42

inconclusive. This is because vibration levels induced by the pump are very small compared to the fan. The ordinary coherence function (r,,) between the inputs, plotted in Figure 3-14, shows that the pump experiences significant vibration from the fan.

    To isolate the influence of the pump vibrations on the on the test section, the vibration data for the case when the fan is switched off is analyzed. The water tunnel speed is set at approximately 0.5 m/s. Along with the vibration measurements, BK microphone measurements are made. The results are shown in Figure 3-16.


         10-2
                                                      YY
                                                      PUMP MIC SPECTRUM
         10-4                                        PUMP MIC NOISE SPECTRUM
                    F - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 


          -6
          10 108-8


          10         ------ ------- ------- ------- ------- ------- -------12
          10  . ------- -----  --------------------------------10-141                                     -       - L    - L     - L     I
                  100      200    300     400     500     600    700     800
                                      Frequency (Hz)



 Figure 3-16 Comparison of the output spectrum with microphone measurements for the
             case when the fan is off and the water tunnel speed is 0. 5 m/s.

    The output spectrum is almost three orders of magnitude less than that of the previous case analyzed. The results show that harmonics as well as sub harmonics are present in both the output spectrum and the microphone data.