The far-field acoustic data for eight full-scale fans tested at a NASA-Lewis outdoor facility were analyzed for the effect of a cut-off design feature where the number of rotor and stator blades are proportioned so that the blade-passage tone will not propagate. Inlet flow distortions, prevalent during static testing, interact with the rotor to generate tones that generally mask this cut-off phenomenon. Distortion tone properties previously evaluated are used to analyze the tones propagating from the aft fan duct. The analyzed data clearly differentiated the fans that are and are not cut-off

Heidmann, M. F.

English

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1975024799
https://ntrs.nasa.gov/search.jsp?R=19750024799 2020-03-22T18:52:31+00:00Z
1. Rel_ort No. 2, Government Accession No. 3. Recipiant's Catalog NO.
NASA T M X-3296
4. Title _ Subtitle 5. Report Dire '
AN OBSERVATION ON 'row£ CUT-OFF IN STATIC September 1975
TEST DATA FROM JET ENGINE FANS s. PerformingO,gsni,,ationCode
7, Author(s; 8. Performing Orpnizltion Report No.
Marcus F. Heidmann E-8409
10. Work Unit No.
9. Performing Organization NemNm_ Adckes$ 505-03
Lewis Research Center
11. ContraRt or Grant NO.
National Aeronautics and Space Administration
Cleveland, Ohio 44135 13. Type of Report and Period Covered
12. Spon_xing Agency Name and Address Technical Memorandum
National Aeronautics and Space ,_Iministration
141Sponsoring Agency C4xle
Washington, D.C. 20546
15. supplementary Notes
16. Abstract
The far-field acoustic data for eight full-scale fans tested at a NASA-Lewis outdoor facility was
analyzed for the effect of a cut-off design feature where the number of rotor and stator blades
are proportioned so that the blade-passage tone will not propagate. Inlet flow distortions, preva-
lent during static testing, interact with the rotor to generate tones that generally mask this cut-
off phenomenon. In this study distortion tone properties previously evaluated are used to analyze
the tones propagating from the aft fan duct. The analyzed data clearly differentiate the fans that
are and are not "cut-off".
17. Key Words (Suggelted by Author(I)) 18. Distribution St0temlnt
Fan noise Unclassified - unlimited
Flow distortion tones S_AR Category 71 (rev.)
Tone cut-off
,,,, ,, ,
19. Security Qmulf. (of this report) 20. Security Cla,if. (of thispa_) 21. No. of Pqes _'_. Price"
Unclassified Unclassified 7 $3.25
°Fu salebythe NationalTechnicalInf0matl0nService,Sprm£faeld,Vfr£moa2;]161
1975024799-002
f,
f_
AN OBSERVATION ON TONE CUT-OFF IN STATIC TEST i
DATA FROM JET ENGINE FANS ;_
J
by Marcus F. Heidmann
Lewis Research Center !
SUMMARY
The far-field acoustic data for eight full-scale fans tested at a NASA-Lewis outdoor
facility are analyzed for the effect of a "cut-off" design feature where the number of
rotor and stator blades are proportioned so that the blade passage tone will not propa-
gate. Inlet flow distortions, prevalent during static testing, interact with the rotor to
generate tones that generally mask this cut-off phenomenon. In this study distortion
tone properties previously evaluated are used to analyze the tones propagating from the
aft fan duct. The analyzed data clearly differentiate the fans that are and are not cut-
off.
INTROIDUCTION
A variety of jet engine fan stages have been designed that incorporate the cut-off i_
principle prescribed by the Tyler-Sofrin theory (ref. 1). According to this theory, the :;
blade-passage tone caused by rotor-stator interactions will not propagate to the far- _
field from either the inlet or aft ducts of a fan when the number of blades and vanes are
properly selected. However, the acoustic results from ground static tests for cut-off
fans usually exhibit a strong propagating blade-passage tone. The practicality of cut- _
off in fan design h_s been challenged because of such results. Recently, however, the
cut-off phenomenon was clearly evident in the noise generated during actual flight by
fans that exhibited strong blade-passage tones during ground static tests (refs. 2 and 3). _
This difference between static and flight behavior is attributed to inlet flow distortions _[
that occur during static testing but are absent during flight. The inflow for static tests
differs from that during flight in that nearby structures and obstacles, ground vorttcies
and flow separation in a flight type inlet can readily distort the inflow during static
tests. Pockets of free air turbulence, which elongate when drawn into an inlet during
1975024799-003
istatic tests, can also give an inflow distortion of considerable duration during static
tests (ref. 4). The interaction of the rotor with such inflow distortions will generate
blade-passage tones regardless of whether the rotor-stator interaction tones are cut-off.
The purpose of this report is to present an observation and data analysis concerning
a difference in the tones generated during static tests by fans which are and are not de-
signed for cut-off. The acoustic data used for this purpose are those acquired from the
Lewis full-scale-fan outdoor acoustic test facility. The observation col_cerns eight
i.83-meter (6-ft) diameter quiet fans previously de_cribed (ref. 5). These fans have no
inlet guide vanes and a relatively iarge rotor-stator spacing and were tested in the un-
suppressed conf._gumtions. The far-field noise for these fans was measured over a
160 ° arc from the inlet axis, with noise propagating from both the inlet and aft ducts.
The specific noise data to be useu are the one-third octave sound power spectra of the
noise propagating from the _nlet and from the aft ducts. These power spectra were ap-
proximated by integration over the inlet and aft quadrants of the measured radiation pat-
tern.
CUT-OFF OBSERVATION
The recent identification of inflow distortions as a common source of blade-passage
tones in static tests has prompted a reevaluation of the Lewis data conc¢_rning cut-off
design features. The rotor-distoition tones generally were suspected of masking the
cut-off phenomenon, particularly in the case of inlet noise, because the tones from such
a source would freely propagate from the inlet duct. The rotor and stator, however,
could obstruct the aft propagation. It was reasoned thst cut-off, the.-'efore, might not be
completely masked for the aft duct. Pursuing this concept, the inlet and aft noise were
examined for differences caused by cut-off. The differences observed are illustrated in
figure 1, which compares the sound power spectra for the inlet and aft noise for two
fans - one designed for cut-off and the other not so designed.
The difference in the spectra for the two cases is the power level of the fundamental
blade-passage tone (BPT) relative to the harmomcs. Without cut-off (fig. l(a)) the fun-
damental tone level is higher than that of the harmonics in both the inlet and aft noise
spectra. Ti_e effect of cut-off on inlet and aft noise is shown in figure l(b), The aft
spectrum exhibits a fundamental tone level which is lower than that of the second har-
monic (2 x BPF), whereas the inlet spectrum Is similar to that without cut-off. Fig-
ure 1, therefore, suggests that cut-off is partially effective in reducing the fundamental
tone propagation from the aft duct. This deduction was verified by comparisons between
other fans _.nd at other fan speeds, although the presence of cut-off was not always as
clearly evidc_t as figure 1 shows it.
2
1975024799-004
1
DISCUSSION AND ANALYSIS
The observation of cut-off illustrated in figure 1 prox4des a basis for analyzing the
Lewis data for the effect of cut-off. Some qualitative properties of the tone levels and
harmonic fall-off rates, however, are useful in performing the analysis.
The level of the distortion tones in the Lewis facility was tentatively established
during the development of a tone prediction procedure (ref. 6). The level of the funda-
, mental tone for distorl2on-rotor tones appears to be equal to that of the rotov-stator
_ tone that is not designed to be cut-off. The significance of this result is that for a fan
I not designed for cut-off, the inlet distortions cause about a 3-decibel increase in the
fundamental tone level emanating from the inlet. The aft noise is relatively insensitive
to inlet distortions for a fan not designed for cut-off.
The data from the Lewis tests were also examined with regard to the level of the
tone harmonics fo:" the rotor-distortion tones and for the rotor-stator tones. Although
the findings are qualitative, it appears that on the average the fall-off rate in harmonic
content with increasing harmonic order is generally much larger for the distortion-
rotor tones than for the rotor-stator tor, es. The difference is roughtly 10 decibels per
harmonic for the distortion tones, compared with 3 decibels per harmonic for the rotor-
stator tones. This tentative result evolved during the development of the tone predic-
tion procedure by assuming various fall-off rates to fit the experimental data (ref. 6}.
The difference in fall-off rate is also partially based on rotor-stator spacing experi-
ments performed on one cut-off fan (ref. 7}. in the tests of reference 7 the expected
reduction in rotor-stator interaction tone levels with increased spacing was observed
only in the level of the harmonics. The results implied that the harmonics are domi-
nated by rotor-stator interaction noise and are relatively free from inlet distortions
effects.
The distortion tone level and harmonic fall-off rate properties can be used to ex-
press more quantitatively the observation of cut-off in the aft noise illustrated in fig-
ure 1. The harmonics in the aft noise spectra was presumed to be due only to rotor-
stator interaction noise. The fundamental tone level of this rotor-stator interaction
noise, when it is not cut-off, was projected to be 3 decibels higher than the ,._bservcd
second harmonic level. This projected level of the fundamental tone establishes its ref-
, erence level. The difference between the observed level of the fundamental tone and
this reference level provides a measure of the effectiveness of .qt-off in reducing tone
level. Figure 2 illustrates the procedure used in calculating this effective difference in
fundamental tone levels.
The effective difference in tone level !s expected to be a ftmctJon of a theoretical i
cut-off ratio 6. The theoretical cut-off ratio is a quantity that can be calculated for !l
each fan and operating condition. It is a measure of whether a tone should or should not i
1975024799-005
I I
propagate. A tone should not propagate when the cut-off ratio is less than unity. An
expression for 5 for the lowest radial order of a spinning mode at the blade pan,sage
frequency for rotor,-stator interactions is given by (ref. 8)
where
B number of rotor blades
Mt Mach number of rotor tip
V number of stator vanes
M* a theoretical cut-off Mach number (based on hub-tip ratio and model pattern)
Mx axial flow Mach number
For the case where Mt is supersonic, the rotor alone may generate a spinning rood?
that propagates. The cut-off ratio for this mode is evaluated by neglecting the stator
vanes (V = 0).
Figure 3 shows the effective difference in fundamental tone level as a function of 5
for the family of Lewis fans. The design speed value of the cut-off ratio for the fans
are identified in the figure. All these fans were tested at underdes/gn rotational
speeds. The experimental data are plotted as a function of the cut-off ratio calculated
for these reduced speeds. Figure 3 shows a reduction in tone level when the calculnted
cut-off ratio is less than unity. Fans that are and are not operating with cut-off are
clearly differentiated. The experimental results basically confirm the cut-off theory.
Theoretically, a relatively abrupt reduction in level should occur at a value of
unity. The gradual and variable reduction with decreasing cut-off ratio observed in
the experimental data partly reflects the propagation of a distortion tone from the aft
duct. These distort/on tones limit the amount of reduction. _hat can be observed.
CONCLUSI(_S
Tone out-off does occur and can be identified in staUc test acoustic data that
are contaminated by the effects of inlet flow distortions. The result confirms that
I
4
1975024799-006
|cut-.off is a quiet ,an design feature that can reduce the noise of operational aircraft _
engines. ._
Lewis Research Center, i
National Aeronautics and Space Administration,
Clevel_nd,Ohio, JulyII, 1975, "
505-03.
REFERENCES
I. Tyler. J. M.; and Sofrin,T. G. : Axial Flow Compressor Noise Studies,SAE
Trans., vol.70, 1962, pp. 309--332.
2. Cumsty, M. A. ; and Lowrie, B. W. : The Cause ofTone Generationby Aero-englne
Fans atl!ighSubsonicTip Speeds and the Effectof Forwar_ Speed ASME Paper
73-WA/GT-4, Nov. 1973.
3. Feller,C. E.; and Merrirnan,J. E.: Effectsof Forward Velocityand Acoustic
Treatment on inletFan Noise. AIAA Paper 74--946,Aug. 1974.
4. Hanson, D. B. : A StudyofSubsonicFan Noise Sources. AIAA Paper 75-468,
Mar. 1975.
5. Heidrnann,M. F.; and Feiler,C. E. : Noise Comparisons from Full-ScaleFan
Tests atNASA Lewis Research Center. AIAA Paper 73-1017, Oct. 1973.
6. Ileidrnmm,M. F.: InterimPredictionMethod for Fan and Compressor Source
Noise. NASA TM X-71763, 1975.
7. Balornbin, Joseph R. ; and St,'diolich, Edward G. : Effect of Rotor-to-Stator Spacing
on Acovstic Performance of a Full-Scale Fan (QF-5) for Turbofan Engines. N&SA
TM X-3103, 1974.
8. Sofrin,T. G. ; ,andMcCann, J. F.: Prattand Whitney ExperienceinC_rnpressor-
iNoise Reduction. Preprint 2D2, Acoust. Soc. Am., 1966.
5
1975024799-007
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1975024799-008


An observation on tone cut-off in static test data from jet engine fans

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