car- bon impurities which deposit on the walls of the driven tube and, as a result, the radiation spectra is over- whelmed by emission from CN Red and Violet hand systems. In a concertive effort to eliminate all sources test gas contaminations, mylar has been replaced by aluminum as a diaphragm material. The diaphragm ruptures due to the rise in pressure within the driver due to the electrical energy input during the capacitor discharge. There is a time lag of 20 to 40 psec be- tween the instant the breaking pressure is reached and the instant the diaphragm is fully open.

So far one set of experiments in the low density regime ( 0.010-0.050 Torr, nitrogen) using the 60 cm driven section has been conducted. These tests were conducted by the senior author in 1986.’’ Tests a t higher density were conducted by Park and Shirai (1- 4 Torr, IIz, AI, K r , 1978),16 and Reller (1-3 Torr, air, 1971).” The shock velocities achieved in these tests are plotted in Fig. 4 as functions of the initial pres- sure in the driven section. At an initial pressure of 0,010 Torr in nitrogen a shock velocity of 13.0 kmfsec have been achieved. This corresponds to an electric energy input of 123 J/cc to the driver gas with an initial charge of 20 kV a t the capacitor bank and a discharge current of 556 kA (Fig. 5). Using the 1.22 MJ capacitor bank, our experience shows that energy inputs up to 686 Jfcc to the driver gas are possible. Although, the shock velocity does not scale linearly with the energy input to the driver gas, theoretical computation^'^ show that, with energy input up to 686 Jfcc to the driver gas, shock velocities up to 20 km/sec in 0.010-0.020 Torr driven gas can he easily produced in this facility.

For ashock velocity range of 4-13 km/sec test times on the order of 700 to 10 psec, respectively, have been obtained.”-’’ The test times as functions of shock velocity are plotted in Fig. 6. From our experience with the 10 cm shock tube we know that there ex- its a threshold shock velocity beyond which the test time in the shock tube suddenly becomes close to zero. This threshold for the 10 cm driven tube is around 13 km/sec. For a 60 cm driven tube i t is likely to be much higher and it would he valid to extrapolate the avail-

4 A I A A 92-3975

able test time data to higher shock velocities. Such a extrapolation is performed in Fig. 6 and shown by solid line. From this extrapolation it appears that test times on the order of 4.0 psec a t a shock velocity of 16.0 km/sec are possible in this facility.

IV. Diagnostics Systems The diagnostics system available a t the facility

consists of 1) emission measuring devices, 2) flow- visualization techniques, and 3) laser based non- intrusive diagnostics.

A. Emission measurements

The emission diagnostic system comprises of 1) a linear intensified 700 active element diode array and a 2-D intensified CCD array with 576 x 384 ac.tive el- ements, both gateable within a time range of 30 11s - 2.5 /AS and both with a 2000-8000 8, spectral respoiise. The diode arrays are mounted a t the image plaiie of a f#/5.3 McPlierson Model 218 0.3 m spectromet~cr. A typical optical set-up for this measurement is shown i n Fig. 7. A special f#/1.5 spectrometer has also been designed for this purpose. For the measurement of the total radiation (2500-10000 A ) a special custom- designed spectrometer is available. Photomultiplier tubes (PMTs) are used to record the total radiation from the test as well as from the driver gas as they pass through the test section. The signal from the I’MTs is used to estimate the test time and to trigger the diode array system a t a given moment during the test history. A typical output from such a PMT is shown in Fig. 8.

Rotational and vibrational temperatures are de- duced from the experimental spectral data, by using the following methodology”. The basic steps f o r the procedure are as follows. After making a reasoiiable guess for the equilibrium temperature, the gas CONI-

position is computed using a suitable computer code. This gas composition, along with the equilibriuiii t,em- perature (T = T, = T, = T,,,, = Teq) is fcd into the ARCRAP5 codeI8 to produce a synthesized spec- trum. The synthesized spectrum is compared wit,li the experimental equilibrium spectrum, especially i n the spectral range of interest. The process is repeat,cd UII- til the synthesized spectrum is identical to tlre mea- sured one. This temperature a t which both spectra match is considered to he the equilibrium tempera- ture of the test gas. Then, for a given vibrational band, a plot of the parameter G ( ~ ) / G ( I ) ~ ~ as a func- tion of the rotational temperature is generated, wlicre G(r) is ratio of the intensities a t two selected points on the rotational-vibrational envelope and G ( I ) ~ ~ is the parameter G(r) for which T, = T e q . Finally, by computing tlre parameter G(r)/G(r)eq from tlre cxper- imental data the rotational temperature of thc !,est gas is read on this theoretical plot. A similar procedure is used for estimation of the vibrational temperature (for details please see Ref. 11). It should be noted that he-

u

fore using the ARCRAP5 and STRAP codes for this regime they will have to be modified as mentioned in Introduction and will be discussed later.

B. Flow-visualization A Nd:YAG laser doubled to 532 nm, with a 25 ns

laser pulse, is used to record holograms of a desired flow field. In order to acquire the holograms the laser light is split into two beams: 1) object beam and 2) rrfcrence beam. Both beams are expanded and col- limated, with the object beam passing through the test section and the reference beam outside the driven tube. Both beams fall onto an Agfa BE56 film plate. The path lengths of the two beams are matched within few centimeters. An etalon is used in the laser path just ahead of the beamsplitter to ensure that the co- herent length of laser light is larger than the path dif- fereutial between the two beams. The film is exposed twice: once during the test and the second reference exposure is performed soon before or after the test. In- terferograms are reconstructed from the single- plate, double-exposure holograms on a secondary reconstruc- tion optical set-up using an Argon ion laser. Such an interferogram of an expanding flow recorded a t the facility is shown Fig. 9. Also shown in the Figure is a synthetic interferogram generated from the flow ficld computations using a 2-D code available a t the

C. Non-intrusive Measurements of Thermodynamic Parameters

Measurement of Vibrational Populations using Raman Scattering: Spontaneous Raman scattering, in which the molecules in each of vibrational and rotational en- ergy levels modulate the incident laser frequency, can provide the needed data on the nonequilibrium pop- ulations a t any given instant. Although Raman scat- tering cross sections are very small, with the use of high power lasers, efficient collection optics and mod- ern niulti-channel detectors, reasonable output signal levels can be achieved a t moderate densities. Results from an on-going test show that, using an f#/5.3 col- lection system and a 250 mJ/pulse KIF excimer laser a t 248 nni, the Raman signal from a population of IO” c ~ r - ~ nitrogen molecules can produce 200 photo- electrons a t the detector, which by use o f a f#/1.5 col- lection system can be increased to about 1200 photo- electrons. Behind a 14.0 km/sec shock into 0.020 Torr air there will be about - 4 x 1OI6 nitrogen molecules in the ground state producing about 5 photo-electrons if the same optical setup is used. Due to high ro- tational temperatures the emission intensities will be spread out over larger number of rotational levels and, therefore, the estimate of 5 photo-electrons may be on higlier side. However, if the laser power is increased to 3J/pulse (dye laser), then the signal level can be increased to about 180 photo-electrons. In any case the signal from spontaneous Raman may be weak in

, /‘

L

fa~i1i ty . I~ ‘ / ‘d

.

L’

5

h 6 W

these regimes and some version of stimulated Raman scattering may he a better choice for this purpose.

Atomic oxygen concentration measurements: The con- centration of atomic oxygen can he monitored by mea- suring emission from excited states or by probing the ground state with single or multi-photon techniques. The lowest energy transition out of the ground state of oxygen is 3P -+ 3S. The ground state is split into three levels at 0, 158 and 227 cm-', so the 3P -+ 3S transition consists of lines a t 130.217, 130.486, and 130.603 nm. This transition is strongly allowed and the lines are relatively broad for concentrations greater than 10l6 ~ r n - ~ . Absorption measurements, therefore, are made far in the wings, where the effects of self-hroadening and foreign gas broadening are not well known. Therefore, a t first it is necessary to mea- sure the lineshape in the far-wing region and then i t will be possible to use this line-profile data for concen- tration measurements.

ArF excimer lasers have a limited tuning range around 193 nm. The output beam can he wavelength shifted in a hydrogen Raman shifter to provide a tune- able light source in the 130 nm region. The proposed layout is illustrated schematically in Fig. 10. The beam of an ArF excimer laser is focussed into a hydro- gen Raman cell. Stimulated Raman scattering occurs in the focal region, generating both Stokes and anti- Stokes lines with aspacingof4155 em-'. The6th anti- Stokes line generated by a 193 nm pump beam is a t 130 nm. Cooling the cell with liquid nitrogen reduces the Q-6ranch width and concentrates population into the J=0 and 1 levels, thus improving conversion efficiency. The output beam will he collimated with a MgFz lens and dispersed with a 20 degree MgF2 prism. An aper- ture selects the 130 nm beam and a narrowband filter reduces stray light. A MgFz flat and solar-blind pho- tomultiplier monitor the reference intensity, and the transmitted beam is directed through the test section. A monochromator with a solar-blind photomultiplier measures the transmitted intensity.

V. DSMC and Fluid Codes

Several DSMC codes are available a t the Aerother- modynamic Branch of NASA Ames applicable to low density environment. Among them are: 1) a code simulating 3-D flowz0 over general body geometries, with rotation, vibrational and chemical nonequilih- rium, and 2) a code for simulating 2-D axi-symmetric flowz1 in nozzles and plumes with rotational, vihra- tional, and chemical nonequilihrium. Also a number of 1-D ~ o d e s ~ , ~ ~ with simulation of rotational, vihra- tional and chemical nonequilibrium, including ioniza- tion and uncoupled radiation are available. A study is currently underway to assess the effects of ioniza- tion and radiation.22 These studies have been very successful and we feel we have improved significantly the physical modeling in Monte Carlo methods. Still,

v

4

availability of experimental data, especially in chemi- cally reacting flows, is a vital requirement for formal verification of these codes and models.

In order to compute the radiation emission from the test gas a computer program named NonEQuilih- rium AIr Radiation (NEQAIR) program was written by Parkz3 at NASA Ames, using the basic structure of a code written by Arnold et al.ld Two computer codes have been written to couple the one-dimensional fluid equations with the nonequilihrium radiation physics (NEQAIR): 1) STRAP (Shock Tube RAdiation Pro- gram) for the shock tube flow behind a normal shock, and 2) SPRAP (Stagnation Point RAdiation Program) for the stagnation streamline in the shock layer over a blunt body. Recently these program have been improved by Whitingz4 and combined into a single code. The improvements consist of 1) generaliza- tion of chemical reaction schemes, 2) improvement in the accuracy of partition functions and radiation cal- culation schemes, and 3) inclusion of the species C, CO, CN, and Cz in the excitation calculations. All of these codes assume Telec = T, = Tu and TR = T in the fluid computations with the average temper- ature, T, = in the rate equations. IIowever, for radiation computations, the NEQAIR code does not assume any given form of population distribution in the electronic states and, assuming a QSS (Quasi- Steady-State), solves for the populations in each pre- dominant electronic level a t each time step. A code named ARCRAP5", which can he used to compute the emission radiation from the test gas for a given set of temperatures and species concentration, is also available. These codes have been successfully used to reproduce and analyze the experimental da ta for mod- erate densities (p1 > 0.1 Torr). As an example, the emission radiation intensity as computed by STRAP for U,=lO.O km/sec in 0.10 Torr air, is plotted in Fig. 11 and is compared with experimental data.

VI. Future plans

A. Facility Upgrade

Spectroscopic measurement^'^ in the 10 cm driven tube have shown that steel walls are a source of serious test gas contamination leading to overwhelming set of spurious atomic lines in the recorded spectra emanat- ing from Fe, Cr and Ni. It is believed that organic impurities such as vacuum oil leaking into the vacuum system or unevaporated cleaning fluid left over dur- ing the assembly process, or simple salty water vapor from the coastal environment, transforms into organic acids under the ultraviolet radiation during the test and, thereby, dissolves the wall material. Emission from so formed steel vapor, which contains thousands of atomic lines, overwhelms the molecular spectra of the test gas. Since aluminum has fewer spectral lines, the test gas spectra taken in an aluminum driven tube contains very few spurious lines emanating from the

6 A I A A 92-3975

contaminants. In view of this finding, two options are being considered: 1) coating the existing stccl tube with a ceramic layer or 2) replacing the 60 crri steel tube with an aluminum tube.

In order t o reduce the impurities during thr load- ing of the driven tube with the test gas (p1 2 0.010 Torr), a new pumping system capable of producing a loWa Torr vacuum will he installed. Turbomolecular and cryogenic pumps are being considered for this pur- pose. The leak rate of such asystem will be maintained below loWa TorrJmin to ensure tha t the impurities are less than 1 ppm.

B . Modification of Computational Tools

As most of the molecules will be dissociated i n this regime, the bulk of radiation will emanate from strong atomic lines. The radiation from these strong lines will tend to get self absorbed, requiring the radiation com- putation to be strongly coupled with the fluid cqua- tion. The bookkeeping will require accounting of more than 2000 atomic line a t each step. At this point, i t will be instructive to recall t.hat for conrp~~t~at ion of nonequilibrium radiation the population of all t.lie dominant electronic states are determined by solving a large set of master equations a t each step. Incorpo- rating the strong coupling between the radiation and the fluid solutions in this environment is going to be a complicated and computationally expensive task

At the same time, due to large boundary laycr. the flow in a low density shock tube can not be simulated by a 1-D fluid model. The 2-D affects need to br ad- dressed. Although, boundary layer stripper platcs can reduce the absorption of uv radiation during the ex- periments, accumulative affects of the 2-D fluid flow on the emission can not be reduced. In order t o model the low density shock tube flow 2-D codes must. I x de- veloped. Efforts are already being made to incorporate strong radiation and ionization in DSMC c o d c ~ . ~ ~

VII. Conclusions

The low density electric arc-driven shock facility a t NASA Ames can produce shock velocities u p to 17 km/sec in a 0.010 Torr test gas with a test tirnc of a t least 4.0psec. The diagnosticssystems availableat the facility are capable of measuring the emission radia- tion, vibrational temperature, rotational temperature, and individual vibrational populations. The diagnos- tic system for the measurement of atomic oxygen i n the flow is under development. Various DSMC and COII-

tinuum codes capable of simulating rotational, vibra- tional, dissociative, chemical and radiative noircqoilib- rium, for moderate level of ionisation and wit.11 uncou- pled radiation, are available for data analysis.

A new aluminum 60 cm driven section and a vac- unm system will be installed in order t o provide spec- troscopically clean test gas. The computer codes needed for data analysis are being modified to incor-

~~ ~~~~

d

porate strong radiation coupling and the 2-D behavior ’_ I of low density shock tube flows. - v Raferences

’Walberg, G., “How Shall We Go To Mars? A Re- view of Mission Scenarios, ” AIAA Paper 92-0481, Jan- uary 1992.

‘Park, C., Howe, J. T., Jaffe, R. L. and Candler, G. V., “Chemical Kinetic Problems of Super-Escape Velocity Mars Entries: A Review and Extension,” To be puhlished.

3Sliarma, S. P., “Assessment of Nonequilibrium Ra- diation Computation Methods for Hypersonic Flows, ” To be published in International Journal of Modern Pkysics C .

“Boyd, I.D., “Rotational-Translational energy trans- fer in rarefied nonequilihrium flows, ’’ Physics of. Elu- ids A , Vol. 2, No. 3,1990, pp. 447-452.

5130yd, I.D., “Analysis of Vibrational-Translational Energy Transfer Using the Direct Simulation Monte Carlo Method, ” Physics of Fluids A , Vol. 3, No. 7, 1991, pp. 1785-1791.

‘Hoyd, I.D., “Analysis of Vibration-Dissociation- Recombination Processes Behind Strong Shock Waves of Nitrogen, ” Physics of. Fluids A , Vol. 4, No. 1, 1992, pp. 178-185.

Temperatures in Recombining Nozzle Flow of Ionized Nitrogen-Hydrogen Mixture: Part %Experiments, ” v Journal of Plasma Physics, Vol. 9, Part 2, 1973, pp. 217-234.

“I’eare, J . D., Georgiev, S., and Allen, R. A., “Ra- diation from the Nonequilibrium Shock Front,” AVCO Everett Research Laboratory, Everett, MA, Research Report 112, Oct. 1961.

’Camm, J. C., Kivel, R. L., Taylor, R. L., and Tcare, J . D., “Absolute Intensity of Nonequilibrium Radiation in Air and Stagnation Heating a t IIigh Alti- tudes,” AVCO Everett Research Laboratory, Everett, MA, Research Report 93, Dec. 1959.

“‘Park, C., “Radiation Enhancement by Nonequi- libriiiin in Earth’s Atmosphere, ’’ J . Spacecrafl and Rockets, Vol. 22, No.1, Jan.-Feb. 1985, pp. 27-36. Also AIAA Paper 83-0410.

“Sharma, S.P. and Gillespie, W.D., “Nonequilib- rium and Equilibrium Shock Front Radiation Measure- ments, ” Journal of Thermophysics and Heat Trans- f e r , Vol. 5 , No. 3, July 1991, pp. 257-265. Also AIAA Paper 90-0139, January 1990.

I2Sharma, S.P., Gillespie, W.D and Meyer, S.A., “Shock Front Radiation Measurements in Air,” AIAA

“Sharma, S.P. and Park, C., “Operating Charac- teristics of a 60 cm and a 10 cm Electric Arc Driven Shock Tube-Part I and Part 11, ” Journal of Thermo- physics and Heal Transfer , Vol. 4, No. 3 , July 1990, pp. 259-272. Also AIAA Paper 88-0142, January 1988.

7Park, C., “Comparison of Electron and Electronic \ ,J

Paper 91-0573, January 1991. _-

-

AIAA 92-3975 7

I4Mirels, H., “Shock Tube Test Time Limitations Due To Turbulent-Wall Boundary Layer, ’’ AIAA Journal, Vol. 2, No. 1, 1964, pp. 84-93.

“Sharma, S.P., Test 31, E.A.S.T. Facility Log Book, April 1986.

“Park, C., Test 26, E.A.S.T. Facility Log Book, August 1978.

17Reller, Jr., J . O., Test 100, E.A.S.T. Facility Log Book, Sept. 1971.

“Arnold, J . O., Cooper, D. M., Park, C. and Prakash, S. G . , “Line-by-Line Transport Calculations for Jupiter Entry Probes, ” Progress in Aeronautics and Astronautics, Vol. 69, Entry Heating and Ther- mal Protection , edited by Olstad, AIAA, New York, 1980, pp. 52-82.

”Sharma, S.P., Ruffin, S.R., Meyer, SA. , Gillespie, W.D. and Yates, L.A., “Density Measurements in an Expanding Flow using Holographic Interferometry, ” AIAA Paper 92-0809, January 1992.

20McDonald, J. D., “Particle Simulation in a Multi- processor Environment, ” AIAA Paper 91-1366, June 1991.

”Boyd, I. D., “Vectorization of a Monte Carlo Sim- ulation Scheme for Nonequilibrium Gas Dynamics, ’’ Journal of Computational Physics, Vol. 96, No. 2, 1991, pp. 411-427.

22Boyd, I. D. and Whiting, E. E., “Comparison of Radiative Heating Estimates Using Particle Simula- tion, ” AIAA Paper 92-2971, July 1992.

23Park, C., ‘Calculation of Nonequilibrium Radi- ation in the Flight Regimes of Aeroassisted Orbital Transfer Vehicles,” Progress in Aeronautics and As- tronautics: Thermal Design of Aeroassisted Orbital Transfer Vehicles, Vol. 96, edited by H.F. Nelson, American Institute of Aeronautics and Astronautics, New York, N. Y., 1985, pp. 395-418.

24Whiting, E. E. and Park, C., “Radiative Heating at the Stagnation Point of the AFE Vehicle, ” NASA TM 102829, November 1990.

-. 5

t /

U

8 A I A A 92-3975

1 o - 8 1 2 4 6 a 1 0 1 2 1 4

S h o c k v e I oc i t y , km/sec

Fig. 1 Computed relaxation time to achieve rotational equilibrium as a function of shock velocity.

Fig. 2 Ames 60-cm electric arc-driven low density shock tube facility.

A I A A 92-3975 9

- :: 6 -

2 5-

4- 0 0 .!= v) 3 -

COAXIAL CABLES TO CAPACITOR

% 0

0 Sharma (1986),l4N2 1% X Park and Shirai (1978),'5

10% H, + 90% Ar

LEXAN INSULATOR

Fig. 3 Schematics of the cylindrical driver. u

s 8 j .- - 7

- 0 Reller (1971),16Air

2 I ' ' ""'I ' ' I ""'I ' ' ' "1111 i; 0.001 0.01 0.1 1 1 0

Driven tube pressure, Torr 4

Fig. 4 Shock velocity achieved at Ames's facility at various initial pressures.

2

0 a,

E Y 1 0 -

S 8

9

.- - 7

- 8 6 a 5 > Y 4 0 0 E 3 v)

2

% Sharma (1986) 1978)' 0 Park and Shirai

A Reller (1971)"

/ , I I. 8 9 1 2 3 4 5 6

1 0 0

Driver energy, J/cc

Fig. 5 Shock velocity achievable as a function of driver energy.

41 2 O

m

Sharrna (1986),' 5N2

Park and Shirai (1978),16 10% H, + 90% Ar

I ?K Reller (1971),17 Air

1 1 0

-

2 3 4 5 6 7 8 I 2 1 I . . . . I

Shock velocity, km/sec

Fig. 6 Test times achieved at Ames's facility as functions of shock velocity.

10 A I A A 92-3975

E’ - 5 0 -

2-1 0 0 -

5

tij - 2 0 0 -

.. c.

c

- - 1 5 0 - a C 0,

b 2 a

- 2 5 0 -

McPherson Model 218 0.3 m Spectrometer

700 Element Intensified Diode Array

Splitter PMT T

30 cm f.1. Mirror 1 3mmSl i t

Driven Tube l j

8 GE Calibration Lamp

Fig. 7 Schematics of the collection optics for the diode array system.

Shock front 1 -

I I Test ends here

\

W

Fig. 8 A typical signal from 1P28 PMT recording the total radiation from driver and driven gases,

Fig. 9 Experimental and synthetic . holograms of an expanding flow.

A I A A 92-3975 11 Solar-blind

6th order antistokes line

L1 I ArF excirner laser (1 93 nrn)

\ J Fig. 10 Opti W

I O 2 c

I

al setup for th measuremen

Shock tube

F atomic o>.-

Shock tube measurements ,/ Sharma et al(l991)

-

-

- Shock-tube measurement Allen et al(l962)

STRAP computation -

:n.

I I I I I .4 .6 .8 1.0 2.0

10-3 I .2

Wavelength, pm

Fig. 1 1 Spectral radiation emission from a 10.2 km/sec shock in an initial pressure of 0.1 Torr nitrogen.