l_ 84- 19787"
NASA Technical Memorandum 83582
The Infinite Line Pressure Probe
David R. EnglundLewis Research Center
Cleveland, Ohio
and
W. Bruce Richards
Oberlin CollegeOberlin, Ohio
Prepared for the
Thirty-ninth International Instrumentation Symposium
sponsored by the Instrument Society of AmericaDenver, Colorado, May 7-10, 1984
N/ A
https://ntrs.nasa.gov/search.jsp?R=19840011719 2020-03-22T00:58:52+00:00Z
THE INFINITE LINE PRESSURE PROBE
I
David R. EnglundNational Aeronautics and Space Administration
Lewis Research Center21000 Brookpark RoadCleveland, Ohio 44135
ABSTRACT
The infinite line pressure probe provides a meansfor measuring high frequency fluctuating pressuresin difficult environments. A properly designedinfinite line probe does not resonate; thus itsfrequency response is not limited by acoustic res-onance in the probe tubing, as in conventionalprobes. This paper will review the characteris-tics of infinite line pressure probes and describesome applications in turbine engine research. Aprobe with a flat-oval cross section, permittinga constant-impedance pressure transducer installa-tion, is described. Techniques for predicting thefrequency response of probes with both circularand flat-oval cross sections are also cited.
INTRODUCTION
Measurement of fluctuating pressure is a commonrequirement in aerospace research. To make suchmeasurements, transducers are often connected tothe desired points of measurement with shortlengths of tubing. In these cases the frequencyresponse of the measuring system is often limitedby acoustic resonance of the connecting tube andtransducer cavity volume. The direct approach toincreasing the frequency response of the measuringsystem is to decrease the length of the connectingtube and minimize the transducer cavity volume.When this approach has been taken to the limits
imposed by environment or geometry, and the fre-quency response is still unacceptable, the infi-nite line pressure probe may be the answer.
The ideal infinite line pressure probe is a non-resonant system. It consists of a long tube whichacts as a waveguide for pressure fluctuations; thetube must be long enough so that the pressurefluctuations are sufficiently dissipated beforereaching the end of the tube that reflections donot result. A pressure transducer set into thewall of the tube senses the pressure fluctuationspassing down the tube and provides the desiredpressure related signal. To avoid resonance, theideal infinite line must be designed so that noreflections of pressure waves occur anywhere with-in the line. This means that the acoustic imped-ance, the product of gas density and velocity ofsound divided by cross section area, must be con-stant. Achieving absolutely constant acoustic
W. Bruce RichardsPhysics Department
Oberlin CollegeOberlin, Ohio 44074
impedance is probably impossible, but approachingthis goal means that care must be taken to mini-mize cross sectional area changes, especially atthe tap for the pressure transducer. Care mustalso be taken to avoid gross changes in gasproperties.
There are relatively few descriptions of infiniteline pressure probes in the technical literature.The earliest work known to these authors was doneat NACA's Lewis Flight Propulsion Laboratory (nowtile NASA Lewis Research Center) in the early1950's and is described in reference I. In thiswork a water-cooled infinite line probe was usedto study combustion instabilities in afterburn-ers. Reference 2 describes the application ofinfinite line probes to measure fluctuating cham-ber pressure in the nuclear rocket (NERVA) programin the 1960's. Reference 2 is a primary sourcefor information on infinite line probes; it pro-vides an extensive description of the infiniteline technique, describes methods for predictingthe response, and shows the effect of variationof design parameters. Reference 3 describes someapplications of infinite line probes in turbineengine testing. Reference 4 describes a more re-cent application in which an infinite line probewas used to measure acoustic noise in the burnerof a YF-I02 turbine engine. Figure 1 shows theprobes from references i and 4. There is remark-able similarity in these probes even though some20 years and many advances in instrumentationseparate their development.
Many applications for infinite line probes in tur-bine engine testing (ref. 3) can take advantage ofthe high natural frequencies and small size of sub-miniature silicon-diaphragm pressure transducers.These transducers have natural frequencies as highas 500 kHz and can be made as small as 1.2 mm indiameter. These advantages make it feasible tobuild infinite line probes with frequency responseapproaching 100 kHz. Such a probe has been devel-oped and put into limited use at the Lewis Re-search Center. The probe uses tubing with a flat-oval rather than circular cross section in order
to permit the pressure transducer diaphragm to bemounted flush with the inside wall of the tubing.
The intent of this paper is to describe the infi-nite line probe and its characteristics in general,
andthento describein moredetail the flat-ovalinfinite line probe. Methodsfor calculatingfre-quencyresponseandphaseshift of probeswithbothcircular andnoncircularcrosssectionswillbecited.
CHARACTERISTICSOFANINFINITELINEPROBE
Beforediscussingtheperformancecharacteristicsof aninfinite line probe,it will benecessarytodescribethegeometryof theprobeanddefinetheparametersof interest. Figure2 showsa schema-tic diagramof an infinite line probeandliststheparamters.Thepressureto bemeasured,Po(t),excitesa pressurefluctuation in theprobeat themeasurementendandthis disturbancepropagatesdownthe line. Thepressuremeasuredbythepres-suretransducer,P_(t), is thepressurewaveafterit hastraveledthedistance_ downtheline.Afterpassingthe transducerthepressurewavecontinuesto thetermination.Fora line of fi-nite lengththeremustbea terminationandthiswill causea reflection; for aproperlydesignedline this reflectedwavewill beweakenoughtobenegligiblewhenit arrivesbackat the trans-ducer. Theterminationis arbitrary fromthestandpointof the line design: it maybeopentothe atmosphere,openinto a volume,or maybeclosed. Thechoiceof terminationdependsuponsafety,environmental,andpracticalconsidera-tions relatedto theexperimentonwhichthelineis used. In somecasesit maybedesirableto in-troducea small,steadyflowof gasat the termi-nationof the line whichwill act to purgetheline of gasfromtheexperiment.Thistechniqueis especiallyusefulto keepreacting gases outof the line and was used in the work of referencesi and 4.
If the pressure at the inlet is given by Pocos mt, then the pressure wave traveling down theline can be described as
Px(t) = Poe-ax cos(mt - bx)
where
Px(t) = pressure at any point x in the lineat time t
a = attenuation factor
b : phase factor
= circular frequency
The attenuation factor is a function of the linediameter, the gas properties, and the frequency.The phase factor defines the phase lag resultingfrom propagation of the wave down the line atphase velocity.
It is important that there is nothing to cause areflection of the pressure wave as it passes downthe line (i.e., the line must have constant acous-tic impedance). In general this means that thecross section area of the line be constant through-out and that there be no changes in gas properties(e.g., a temperature gradient) within the line.
In real probes this means no crimps or burrs atplaces where tubing sections are joined, no sharpbends, no sharp changes in tubing shape, and espe-cially that the transducer tap volume be small orthat the transducer be coupled to the line in sucha way as to minimize a change in the impedance ofthe line. This may be accomplished with a short,small diameter passage (or a group of such pas-sages) coupling the line to the transducer. How-ever it must be recognized that this system ofpassages coupled with the transducer cavity is initself an acoustic filter which will limit fre-quency response.
Analysis of Frequency Response
There have been numerous studies on the response
of pressure tubing and pressure measuring systems.Of the many available, the analytical techniquedescribed in reference 5 has been found to be es-pecially useful in analyzing infinite line probes.This technique can analyze series-connected sys-tems of tubes and volumes. It allows one to cal-culate the sine wave pressure amplitude and phaseangle in any volume in the system relative to thesine wave pressure amplitude at the input to thesystem. Thus it provides information in thestandard amplitude ratio and phase angle versusfrequency format.
For the conditions encountered in turbine enginetesting, the frequency response predictions fromthis analysis are in good agreement with measureddata. Comparisons of predicted and measured re-sponse are presented in references 5 and 6. Theanalytical technique of reference 5 was used inthe work of reference 2.
The assumptions upon which the reference 5 analy-sis is based are the following:
1. No steady flow in the line.
2. The magnitudes of the pressure, density,and temperature fluctuations with time are smallcompared to time-averaged values. (When fluctua-tions are not small, wave shape distortion andamplitude-dependent attenuation can result. Ref-erence I discusses these effects.)
3. The diameter of the line is small comparedto the length.
4. The diameter of the line is small comparedto acoustic wavelengths of interest.
5. Flow is laminar throughout the system.
Effect of Parameter Variation
Figures 3 to 9 have been prepared to illustratethe calculated performance of an infinite lineprobe and the effects of variations in parameters.These figures are similar to those presented inreference 2 except that the gas conditions andprobe sizes are more representative of turbine en-gine testing than are the NERVA conditions (gase-ous hydrogen at 37 atmospheres and temperatures ofI00 to 300 K).
Figure3 showstheperformanceof a baselineprobe. Theprobeis a 2 mminsidediametertube2 meterslongandclosedat the end. Atransdu-cer is placed4 cmfromtheentrancewith atrans-ducertapvolumeof 0.45mm3. Thegasconditionsarethoseof air at 1 atmosphereand300K. Theprobeperformanceis plottedasamplituderatioP_/Poandphaseshift versusfrequency.Theam-plituderatio scaleis expandedsoasto showtheeffectsof reflections. Theamplituderatio is1.00at lowfrequencyandis onlyslightly atten-uatedto 0.90at 10kHz. Theoscillation in theamplituderatio at lowfrequenciesis dueto re-flections fromtheendof the line. Theamplituderatio oscillationsabove5 kHzaredueto pressurewavesreflecting betweenthetapvolumeandthemeasurementendof the line. Thephaseshift in-creasessmoothlyandbecomeslargeat frequenciesat whichthetap lengthrepresentsmultiplewave-lengths.
Figure 4 illustrates the effects on amplitude ra-tio of changing probe design parameters. Figure4(a) illustrates the effect of increasing the linediameter from 2 to 2.5 mm. With the larger diam-
eter there is less attenuation of pressure ampli-tude and the amplitude ratio oscillation due toreflections from the end of the line is larger.The oscillation due to reflections from the tapvolume is about the same because it is affected
by two opposing factors: there is less attenua-tion because of the increased diameter, but therelative discontinuity at the tap volume issmaller, causing a weaker reflection.
Figure 4(b) shows the effect of increasing the
length of the line from 2 to 4 m. The oscillationdue to reflection from the end of the line is re-
duced; the oscillation due to reflection from thetap volume is unchanged.
Figure 4(c) shows the effect of reducing the tap
length from 4 to 3 cm. The magnitude of the os-cillation due to reflection from the end of the
line is reduced somewhat because the pressure tapis closer to the pressure node which must existat the open end of the probe. However, the majoreffect is that the oscillation due to reflection
from the tap volume shifts to a higher frequencyband.
In figure 4(d) the tap volume is changed from 0.45to 1.35 mnri and to zero. There is no change inthe oscillation due to reflections from the end of
the line. However, changes in tap volume alterthe strength of the reflection from the tap volumethus affecting the magnitude of the oscillation inamplitude ratio at high frequencies. With zerotap volume there is no reflection and the oscilla-tion in amplitude ratio in the higher frequencyband vanishes.
The effects of changes in gas properties areillustrated in figure 5. Figure 5(a) shows theeffect of increasing the average pressure levelfrom 1 to 1.5 atmospheres. The increased pres-sure results in a lower kinematic viscosity (_/p)and the attenuation in the line is reduced. Thus
the magnitudes of the oscillations due to reflec-
tions from both the end of the line and the tap
volume are greater.
The last of the baseline illustrations, figure5(b), shows the effect of temperature gradientsalong a probe. Case A shows the effect of a 500 Klinear gradient over a length span from X : 0.5to X = 3.5 cm. Case B shows what happens if that500 K gradient exists within the first cm of theprobe length. The response for case A shows thattemperature gradients of this kind are undesirable(note the change in scale of amplitude ratio onthis figure). However, the case B result showsthat by forcing the gradient to the very front endof the probe, the effect of the gradient can bepushed to a high frequency band, possibly out ofthe frequency range of interest. Positioning ofthe temperature gradient may be accomplished by acombination of water cooling and a slight purgegas flow down the line.
THE INFINITE LINE PROBE WITH FLAT-OVAL TUBING
Design Details
The objective in developing the flat-over probe
was to get the highest possible frequency responsein a configuration compatible with turbine enginetesting. The natural frequency of a silicon-diaphragm pressure transducer with 1.2 mm diamterand a 1.6 atmosphere pressure range is approxi-mately 500 kHz, and the flat (within 5 percent)frequency response limit is approximately 100 kHz.The task was to design an infinite line probe touse such a transducer and not limit the inherent
response of the transducer.
There are two size limitations imposed on thedesign. One is that the internal dimensions ofthe line must be small compared to the free spacewavelength for the maximum frequency of interest(roughly 3.4 mm for I00 kHz in room temperatureair). The other limit arises from the anticipatedapplication, which was to make total pressuremeasurements in compressor blade wakes. For thisapplication a probe configured to measure totalpressure would be mounted radially behind a com-pressor rotor. Optimum resolution of the bladewake required that the probe opening be small(less than I mm) in the blade passing direction.
A probe design based on tubing with a flat-ovalcross section seemed best suited to meet theserequirements. This cross section would providereasonable area while maintaining a small dimen-sion in the blade passing direction and provide aflat surface so that the pressure transducer canbe installed with minimal tap volume. The meas-uring end of the resulting probe is shown in fig-ure 6. The inside dimensions of the flat-ovalcross section are 2.5 by 0.8 mm. The head of theprobe was made from two rectangular plates w_ichwere electric-discharge-machined (EDM'd) to givethe correct passage shape when the two pieces weremated together. The EDM'd passage has a gradual90 ° bend (centerline radius of curvature is 2.7mm) so as to make the probe opening normal to theaxis of the probe body (i.e., to allow measurementof total pressure). The remainder of the infinite
line is madefromflat-oval stainlesssteel tubingsupportedwithin1/4 inchdiametertubingwhichformsthebodyof theprobe. Thistubingwascarefullymatedto theEDM'dpassagein theprobeheadsoasto avoiddiscontinuitiesin the insidesurfaceof the line. Thetotal lengthof the lineis 63.5cm;this lengthwasdeterminedassuminguseof theprobein air at 1 atmospherepressure.Theprobeheadwascarefullyshapedwitha fileto achievethe shapeshownin figure 6 after theprobewasassembled.Thebacksideof thetrans-ducercanbeseenin figure 6 asthedarkobjectonthe sideof theprobehead.Thedarkmaterialis epoxycoveringthetransducerleadwires.Thepressuretransduceris a flat configurationwith thediaphragmat theendof a short (0.3mm)cylinderwith its axisnormalto the flat platewhichformsthemountingsurfaceof thetransducer(seefig. 7). Thetransduceris mountedwithsil-iconerubberto theoutsideof theprobeheadsothat thecylinderholdingthediaphragmprotrudesthrougha hole in theprobeheadandpositionsthediaphragmflushwith the insidesurfaceof theflat ovalpassage.Thedistancefromthe centerof thediaphragmto the probeopening(i.e., thetap length)is 1.2mm.If thediaphragmismountedperfectlyflushwith the insidesurfaceof the passage,thetapvolumeis zero;with anuncertaintyin diaphragmpositionof 0.05mm,themaximumtap volumeis 0.09mm_.
FrequencyResponse
Amajorproblemin developingprobesfor theseveryhighacousticfrequenciesis that measuringtheir frequencyresponserangesfromdifficult toimpossible.In thecaseof this probe,noattemptwasmadeto measurefrequencyresponse.Instead,reliancewasplaceduponanalyticalpredictionsof frequencyresponse.Thedifficulty with thisapproachis that theanalysisof reference5 re-quiresa circular crosssection.
A studywasundertakento find a solutionthatcouldbeusedfor theflat-oval crosssection(ref. 7). Earlyin this studyit wasdeterminedthat a solutioncomparableto that in reference5couldnotbeobtainedfor tubesof noncirculargeometry;however,a solutionfor planewavestravelingbetweeninfinite flat plateswaspos-sible. Thisflat plate solutionsuggestedthat,for theprobesizesunderconsideration,thefre-quencyresponseof theflat-oval tubingwouldbenearlyidenticalto that of anequivalentcirculartubewitha diametergivenbyfour timestheareaof the flat-oval crosssectiondividedby itsperimeter.Thereference5 analysisfor circulartubingusingthis equivalentdiametercouldthere-fore beusedto predictthefrequencyresponseofflat-oval probes. Note,however,that the actualcrosssectionareamustbeusedat placesin theequationswherethe areaandvelocity areusedtocomputethevolumeflow rate. Forthe flat-ovalsectionusedin this work,theequivalentdiameteris 1.20mm.Thisequivalentdiameterconceptisquite similar to thewell-known"hydraulicdiame-ter" usedto characterizeliquid flow in openchannels.Thestudypresentedin reference7 in-
cludesa comparisonof predictedandmeasuredfre-quencyresponsedata. Threedifferent tube-and-volumesystemsweretestedin whichthetubeswerelengthsof theflat-oval tubingusedto maketheinfinite line probe. Theexperimentalresultsagreedcloselywith thepredictions.Reference7 alsopresentsanapproachto calculat-ing theresponseof tube-and-volumesystemswhichmaybepreferableto thereference5 approachforsomeusers. Thereference 5 method uses a recur-sion equation by which the pressure amplitude andphase angle in one volume may be determined rela-tive to other volumes in a series-connected stringof tubes and volumes. The alternate approach ofreference 7 uses a product of matrices similar tothe ABCD matrices used in solutions of electricaltransmission line problems.
Figure 8 presents the calculated frequency re-sponse of the flat-oval infinite line probe de-scribed in this report. The probe parameters andgas conditions used for the calculation are listedin the figure. With the short tap length (1.2 mm)the attenuation is very low, even at high frequen-cies. The calculated results are shown for twovalues of tap volume: zero and 0.09 mm3. It isquite apparent that minimizing the tap volume inthe mounting, of the transducer is important.
Test Experience
Although the flat-oval infinite line probe was in-tended for measurements in blade wakes, its majoruse to date has been in an aeroelastic fan flutterexperiment. In this experiment the objective wasto measure the dynamic pressure field upstream ofthe fan during flutter and nonflutter operatingconditions. The probe was mounted upstream of thefan of an F-IO0 turbine engine and was positioned180 degrees from the flow direction so as to pointdownstream toward the fan. The engine had addi-tional instrumentation to detect and measure theeffects of flutter. This included a photoelectricscanning system to detect nonengine order vibra-tion of the blade tips, dynamic strain gages onthe fan blades, and miniature pressure transducersin the fan casing over the tips of the fan blades.
An example of the pressure data obtained from theprobe is shown in figure 9. Figure 9(a) is thefluctuating static pressure versus time; the pre-dominant frequency in this signal is the fan bladepassing frequency, 4631 blades per second. Thehigher frequency portions of this signal are partof the repetitive structure of the pressure field.Figure 9(b) shows the frequency spectrum of thissignal. The predominant peak is the blade passingfrequency at 4631Hz, and harmonics out to theseventh are apparent.
Another test using the flat-oval infinite lineprobe is reported in reference 8. In this testthe response of the infinite line probe was com-pared with that of a frequency compensated dragforce anemometer. The compensated frequency re-sponse of the anemometer was calculated to be flatwell beyond the fundamental natural frequency(42.8 kHz) of the anemometer beam. The comparison
wasmadebyusingeachinstrumentto measuretheprofile of shockwavesexiting a smallshocktube.Figure10showsthat thetwowaveformsarenearlyidentical.
CONCLUDINGREMARKS
Thispaperhasdiscussedthe infinite line pres-sureprobeandits potentialfor fluctuatingpres-suremeasurementsin turbineengines.Someexam-plesof applicationsof infinite line probesinturbineenginetestingweregiven. Thegeneraloperatingcharacteristicsanddesignparametersof the infinite line probeweredescribed.Meth-odsfor analyzingthefrequencyresponseof infin-ite line probeswerecited. Finally, a specificprobedesignedfor veryhighfrequencyresponsewasdescribed.
In thediscussionof the generaloperatingchar-acteristicsof the infinite line probe,thecal-culatedresponseof a baselineprobedesignwaspresented.Thentheeffectsof changesin probedesignparameterswereillustrated in a seriesoffrequencyresponsediagrams.Thispart of thediscussionis verysimilar to the presentationmadebySamuelson(ref. 2) in 1967. It mustbeacknowledgedthat Samuelsonpresenteda verycom-plete anddefinitive treatiseonthe infinite lineprobe. However,theexamplespresentedin refer-ence2 werefor nuclearrocketapplicationsandarenoteasily relatedto turbineenginetest con-ditions. Thediscussionpresentedherewasin-tendedto provideillustrations for thesesomewhatdifferent conditions.
Finally, applicationof this informationto spe-cific measurementproblemsshouldbeconsidered.Thedesignof infinite line probesis complicatedbythelackof approximationequationscomparableto the linearizedsecondordersystemequationsusedto describetheresponseof simpletube-and-volumepressuremeasuringsystems.Forthis rea-sonrelianceonthecomputer-basedanalysispro-cedurescitedhereinis almostmandatory.Oncetheseanalysisproceduresaremadeoperational,however,the advantagesaffordedbytheir useareconsiderable.Thefrequencyresponseof agreatvarietyof configurationscaneasilybecalcula-ted, including,if necessary,noncircularcross
sections. In addition,theaccuracyof theseanalysesshouldbesufficient to minimizeor, insomecases,eliminateexperimentaldeterminationof thefrequencyresponseof pressuremeasuringprobes.
REFERENCES
i. Blackshear, P. L.; Rayle, W. D.; and Tower,
L.K.: Study of Screeching Combustion in a6-1nch Simulated Afterburner. NACA TN 3567,1955.
2. Samuelson, R. D.: Pneumatic InstrumentationLines and Their Use in Measuring Rocket NozzlePressure. Report No. RN-DR-0124, NuclearRocket Operations, Aerojet-General Corporation,1967.
3. Fischer, J. E.: Fluctuating Pressure Measure-ments from DC to Over 100 kHz in Jet EngineTesting. Instrumentation in the AerospaceIndustry, VoI. 17, Instrument Society of
_1971, pp. 117-123.
4. Reshotko, M.; Karchmer, A. M.; Penko, P. F.;and McArdle, J. G.: Core Noise Measurementson a YF-102 Turbofan Engine. American Insti-tute of Aeronautics and Astronautics, AIAA77-21, 1977.
5. Bergh, H.; Tijdeman, H.: Theoretical and Ex-perimental Results for the Dynamic Response ofPressure Measuring Systems. Rep. NRL-TR-F.238,National Aero- and Astronautical Research
Inst., Amsterdam, 1965.
6. Nyland, T. W.; Englund, D. R.; and Anderson,R.C.: On the Dynamics of Short PressureProbes: Some Design Factors Affecting Fre-quency Response. NASA TN D-6151, 1971.
7. Richards, W. B.: On the Propagation of SoundWaves in Tubes of Non-Circular Cross Section.NASA TM to be Published.
8. Fralick, G. C.: Extending the Frequency ofResponse of Lightly Damped Second Order Sys-tems: Application to the Drag Force Anemome-ter. NASA TM 82927, 1982.
,-- TO 15 m OF 114 in./
f,' COPPER TUBING
91 cm
1
\_- TRANSDUCER AND
PREAMPLIFIER
IIIII
I- COOLING WATER
SOUND IN,1/4 in. TUBE
(a) Reference I probe.
TEST
DUCT "
FEED-THRU
PRESSURE
FLOWREGULATOR TAP
O.635 cm diam
r-SENSING,,: / TUBE
N!
SUPPORT BLOCK
FVENT
THERMOCOUPLE
30 m COIL
_OF TUBING\
PURGE FLOW
(b) Reference 4 probe°
Figure 1. Infinite line probes used in afterburner and combustor testing.
, ,-TRANSDUCER
TAP VOLUME, VT __-'_, " D TERMINATION
F-'IPolt) -- _ ,41:, ,
L_.]
i:--x L "-ID - INSIDE DIAMETEROF LINE
L - TOTALLENGTHOFLINE
VT
= TAP LENGTH,LENGTHFROMMEASUREMENTENDOFLINETOTRANSDUCER
- TAP VOLUME, VOLUMEAT TRANSDUCER/LINEINTERFACE
GAS PROPERTIESOF INTEREST
Po(t) - FLUCTUATINGPRESSUREAT MEASUREMENTENDOF LINE
Px(t) - FLUCTUATINGPRESSUREAT POINTx IN LINE
P_(t) - FLUCTUATINGPRESSUREAT TRANSDUCER
T - GAS TEMPERATURE
p - GAS DENSITY
y - RATIO OF SPECIFICHEATSCp/Cvp - GAS VISCOSITY
C - SONICVELOCITY, yP/p
u - KINEMATICVISCOSITY, pip
Figure 2. Schematic diagram of infinite line with definition ofparameters of interest.
CD(7)
CD(2)CDCD
CD0
-m
C]
A
0 CD C] CD 0
I --_ CSJ 0")I I I
6S3 W030) 3SWRd
>-
W
0WnlEL
c-
°I
__ E r-
E
O °
°_LL.
CO0
,--4
I I I I I I
.,.,r.___...._..-_
-"_...,._....
"_ 0 (.0 ('X.f CO0 C:) O_ 0") O0
• o • • •
011UN 301]I17dWU
_X"" 1' _--=-' r_- b d
_m_ _E "_Z'0 (.0 0_1 CO
0 0 O_ O_ O0
OIIU_I ]OFIII]dl_U
0
I I
00 00
,- ,o "--
L. mm
._.E _L.L.
1.08
°I 04l---(I:
n,-1.00ILl
.96_._113_
"- 92_ •
.88
I I f IIlil i i I I IIIli_ I I I IIIII I I I I I I Ill
P AIR
_tm V illlliHIii,
vlTIIIIII"F" I I I Ill,,| I I I llllll i ' i li'i'l I I i i.lil
(a) Average pressure.
1 20
_o1110 l
(IE
l.O0-Wc-_
i--- ._u -I--..4
__ICL
z 80QZ •
•701
I __........,........,........,._,.,....,
" /i"-'" " '-'i
]LOCATIONOF 500K TEMPERATUREGRADIENT
CASEA: BETWEENx - 0. 5AND x = 3. 5 cmCASEB: BETWEENx - 0 AND x --1. 0 cm
I
I I I IIII1_ I I I IIIIII I I I IIIIII I I I IIIIII
10 100 1000 10000FREOUENCY (HZ)
(b) Temperatu re gradient.
Figure 5. Effects on frequency response of changes
in gas properties. Baseline cor_ditions: D, 2 ram;L, 2m; _, 4cm; VT, 0.45mm"; airatlatmand300 K.
Figure5. - Measuring end of flat-oval infinite line probe.
,_---2 mm ------,
JDL.Oj/
L 1. 25 mm DIAPHRAGMMOUNTED FLUSH IN ENDOF 1. 5 mm DIAM. CYLINDER
,- EPOXY/ COVERING
LEADWIRES
r SILICONE RUBBERI
F'- PROBE HEAD / ADHESIVEI /II I
Figure 7. Pressure transducer configuration and transducermounting in flat-oval probe.
°1 04I---I •
I--(3:
_I.00bJE_
_- .96.._1
_ 92(I2 •
0
Or)UJ"' -90¢'v
U..I
,-,_-180
LL]COoz-270-r-13_
-360
, ,,,,m I i t ,,t.q _ i IIllft I I itltllt I ! I tllill
VT,mm 3
O.09 - FLAT-OVAL PROBE
a) 0 - IDEAL FLAT-OVAL PROBE, tilL,Ill I I,It.,I , ,t*..I I I,ll_ltl I IIIIIll
.... i.,. I . , i ..... i ! ..... i I
(b! ..... ,,ta a tltJlJl * t i*t*J*l * * *st*a*l t * tJ*tatJ
100 10000FREQUENCY [HZ)
Figure & Calculated frequency response of flat-oval infinite line probe. Probe parameters:
equivalent D, 1.20mm; L, 0.63m: _, 1.2mm;air at I arm and 300 K.
3_IFISS3_Id
r'_ o,
:_,.,r_
30VllOA lnaln0
o'.
_- o,,
E
t--°_
_ ,_...._- 0
--E
I.L
78 ps
(a) Transient response of compensateddrag forceanemometer to shockwave.
63 mV
/8 ps
(b) Transient response of flat-oval infinite line probeto shockwave.
Figure 10. Comparisonof transient response of flat-ovalinfinite line probeand compensateddrag force anemometerto shockwave.
1. Report No.
NASA TM-835824. Title and Subtitle
2. Government Accession No. 3. Rec,p,ent's Catalog No.
5. Report Date
The Infinite Line Pressure Probe
7. Author(s)
David R. Englund and W. Bruce Richards
9. Pe_o_ing Organization Name and Address
National Aeronautics and Space AdministrationLewis Research CenterCleveland, Ohio 44135
12. Sponsoring Agency Name and Address
National Aeronautics and Space AdministrationWashington, D. C. 20546
6. Peflorming Organization Code
505-40-1B
8. Pertorming Organization Report No.
E-1973
10. Work Umt No.
11. Contract or Grant No.
13. Type of Report and Period Covered
Technical Memorandum
14. Sponsoring Agency Code
15. Supplementaw Notes
David R. Englund, Lewis Research Center; W. Bruce Richards, Oberlin College,Physics Dept., Oberlin, Ohio 44074. Prepared for the Thirtieth InternationalInstrumentation Symposium sponsored by the Instrument Society of America,Denver, Colorado, May 7-10, 1984.
16, Abstract
The infinite line pressure probe provides a means for measuring high frequencyfluctuating pressures in difficult environments. A properly designed infiniteline probe does not resonate; thus its frequency response is not limited byacoustic resonance in the probe tubing, as in conventional probes. This paperwill review the characteristics of infinite line pressure probes and describesome applications in turbine engine research. A probe with a flat-oval crosssection, permitting a constant-impedance pressure transducer installation, isdescribed. Techniques for predicting the frequency response of probes withboth circular and flat-oval cross sections are also cited.
17. Key Words (Suggest_ by Author(s))
Pressure measurementDynamic pressure
18. Distribution Statement
Unclassified - unlimited
STAR Category 35
19. Secudty Cluslf. (of this repoR)
Unclassified_. Security Cluslf. (of this page)
Unclassified21. No. of pages 22. Pdce"
*For sale by the National Technical Information Service, Springfield, Virginia 22161
