validation of caa prediction of noise radi- ated from turbofan intakes

16 th International Congress on Sound and Vibration, Kraków, Poland, 5–9 July 2009
4
TCS
Baffle
Figure 3. Intake of 1/3 scale model fan rig with acoustic baffle and Turbulence Control Screen (TCS).
4. Mode Detection Data
Transducers ring Intake barrel
Transition casing
Fan case
Cylindrical section
The measured data obtained from the pressure transducers mounted in the mode detection ring
are converted to wall sound pressure levels of the circumferential duct modes using the method dedescribed
by Rademaker et al. 7 .
5. Tone Predictions at the Blade Passing Frequency (BPF)
Spinner
Predictions at BPF were performed for the fan rig described above. Predictions of the far-field
sound pressure level (SPL) were made for hard-walled and lined configurations at a number of fan
speeds. The source is assumed to consist of a multimodal base and a single BPF tone at higher fan
speeds. The distribution of acoustic power in the multimode source is modelled by assuming equal
power per mode for all cut-on modes. When the fan tip speed is supersonic, an additional rotorlocked
component is included which corresponds to the first radial BPF tone for which (in this
case) m = 24. All of the cut-on modes which are included in the mutimodal content, as well as the
'm = 24' mode, are assumed to be uncorrelated. At subsonic tip speeds (50% to 70% fan speed), the
‘m = 24’ mode is cut-off and the source model then consists only of the multimodal, equal power,
component. Above 80% fan speed however, the ‘m = 24’ mode is cut-on and the fan source contains
both contributions. In all cases and at all frequencies, far-field directivities are computed for
each of the modes present at the fan plane, and these are summed in the far field as contributions
from uncorrelated sources. The sound pressure level is then calculated as;
p 1
SPL = 20. log10
. , (1)
2 pref
where p 2 is the root mean square pressure and Pref is a reference pressure, 2.10 -5 Pa.
Measured data are taken for the case of a hard (i.e. no liners) and a lined intake. The hardwalled
case is used to calibrate the source model. Fig. 4 shows mode detection data for a hardwalled
intake taken from the ring of transducers close to the outlet flare (see Fig. 3). The measured
SPL on the wall is sampled at a frequency which corresponds, at each fan operating speed, to an
'Engine Order' (EO) of 24, corresponding to BPF for a 24 bladed fan. At each fan speed the measured
data is decomposed into components corresponding to each circumferential mode number m.
Fan