264 Lecture 14 
Fig. 14.3. Effect of a circular hy- 
drophone on the received power 
spectrum of flow noise. 
[= ieee] 
nd 14 
10-2 
13 
RELATIVE HYDROPHONE SENSITIVITY 
w87U —— 
The above theory is only partially borne out by the experiments as will be 
shown later. It has been assumed that the turbulence is homogeneous, so that 
it can be described by its space- and time-average microscopic properties. 
However, for the frequency analysis, filters are usedthat have a relatively great 
bandwidth and consequently a very short integrationtime. The frequency analysis 
therefore yields results that correspond to averages over very short time in- 
tegrations. For such short integrations the turbulence is far from being homo- 
geneous. Every individual bump of turbulence is recorded separately: the filters 
have forgotten all about the first bump of turbulence when they receive the 
second, and the cancelling-out effect described above seems to be limited, on 
the average, to the volume of one such bump of turbulence. This volume is ap- 
proximately the same as the volume over which the turbulent velocities are 
correlated. 
The analysis of turbulence may usually be designated as an analysis of the 
macrostructure of the turbulence; in contrast, an analysis of the flow noise 
seems to correspond to an analysis of the microstructure of the turbulence. 
14.5. NOISE GENERATED BY THE SURFACE ROUGHNESSES 
As the velocity of the vehicle increases, the roughnesses become more ef- 
fective. Experimental results show the roughnesses shed eddies whenever they 
