Acoustic mode measurements in the inlet of a turbofan engine

Turbomachinery noise propagates in aircraft jet engine ducts in a complicated manner. The measurement of this propagation is useful both to identify source mechanisms and to design efficient linings. A practical method of making these measurements has been developed using linear arrays of equally spaced microphones mounted flush with the duct wall. Circumferential or axial arrays are analyzed by spatial Fourier transform, giving the sound level as a function of the spinning order or axial wavenumber, respectively. Complex demodulation is used to acquire data in a modest bandwidth around a high frequency of interest. A joint NASA/Boeing test of the system used 32 microphones in a JT15D turbofan engine inlet. A 400-Hz bandwidth centered at blade passage frequency and at one-half blade passage frequency was studied. The theoretically predicted modes were clearly seen at blade passage frequency; broadband noise at one-half blade passage frequency was biased toward modes corotating with the fan. Interference between similar modes was not a significant problem. A lining design study indicated that a 15% improvement in lining efficiency was possible for this particular engine when mode data were used. The technique has proved reliable and useful for source diagnostics and lining design.