Response of a Vibrating Plate in a Fluid 



1TI,o>)llJS 





Fig. 2. . Power Spectral Density of the Wall Pressure 

 Fluctuation 



the shock. The normalized power spectral density found upstream 

 of the shock corresponds to the zero pressure gradient, and peaks at 

 coS/Ug — 2 while downstream the spectral density is modified in the 

 region below the peak. It is significant that by altering the local 

 flow conditions , only the low frequency ends of the spectra are 

 appreciably affected. It is noticed that the pressure flucutation 

 measurements at x/6 — where the shock impinges show a notice- 

 able deviation from, the general pattern in the higher frequencies. 

 This Is attributed to an Intermlttant signal superimposed on the 

 regular pressure signal as seen on the oscilloscope. It Is possibly 

 due to the characteristic fanning of the shock as It goes through the 

 boundary layer. 



Measurements of the cross -correlation are shown In Fig. 3. 

 The cross-correlation characteristics are a function of position 

 downstreami of the shock. The cross-correlation between positions 

 x/6 = 0.33 and x/6 = 3.80, the farthest apart, has characteristics 

 similar to those found at zero pressure gradient boundary layer In 

 that the ratio between the convection velocity and the freestream 

 velocity Uc/Ue = 0.72 and that the correlation between those two 

 points Is still significant. The cross -correlation of the shortest 

 distance between x/5 = 0.33 and x/6 = 0,75, shows that the con- 



481 



