Multiple pure tone noise generation and control

Benzakein, M. J.; Kazin, S. B.; Savell, C. T.

doi:10.2514/6.1973-1021

Cited by 6 publications

(1 citation statement)

References 4 publications

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“…One is to absorb the sound using an acoustic liner or splitter in the inlet duct (ref. 1). Unfortunately, adding these sound suppression devices can add weight and length to the inlet and/or reduce aerodynamic efficiency (ref.…”

Section: Introductionmentioning

confidence: 99%

Shock Characteristics Measured Upstream of Both a Forward-Swept and an Aft-Swept Fan

Podboy

Krupar

Sutliff

et al. 2007

Volume 6: Turbo Expo 2007, Parts a and B

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Three different types of diagnostic data-blade surface flow visualization, shroud unsteady pressure, and laser Doppler velocimeter (LDV)-were obtained on two fans, one forward-swept and one aft-swept, in order to learn more about the shocks which propagate upstream of these rotors when they are operated at transonic tip speeds. Flow visualization data are presented for the forward-swept fan operating at 13831 rpm c and for the aft-swept fan operating at 12500 and 13831 rpm c (corresponding to tip rotational Mach numbers of 1.07 and 1.19, respectively). The flow visualization data identify where the shocks occur on the suction side of the rotor blades. These data show that at the takeoff speed, 13831 rpm c , the shocks occurring in the tip region of the forward-swept fan are further downstream in the blade passage than with the aftswept fan. Shroud unsteady pressure measurements were acquired using a linear array of 15 equally-spaced pressure transducers extending from two tip axial chords upstream to 0.8 tip axial chords downstream of the static position of the tip leading edge of each rotor. Such data are presented for each fan operating at one subsonic and five transonic tip speeds. The unsteady pressure data show relatively strong detached shocks propagating upstream of the aft-swept rotor at the three lowest transonic tip speeds, and weak, oblique pressure disturbances attached to the tip of the aft-swept fan at the two highest transonic tip speeds. The unsteady pressure measurements made with the forward-swept fan do not show strong shocks propagating upstream of that rotor at any of the tested speeds. A comparison of the forward-swept and aftswept shroud unsteady pressure measurements indicates that at any given transonic speed the pressure disturbance just upstream of the tip of the forward-swept fan is much weaker than that of the aft-swept fan. The LDV data suggest that at 12500 and 13831 rpm c , the forward-swept fan swallowed the passage shocks occurring in the tip region of the blades, whereas the aft-swept fan did not. Due to this difference, the flows just upstream of the two fans were found to be quite different at both of these transonic speeds. Nevertheless, despite distinct differences just upstream of the two rotors, the two fan flows were much more alike about one axial blade chord further upstream. As a result, the LDV data suggest that it is unwise to attempt to determine the effect that the shocks have on far field noise by focusing only on measurements (or CFD predictions) made very near the rotor. Instead, these data suggest that it is important to track the shocks throughout the inlet. IntroductionNarrow band noise spectra of high bypass ratio turbofan engines operating at subsonic fan tip speeds normally contain tones at harmonics of the fan blade passing frequency (BPF). If the rotational speed of the fan is increased so that the relative tip speed becomes supersonic, extra tones normally appear in the spectra at multiples of the shaft rotational speed (multiples of the once-p...

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Section: Introductionmentioning

confidence: 99%

Shock Characteristics Measured Upstream of Both a Forward-Swept and an Aft-Swept Fan

Podboy

Krupar

Sutliff

et al. 2007

Volume 6: Turbo Expo 2007, Parts a and B

View full text Add to dashboard Cite

show abstract

Spinning mode analysis of the acoustic field generated by a turboshaft engine

Blacodon

Léwy

1992

Journal of Aircraft

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Acoustic spinning-mode analysis by an iterative threshold method

Blacodon

1995

Journal of Aircraft

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Spinning-mode analysis methods are commonly used for studying the spatial structure of the acoustic field inside ducts. In some applications, the background noise is very high, so that the acoustic modes may be overwhelmed by spurious modes in the modal spectra. The problem may be solved by methods that greatly reduce the background noise. Four methods studied in a previous work are briefly summarized in this article. The method providing the best reduction of the background noise is inaccurate in certain cases. An iterative threshold technique is proposed to eliminate this drawback. It is compared to the four other methods using numerical simulations and mode measurements in the nozzle of a Turbomeca TM333 turboshaft engine. Nomenclature/ = frequency G, = gain in signal to noise ratio of method / = 10 log[(S/N) OUT /(S/N) IN ] / = unit matrix Im[w(/, m)] = imaginary part of o>(/, m) J = number of microphones L = number of data blocks M N = set of noise modes M s = set of acoustic modes, signal m = angular wave number of spinning mode order N(f, 0) = frequency Fourier transform of Ai(r, 0) n(t, 6) = background noise Re[o>(/, m)] = real part of w(f, m) 5(/, 0) = frequency Fourier transform of s(f, 0) s(t, 0) = acoustic pressure, signal T Vmin = the smallest power spectral density on a noise mode Tr[V(f)] = trace of matrix T(/) t = time y(/, 0) = frequency Fourier transform of y(t, 0) y(t, 0) = time history of acoustic pressure (signal -+ noise) measured by a microphone at angle 0 T/v(/) = cross-spectral matrix of N(f, 0) r s (/) = cross-spectral matrix of S(f, 0) r y (f) = cross-spectral matrix of Y(f, 0), see Eq. (3) 8 = Kronecker symbol 0 = angle 6j = location angle of microphone y [ = 277(y -!)//],/= 1,2, . . . ,7 cr 2 (/) = noise power spectral density <£,(/, m) = wave number spectrum at frequency/by method / o>(/, m) = angular Fourier transform of Y(f, 0) * = complex conjugate + = transposed complex conjugate Presented as Paper 92-02-041 at

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Multiple pure tone noise generation and control

Cited by 6 publications

References 4 publications

Shock Characteristics Measured Upstream of Both a Forward-Swept and an Aft-Swept Fan

Shock Characteristics Measured Upstream of Both a Forward-Swept and an Aft-Swept Fan

Spinning mode analysis of the acoustic field generated by a turboshaft engine

Acoustic spinning-mode analysis by an iterative threshold method

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