KluyVJrwcJ 1 - D~LFT
van
KARMAN
INSTITUTE
FOR FLUID DYNAMICS
6 SEP.191f
TECHNICAL NOTE 100
DESIGN AND CALIBRATION OF FOUR PROBES FOR USE IN THE TRANSONIC TURBINE CASCADE TESTING
C. SIEVERDING and L. MARETTO von Karman Institute
F. LEHTHAUS and O. LAWACZECK DFVLR-AVA Göttingen
MAY, 1974
~A~
-~O~-
RHODE SAINT GENESE BELGIUM
~VW
TECHNICAL NOTE 100
DESIGN AND CALIBRATION OF FOUR PROBES FOR USE IN THE TRANSONIC TURBINE
CASCADE TESTING
C. SIEVERDING and L. MARETTO von Karman Institute
F. LEHTHAUS and O. LAWACZECK DFVLR-AVA Göttingen
LIST OF SYMBOLS
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~LIST OF FIGURES •
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• • iiSUHMARY
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i i i1. INTRODUCTION
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• 12. PROBE DESIGNS •
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33. CALIBRATION TESTS IN THE TRANSONIC WIND
DFVLTI/AVA
-TUNNEL OF THE GOTTINGEN •
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• • 53.1 Test set up and c alibrat ion procedure
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• 53.2 Test results
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• 64. TESTS WITH THE nEEDLE-PROilE AT TRANSONIC MACH
NUMBERS In THE VKI HIGH SP.Kl::D CASCADE TUNNEL 13
4.1 Test set up and calibration procedure • • 13
4.2 Test result s • • • • • • •
•
• • • • • • 144.3 Invest igat ion of the bow wave generat ed
by a pitot tube at M
=
1.06 • • • • • • • 155. INFLUENCE OF Re-NUMBER ON CONE-PR013E. TTIUNCATED-OONE-PROHE AND WEDGE-PROBE AT
111 = 1.64 and 2.2 •
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••
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• • • • • • 175.1 Test set up and calibrat ion procedure •
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175.2 Test result s
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• • • • • • • • 186. CONCLUSIONS • • • • •
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• • • • • 19REFERENCES
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• • • • • • • • • • 20k t M P RE s
v
a Tl \I o 1 2 s L R REF PIT LIST OF SYMBOLSspecific heat ratio reference length
Mach number
pressure
Reynolds number
sensitivity of directional probe flow veloc ity
incidence angle blade efficiency kinematic viscosity SUbscripts total upstream downstream st at ic left right isentropic reference pitot
- ii
LIST OF FIGURES
1 Four probe designs and photograph of the probes
2à-b Probe hol der for simultaneous calibration of 5 probes in
AVA-Göttingen transonic wind tunnel
2c Schlieren photographs at M
REF
=
1.413a Photograph of probe holder for single probe measurements
3b Single probe mounted in AVA-Göttingen transonic wind tunnel
4 Reynolds number as function of Mach number in AVA wind tunnel
5 Correct ion factor for pitot probes as a function of incidence
angle and Mach number
6
Correct ion factor for pitot probes at zero incidenee versusMach number
7 Correct ion factor for statie pressure probes as a function
of ineidenee angle and Mach number
8a Correction factor for statie pressure probes at zero incidence
versus Mach number. Comparison between simultaneous calibra-tion of 4 probes and calibracalibra-tion of each probe separately
8b Comparison of measured cone statie pressures with
corres-ponding theoretical values at zero incidence
8c Correct ion factor for statie pressure in case of using the
directional tubes of the NEEDLE-probe for statie pressure measurements (at zero incidence)
9a-d Variation of the differential pressure of the direetional probes as a funet ion of ineidence angle and Mach number
ge Variation of the sensitivity of the direetional probes of
the NEEDLE-probe and the TRUNCATED-CONE-probe as a funetion of Mach number
10 Modified test seetion of VKI high speed cascade tunnel C-2
for ealibration at transonie Mach number
11 Comparison of statie pressure measurements by wall tappings
with statie pressure measurements at midspan of test seetion by AGARD needle probe
12 Comparison of correct ion factors for the statie pressure of
the NEEDLE-probe as obtained at VKI and AVA
13 Probe arrangement for the investigat ion of the bow wave
generat ed by a pit ot tube
14 Pressure rise through bow wave measured by NEEDLE-probe and
wall t app ings
15 VKI supersonic wind tunnel S-3
16 Reynolds number influence on probe eharaeteristies
SUMMARY
The work described herein concerns the design and the testing of four probes for the investigation of the flow
behind straigh.t transonic turbine cascades. The probes have
been designed and manufactured at VKI. The calibration tests
were carried out at the DFVLR - AVA - Göttingen and at the
von Karman Institute. The probes were investigated in the
Mach number range 0.8 < M ~ 2.2 for the range of incidence
angles _12° < ~ < +12°. Special attention is given to the
calibration in the transonic domain. The influence of Reynolds
numbers on the probe calibrations is studied at M
=
1.64
andINTRODUCTION
Contrary to measurements 1n subsonic flow the total pressure measured with a pitot tube in supersonic flow includes the shock losses in front of the pitot probe. This shock is assumed to be normal in the vicinity of the opening of the pitot probe. The shock strength is of course a function of the !.fach nwnber and therefore the static pressure has to be known
in order to evaluate the true total pressure in the outlet flow. Rence, an error in the static pressure influences directly the total pressure.
The blade performance of turbine cascades is charac-terized by the ratio of total pressure downstream of the cascade P 02 to the total pressure upstream of the cascades POl or by the blade efficiency
K
-Ps 2 K-l y2 1-
( - )
2 P 02 11 os-
os y2 .-
K 2.1S K-l P s2 1-
( - )
POlIf we assume for example that the static pressure in the outlet plane of a shock free two-dimensional nozzle is consistently
measured too low by 1
%
of the real dynamic pressure (P 02 - P ),82
then the nozzle e f1'i ciency would increase lin early from
11 = 100 % at f-12 = 1 to 11 = 101 % at M2 = 2.0, while P 02 /P Ol
would increase in a parabol ic way from 11 = 100 % at M2
=
1.0to 11
=
103.2 Ilf /0 at H2=
2.0.Apart from measur1ng .errors which can be directly
attributed to errors in the readings of the pitot pressure and static pressure. the most frequent ones are due to the fa ct that the pitot pressure and static pressure are measured in two
- 2
-The stro~pressure variations in the outlet flow field of
tran-sonic turbine cascades require in fact that the total. statie and directional pressure be measured in the same field point. This is obviously not achievable and has to be replaced by the requirement of the probe head being very small compared to the cascade dimensions. If the flow is two-dimensional. it is pos-sible to measure the total. statie and directional pressure on
the same stream surface. which allows to use 2 or 3 separated
pressure tubes aligned parallel to each other with all measuring holes lying on a line normal to the flow direction. In any way. the success of obtaining reliable cascade data is very closely
linked to the ability of reducing the probe dimensions. Minia-turizing of the probes is also mandatory in order to limit an
alteration of the outlet flow field at transonic Mach numbers by the presence of the probe. Furthermore. the probe parts ahead of the measuring holes have to be kept to a minimum.
The a~m of this note ~s not the presentation of new
probe concepts but rather a comparison of the characteristics of various probe designs which have already found applications in cascade testing and which are susceptible to present an ade-quate anSwer to the particular problem of transonic turbine cascade testing. An effort has been made in view of reducing the prooe dimensions to a minimum size.
2. PROBE DESIG~S
•
The design of a probe for the use 1n the outlet flow field of transonic turbine cascades is, in most cases, a compro-mise between the need of reducing the probe dimensions. the
re~uirement of short response times and good probe
charaeteris-tics. The fOllowing probe characteristics are desirabIe :
1. The pitot pressure does not need any correction in the whole
Mach number and incidence range.
2. The correction of the statie pressure 1S aeeessible to
theoretical considerations in order to limit the calibration work.
3. The directional probe 1S sensitive to flow angle variations and has a linear calibration curve over a wide range of incidence angles.
Four probes have been designed and manufaetured at VKI (Fig. 1).
The predominant factor in all four designs was the choice of the statie pressure probe and the position of the statie
pres-sure holes w~th respect to the pitot tube opening. According
to their characteristic features, the probes are called
1. NEEDLE-probe,
2. CONE-probe,
3. TRUNCAT~D-CONE-probe.
4.
WEDGE-probe (AVA).The NEEDLE-probe consists of a long needle statie pressure probe
and separated from i t . a combined total-directional probe.
The length between the cone head of the statie pressure tube :
(cone angle 15°) and the measuring holes are 12 x d (d
=
1.5 mm)while the length between the measuring holes and the probe stem
amounts to 15 x d. The two tubes of the directional probe lie
on both sides of the pitot tube, their openings are cut under
an angle of 35° with respect to the tube axis. All measuring
holes of the needIe probe lie on a line normal to the flow direction.
4
-The CONE-probe measures the statie pressure on a 30° cone by means of 4 measuring holes (4 x 90°) at 1.3 mm from the eone
head. The total-direetional probe is the same as for the nee41e probe. The pressure holes on the eone line 0.6 mm behind the openings of the total-direetional probe.
The TRUUCATED-CONE-probe consists of a eombined total-static pressure probe and. sep~~ated from i t . a directional probe. Tqe eombined total-statie pressure tube is made out of two eo-axial tubes : the inner pitot tube and the outer statie pressure tube with the truncated eone of 25° eone angle. The statie pressure
is measured on the eone by 6 x 60° holes at 1.5 mm from the tip. The openings of the direetional tubes are lined up with that of the pitot tube. The directional tubes are cut under an angle of
40° with respect to the tube axis.
The WEDGE-probe has originally been developed at the AVA-Göttingen
(Ref. 1). The probe shown in figure 1 is a somewhat modified version with redueed dimensions. Contrary to the othe~ probes.
the wedge-prob~ incorporates the holes for the pitot.statie and
directional pressure in one unit only. The pitot tube is situated in the center of the wedge and eaceeds the wedge edge by 0.1 mmo The directional holes lie on the upper and lower side of the
we~ge at a distance of 1.3 mm behind the wedge edge. The statie
pressure is measured on both sides of the wedge behind a step which is situated at 1.1 mm behind the wedge edge.
It should be noted that the directional elements of each probe can of course also be used in a zero balancing mode for the determination of the statie pressure.
3. CALIBRATION TESTS IN THE TRANSONIC WIND TUNNEL OF THE DFVLR/AVA G5TTINGEN
The main body of the calibration work was carried out at the DFVLR/AVA under contract for VKI. The tests were made in
the large transonic wind tunnel (test section of 1 m x 1 m).
This tunnel was chosen because of the possibility of varying the tunnel Hach number cont inuously in the range of interest,
i.e., 0.8 < M < 2.2 and because the large test section would
allow to test all 4 probes simultaneously.
3.1 Test set up and calibration procedure
The ÄVA transonic wind tunnel is a closed loop wind tunnel with continuous operation. The tunnel has a transonic test sect ion with perforated walls for the Mach number range
0.8 < M < 1.2 and a supersonic test section at the nozzle exit
for the Mach number range 1.2 < M < 2.21. The dimensions for
both test sections are 1 m x 1 m. The figures 2a and 2b show a
schematic sketch and a photograph of the probe holder with the 4 VKI probes and an additional AVA probe mounted in the center of the probe holder. Some schlieren pictures (Fig. 2c) illustrate
the shock patterns at a Mach number of M
=
1.41.Because of certain anomalies ~n the tets results, it
was later deemed necessary to repeat the calibration with each probe separately. Figure 3a shows the probe holder with one single probe and Figure 3b the set up in the wind tunnel.
The tes~ were run in the transonic Mach number range
(O.8 < M < 1.2) at a stagnation pressure Po :: 1 ata and in the
supersonic range (M > 1.2) at Po :: 0.8 ata. The Reynolds number
in the test section based on a reference length of 1 m is of the
order of 0.75 x 107 to 1.4 x 107 • The relation between the
Reynolds number and the Mach number is presented in Fig. 4. The
referenc~ values of the total and static pressures in the test
6
-are known from previous ca1ibration tests of the empty test sect ion.
3.2 Test results
3.2.1 ~i~nific~n~e_o! ~l~c~a~e_e!f~ci ~f_PLo~e
~n~ ~r~b~ hold~r_f~r_the_sim~lia~e~u~ calibration of several probes
---_
....
_---As mentioned before, two test series have been Tun 1. a simultaneous calibration of all probes mounted together
~n the test section (referred to as "common calibration" in
figures
6
and8)
and2. a calibration of each probe mounted separately in the test
section (referred to as "single calibration" in figures 6 and
8).
The necessity of the second test series became apparentaf ter a quick critical check of the test results of the "common calibrat ion". It was found that
1. The calibration curves for the AVA-probe showed eonsiderable differences in the transonic domain in eomparison with
previous calibration results.
2. The ealibration curve for the statie pressure of the NEEDLE
probe indicates excess~ve errors in the transonie range (up
to 22
%
of the dynamic pressure at an angle of attaek a=
0)and still unexpected high errors in the supersonie Maeh number
range (3
%
of dynamic head at M=
2.2 for a=
0).From a detailed investigation performed at the AVA, i t was con-cluded that the simultaneous calibration of all probes was not a suitable test procedure. As possible error sources, one must consider :
1. interference effects between the probes ,
2. slight vibration of the probes due to an insufficient rigidity of the probe holder.
The last point is of particular importance in the transonic domain. It is obvious that the variation of the flow field in
the environment of the probes due to the blockage· effe~t of
probes and probe holder is much stronger and reaches much further upstream than in the case of the calibration of a single probe. Furthermore. it is evident that the blockage effect for given dimensions of a probe and the probe holder depend on the dimensions of the tets section. Two questions then arise
1. what is the appropriate environment for the probes during
the calibration procedure (size of test section) ?
2. do we have to measure the reference values for the total and statie presure inside or outside of the influence field
of probe and probe holder ?
These questions need an adequate answer for the sake of testing turbine and compressor cascade at transonic outlet flow Mach n umbers.
The test resul~ of the "common calibration" are not
presented in detail. The main results will. however. be compared with those of the "single calibration" in order to underline
the significance of the probe blockage effect on the test results.
3.2.2 Single probe calibration
---The results of the single probe calibration are shown
in figures 5 to 9. The data are presented in the following form
Total pressure probe (PO)REF (PO)PROBE = f(a)
( Po P )
S REF
(PS)REF (PS)PROBE
f(
cd
=
Statie pressure probe
( Po - P ) s REF P L - P R f (Ct ) = Direetional probe (po
-
P ) S REFSensitivity of directional probe S
=
8
(po -- P R P ) s REF per degree incidence anglewith the tunnel Mach number MREF as free parameter and
(p 0) REF
I
( Ps )REF
J
real total and static pressure ~n the test section
(p Ot) ..,
p ~ PROBEI
'>
(p s ) PROB}!;
j
measured total and static pressure in the test sec-tion by probe to be calibrated. In supersonic flow Ppit represents thetotal pressure behind the bow wave (nojmal shock) in front of the probe head
} direct ional pres.ures
a flow angle with respect to probe axis
The calibration covered the incidence angle range of -12°<a<+12°
and the Mach number range 0.8 < M < 2.2.
!~~~!_~~~~!~~~_~~~E~~~~~~: (Figs.
5
and6):
The measured totalpressure of the
4
probes is practically independent of thevaria-tion of the incidence angle eover~d in the tests in the whole
Mach number range. The only exception is the wedge probe at an
incidence angle of 12° in the Hach number range 0.8 < M < 1.33
which shows a slight pressure drop. The pitot tubes measure
the correct total pressure at subsonic Mach numbers. The
dif-ference between P
pit and (PO)REF at supersonic Mach numbers
follows very closely the normal shoek relations up to M
=
1.8
(Fig.
6).
For Mach numbers M >1.8,
the measured values startto fall below the theoret ical curve ,Th ich indicates that the shock losses are somewhat less than those corresponding to a normal shock.
§~~~~~_~~~:~~!~_~~!!~~~~~~~ (Figs.
7
and8):
The shape of thecalibration curve and the maenitude of the deviation of the
measured static pressure from the reference statie pressure vary significantly for the various probe designs. The calibration
zero ineidenee angle axis and have a simple parabolie shape. The curves for the TRUNCATED-CONE-probe are in general symmetrie to the zero ineidence angle axis. but their shape can only be appro-ximated by a higher order equation. The asymmetry of the CONE-probe ealibration curves might be caused by a slight asymmetrie
position of the
4
pressure holes on the cone envelope or byimperfections of the eone nose.
The error of the statie pressure measurements at zero incidenee is plotted in figure 8a in funetion of the Mach number. This figure also includes the results of the simultaneous probe ealibration. The test results for the single probe ealibration
outside of the transonie range. i.e •• M > 1.2. are very
eneou-raging, The tests eonfirm in faet that the NEEDLE-probe needs
only very little correct ion (maximal error of 1
%
in the range1,2 < M < 2,2) and that the curves for the CONE-probe and
TRUNCATED - CONE-probe follow very elosely the theoretieal value of the statie pressure on a 30° and 25° eone. A comparison of the theoretical and experimental curves is shown in figure 8b.
The differenee for the 25° eone amounts to about 0.6 to 0.8
%
in the Mach number range 1.3 ~ M ~ 2.1 while for the 30° eone
differences of 0.3 to 0.6
%
are recorded. The higher differeneesfor the 25° cone have to be attributed to the blunt nose. The
relation of the statie pressure error versus Mach number is not
direetly accessible to theoretieal considerations in the case of the wedge probe. however. the smooth behaviour of the measured calibration curve will be helpful for future calibrations of
similar probes. It should be noted that the measuring points for
M
=
1.63. whieh all lie far off the curves. are obviously wrong.as this phenomenon oecurs systematically for all probes.
As mentioned earlier. the direetional element of eaeh probe might also be used for measuring the statie pressure. In this case. we use
p
- 10
-Figure 8c represents the case where the directional tubes of the NEEDLE-probe are used to measured the statie pressure at zero angle of attack. It is obvious that the ineensitivity in
the Mach number range M
=
1.0 to 1.3 excludes the use of sucha probe in this doaain. However. for M > 1.4, the statie
pres-sure correction factor depends not only strongly on the Mach
number, but moreover, it is a linear function of the Mach number.
The curves for the simultaneous probe calibration
deviate in the range M > 1.2 in general by 1
%
to 2% from
the single probe calibration. Possible reasons are interference effect s and vibrat ion of probe holder as already ment ioned.
The transonic regime 0.8 < M < 1.2 is characterized
by very high statie pressure errors for all probes. The influence of the different blockage effects for the single anf common
cali-brat i on is c lear ly demons t rated. In the e ase of the NEEDJ.E-probe,
the different blockage effect is expressed by a reduetion of
the st~tic pressure err6r from -22% to -16% at. M
=
1.05 and areduction from -14% to
-7%
at M=
0.8. The remaining error is,however, s t i l l much too high in order to attribute it simply to the NEEDLE-probe. If the statie pressure tube of the
NEEDLE-probe indieates errors in the transonic domain signifieantly
different from those in the subsonie domain, these differences are que either to small normal shocks formed on the statie pres-sure tube itself somewhere ahead of the measuring holes or to the interferenee of the bow wave, generated by the pitot tube, with the statie pressure tube ahead of the measuring holes.
However, normally tor slender bodies like those considered here,
these effeets should oecur only for M > 0.9 - 0.95.
It must therefore be concluded that also in the case of the single probe calibration, the high errors reeorded for
the NEEDLE-probe in the range 0.8 < M < 1.1 are due to strong
upstream effects of the probe holder. The faet that MREF is
based on m asurements with an empty test seetion entails that the statie pressure measured by the NEEDLE-probe is eompared to
a reference statie pressure obtained in a different flow field.
Hence. the blockage effect of the probe holder ~as to be ~e~~ced
further or the reference statie pressure has to be measured within the influence field of the probe holder. These conclusions are of course also applicable for the other probes.
~~~=~~i~~!!_E~=~~~E=_~~=ff~~i=~~: The variation of the directional
pressure coefficient in function of the incidence angle and the
free stream Mach number is plotted in figure 9.
The two ma~n requirements for a directional probe are
1) a good sensitivity and
2) a linear relationship between the directional pressure coef-fic ient and the Mach nwnber.
Both characteristics depend mainly on the incltided wedge angle (or equivalent wedge angle for a side by side arrangeaent of
single tubes). If the directional probe is a combination of single
tubes (NEEDLE-probe, CONE-probe, TRUNCATED-CONE-probe) both the
sensitivity and the linearity depend furthermore on the alignment of the holes with respect to the stream surface and the arrangement of the tubes to each other (tube P
L and tube PR side by side or
separated by a pitot tube).
The best linearity is obtained with the directional elements of the NEEDLE-probe and THUNCATED-CONE-probe. The cali-bration curves for both cases can be approximated with good
ac-curacy by a straight line in the angle range of ~ 10°. The
sen-sitivity of these probes, expressed by the slope of the curves
in figures 9a and 9c is plotted in figure ge. Bot~vurves show ~a
st rong Mach number dependenc e in the· range 11 = 0.8 t o l . 6 (almost
linear relationship). This dependence is, however, greatly reduced
for M > 1.6. The difference between both curves is mainly due to
the difference of the equivalent wedge angle (70° for NEEDLE-probe. 80° for TRUNCATED-CONE-probe).
The directional element of the CONE-probe is the same as for the NEEDLE-probe, however, a non-alignment of the openings
12
-of the direetional tubes with respect to the stream surface produces a slight S-shape o~ the calibration curves (Fig. 9b).
The irregularities of the calibration curves for the WEDGE-probe (Fig. 9d) are difficult to explain. The range of incidence angles in which all curves ean be approximated with a sufficient accuracy by a straight line is limited to a = +
6°
to 8°.
The variation of the zero error (defined by the inter-section of the calibration curves with x-axis) for a change in
M
REF is probably due to a lack of aeeuraey in the geometrie
4.
TESTS WITH THE NEEDLE-PROBE AT TRANSONIC MACH NUMBERS IN THE VKI HIGH SPEED CASCADE TUNNEL C-2Based on the conclusions concerning the calibration
results in the transonic domain as obtained in', the AVA transonic
wind tunnel, an additional test series was run in the VKI high speed cascade tunnel C-2 to study more closely the characteris-tics of the NEEDLE-probe in this range. The use of this tunnel presents the advantage of calibrating the probe in an environ-ment which is very similar to the outlet section of transonic
turbine cascades tested in the same ~unnel.
4.1
Test set up and calibration procedureThe C-2 tunnel was equipped for these tests with a
convergent nozzle which is followed by a sudden area enlargement.
The test section dimensions at the nozzle exit are 135 mm x 50 mm
and behind the enlargement 195 mm x 50 mm (Fig. 10). The Mach
number behind the nozzle exit depends on the settling chamber pressure and the distance of the measuring plane with respect to the nozzle exit.
The NEEDLE-probe was tested 1n the Mach number range
0.5 < M < 1.05 at zero incidence. The reference statie pressure
in the test section was measured by a pressure tapping on the
tunnel side wall, in presence 'of the NEEDLE-probe. The quality
of the side wall pressure measurements was checked before by comparing them with the measurements of a single AGARD needle
probe in the center of the test section. Figure 10 shows the
arrangement of the probes in the test section. The reference total pressure was measured by a pitot tube placed just upstream
of the nozzle exit. The relation between the settling chamber pressure and the reference total pressure 1n the test section was measured once for all before starting the calibration tests.
- 14
-4.2 Test results
The eomparison of the reference wall statie pressure with the AGARD needle measurements at mid-span is presented in figure 11. The differenee between both measurements is very smalle
A max~mum deviation of 0.7
%
of the dynamie head is reeorded atM - 1.05.
A eomparison of the side wall pressure distribution in the measuring plane with and without probe in the test section
shows a negligible bloekage effeet of the single AGARD needle probe while the presenee of the NEEDLE-probe alters eonsiderably
the flow field for M > 0.9. The reason is that the shaft of the
AGARD probe is about twiee as far behind the measuring plane as the shaft of the NEEDLE-probe.
The reference statie pressure was measured on the same side wall pn which the NEEDLE_probe was mounted (Fig. 10). There-fore, the wall pressure is influeneed in the same way by the probe shaft as the needle statie tube in the free stream.
The ealibration results are presented in figure 12 whieh also shows, for eomparison purposes, the calibration curve obtained in the AVA transonie wind tunnel. The statie pressure errors obtained at VKI are eonsiderably smaller than those at
AVA. The VKI values recorded at M
=
0.8 and 1.0 are -1.2%
and-3
%
instead of-7
%
and -13 '% found at AVA. However, aeompari-son of the statie pressure measured by the NEEDLE-probe and the wall pressure measured without the presence of any probe in the test section, showed very similar results as those obtained at AVA. One should therefore insist on the faet that the differences
in figure 12 are due to the differenees in the referenee statie
pressure. ~.e., whether the reference statie pressure is measured
4.3 Investigation of the bow wave generated
by a pitot tube at M = 1.06
In order to study more close1y the strength of the pitot tube bow wave at the intersection point with the needle
statie pressure tube of the NEEDLE-p~obe a test series was run
in which the bow wave of a pitot tube of 2 mm diameter was
traversed at a distanee of 16 mm by a single AGARD needle probe with a tube diameter of 1.5 mme The AGARD probe was introduced through the s ide wall, the pit ot probe from the bott om of the tunnel. Both probes were placed at mid-stream (Fig. 13). The
wa1l pressure distribution was measured parallel to the AGARD
needle probe. The results are presented in figure 14.
Both the AGARD probe and the side wal1 pressure tappings
measure the Mach number ahead of the shock as M
=
1.06. The shapeand position of the bow wave as taken from schlieren photographs
at M
=
1.06 are drawn to scale in figure 14. In the case of zerodisplacement of the AGARD needIe probe (i.e., statie pressure measuring holes aligned with pitot probe opening) the bow wave would intersect with the needle tube at a distance of 3.0 mm ahead
of the measuring holes. In that case. the Mach number measured
with the AGARD probe behind the bow wave is MAGARD
=
l.O. Inorder to correct the measurements behind the bow wave to the
undisturbed flow conditions ahead of i t , the statie pressure read
by the AGARD probe needs a correction of 7
%
of the dynamie head(po -
Ps)REF' The side wall pressure tappings which areapproxi-mately twiee as far from the pitot probe than the AGARD needle probe are much less influenced by the bow wave. The Mach number
changes on1y from M = 1.06 ahead of the shock to M = 1.04 behind
the shock.
The fact that there is a non-negligible influence of
the bow wave on the wa1l presure measurements indieates that the
reference wall statie pressure for the calibration tests in figure 12 also does not correspond to the undisturbed flow con-ditions ahead of the bow wave. The influence of the bow wave on
- 16
-the wall pressure ~easurements is, however, not necessarily the
same in both cases because of the difference in the frontal area of the shock generators.
The test results allow to conclude that an error of
the static pressure measurement of
7
%
at M=
1.06 presents amax~mum value for the NEEDLE-probe; t he real value should lie
5. INFLUENCE OF RE-NUMBER ON CONE-PROBE,
~RUNCATED-CONE-PROBE AND WEGDE-PROBE AT M
=
1.64 and 2.2(VKI 8UPER80NIC WIND TUNNEL 8-3)
The calibration tests in the closed loop AVA transonic wind tunnel were carried out :for the transonic Mach number range at an absolute total pressure o:f Po
=
1 atm and :for the superso-nic Mach number range at Po=
0.8 atm. (Fig.4).
However, the probes are intended to be used in the VKI high speed cascade tunnels which are o:f the ~lowdown type with exhaust to atmos-phere. These tunnels are operated :for transonic turbine cascade testing with settling chamber pressure o:f the order o:f 2 to 4 atm. An additional test series was there:fore per:formed in the VKI supersonic wind tunnel 8-3 in order to investigate possible Re-number e:f:fects on the probe calibrations. The tests were 1 imi ted t 0 high supers on ic Mach numbers (M=
1 .64 and 2.2)because o:f the rather small dimensions o:f the 8-3 tunnel. This test series did not include the calibration o:f the NEEDLE-probe.
5.1 Test set up and calibration procedure
The supersonic wind tunnel 8-3 (Fig. 15) is,like the high speed cascade tunnels,of the blowdown type :facility with exhaust to atmosphere. The inlet duct consists o:f a straight :fixed lower nozzle block and a contoured interchangeable upper nozzle bloek. Two contoured nozzle blocks :for M
=
1.64 and 2.2 were used :for th is test series. The test section measures 60 mmx 50 mme A variable supersonic dif:fuser downstream of the test section allowed to obtain a fully developed supersonic flow
in the test section :for both nozzle con:figurations at an absolute pressure in the settling chamber as low as Po
=
2 atm. The tests were per~ormed in the range of Po=
2 to4
atm.The refe~ence static pressure in the test section was measured by an AGARD needle probe and the re:ference pitot pressure by a single pitot tube with an outer to inner diameter ratio
18
-*
~
3. The stems of the reference probes as weIl as those ofthe probes to be calibrated did in no case alter the flow condi-tions in the measuring plane. Each probe was put separately into the tunnel. The repeatability of the test conditions was checked by a pressure tapping on tha lower nozzle block in the measuring plane.
5.2 Test results
The test results in figure
16
present only the datafor zero incidence angle. The graph shows both VKI and AVA data. In case that the VKI and AVA results were not obtained at the
same Mach number, the AVA dat a in figure
16
pre sent interpolatedvalues from the calibration curves in figure
7.
It is very difficult to draw any definite conclusions
about the influence of the Re-number on the various probes. The
problem lies in the fact that no comparative data are available between the VKI and AVA results for the same Re-numbers. Hence, we are unable to properly eliminate the differences which are
inherent to the use of two different calibration tunnels, two different measuring systems, etc. Nevertheless, the tests ind icat e that
1) the CONE-probe and the TRUNCATED-CONE-probe are not at all or
only very little influenced by a change in Reynoldsnumber in
the investigated Mach number range;
2) the WEDGE-probe is very sensitive to a Re-variation at M -
1.64
At M
=
2.2 the Re-effect is trongly reduced but seems s t i l lto exist.
It should be noted that the present VKI data of the WEDGE-probe
(open circles) are confirmed by 2 data points (open triangles) from earlier VKI tests with a very similar wedge probe.
Obviously, these tests are only of very restricted value. More testing is needed 1n order to cover a wider range of Re-numbers and Mach numbers.
6. CONCLUSIOl'iS
- The pitot pressure readings of all probes follow very closely the normal shock relations in the supersonic domain up to M
=
1.8. For M > 1.8, the measured bow wave losses are smaller than those corresponding to a normal shock. The deviation increases withincreasing Mach number.
The pitot pressure of all probes ~s independent of an incidence angle variation for the range of ~ 10°.
- In the supersonic Hach number range, the NEEDLE-probe needs at zero incidence less than 1
%
correct ion for the static pres-sure. The statie pressure measured with the CONE- and TRUNACTED-CONE-probe follows at zero incidence the theoretical values within 0.3 to 0.8%.
The WEDGE-probe (AVA) requires very highcorrect ions but all the c alibrat ion curve s are of s imple para-bolic shape.
- The calibration ~n the transonic regime requires much more attent ion. This problem is not only the problem itself of per-f'orming measurements in the transonic domain, but it has to be
seen in connection with the final purpose of the calibration, i.e., the measurement of the performance of a transonic turbine cascade at transonic outlet Mach numbers with a relatively big probe at a relative short distance from the trailing edge plane. In fact, the blockage effect of the probe stem and probe holder can be compared to the effect of a local back pressure valve which might not only modify the outlet flow field but also the complete passage flow. Hence, it would not only be advisable to calibrate the probes in a tunnel of similar size as the cascade tunnel in order to simulate the same blockage effect, but also to measure the reference pressures in presence of the probe to be calibrated. i.e., within the modified flow field. because
this is the one which is also going to exist at the cascade exit.
- The principle used for the directional probes of the NEEDLE-probe, CONE-probe and TRUN C.ATED-CONE-probe gi ve in general sat i
s-- 20
-factory results in the incidence angle range of ~ 10°; however,
the linearity of the curves depends strongly on a precise align-ment of measuring holes with respect to the stream surface. The relatively small wedge angle of the WEDGE-probe limits the linear
range to ~
6°
to8°.
.
_ Contrary to the CONE-probe and TRUNCA~ED-CONE-probe. the
WEDGE-probe characteristics are Re-number dependent •
REFERENCES
1. AMECKE. J. and LAWAC~ECK, 0.: Aufbau und Eichung einer
neu-entwickelten Keilsonde rUr ebene Hachlaufnessungen. insbesondere im transsonischen Geschwindigkeitsbereich.
---7
+=
_ ..
~=-m
----,
' ._ - . . - - - I -~. R-'-r
'*
r}L~
3' FOR MOUNTING IN TRANSONIC WINp TUNNELAVA- GÖnlNGEN
/
;'
=L:J---,
- - - - ( _1 __ .----j;
L-_r~--~~----~
28 34 NEEDLE PROBE PL~$
P~ ï..!iTYn~ Ppif b '" PR PPIT RL ' ~ 1---1. 25 -l
HOLES '_9000"'~"""..,.-;r:/r.;.,.'7/ 7;?;':::;>;;"7;7;'1""~""~:S:~""'~""::l
! I ~>'j9)j I-112.
1 ~ DETAIL B CONE PROBEt
HOLES 6.600~~
t
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t
Q A~. o~~~~2<~ifflP;
I
V
//./
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·
_
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.
--%-~+--DETAIL CTRUNCATED CONE PROBE
1=
::~ -l0~~~
WEDGE PROBE (AVA)FIG. 1
~
~
NEEDL E PROBE
CONE PROBE
TRUNCATED CONE PROBE
WEDGE PRO BE (AVA)
/
/
Ppit
FIG.2-a
80
1
CONE PROBE AVA -PROBE NEEDLE PROBEPROSE HOLDER FOR SIMULTANEOUSLY CALIBRATION OF 5 PROSES.
FIG.2-c
T~ANSONIC WIND TUNNEL
FIG.3
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