1.0 























08 

 0.6 































































04 





[ 





















\ 



















0.2 





V 



\ o 











































^ ~i 



-~^~~. 



--^^ 



^-o-4-^>— < 





>- A 



0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 



0.8 Ug 



Figure 6 — Longitudinal Coherence of the Pressure Fluctuations 



so that 



/ 



vHt) = p {f)df 



and can be computed in terms of the measured data P ^(f), P2(f)i ^^nd IPjjl^ ~ ^12 "*" ^12^* 

 Perhaps of greater interest is the quantity 



|Pl2l^ ^ 



which can be taken as a measure of the coherence of P2 (^) '^^^^ Pi(^)' This quantity is 

 plotted in Figure 6. 



There are several important limitations in the data presented in Figure 5. One trend 

 in the data that is desirable to investigate is the effect of the parameter f 8*/Uq. Unfortu- 

 nately, the transducer used for the cross spectral density measurements had a usable frequen- 

 cy range up to only 2000 cps. Consequently, only the flat part of the spectral density, as 

 shown in Figure 4, could be investigated, The-upper limit of 2000 cps set an upper limit on 

 f SVUq between 0.083 and 0.33. As can be seen in Figure 4, this does not permit investiga- 

 tion of the cross spectral density of the pressure fluctuations in the range where the spectral 

 density is rapidly decreasing with frequency. With reference to the velocity fluctuations in 

 the boundary layer, the region investigated corresponded to the region where the velocity soec- 

 tral density is described by the - 5/3 law where inertia forces dominate. It would be interest- 

 ing to procure data for the cross spectral density in the range where viscous forces dominate. 



11 



