272 Lecture 14 
60- HYDROPHONE NO.6 HYDROPHONE NO.7 HYDROPHONE NO.9 |HYDROPHONE NO.8 
(stagnation) (45 deg) (side) (rear) 
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10 ee LOOMO MOO OMG 
DECIBEL SPECTRAL LEVEL (dbs) 
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| 2 3 4 | 2 3 4 | 2 3 4 | 2 Se 
DEGREE OF SURFACE ROUGHNESS 
CODE. | POLISHED 4 NO 24 GRIT 
2 SPRAY PAINT GO POINT OF MEASUREMENT, JOINTS UNWAXED 
3 NO. 120 GRIT * POINT OF MEASUREMENT, JOINTS WAXED 
Fig. 14.11. Comparison of flow noise for various degrees of surface roughness at high and low frequencies. 
and increasing the high-frequency noise spectrum, at least to some extent, at 
the expense of the low-frequency spectrum. At high frequencies the nose hydro- 
phone (No. 6) is particularly sensitive to surface roughness, and a high polish 
gives much better results than spray paint. The flow velocity at the circum- 
ference of the nose hydrophone is about 2 ft/sec. The Reynolds number obtained 
by the height of the roughnesses (0.001 in.) and the velocity that would be 
expected at the edges of the hydrophone is 25 and, hence, five times larger than 
the values required for shedding eddies. This Reynolds number would correspond 
to a noise level of roughly —60 db (re 1 d/cm’), as could be concluded from the 
rotating-cylinder measurements. Hydrophone 7, the 45° hydrophone, is not very 
sensitive to very small surface roughnesses over its area; the noise level is 
mostly radiated noise which masks the noise that would be produced by small 
surface roughnesses. However, the noise generated by coarser roughnesses 
over the hydrophone area seems to mask the radiated noise completely. At hy - 
drophone 8, on the other hand, the boundary layer is already relatively thick, 
and, as a consequence, the high-frequency boundary-layer noise is very weak. 
Therefore, most of the high-frequency noise observed at this hydrophone is due 
to the surface roughness, and polishing leads to much better results than the 
spray-paint finish. 
