· .'

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Charl.es G. Miller

NASA Langley Research Center Rampton~ Virginia 23665

rl' Presented at the AIAA 10th Aerodynamic Testj~g Co~erence

Paper No. 78-168

San Diego, California April 19-21. 1978

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/,..-- ''''\ , A CRITICAL EXAMINATION OF EXPA..'iSIO~P:ERroR.J,t.ANCE

Charles G •.. Mill.er* NASA Langley .Research Center

Hampton. Virginia 23665

Abstract

An experimental study of the performance of the expansion tunnel for various test gas~s and range of quiescent acceleration section and nozzle pressure, nozzle geo=etric area ratio, and nozzle axial station has been performed. Flow diagnostics used to ex~ine expansion tunnel flow characteristics vere tice histories and profiles of pitot pressure and axial component of flow velocity. The purpose of this study vas to ~eter­mine expericentally what l~itations might restrict predicted operational flexibility and the advan­tages and disadvantages of this mode of operation as cOl:lparei to the expa.'1sion tube. Results are presented which demonstrate the ~xpansion tunnel offers several adv~tages over the expansion tube, but the severity of the disadvantages of the tupnel makes the expansion tube mode of operation the more desirable for performing hYPersonic­bypervelocity aerothermodynamic studies of

. proposed entry configurations.

Nomenclature

nozzle entrance diameter. m

pressure, N/m2

pitot pressure, N/m2,

tube radius, m

nozzle exit radius. m

time after flow arrival, s

temperature, K

axial component of velocity measured downstream of· nozzle exit, m/s

incident shock velocity, m/s

horizontal distarice from tube or nozzle centerline. m

vertical distance from tube or nozzle centerline. m

axial distance from the plane of the tube exit or nozzle exit

densi ty. '~g/m 3

time interval betveen initia~ion of tertiary diaphragm opening and flov arrival at the diaphragm

-Aero-Space Technologist, AerothermodynB!!lics Branch, Space Systems Division. Member AIAA.

Subscritlts

c tube or nozzle centerline

e acceleration section exit

n state of quiescent nozzle gas

1 state of quiescent test gas

4 heli~.driver gas condition at time of primary diaphragm rupture

10 ·state of quiescent accelerati9n gas

Introduction

Tne launch of Sputnik by the U.S.S.R. 2 decades ago brought viaespread attention to the need ~or ground-based facilities that could provide. infon::ation on the level o:f' hea:ting and aerodynamic performance of vehicle~ entering the atmosphere o~ Earth. This event helped st:Umil.ate the development of different types of hypersonic facilities~ each of which simulated or duplica:tei certain aspects of the entry problem. One facility that emerged from this develop::;ent used an mlsteady expanSion wave as an energy addition mecl!anism to generate bypersonic and ~rvelocity :nov and ;;as called an- expansion tube.I A numb->-r o:f' expansion tubes originated in the early 1960's as experimentalists sought to deten:dne vtat real-life liXlita!ions might restrict the predicted performance.' Folloving a period of exper:iLental and theoretical study of expansion tube perl=ce, a theoretical analysis3 vas performed for a ~ister facility . which held pro=ise for reducing some of the drav­backs of the ex:-...a:nsion tube. TItis facility. called an expansion tunnel .. 3 :is simply an expansion tube With a nozzle positioned at the exit of the acceleration section. _ The advantages of the expan­sion tunnel. as COl:1pared to the expansion tube, were judged to outweigh the disadva..'1tages. 3 and this conclusion led to the design and fabrication of the Langley Expansion Tube/Tunnel, which became fully operational in 1972.

The Langley Expansion Tube/Tunnel vas initially opera.ted as an expa.nsion~~ _and its development consisted, in part, 'or the study of some of the many combinations of options to learn Vhat limitations might restrict the predicted operationnI flexibility. (For example~ see Hers. 1.-8_) These tests demon­strated that uniform flow or 200 to 300 microsecond duration could 1).~ "en!:rated at the accelerat~on­sect~on exit (test section) ror a number of test gases. The flov quality vns deemed acceptaglc Aor aerotbermodynamic studies ahout test models .1, and to serve as the hypervelocity-hypersonic entrance conditions for the nozzle of the expansion tunnel. Hence. fblloving these expansion tube tests. the facility vas converted to an expansion tunnel by the addition of a 100 half-angle conicaJ

"'"iloiiIe at the acceleration section exit. Naturally, ~mary purpose of this conversion was to

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determine experimentally the advantages and dis­advantages of this mode of operation as compared to the expansion tube.

The purpose of this paper is to present recent results obtained in the Langley Expansion Tunnel for CO2, air, argon, and helium test gases and range of quiescent acceleration section and nozzle pressure, nozzle geometric area ratio, and"

,Dozzle axial station. Results were obtained vith and vithout an electromagnetically-opened tertiary diaphragm.9 (This diaphragm vas positioned at the exit of the acceleration section and per:itted the nozzle to be evacuated to a lover initial pressure than the quiescent accel~ eration gas.) Flov diagnostics used to examine expansion t~el flow characteristics were time histories and profiles of pitot E!~~ ~<E6mEonent of fiov velocitx. Fl~~ocity vas measured using a unique pnotoionization technique developed by Wilfred J. Friesen of 'the Langley Research Center10 and es~imates of free stream density and density profiles vere also inferred fro~ th~s veloc~tymeasurement technique. COmparison of expansion tunnel flov characteristics, vith published and heretofore unpublished expansion tube data is performed, and the adva.'1tages and disadvantages of these tvo modes of operation are discussed. A judgment is made as to the more favorable mode of operation for performing aero­thermodynamic stUdies of proposed entry configurations.

Facility and Auparatus

Expansion Tube/Tunnel

The Langley Expansion Tube is basically a 15.24 cm di~eter tube divided into three sections by tvo diaphragms; thus. this facility may be vieved as a shock tube with a constant di~eter tube section added to the down­stream end. (A detailed description of the basic

, components and auxiliary eqUipment of the Langley Expansion Tube is presented in Ref. 4.) The driver gas is introduced into the upstream, high­pressure section and the intermediate section is filled vith the desired test gas. The downstream section is referred to as the expansion or accel­eration section and is filled vith the accelera-. tion gas. Thick steel diaphragms separate the driver and intermediate sections, vhereas a thin ~lar diaphragm separates ~he intermediate and acceleration sections. The expansion tunnel is Simply an expansion tube with a nozzle installed at the exit of the acceleration section (Fig. 1). A third or tertiary diaphragm separates the accel­eration section and nozzle so the nozzle maybe evacuated to a lower quiescent pressure than the acceleration section.

The idealized operating sequence of the expan­sion tube is shown schematically in Figure 2. Upon rupture of the primary diap'hragm, an incident shock propagates through the test gas and encounters and ruptures the low-pressure secondary ~iaphragm. A secondary incident shock propagates 'into the acceleration gas and the shock-heated test gas undergoes an isentropic, unsteady expansion as it passes through the upstream expansion vave generated upon rupture.of the secondary diaphragm. This expansion process cenerates hypersonic and bypervelocity ~lov at the acceleration section

exit from the lov Mach number shock-tube flow vhicb encounters the secondary diaphragm. For the

, expansion tunnel, the test gas undergoes an isen­tropic steady expansion in the nozzle following removal of the tertiary diaphragm at the accelera-' tion section exit.

An electromagnetically-cpened tertiary diaphragm~ Vas ef!Pioyed.-'.rhecOncept"is to use electrom~netic repulsive forces in a vire to rip open a Mylar diaphragm and rapidly vithdrav the ~lar from the flov path. In this study, the copper vire vas 13 AWG (American Wire, Gage), the My""1ar va.=_12.7 jJm ~iCk, and the .;nergy __ storage system t Whlch consisted of two capacitors, each rated at 5 kV and 100 llf} vas ch~d to 3.2 kV. As observed inI~e 3, this volta£c produced opening times of 100 to 900 ps vithout ~e breakage. ----' Survey Rakes

~i~t-pressure profiles at the expansion tunnel test section were obtained vith a 25-probe survey rake fabricated from stainless steel and lja"""ing~a probe spacing of 2.29 COL T"ne centerline of "the center-rake probe vas coincident vith the expansion tunnel centerline, and the rake co~d be rotated 3600 about the centerline. The strut vas designed to permit the rake tolDe positioned at locations upstream and downstream of the nozzle exit. The outside diameter of each ~robe at the sensing sur:face-vas-Q.19 em and a perforated disk arrange­ment1 vas used to protect ~~ransducer trom particle contamination in the post=test flow_ Pitot-pressure p~files at the exit of the expan­sion tube vere measured vith a scaled-do_~ version ot the 25-probe survey rake. The eleven probes of this smaller rake vere spaced 1_ 78 cm apart and. the

. pressure probes were the same as employed in the larger rake.

of the axial cO:::Ponent of the rflow using a l7-probe, vertically

mo -----j fabr~cate ess steel, the center-rake probeoeing coincident with the t~el centerline. and the probe spacing vas 3.81 em. Sixt,een ien probes (described in a subsequent section) vere ipstalled in this r"'-ke.

Lnstrumentation

Pitot Pressure

Pitot pressures at the nozzle exit and expan­sion tube exit were-measured using co~~ercially available, nliniatu...-e quartz (high-impedance) pressure transduc£YS. These transducers vere acceleration-co~ted ~nd had rise times of approximately 1 to ]. lls. F.tlch transducer . and its corresponding ch'irg:e a::;plifier vas cal:ibrated statically o.i'ter i=tallatlon in the pitot ~robe and at frequent in:t.ervals during the study; thuS, the transducer, cb2rge amplifier, connecting cables. and oscilloscope used to record the output signal were cali~ed as a single channel. Thermal protection of each transducer took the form of a circular ~iece of electrician tape placed over the s~ing surface.7

A recent study at the Langley Research Center by John A. 'Moore denonstrated that pitot pressure

of the_~:lQ@sion J;,Y):>e_ could be -=:=-=='---':-::-Within- 8 percent with thes~ _ quartz pressure transuucers. This uncertainty

/ corresponds ~3E:tic) calibr~ion onhe Ii::? : pressure tra."lsducers vith an applied pressure

I') .g ". ,. of 138 kll/m2• which is approximately the .Iv , \~ -magnrtUcIe of the pitot pressure measured

ftJ I during the test-flov period of the e::,.-pansion tube. For the expansion tunnel, the magni-tude of pitot pressure is roughly 0.01 times that for the expansion tube, thereby a~proach­iiigthe lo~limft~ useful rang~ of operation of these pressure transducers. The expansion tunnel pitot pressure level is approximately the sane pressure level as experienced on the ~~l of the acceleration section during the test-flow period. Static calibrations of tube-vall nressure transducers have been checked7 by running the expansion . tube as a lov-perfo~ance shock tube such that the pressure immediately behind the incident shock could be predicted accurately. A compar­ison involving 21 pressure transducers revealed the maximum difference hetveen the static and dyna.r:ic calibrai::_igg for anY,Df the transducers vas less than 14 perc~. For the present study, measured pitot pressures at the tube exit and nozzle exit are believed accUrate tQ within 10 percent and 20 percent, respectively.

· Velocity

Incident shock velocities in the intermediate · section and the acceleration section are routine~y measurI~ using a microw~ve interfero~eter system and time-of-arrival measurements.1S Both methods are employed fOr each test gas because the electron density of the shock front for certain test gases may not be s'~ficient to ~rovide reflection of the micro.~ve signal. l2 Fo~ the ~resent tests, the test gas-acceleration gas ~ce ve~ity at tbe acceleratiop'spctjon exit ,is essentially equal to the incident shock velocity into the aecelerat10n gas. b

The axial compone~t of the flo~ velocity dovnstream of the nozzle exit vas measured using

· a technique developed by Wilfred J. Friesen of the Langley Research Center.~O,13 This method used tvo ~indow1ess. collimated ultraviOlet ligh~ sources10,I3 spaced 3.81 em apart. These light source~ vere of the capillary discharge type,lj and the pulse-duration of the light vas approxlmately 0.3 llS. The light sources vere pm:sed sJ.multaneously ·to rod1lce tvo parallel

on ze col~s in the flov vhich vere detected downstream or the light sources by ion probe~. The axial component of flew velocitY-vas 'inferred from the time interval between the arrival of the tvo ionized colunu,s (which were svept dO-7lstream vith a velocity equal to the flo'll velocity) at the ion probe. Thus, the velocity represents an average value oyer tl]e .3 .. 81 cm interval betveen the the light sources!,.. The Ion probe consisted of a 15° balf-angle cone forebody, vith a base diameter equal to 1.015 cm. folloved by a cylindrical afterbody baving a diameter of 0.95 cm. The cone ~as 'fabricated from stainless steel, and the ion sensor vas a brass ring sandwiched between nylon insula~ors; the center of the sensor vas 2.78 mm do.~stream of the cone­cylinder junction. The ion sensor~ were locate? 20.3 cm downstream of the nozzle exit~ The most

3

-,

.upstream light source vas located 11.82 c~ Clo.":lStrea.::l of the nozzle exit. . . ,

Output signals from the ion probes were recorded from oscilloscopes wi~h a camera. Oscilloscope tine $Veeps were calibrated before, during, and after the test series, and corrections vere applied to the data if necessary. The output signal from an ion sensor mounted flush vith the nozzle vall and at the nozzle exit vas fed into a tbree-channel. time-delay generator and the output froc this generator vas used to pulse the tva light sources simultaneously, as ve11 as trigger the oscilloscopes. This arrangement permitted the axial co:ponent of the flov velocity betveen the tvo light sources to be measured at different ti~es in the flow sequence. Based on estimated uncertainties in time and distance measurements, the inferred velocity is believed accurate~ within 2.5 percent. j

De::sity

Initial attempts to measure free stream density do.":lStream of the nozzle exit using an electron be~ to produce fluorescence in the test flov proved unsuccessful. 7 Rovever, by comparing the charge collected at the ion~p~obE!!;;_, .. i~-'ti:~tes of vertical d~ity profiles, "nortlalized by the nozzle cenli:r:",, __ line density, were obtained by Wilfred J. Friesen. Toe charge acqm.red by a fluid element (and hence trensferTed~to--a' given ion probe) is proportional to the density of the fluid element and t~~ose of pho~oionizing radiation received from the ultra­violet light source. Since the variation of the ranation dose I!.-,=!o-=_s,_the f~ow, depeIlds ori.t~ dist~ibution of the fluid density and the optical propert~es of the fluid, the distribution of the fluid density can be deduced approximately from the charge col1ectei by the various ion probes. In carrying out this a.~alysis, it vas a~sumed that (1) the density profile is s~~:t;:rical aboyt the noz:le centerline, (2) the effe£tive ~~~niza­tien cross section is constant along the light pat-h, and (3) ali Joon probes sa.'lIple equ'aI-seg;Inents of the ionized column. According to this analysis. the density varies as the geometric mean of charges collected at sJ~etric (about the flo'll centerline) pairs of ion probes. ThE: magnitude of the.,g,ensity ( vas obtained by equating t~al of the de;sr~s '- . ion just dovnstream of the nozzle exit to the integrated dens1ty of a constant property gas column vhich produced the sa.'lIe trans­mi~sion of ultraviolet light as the nozzle flo'll. The opt1cal transmission of the nozzle flo'll was. measured by tvo vindovless. nickel photodiodes positioned opposite the ultraviolet light sources and near the edge of the flow. The optical tra.'lsmission of the reference gas column vas obtained from a ztatic calibration carried out in the nozzle dump tank over a range of pressures.

Er-~sion Tunnel

The present tests vere performed vith an unhe~ed .he1ium driver pressure of approximately 33 MR/m2 • ~. ~ide, ~, az:d ~ vere employed as the test gas at a qU1escent pressure of 3.45 &~/m2. The acceleration gas vas the same as the test gas~ but at quiescent pressures of only-Y.l to~54.0 N/m2 • The 11.7 m long

,I • • .. 1'_- .. '

._ ..•.. _ ... __ ... - ---------.-~- .--... ------.., intermediate section vas se arated from the n. ong acceleration section by a b.35llll1 t~MYlar diaphr~gm vhich vas opened by the floy. The entrance diameter to the.10o half-angle conical nozzle vas varied from 5.1 cm to 9.5 cm by ~!n!S itlterchal't&e~bJ:e . nozzle entrance sections., The sharp leading edge of the~ntrance section vas inspecte-Uor d8.lllage'iirtereach test 8.Ild ~.enerally

Post-Test Flov Pressure Level

'fIreplaced after five to seven r1,;!ls. The. nozzle . ~ exit diameter vas 63.5 cm; thus, nozzle geomet;:.!!=

~ar~os from 156 to 44 vere tested. Tests

One advantage of the expansion tunnel as com­pared to the expansion tube is a reduction in pressure loads on test models dur~,i:..h~~j<;-~~t f1~. period.1 Tnis advantage has gained additional si~ficance vi~h the recent availability of sub­miniature (1.5 ~ di~eter) semiconductor pressure transducers, vhich provide the opportunity to measure detailed pressure distributions on relatively small models. Since shakedovn, the Langley Expansion Tube has been operated routinely v I i> were performed vi th the 25-probe survey rake

11.v6\.p positioned 20'l..cm .ll:::'~ 10.~ cm -:¥f:t:r~am,_QLthe lP;;;r} .i ~oZZle exit (Z :: -20.3 and z = - 0.2 cm), a.:t_the

with an unheated helium driver gas at a pressure of , approximately=33 tlB/m2, and this driver condition _A ... ~ {Wi' vas carried over to the expansion tun.'1el ::lode of .;:5 u­

operation. For this driver condition. a :aximum ::;" ~ .U) exit. 8.Ild 10.2 cm and 20.3 cm dovnstream o..Lthe

( J {\ ~ i exit. M~/l ,_1'''' I. Because of the f;!,nit~~~1ng tiI:te, of the

§elf'-opening_tertiary. diaphragm. the opening must be synchronized vith the flov arrival at ~he

. diaphragm. The output signal from a thin-film resistance gage located 9.25 m upstream of the tertiary diaphragm vas supplied to a time-delay generator connected to the energy storage system of the self-opened diaphragm. This arrangement permitted the time interval betveen initiation of the diaphragm opening and arrival of the flov at the diaphragm station ~T to be varied. Values of' AT vere inferred using a nominal value of the time required for the incident shock in the accel­eration gas to travel the distance betveen the

. ~ocation of the resistance gage and.the diaphragm location. (Electrical disturbances generated by the discharge of the ener.gy stOFage:-:5y:stem. cr~ated di~urbances on.the output of time~~f~arrival--7 instrumentation and on ~icro~ave measurements, thereby prohibitingmeasur~ent of the incident shock velocity in the~vi~inity of the acceleration

. sect-ion exit,-'ominal values of 1695 \.Is and 1790 \.Is for air and C02 test gases vere obtained from. tests vithout the tertiary diaphragm.)

Expansion Tube

The present expansj.on tube tests vere also perf'ormed vith-~um driver, and the pressure vas approximately 33 1.m/m2 fer all but a fev tests. Dry-air, carbon dfoxide. argon. and helium vere employed as test gases at quiescent pressures from 0.7 to 3.45 ~~/m2. The acceleration gas vas the same as the test gas, and the quiescent pr~ssure ranged from 1.6 to 26.5 N/m2 • The intermediate section length and acceleration section length vere less than for the expansion tunnel, being 7.5 m and 14.1 m, respectively. These sections vere separated by a 6.35 ~ thick MYlar diaphragm. The ll-probe survey rake vas positioned horizontally 0.8 cm to 21.1 cm dovn­stream o~ the tube ex!t.

Results and Discu~sion

,._This paper presents selected segments .fr.olli'.:

pitot :pressure of roughly 14 1.fN/m2 is experienced 'Z(f1rO),,--­over a 4 ~s time period d~he post.:teSi flov for the e~s~on tuce, compared to about 0.3 Mrt/m2 for the expansion tunnel. 7 The high­pressure post-test flov of the expansion tube imposes stringent requirements on the design and fabrication of test ~dels, model support systems, and inst~entation. Recent tests in the expansion tube demonstrated that co~ercially available se:iconductor press~e transducers could not survive the 14 !~/n' post-test pressure pulse; tht:.S. the need for a 33 M!:/m2 helium driver pressure vas reevaluated. To reduce the post-test pressure pulse. tests ve;;'e performed with a 13 MN/!;2 h~ driver .Rressure 8.Ild C02 test gas. Pitot pressure ti~e histories are presented in Figure 4 for a 33 f.'!N/m2 and 13 t-li/n2 helium driver pressure. The ra '0' of driver ressure to uiescent test gas Ptes e'vas the s~e for the t .... o values of'~drl:vg I", :pressure. as vas the "we of quiescent aeceleratJ:on (~'J gas pressure. The lover ar~ver pressure produced /VN",/ an in~ase in the quasi-steady test-flovj?eriod. /" Pitot pressUre profiles presented in Figure 5 •

. vhich correspond to the data presented in Figure 4, illustrate the existence of ~~ inner core of nearly constant pitot pressure vith a diameter ~oxi­mat ely 0.6 times the tube diamete~both driver pressures. Thus. no cegradat~on ~n flov quality vas observed vith the lower driver pressure, and the only apparent cisadvantage of this lover driver pressure vas a decrease in schlieren quality due to the lover flov density. Even at this reduced driver pressure, vhieh yields a maximu.'l1 ~ost-test pitot pressurl! of"approxi::ately 5.5 l-rH/m , the su.-vival of the s~iconductor pressure transducers is questionable. Until the survival of these pressure transducers in the expansion tube is real­ized. which may be possible with a hydrogen driver gas, the lover post-test pressure pulse of the expansion tunnel represents an advantage. Because thin-film resistance heat-transfer gages that face into the flov are destroyed during the post-test flov in the expansion tube and the expan­sion.tunne1.1 neither mode of operation offers a

'significant advantage in. the measurement of heat­transf'er rate.

the matrix ot data obtained wi ththe Langley: ,::;.; .... : Tertiary Diaphragm Expansion Tunnel. The best flov conditions',' which "'.> correspond to the combination of parameters .. ," . Tests in th tunnel13

:providing the maximum quasi-steady test flov decons~t~r~~~~~~~~~~~~~~~~ duration and uniform flov across the test core. flov quality vas improved significantly vhe~ the are establiShed. F10v characteristics of the nozzle-vas evac~ed to a much lower pressure than expansion tunnel are compared to those measured the acceleration section;-in agreement vith . in the expansion tube and the advantages and dis-' prediction. l4 To evacuate the nozzle requires advantages of the tvo modes of operation '(tunnel installation of a third diaphragm at the exit of the and tube) are discussed.' acceleration section. This third or tertiary

! .

·- •• ~:~ f';....:-~~-

diaphragm ~ust be self-opened to eliminate a reflected snock att""henOZZIeeln:f8.ric-e~uch a~ would occur vith a flov-opened diaDhragm~,l H~ce. initial tests in the expansion tunnel utilized the electromagnetically-opened diaphragm described in a previous section. These diaphra~s were time cons~ing to ~abricate. plagued with pin­hole leaks, unreliable in that the wire often tore· loose from the I-lylar during the opening process, and presented the possi~ility of diaphragm vires being swept do~stream ~d d~aging the nozzle entrance or survey rake. Because of this tertiary diaphragm, the test repeatability of the expansion tunnel vas peorer and the run frequency less than for the expansion tube mode of operation. Thus, a critical exacination of the performance of this tertiary diaphragm vas performed. .

Because of the finite opening time of the tertiary diaphragm, the opening must be synd:ro­nized. vith the flov arrival at the diaphragm. If the diaphragm is opened too soon in the flov sequence, the flov viII experienc~ a density gradient in the acceleration gas in the vicinity of the nozzle entrance due to an expansion wave propagating into the quiescent acceleration gas. If the flov arrives prier to removal of the dia­phrag:n fro::: the nozzle entrance, a reflected shock vill be produced resulting in a degradation of nozzle flov quality.9 The effect of diaphragm opening sJ~chronization on pitot pressure time

.liistory measured at the exit of the nozzle is shown in Figure 6 for air test gas. These pitot pressure time histories were obtained with the quartz pressure transducer mounted flush with the end of the pitot,pressure probe to provide maximum response to a pressure change. To minimize the chance of destroying the unprotected ~ransducer, the probe vas positioned 6.9 cm off the nozzle centerline and out of t:'e bore of th~ nozzle--­

,emrance. For tests wLh the tertiary diaphragm, the only variable in Fi€UI"e 6 is the time dit'fer­ence betveen . .. . on of the .£i?.pl!J;:.qgnl,_Oll-ening and &rrival 0 -w at the dia~h:!"3.;;:m f.r. The p~tot pressure history measured at the nozzle exit for the case of no tertiary diaphragm reveals the existence of initial peaks in pitot pressur~ -which are attributed toa relativ~ly strong shock system fo~ed -within the nozzle,14 followed by a quasi-stea~ pitot pressure period of approximately 400' lls and a subsequent -ores sure decrease characterized by the ap?~arance of high-frequency variations in pitot pressure. For the tvo largest time intervals presented in Figure 6, an initial peak in pitot pressure such as observed for the case of no tertiary dia~hragrn is absent; however, a quasi-steady pitot pressure period' of ~OO to 350 lls exists and the magnitude of this pitot pressure is essentially equal to that measured for the nO-diaphragm test. This indicates that ttle nozzle flow Is fully started for the case of no tertiary diaphragm, despite the higher initial nozzle pressure, and usage of the tertiary diaphr.agm diminishes the magnitude of the starting shock. Also, the useful quasi-steady flow period for the c~se of no tertiary dia hra a earc to excee at obtalne with the tertiary di~agm. ~

The t.~ largest ti:e intervals exceed the d~ll.~!:.l'l~ 'Opening time determined from, bench tests, implying that the diaphragm is removed completel~rom the tube r~or to flow arrival. When the time interval is approximate y equa 'to

.... ~ .. , .. ..-._.... . ........ _._ ...

the diaphragm opening time (M :: 760 lls) and the time available for the spreading of the density gradient in the quiescent acceleration gas is reduced, the pitot pressure experiences a mere pronounced initial pressure increase. Fo~owing

this initial increase for 6.T::: 760 llS ,_ the.:pj.tot pressure is nearly constant ro~proxilllately 450 us; howe!er, the m~-nitude of the fluctuatIcins.-'-in pIfot pressure is greater tEan those observed for the two largest time intervaIs and for no tertiary diaphragm. For til::e interVals less than 76o_us, thus. less tha.~ the diaphragm opening time. a degradation in pitot pressure time history oc~~s. For the smallest tiI:!e interva1~ VhlCh is consider­ably less than the diaphragm opening time. the pitot pressure time histories resemble that o~ a ~low-opened tertiary diaphragm, indicating that the diaphragm was not removed sufficiently at the time of flov arrival to avoid producing a distur­bance at the nozzle entrance. That reasonably good ~uality pitot pressure time histories vere measured for t:i:::e inte:rvals nearly equal to or somewhat less than the opening time is attributed to the close position of the nozzle entrance to the diaphragm. (The tertiary diaphragm vas positioned 1.3 cm upstream of the nozz~~ntrgo~~.)

. Tziis perllll'ts the dl.apnragm 'to 0 en cle ap ox en ce diameter of 8.9 em. instead or the tube diameter of 15.4 cm, to avoid significantly disturblng'the Dozzle-entrance ri,;w.

Tne degradation of flov quality, as ini'erred tr~ pitot pressure time histories, with decreasing ~. was not clearly evident -rrom~ocity ~easyre­ments. Vertical velocity profiles measured just do.~stream of the nozzle exit and at a test time of 325 ~s are shown in Figure 7 for CO2 and air test gases. These profiles were obtained for the case of no tertiary diaphragm and with the selr­

·opening diaphr~ over a range of f.T. Tee velocity profiles for air test gas (Fig. 7(b» were obtained with the same nozzle entrance di~eter and quiescent acceleration gas pressure as the'pitot-pressure time histories presented in Figure 6. Results -Jith and without a tertia.~ diapr~agm reveal the existence of a core of nearly constant velocity for both test gases. the diame:t.e.l:,,_of_vhichis- approxi­mately 0.1 times the r>ozzle exit....Jtiameter. The Yeloc~ty remains fairly unifo~ across the core for the present range of AT. including values appreciably less than the diaphragm opening t~e. There is some indication that velocities measurea vith the tertiary diaphragm are less tha.~ ~hose measured without a diaphragm, and the velocity decreases vith decreasing 6., implying a loss of energy at the tertiary diaphrar,m location. HO\lever, the relat.:'vely small differences in velocity prohibit a definite conclusion. Large oscillationn in I'itot pressure \lere observed \lith CO2 and ulr test ~ascs t·or ~T less than 360 ps, or so. B!lned on these Tii t.ot pressure meH.5Urements. the flov qualIty .as deemed unsatisfH.ctory for model testing; however, no stl·:=h conclusion can he drnvn from the velocity measurements, \lhich illustrate no pronounced degradation in rlow quality with decreasing 6.. for values as low as 90 llS. {Tests in the Langley Pilot, Model Expansion Tunnel shoved no significant differences in velocity profiles measured at the nozzle exit betveen a run in vhich -the tertiary diaphragm vas inadvertently opened by the flov and runs wen the diaphragm opened properly. )13

__ ...... _ ..... ....0..;.... .~--' .',,*# -"" -' ... --",,~---. '.

~----- ------------The r~ults of ...!1_gu:-es~-I~hich shov

. ": that. no. sacrifice in flov quality is incurred by elimination of the tert~ diaphragm, contradict prediction.~4--Predictio~ indicates that exces­sively long nozzle starting times vould be required if the nozzle vas not evacuated initially to a pressure much lover thao the quiescent acceleratien gas pressure. A possib~e explanation for the discrepancies be~veen ex?eriie~t and prediction is the ter~Tiir.t~A~~~€p_."I..:-~~s~"lll!!~d.to .open instantane.ously,~~ vnereas the actual opening time vas apprexinately 700 to 900 lls;B:nd--'fhe .Jl.o.z_zle nev 1S not .one c.il:lensic::a.l as assumed-:" Several tests vith the self-open~ng~rt1arY diaphragm vere perfor:ed to dete~ne the range .of the ratie of initial nozzle ~ressure to ouiescent acceleration gas pressur~, Pn/PIO that provides reasonablr good :flev qusl.i ty • Pi tot pressure time histories for C02 test gas a.~d· ~T equal te 655 us are shovn in Figu:e 8 for Pn/P10 less than unity, c.orrespending te an exllaDsron \/'ave pro~lnte the ~uiescentacceleration gas,at ambient speed of seund u;on diaphragm rupture, and ~/pl0 greater th~ unity, corresponding to an expansion vaveJ?ropaga.ting into the quiescent

, nezZle gas. Distinct starting shocks are .observed : 1 fer all values of Pn/PlO nth the_,~!l..<?ck _strength. II~I increasing vith increasi::g pressure ratie. The ('j p J TIl pitet pressure tl.l:\-e=lust.ory anct'magni tude 'fer

; Pn/PlO equal te unity is similar to that .observed : fer ne tertiary diaphrag=,7 implying the absence

'o~ significant disturbances due te the actual opening precess .of the diaphragm. Comparing time histeries and press~e ~agnitude during the quasi-steady flov period, the results .of Figure 8 demenstrate that reasona~ly geod flev quality may be generated for values .of PO/P10 as high as 1.5.

Tests vith the tertiary diaphragm revealed an interesting pheno~ena that must be censidered in analysis of the pitet pressure data. SJFifi­cant pressure lag may exist fer pitot pressure p'rebes in which the nressure transducer is protected from part1~le cent~1nation 1n the post­test flov b7 a perforate~ disk arrangement. 7 This effect is illustrated in Figure 9, vhere pitet pressure t~e histories ~or a flush-meunted pressure transducer and a protected pressure transducer are shevn for tests vith ne tertiary diap'hragm a~d the self-o;ening diaphragm fer AT = 1160, ~s. The protective arrangement use~in this study did not pr.oduce a pressure las-effect in expansien tube tests, ;;here the pitot re sure vas apprex1~a e_ '-es ~at measursd at the tunnel exit, nor in cali:ration shock-t~ tests f~itet pressures as low as 8 te 10 kN/m2•

. Hevever, by reducing the magnItude of the starting nezzle shOCk, the self-o;ening diaphragm data reveal that erroneous conclusions may be dravn ~rom pitot'pressure time histories if the protecter

~I .' 1 ir/)

, j i- .'

01-~ ': ~ . I .;

~~r. arrangement is used. For exa::tple. the present pretecter arrangement and quasi-steady'pitet pressure magnitude of about 2.5 kllim2 yields a pr~ reughlz ~Q £.0 ~ or ne¥'ly equeL.t t~ the quasi~steady flev duratien.

Fer the r~~n~rpan~en tunnel res~ts presented herein, the tertiary diaphragm vas remo~ith this diaphragm remeved, t~ opera­tlen .of the expansion t~"~e1 ~s no mere difficult than for the expansion, t~be, the primary diff'er~

'ence being the inspection. repair, or replacement of' the nezzle entrance· leading edge af'ter each

.... ~ e. _. ____ ~_-->.--.-.-.. -~ T:'lfS .. ~~ .... , .•• ,...J.:~,": ':' ....

"7·: .f . ., '" "" .......... ~_.

--_.- ------------------- -.-- --------, test. Thus, the run trequency and test repeat­ability for the tva modes '.of operation vere essen~ tially the same. Alse" pitot pressure measurements vere free .of pressure ~ag effects.

Quiescent Nozzle or Acceleratien Section Pressure

In the. sea...-ch :for opti= f~ov conditions in the expansion tube or expansion· tunnel, the time history .of centerline pitet pressure is generally used to infer the quasi-steady test flov duratien fer the various combinations of parameters tested. Te fu..-tner optimize nov cenditiens, pitot press'.lre profiles, velocity t:il!le histories~ and velocity prof'iles are ~easured :for cembinatiens providing quasi-steady fle'ol. The :final state after the unsteady exr::!msien in the expansien tube .or expan­sion tunnel is depend~ on several facters, one of ~he more il:lportant being the density (.or, fer ambient te~perature~ the pressure) of' the quiescent acceleratien gas. The e:ffect of quiescent acc~l­:::ation gas pressur~~ Plo (esual to initial nezzle pressure Po) en pitot pre~suoce time history at the nozzle exlt has been p..'"esented7 ~er C02 and air test gases. At the time these results vere presented, initial atte~pts to establish nozzle 1"1.0'01 ~~th -argon and helium test gases preved unsuccessful. Reaults :from additienal tests nth these tvo test gases are shovn in Figure 10, "here centerline pitot pressure time histories are presented fer a range of quiescent nezzle pressure. The initial peaks in pitet pressure observed ~or air and C02 test gases and attributed to a starting shock system are absent for the lovest and highest value .of Po tested fer argon and all but the highest -value fer helium. The results trom tva t~sts are shovn fer argon and Pn equal to 3.1 N/m , because one test demonstrated an initial peak in pitot pressure and the other did not. In some instances, pitot pressure probes vithin the bere .of the' nozzle entrance vere damaged during the post-test flov period. This damage became evident in the next run and .often appeared as pressure lag d11e te a. damaged protecter disk. Hence, the pitet pressure time histories of probes adjacent te the center prebe "ere examined and the trends illustrated in Figure lOCal for argon vere alse i~ustrated by the adjacent probes. A value .of Pn equal to 3.1 N/~ may be marginal for generation .of a starting shock s~~tem. The opt~ value of qujpsCg nezzle pressure 'fer argon ~ deduced te be a !"Oximatel . N m an apnrOXiinately 33 N/m fer helium. As observed 'fer C02 and air,1 the magnitude of the centerl.ine pitot pressure during the period of-~xasi-steady flev is nearly censtant over the ran~e of Pn examined for argon. Valu~of Po yielding the maximum test-flov) pe!!~dt:o.La givel1 test gas in the expan~~Cl?~ are rou~hly half the quiescent. acceleration gas pr~sure yielding maxi::lUm test f"lovs in the expan­sion tube. Thu5. optimum expansion tube flav coiidltiolls •. 1lhicll .. l?erve 1M3 the nozzle entrance coiiditiens, de not yield ept.imum expansion tunnel flev cendi t ions. -:z.: ---.. . ..... --... -... ~ ... ---~

Attempts to measure ve~ecity time history at the nozzle exit for argen test gas vere .only partially successful. Hovever, velocity time his­tories for C02 and air test gases ver~ measured for the quiescent nozzle pressure providing the maximum test-flov peried and are shevn in Figure 11. These velocities represent the average axial

.... ----~ -. -.-~~ -~~. -- ----_ .. ---_.- ._---- < .--_ ••• _---......

: ;

...... _-

component of flov velocity over the interval 11.82 to 15.63 cm do.~streac of the nozzle exit, and are nondimensio~alized by the velocity measured at the nozzle entre-~ce using the micro­vave tech.~ique.l1 These values of flov velocity represent ~~ average of the velocities obtained from the center five ion probes for a given test and the average of two tests. Also shovn are corresponding pitot-pressure time histories.

Values.of flov velocity at the tW9 ear~l~?t times agree within the experimental uncertainty (denoted by barred s~bols), whereas a decrease in velocity occurs between the earl~nd~.e

s r-es for both test gases. Thus. the flow lreloc~ty is essentially constant during the later half of the quasi-steady pitot-pressure period and decreases during the period of decreasing pitot pressure and high-fre~uency variation in pitot pressure. The flov velocity increases as it traverses the distance betveen the no~e e~rance and exit. indicating a conversion of internal energy (decrease in static enthalpy) into ~netic ene~ as ex~ected. This increase, vhich vas not observed in the _L~ey Pilot Model . Expansion Tunnel.13l~s 4 to 5 percent for air and 2 percent for C02' For hypersonic flov,. the pitot pressure is approximately equal to pU2; bence, the results of Figure 11 ~ply that free stream density and velocity are essentially invariant with time Just dovnstream of the nozzle exit for about 300 VS folloving the initial peak in pitot pressure. These results also confirm that the flov density undergoes an appreciable decrease in magnitude as the tlov traverses the nozzle, v~tb the density at tbe nozzle exit being about 0.02 ~imes that at~he nozzle entrance for both air and C02 test gases. (The ratio of density at .~he nozzle entrance to that at the nozzle exit is approxi=ately equal to the nozzle geometriC area ratiO.) The relatively small increase in flow velocity and large decrease in density as the nO\/' traverses the nozzle-··:rsexPected:-Since-~ost of the flov energy is kineti~ at the nozzle entrance, and the continuity of mass and energy

.relations shov that the density varies inversely with the nozzle area ratio.

V~locitr profiles dovnstrea~ of the nozzle exit are sbovn in Figure 12 for C02 and air test gases and several values of qUiescent nozzle pressure. These values ot velocity represent the average of tvo runs. A eore of uniform velocity _ exists about the nozzle centerline and the diameter is ~oxloately 0.7 times the nozzle exit diameter for C02 and 0.65 to 0.7 times the exit diameter 1'0~ air for the range of p examined. The average velocity for the ceRter seven prGbes is 1.01 times the measured entrance velocity for the lovest value of Pn vith C02. and 1.04 times the

'entrance velocity for the highest value. This trend is expected from con::;ideration 'of the continuity of energy relation.. The ra"99 of exit ve~~it~~_entrance velocity Varies from 1. 03 to 1.05 over the range of Po ~or .. a!r-:-------"-

Vertical pitotrp~sure profiles measured at the nozzle exit for C02 and air test gases shoved the test core (region of uniform pitot pressure) diameter vas approximately half the nozzle exit diameter. T. Thus~ the test corg diameter inferred rrom-pitot pressure profiles is less than that inferred from velocity profiles. The pitot

J

pressure profiles also illustrate that __ a region of relatively higb ~itot pressure surrounds the te' core; e p~tot pressure diminishes rapidly betveen the high pitot pressure region and the tunnel vall. It is interesting to note that a peak in pitot pressure vas also observed in boundary layer surveys on a sharp 40 vedge in heliUm flov at a local Mach number of about 11.15 Predictions revealed15 that this peak vas the result of self-induced bluntness producing a shock inflection point, resulting in a line of maximum local total pressure within the flow field. The boundary layer surveys on the vedge showed the pitot pressure boundary layer thickness. which closely correspond.s to the thermal boundary layer thick.~ess~ vas 1.5 to 2.3 times the velocity boundary layer t~ckness. For the present tests, the nozzle vall boundary layer thickness inferred from pitot pressure profiles is approxi~ately ~.4 t~es tEe boundar'}" layer thickness. in~om velocity prof~les.

The vertical·profile of the axial component of flow velocity and the cJ?rresponding densit~ pro­file inferred from the charge collected at tbe ion Probes is sbovn in Figure 13 for C02 test gas and quiescent nozzle pressure of 2.1 N/m2• The

'velocity and density are nondimensionalized by the nozzle centerline values, and the profiles vere obtained during the period of quasi-steady pitot pressure. Valges of fr:e.e.. stream .d.e~sity_are vithin 20 percent in aJ:eg-ion--abeu;t the nozzle center1ine. and the diameter_q~_t~~~egion is about half the Dgzzle exit diameter. A toroid of hijgh-density flaY s~~ounds this inner ~. of reJ.atively uniform density, but tbe flov velocity is constant across this region and tbe toroid. Out­side the toroid, the density diminishes rapidly in the direction of the nozzle vall. Vertical profiles of flow velocity end density for air vere similar to those observed for CO2 , These findings are i.tLagree­ment with measured pitot-pressure p~o~iles 7 A sT~sion tunnel flow model vas o"bserved in the L~ot lobdel Expansion 'I1lIlnel. 13 The toroid of hjth-de-sj~ flOW is believed to be caused by embedded shocks (veak disturbances), ~s prediCted lOr hIPersonic ~J~ in conical nozzles. 1o As a -~of interest, the value of centerline density obtaine~Lfr.Q!!Lmea~g-:tlle charge collected by the ion probe is about ~ice the density inferred-"fr2m t~e measured veloeity and Ditot pressure. In viev of the assumptions required to obtain a Value of density from the measured charge, this difference in centerline densi~ of a factor of 2 is believed to be-quite reasoni51e. ~ -

A primary advantage expected with the expansion tunnel mode of o~enl.tion is a. larger test core pia­meter. For the expansion tunnel, the test core diameter is approxinately 0.5 times the nozzle exit qJameter, or 3?,.c!ll. The size of the test core-in the expansion tube is shc.wn in Figure 14. where pitot pressure profiles measured 5.6 cm dovnstream of the tube extt-are presented for C02' air, ar~on_ and helium test gases. The driver pressure and quiescent test gas ~essure PI are the same as employed in the exp~sion tunnel tests. A range of quiescent acceleration gas pressure was examined for each test gas sma with the exception of helium. the optimum value o~ quiescent nozzle pressure falls vithin·this range. Because of the variation of pi tot pressure with time· for certain values of PlO-7 the prbfiles were ~ at times of 100 ~s and 300 lis. Readings"W!re made at 300 liS only i£ the

• <I ~'.'

, :.t:

.. ·centerline pitot pressure vas free of large 1 pressure oscillations and the "dip" condition (a decrease in pt occurring af~er'a period of near quasi-steady flo~). The t~~et:er for ... COz.. and air is 0.5 to 0.7 times the tube d~et.er;··:".'· Qr 7.6 to 10.7 cm, for all values of :PIO' Thusi'·>'" the noz::le entra.'1ce flov is uniform for nozzle entrance di~eters eQua~ to or less than 7.6 cm. The pitot pressure p;ofiles for CO2 and air indicate pcssible flo~ aSJ~etry at the tube exit. The core diameter for argon and t~o lo~er values of PIO for helium is half the tube diameter; the core diameter nay increase vith PIO for helium. Only helium test gas demonstrates steady flov for the middle value of PIO tested. This middle value is near the optimum'value determined from centerline pitot pressure time histories. For ;he m~e value of PIO for each test gas, itot pressure tine histor~es s o~e arge oscillations, in P1tOt pressure occur later in the flov for the center xrobe than for adjacent probes. _~s implies that a turbulent flow region moves irrvard

'£rom the tube vall vith time, and eventually fills the tube.

Toe test core diameter of'the expansion t~el . is four times that of the expansion tube, a.Iid this represents an advantage in model construction a!ld instrumentation for tunnel testing. Hovever, several disadvantages e~st for testing models in

~the expansion tunnel: (I) The degree of the~o­chemical nonequilibrium flov vithin the shock layer about a ~del is expected to be nore-severe for the expansio!l tunnel. To m~intain the same binar.r lav scaling parameter6 (product of density and shock standoff distance) as the expansion tube, the test core di~eter of the exoansion tun.'1el vould have to s:cco::nodatemodels. 5Otim~;-ia;g;rthan t;sted iIcthe expansion tube. "This ':increase in'model size is required to compensate for the tunnel free stream density being only 0.02 times the tube free stream density. (2) Attempts to measure shock shapes o~ a flat-faced cylinder positioned at the tunnel exit with a single pass and double pass schlieren ~ystem vere unsucces~ful due to the lov value of free stream density. [ This inability to measure shock shapes represents the loss of an fmportaijt diagnostic in the study of real-gas flov fie1ds. c,8 (3) Ex5sting pressure transducers must be operated at maximu."ll sensitivity to measure pitot pressure in the expansrontUIlnel. These trans­ducers are not capable of accurately measuring lover pressures such as occur on the surface of slender models. (4) ~m flow conditjons at the expansion tunnel exit have not· been dete~ined

'- ~e to the inabIlity to measure three flov quanti­ties simultaneously in the vicinity of the test section. Attempts to measure free stream density using ~'1 electron beam17 proved unsuccessful. The capability of measuring pitot pressure and stagna­tion point heat-transfer rate simultaneously has been de:onstrated. 7 However, large uncertainties exist for the pitot pressure, due to the method gf measurement and possible rarefied flov effects,l and accurate interpretation of the convective heat transfer rate may be difficult due to flov chemis­try and rarefaction effects. Also, the combination of pitot pressure, heating rate to a sDher~and

"flov velocity is ~des~le for accurate deter­m~~IL.~test ~ cOnditions. 19 A rough estimate of expansion tu.'1nel flov conditions may be obtained from isentropic, inviscid, one-dime.nsional 'predictions for thermochemical equilibrium. 20

s

.........

Nozzle Entrance Diameter

The geometric area ratio of a conical nozzle may be conveniently varied by utilizing inter­changable nozzle entrance sections of diff~ent entrance diam~ter. Ideally, this variation in geometric area ratio results in a corresponding variation in the flov expansion vithin the nozzle, thereby producing a range of test flow conditions ~or a given nozzle entrance condition. Thus, the expansion tunnel possesses additional versatility, in comparison to the expansion tube, for generating & range of free stream conditions.

Horizontal itot ressure nrofiles an~y tical veloc} t profU.e.s...JiU:.e,~Sr.mzIL.in..iigur-es--15· and 16, respectively, for C02 and air test gases and a range of nozzle entrance diameters. Nozzle entrance diameters from 5.1 cm to 9.5 cm vere tested, yielding geometric nozzle area ratios from l5J to 44. The results of Figures 15 and 1 represen vhe ~erage of tvo tests for each entrance di~"lleter. and may be compared to the results of Figure 14, which represent a geometric area ratio of unity. The magnitude of the pitot pressure across the test .core decreases vith decreasing entrance diSJ:Ieter, as expected from the conservation of oass relation. (The one-dimensional conservation of mass relation shovs that the ·pitot pressure at the nozzle exit is inversely proporti~o-tEe=effective nozzle area raUo, vfi1cn 1S somevhat sl:laller than the..,g.eOJll.e::tJ:'j,c area ratio. Hovever. the pitot pressure magnitudes of" iigures 14 and 15 reveal that the pitot pressure at the nozzle exit is. roughly, inversely propor­tIonal to 1.5 to 2 times the geometric area rat~ Hence, the one-dimensional· mass relation is not valid for the present conditions.) The ~ter of the core of essentially const~~t res sure is near y cons ant for the r~'1ge of entrance diameter ex~ned. be1ng approximately 0,45 times the nozzle exit diameter for the smallest entrance diameter and approximately 0.50 times the exit diameter for the largest entrance diameter for both test gases. The velocity profiles of Figure 16 illustrate the flov velocity across the test core is essentially constant for the present range of nozzle entrance diameter. This trend is expected. since most all the flov energy is kinetic and the contribution of ch~'1ges in internal energy of the test gas during expansion become very small. The velocity test core diameter decreases fromapprox­ima.tely g.7 times the nozzle exit dtameterforthe largest entrance diameter to about 0.5 times the exit di~~eter for the ~allest entrance di~~eter. Thus. the velocity boundary laye~ thickness approaches the pitot pressure boundary layer thick­ness with increasing geometric nozzle area ratio.

Axial Station

A major concern with the use of conical nozzles is source flow effects. Pitot pressure profiles measured at stations upstream (denoted by negative z) and do~~stream of the nozzle exit are shown in Figure ~ for C02 and air test gases. These results represent the average of tvo tests and no correction is applied to the pitot pressure due to the good run-to-run repeatability of the flow velocity at the nozzle entrance. The magnitude of pitot pressure within the test core decreases with increasing distance from the nozzle entrance, indicating the existence of source flow effects. For the range of-axial station examlnea;-t1le--~$t

core diameter increases from 0.45 times the nozzle exit diameter to 0.50 times the exit diameter be­tveen the most upstream station and most downstream stations.

For comparison, pitot pressure profiles "measured dO'\ffistream of the expansion tube exit are presented in Figure 18 for C02. air, and helium test gases. These results also represent the average of twa tests, and run-to-run repeatability in the velocity, Us ,10,e inferred from time-of­arrival measurements was with 0.8 percent for all test gases. For stations within 11 cm of the tube exit, the pitot pressure prorTles shov the existence of a core of uniform pitot pressure. The ma~itude of pitot pressure within the core is essentially constant with distance dOw~stream of the tube exit. A decrease in test, core ' diameter occurs for axial-stations exceeding II em, althbughthe Pitot-'pr~~s;;:~-;ithi;-the core rema~nsnearIY constant iith axial station.

The results of Figures 17 and 18 are compared in Figure 19. where the average pitot pressure of

.l the center seven probes of the survey rake used in i\ the expansion tunnel tests and the average pitot

Jo pressure of the center three probes in the ~, ~ expansion tube tests are plotted as a function of

\,rV distance from the tunnel or tube exit. These W' /, values of average pitot pressure are ~ (nOndimenSiOnaliZed by the average pitot pressure

~ ! 'measured at or near the tunnel or tube exit. The y r~ults of Fi~~e 19 demonstrate the absence of any

6' 'I ~ significant variation in flov properhes w:tth aual

~IVr\ stat10n downstream of the tube exit, whereas.

• rather large axial variations in flow properties ~ are observed lor the expansl:0n tunnel'. For C02 .p II and air test gases, the pitot pressure decreases

\, • by a factor of approximately 1.75 for the range or

~ \11/ axial station examined. This magnitude of source

r interpretation or measurements performed on a test ~ X~~~ flov effects will introduce uncertainties in the

~ model.

Concluding Remarks

An experimental study of the performance of the Langley Expansion Tcmnel for various test gases (primarily C02 and air) and range of quiescent acceleration section and nozzle pressure, nozzle geometric area ratio. and nozzle axial station has been performed. Pitot pressure time histories measured'vith and without an electromagnetically­opened tertiary diaphragm revealed the nozzle flow fully starteg without the diaphra~ (initial nozzle pressure equal to quiescent acceleration gas ,pressure). and the quaSi-steady pitot pressure period exceeded that obtained with the tertiary diaphragm; both observat' ons contradict-preclidlon. Pitot pressure and flow velocity time is or~es demonstrated free stream density and velocity are ess~ntially invariant with time for about 300-~s follov1ng the nozzle starting process"and test~ times for the expansion tunnel exceeded t~e for tne expansion tube with the-same-dri ver~quTescent 'eest gas, and quiescent-acc;l~ra.tIon gas pressures. The now vel~ i:lcreases slightrrtl-t-o~ 5 percent) as the flow travels between the nozzle ,entrance and exit, whereas the flov d£nsity decreases drastically (density at nozzle exit

. ~2 times that,at nozzle entrance). J?~~ pressure profiles reveal t~ of a uniform core about the nozzle centerline. the

: .. ....... ~

dianeter of which is approximately half the nozzle exit diameter or £our times the core di8;1!eter ?bserved in the expansion tyPe. ,Velociu pr~iles also illustrate the existence of a uniform core, but the diameter is about 0.7 times the nozzle exit diameter. These profiles andl~-density profiles inferred from the velocity measurement tecr~ique shoy a toroid of higher density floy surro~s an inner region of lover. uniform density. ~is

toroid of high density flow is attributed to disturbances as predicted for hypersonic flov in conical nozzles. AX1al var1atlozi-of the 25-probe pitot pressure survey rake upstream and downstream of the nozzle exit revealed the existence of significant source flov effects, characteristic o~ conical nozzles.

Expansion tunnel results were compared to expansion tube data to determine the advantages and disadvantages of the two medes of operation. Pril:lary disadvantages of the expansion tunnel are as follows: (1) Exi~~nce ot severe sQUbce flo~ effects. (2) Accurate determination of free stream and postnorm~_shock flov properties is not

'possible, due to the current inability to simul­taneOusly measure a sufficient number of flow quantities. in the vicinity of the nozzle exit vith available flov diagnostics. (3) !"ne lov free stream density will result in nore pronounced non­equ11ibri~~ flow effects in the shock layer of blunt test models. (4) The low value of flow density prohibits floy visualization using conven­tiona::r'techniques. Important a~ges of' the expansion tunnel are: (1) Test core diameter is rour times larger than for the expansion tube; and (2) Fost::test pressure loads are greatJ,y.reduced. With the tertiary diaphragm removed frem the expan­sion tunnel., the run frequency and test repeat­ability for the two modes of operation were essen­tially the same. Recent expansion tube tests d~onstrated the quasi-steady test tine can be increased with no degradation in f10~ quality by operating at a lower quiescent test gas pressure. and this longer test time is comparable to that of the expansion tunnel. In general. the larger test models for th~ __ ~)(~ansion ~unnel do net-offer a Significant" advantageIn- convectiveheat1ng measure­ments using thin-film resistance gages; also. the development of SUbminiature semiconductor pressure transducers (which'can be used only at the higher model surface pressures of the expansion tube~ because of their relatively lov sensitivity). minimizes advantages orfered with a larger model in the tunnel.

In the opinion of the author. the expansion tube mode of operation appears to bJLJ1,9~ desirable than the expansion tunnel mode of opera­tron for performing hypersonic-hyperve1ocity rear­gas studies of proposed entry configurations.

References

~impi. R. L., "A Preliminary Theoretical. Study of the Expansion Tube. A New Device for Producing High-Enthalpy Short-Duration Hypersonic Gas Flows," TR R-133. 1962. NASA.

; ....

. , : .

.~ .

I

2Anon , Fourth Hypervelocity Techniques Sy::.posiU!ll; "Advanced Experimental Techniques for Study of Eypervelocity Flight," Arnold Engineering Development Center, 110ve!:lber 1965.

3.rrbpi, R. L. and Callis, L. E., "A Perfect­Gas Analysis of the Ex;~~sion TQ~nel. a Modifica­tion to the Expansion Tube," TR R-223. April ~965,. NASA.

"Moore, J. A., "Description and Initial Operating Perfor.nance of the Langley 6-Inch Expan­sion Tube Using Heated EeliU!ll Driver Gas," TM X-3240, Septe:nber 1975, NASA.

5Moore , J. A., "Description and Operatipg Performance of.a Parallel-Rail Electric-PIc System With Helium Driver Gas for the Langley 6-Inch Ex:?ansicn Tube," TN X-344B, December ~916. NASA.

6Miller , C. G •• "Shock Shapes on Blunt Bodies in Hypersonic-H:rperve2ocity Helium, Air, and CO2 Flows, and Calibration 3esults in Langley 6-Incn Expansion Tube," TN D-1800. February 1975. NASA.

. 7Miller• C. G •• "Operational Experience in the Langley Expansion Tube With Various Test Gases and Preliminary Results in the Expansion Tunnel," AIAA Ninth Aerodynamic Testing Conference, June 1976.

8Miller, C. G., "}. Comparison of Measured and Predicted Sphere 'Shock Shapes in Hypersonic Flows With Density Ratios Fro!:! 4 to 19," TN-D-8076. December 1975, NASA.

91400re, J. A., "Measured Opening Character­istics of an Electromag3etically Opened Diaphragm ~or the Langley Expansion Tunnel," TM X-3378, August 1976, NI..SA.

1°Friesen, 'it. J., "Use of Photoionization in Measuring Velocity Profile of Free Stream Flow in Langley Pilot :':cdel Expansion Tube," TN D-4936. December 1968, ,jASA.

llLaney, C. C., "~~crowave Interferometry Technique for Obtaining Gas Interface Velocity Measurements in an Expa~sion Tube Facility," TM X-72625, November 1914, NASA.

12Miller , C. G. and Jone!!, J. J., "Incident Shock-Wave Characteristics in Air, Argon.·Carbon Dioxide. ~~d Helium in a Shock Tube With Unheated Helium Driver," TN n-8099, December 1975, UASA.

. --13Fricsen. 'it. J. and Moore, J. A •• "Pilot

Model Exp~~sion Tunnel Test Flow Properties Obtained From Velocity~ Pressure, and Probe !1easurements," TN D-1310, November 1973, NASA.

l"weilcuenster, K. J., "A Finite~Difference -Analysis of the liozzle Starting Process in an Expansion Tunnel," TN D-8105, December 1975, NASA.

10

15 Watson, R. D., "Wall Coo~ing Effects on Hypersonic Transitional/Turbulent Boundary Layers ,at High Reynolds Numbers, It AlA}. Journal, Vol. ~5, October 1977. pp. 1455-1461.

16Callis, L. B., "An Analysis of SupersoniC Flov Phenomena in Conical Nozzles by a Method of Characteri~tics." TN D-3550, August ~966. NASA.

11 Hoppe, J. C., "Electron Beam Fluorescence System to Measure Gas Density in ~pulse Facili­ties," ISA 20th International Instrumentation S~posium. ¥;ay 21-23, ~974.

1~ailey, A. B. and Boylan, D. C., "Some Experi;nents on Il:lpact-?:ressu:::-e Probes in a 1ow­De!lsity, Hypervelocity Flow." AEDC-Tl.-61-161. U.S. Air Force, December 1961.

19Miller , C. G •• and Wilder, S. E., "Real_Air Data Reduction Procedures Based on Flow Par~eters Measured in the Test Section of Sunersonic and Hypersonic Facilities," TN D-66lB,-¥~Ch 1973, NASA.

20Miller, C. G. and Wilder. S. E., "Program and Charts for Determining Shock Tube, Expansion Tube, and Expansion Tunnel Flow Quantities for Real Air, n TN D-11.52. Fe,!:>ruary 1975. NASA.

..

.. :~,:.,: ..

... _-_. ----.--------_._ .. _--.., .

. . DRIVER VESSEl PRIMARY ACCElERATION TUBE

~COAXIAtCow:CZlTRaIAPHRAGMS . AtTtRNATEPOSITIONS

FOR ELECTRIC- DRIVEN FOR SECONDARY ARC DRIVE . . TUBE _ DIAPHRAGM

" .. ~~-:!@=;.-'~~.~~,.".~ -:lr.:::.;:)1 L" .. ' .. =- =~ ~ lk''<",1 c~~i i:-::»~.:-a- --:-:--::---~--'if9: .,. --:;;--....,.... ~ £

1 U I I TO lERTI,l.RY 0 5 10 ROUGHING

DIAPHRAGM DISTANCE FROM PRIMARY DIAPHRAGM. meter~PUMP

NOZZLE NOZZLE . TES~ON DIFFUSION PUMPS" ,~; TANI( ............. ""'!. "'- \. _ :"'f't

. """..:;.-1:".-~ DUMP TANK ~;jJ) .... - ~~ ,... 25 30 35

DISTANCE FROM PRIMARY DIAPHRAGM. met2rs

Figure 1. Sketch of Langley Expansion Tunnel.

PRIMARY DIAPHRAGM

DRIVER SECTION

Figure 2.

NC I DENT SHOCK

~ ITST GAS IN ACCELERATION CD . SECTION (FREE StREAMI 1 ~ QUIESCENT ACCElERATION GAS

INCIDENT @ f~NM~n'~6'c~AI~6EHINO SHOCK ACCELERA TI ON StenON

I NTERM£DIA TE ACCELERATION SECTION SECTION

DISTANC£-

Scheoatic diagram of expansion-tube flo\/' sequence.

2.0 013 Awe 015 Awe

OPENING TIM£.. ms

SOliD SYMBOlS DENOTE BROKEN WIRE

L2

• 8

LINES DENOTE SECOND­ORDER CURVE FIT

. • 6,!-1--'---!Z:----L--t3---'-----:l CAPAC nOR V<X.TAGE. tV

Figure. 3. Measured tertiary diaphragm. opening time as a function-of capacitor voltage.

---.~--- ."._--_._---- _ ....

T~it , ..... , ''0 t:~ :~J ~f':! t,;...

J"7 . ~.: ''''l !':~ ... ~ .... ",.:--_

.._---_._-------------

160

o

Figure 4.

120

20

Figure 5.

!

FLUSH !SEMICONDUCTOR), xlr = o.z

200 400 to liS

E~fect of driver pressure (P4!Pl constant) OD expansion tube. pitot-pressure time history for CO2 test gas, P10 = 3.2 N/m2 •

o

o

o

o 0 o 0

2 PC' MN/m

o 32.7 o 11Z

~~ ..

o o

o 0

PI' tN/m2 Us,l' m/s Us,lO, ~, m/s O. 69 3029 4560 0. 28 3066 C75

o o

Effect of driver pressure (P4!Pl constant) on expansion tube pitot pressure distribution for CO2 test gas, PIO = 3~N/m2 • z :' 1 cm, .r = 7.6 cm •

"---_ ....... __ .- -_ .... ------_.

I i j.

1 ! ! .

, .

.'

' ...... -•••• -.01; \ •• " ... .

4

2 Pt. kNfm 2

0

4

2 Pt. kNfm 2

2 Pt. kNfm 2

o

.. _ .... ""

~ARY Cd) ~ .. r :::: 760 IlS OIAPHRAM

Ql) 6'T ::: 1160 liS

I

(f) 6T:::: 360 liS (c) 6 T:::: %0 liS V

Figure 6~ Pitot pressure time histo~ for the case of no tertiary diaphr3.gllI and for various time intervals "between

• initiation of tertiary diaphrag:! opening and flow arr! val at the diaphr8.glll. Air test gas, flush­mounted pressure transducer,

6.0

5.6

u: S.2 tmls

U

(4

~igure 1.

---'

p~O = 3.2 N/m2 , d* = 8.9 cm, z = 0, x/R = 0.2.

AT, liS o NO TERTIARY DIAPHRAGM o 855 o 655 A 455 £::,. 2SS ~ 90

o .2 .4 .6. .8 LO y/R

Velocity distributions for the case of no tertiary diaphragm arid for various time intervals between initiation of the tertia~ diaphragm opening and flov arrival at the I diaphragm for CO2 and air test gases •. ~ d* = 8.9 em, t = 325 ~St R =.31.75. I

i ______ .. _. __ ... ____ . __ . __ . ___ .--1

---------. -AT. liS

o NO T£RTlARY DIAPHRAGM

.. 0760 0560

6.0 A 360

·0 tl.16O

5.6 g~!~ 0 goS 0 °o~ 0 0%.

A tl. A§ (>0

5.2 a ES 0 !

u. 0 tmlS

4.8 ~ 0

4.4 ." AIR. PIG = 3. 2 N;m2

4.0 -La -.8 -.6 .4 .6 .S LD

Figure 7. Concluded.

6

"t.e· 4

tN/mi 2

0 6

"t.e· • t;Wm2 .

2

Figure 8.

lit. tN/m2

Pt. tNfm 2

:'Figure 9.

..

fa! Pn = 0.5 PlO

tJ It" = LS "10

Effect of initial nozzle pressure O~ time history of centerline pitot pressure for C02·test gas. Plo = 2.~ N/m2, d* = 8.9 cm~ z = o. lrc = 655 llS.

6

I

2

0

6

til NO tERTIARY DIAPHRAGM

hOOO

• P1WTECTOR DISK

2

o ." .6 .8 1.0 t-- .: -. :

fUlSH MOlM'

PROTECTOR DISK

, l'itot pressur~ tlme'hfstoriesfor a

flush-mounted pressure transducer and a transducer having a protector disk. Air test gas, PIO = 3.2 N/m2 , d* = 8.9 em, Z = o.

l >.' : .....

~ .. :.:: ~;.:' ~: .. '.' 6 ..... '

: 2 "te' tH/m

•

2

Figure 10.

4

2 Pp kN/m 2

1

Ll

..!L u Ue

~. Figure 11.

Ca) ARGOO

Ill) HRIUM

'n = lL3 N/m2

P =11N/m2 n

Erfect of initial nozzle pressure on time histor,y of centerline pitot pressur~ for argon and helium test gas,s. PIO = Pn. d* = 8.9 c~. z = o.

fal PITOT PRESSUR~

II) VELOCITY

Measured pitot pressure and average axial component of velocity as a function of time for air and C02 test gases. Bars on symbols denote uncertainty in measured velocity. d* c 8.9 em, Oe = U 10 • s. ,e

. . • ____ •• ___ ._ ••• _ .... ___ ._ ..... _._._ ... __ 0 ____ 0. __ • ______ _

• _;....-_. - ___ ••• o ..... _ .....

• e

5.6

5.2 U.

tmls U

U. bills

6.0

5.6

5.2

(,1

8

8 o

2 Pn • N/m Us. 10, e' m/s

o 1.1 5365 o 2.1 5051 o 12 4S89

-.2 0 .2 .4 .6 .8 LO ylR

"AIR

0000 0 0

~8~Sfsl88§

Us. 10, e' mrs 5700 S4'4

5304

o

o o

.(~o -.1 -.6 -.4 -.2 0 .2 .4 .6 .8 LO 11R

.~ ........ : Figure 12. Effect of initial nozzle pressure on

velocity distribution for C02 and air test gases. P10 = Po,

I Figure 13 •

d* = 8.9 em, t = 325 ~s. R = 31.75 cm.

3 o UlUe o pIPe

y/R

-... :.:----~~ Vertical Velocity a¥'static densit~ . profiles do;mstream-of-the--no2z:fe

exit for C02 test gas. Pn = P10 = 2.1 N/m2• d* = 8.9 cm, R = 31.75 em.

r' . -":,. ~

I

! ., :!

';

i .. .. ,

. 1 I

1 1

I.

160

140

13)

100

80

60

40

20

0 -LZ

160

80

80

Figure 14.

F".+ ;:". # '" : ... ",,* This ~"": •• f"~~" f""- .~- .. :! ofJ: - .. '''~l'' ... .:~~ .;~-,. - . .. " '. - .... -~ of ."oj $" .......... !. _~

,----------------------------------------------. • I

<><><><><><> <>OOi1 o c:!d

aogg~8 9 C1 d

0 00 00 0

0

~ <> g (altOZ

0

-.& '.4 0 .4 .8

o. cJ

o

'lIr

<> GIl AIR <> B d

r::::

o

cf cf

o o

o d

o

PIO' Nlm o 16 o 12 04.1

1.2

<>

2

<> d

d <> o C1

o

o C1

o

d o

o

-.8 -.' o .•

t:) ARGotl

dO <><><><><> do g 0 000

cJcJC1 Clef

<> o 00 00 0

o

C1

-.8 -.4 0 .' xlr

x/r

2 PIO' Nlm o 2.7 o 7.S <> 10. 9

.8 1.2

Us..10. e' mls 5217 S011 4883

<>

.8 L2

Us. 10. e'· mls 5302 4951 4925

Effect of quiescent acceleration ga~ pressure on expansion ttcl>e pitot-

. pressure distribution ~ several test gases. PI = 3.45 kN/m2, z = 5.6 cm, r = 7.6 cm. Unflagged symbols read at t = 100 ~s. flagged sYmbols' read at t = 300 ~s.

60 <> 0

d

, .

o

-lIS) Hallr.\

<> g C1

0

-.4

. ,

~ g Cf ~

8- a <> C1 g

0 0 0 0 C1

2 . '10' N/m Us. 10. e' mls o 5.3 1312 o 16. 0 6ClO4 0 <> 26.S 6605

o xIr

_& L2

. Figure 14. Concluded.

2 . Cal CO2, Pn :: 2.1 N/m • Us. 10. e :: S014 ! 105 mls

s

6

Figure 15.

8 <>

Ife. tm o 9.5 o 19 <> 7.6 .0. 6.35

de. tm' o 9.5 o 19 07.6 .0. 6.35

Effect of nozzle e~nce diameter on pitot pressure distribution for C02 and air test gases. PIO = Pn~ z = 10.2 cm, t = 200 us, R'" 31.75 cm.

. .

i .

------------.. -,--- -- : ----- -------_ .. -- '-'---"-." --- ----.--------

- --~-:....-- .. ---------;.;~~~;.~: ,- .. ",,' Tn., ,,!oJ .... , *:: b"!'" ., ... ~ :"":"!''': • ..,

77-;' ~~ "$ ~ ... ~.":.-:-~ '. :.~ ..... r =-: ::'. '.:'~ -: r: -- ..... ';" .' . ..." , •. ;' .. -.-=_ ..... _~";I'_- lII:0l ...

.. t COi, Pn" 2.1 N/m2

• Us.lO,e = S113 !. 70 m/s

d·, em 0 9.5 0 8.9 0 7.6

5.6 A 6.35 ~ 5.1

5.2 e@g~~9Bs~~~~ 8 0

u, u 8 ~. A 0 . tmJs il. A ~

U ~

.e.O -LD -.8 -.6 -.4 -.2

• , AIR. p = 3.2 Nhn2 U 10. :: 5406 + 67 m/s n ' s. e -

6.0

5.6 a!~g~~f~R~~gg ° 5..Z A

~A 0 de, em U, tmIs 0 9.5

U 0 1.9 0

<> 7.6

U ~ A, 6.35 ... to. .5.1 ~

.e.~1.0 -.8 -.6 -.4

Figure 16. Effect ot nozzle entrance diameter on velocity distribution for C02 and air test gases. PIO = Pu.

7

6

5

~

2 Pt' tNlm

)

2

. '

t = 325 llS, R = 31.75 cm.

fal CO2' Pn :: 2.1 N/mZ

0 z. em o -20.3 0 0 <> 20.3 0

0 00

0 0 <>

0 0 0

0 OOgo8 08§ogoooo 0 0 0 0 0 000

000000000000000 0

Effect of nozzle axial station on pitot pressure distribution for C02 and air test gases. PIO = Pn' d* = 8.9 em. t = 200 ~s. R =.31..75 em •

7

6

5

. 2 . "t, tN/m 3

2

o

• 0 o

•. ~ ..... -.... --~'"

III AIR. Pn·: 3.2 N/mZ

Z. C11l o -211.3 o 0 <> 211. 3

o

o

° <>

. Figure 17. Concluded •

120

100

80

2 pt. kN/m ·60

40

20

100

80

60 ; 2 : 't' tN/m

40

20.

; Figure 18.

6 § til ~ ~ i ~ ~ h. z. em t:.. 0 L2

0 6.2 <> 113 A 16.4 ~ 21.4

Ca) CO2, pt = 0. 69 kN/m2 A

~ PIa = 3.2 NlmZ 0

0 Us. 10. e = 5072 ~ Z1 m/s

0 .4 .8 LZ x1r ----..:. .... _----_.

i ~ 3 t R 0 h. g 0 til

z. em <> 0 1.2

A 0 6.2 A OlD il..

~ I::. 16.4 to. 2L4

tlJ Am. '1 = G. 345 tN/m2 0

Z

~ ~ PI0 = 6.7 N/m

US;lo.e ~ 556> ! 32 m/s

-.4

Ef~i'e of' axia1 station on expansion ube pitot-pressure distribution

wnstream of tube exit for C02' air, and helium test gases.

-. --_ .. __ ._._. - ----_._-----------

----_._-----. 10

60

Figure 18.

L

14

L2

J: _t_ LO liV%=O

.8

.6

z. em OL2 o 6.2 011.3 !!;. ~ a t:.16.4a~QIOl'l::l t.. 21.4 2t e

& IcIHELlUM.·PI = 3.45 ~m2 PIO = 16 N/m

Us.lQ. e = 6962 ! 57 m/s

-.8 -.4 .. o 'lir

Concluded.

o

o o

o

TEST GAS o AIR o CO2 <> AIR b. CO2 b. He

FACilITY TUNNEl TUNNa TUBE lUBE TUBE

%. CIII

\

o o

.4

o

o

~l,)rS';.·l,:~.~

. '~'~F'f>'?:'7: '''~'.- ,-.:' .'C·r .

1.2

Figure 19. Variation of average pitot pressure . across test core vith axial location for expansion tube and expansion tunnel.

.- -- _._-- --.,-....... --_._._------_ .... _---_ ... ----_ .. _--- .

•

- i

., !

----------------

•