266 Lecture 14 
pu,> 8" 8" =0.105" AT 100 FPS, DIA OF HYDROPHONE 
LESS THAN 0.125" 
db M. HARRISON 
“ia 
= A 
WILLMARTH 
rey MA 
@ =50 FPS 
O = 100 FPS 
A =.200 FPS 
Pale DIA HYDROPHONE 
ate 
Sea 
= 
1" DIA HYDROPHONE we SS 
Nan 
10 100 
Ol | #/(U,/ 8°) | 
Fig. 14.5. Flow-noise spectra in dimensionless variables showing the effect of hydrophone diameter. 
boundary layer is turbulent or laminar. However, the eddies decay rapidly in 
the laminar boundary layer, whereas in the turbulent boundary layer the eddies 
shed by the surface roughnesses increase the intensity of the turbulence in the 
boundary layer. 
14.6. EXPERIMENTAL RESULTS 
Measurements of the boundary-layer noise have been performed in the test 
section of the Garfield Thomas Water Tunnel at the Ordnance Research Labora - 
tory and in Key West with the aid of two cigar-shaped buoyant units. Because of 
the great intensity of the flow noise, machinery noise is masked by flow noise 
and measurements can be performed in water tunnels, The curves shown in 
Fig. 14.4 were obtained in the test section of the Garfield Thomas Water Tunnel, 
which has a diameter of 122 cm. Hydrophones were mounted flush with the wall 
and the noise levels were analyzed in a frequency range from 60 cps to about 30 
kc. Figure 14.4 shows the noise levels as functions of the frequency for various 
speeds. Figure 14,5 shows the same curves and additional ones for a larger 
hydrophone diameter represented in dimensionless variables, The curves obtained 
for a particular hydrophone size are almost coincident and the difference in the 
levels obtained for the two hydrophone sizes is about 12 db as predicted by the 
theory. Figure 14.6 reproduces the result of anarrow-band analysis of the noise. 
Machinery noise and wall vibrations would have generated spectral lines; how- 
ever, such lines do not appear in the spectrum. 
