Power spectra of pressure fluctuation, bump motion and shock wave movement

Time-variant signals and corresponding power spectra of pressure fhctua – tions, bump top motions and shock wave movements are shown in Figure 6 for three bump oscillatory frequencies (10Hz, 75Hz and 200Hz). Both pres­sure fhctuation and shock wave motion signals seem to follow the shape of the sinusoidal signal generated by the bump displacement at the oscillatory fre­quencies of 10Hz, 75Hz and 200Hz. At these three excitation frequencies, the pressure fhctuation and shock wave motion power spectra show the same clear fundamental harmonic. The bump top location movement power spectra con­tains one supplementary higher harmonic component that is not shown here. It does not exist in the power spectra of the pressure flictuation and shock wave motion signals. It is interpreted as being linked to external mechanical vibra­tions coming from the oscillation drive train and the wind tunnel. All three oscillations seem to be of a sinusoidal type after ensemble averaging posttreat­ment.

2.2 Schlieren visualization results

Figure 7 characterized the measured oscillations of the shock wave up to k=0.294. The mean location of the shock stays the same for all excitation frequencies. Moreover one can notice that the amplitude of the shock wave oscillations increases slightly from 0.015 to 0.294. The first bending mode shape at k=0.015 is characterized by a phase lag towards bump motion close to 315Deg., and the phases range between 30Deg. and 90Deg. for the second bending mode shape from k=0.03 to k=0.074. The phase decreases signif­icantly from 270Deg. to almost 0Deg. at reduced frequencies higher than k=0.089 for what has been considered as a third bending mode shape.