m 
~ 
n 
> 
c 
Q 
= 
N 
SS 
> 
Q 
3 
Q 
i) 
vu 
= 
Q 
Q 
2 
i) 
No 
nN 
N 
| 
| 
{ 
| 
i 
} 
l 
To we ee = : Sts es em SSS oe SAEs 
2G eS a SS SS SS SS SS = 
sie fe Nf 20 CYCLES SE eo 
7 2s lige reery, fee ees i ee oe ee ee a A 
a a * = - A is abe “4° 4 
Sd =v == ; 4 
aE ed es Sere oe et ee arc Oe ae ae ee Se 
J ae@rais 4 ° a = oe. or ere c UES ar 
eek eee 
iio: ==. == sa 2S SSS Se So SS 
ae meer oes lee SS Se 2 ee ee Ge Se Ge * 2 a sere 7-7 ar G 
es Oh oie 4 7 hPa its! Pe oD ee oan ai Le one 
Ba | 
it 
6 
1 
f 
Ba 
28 
A | 
it 
si 
I 
: 
l 
jgsue 
MIL 
(ith 
HV 1% 
bh 
2 
li 
ny 
ol 
AI 
fo) 
/ 
oO} 
{ 
°) 
ad 
oO 
On 
So 
oa 
Oo 
N 
iy 
o 
a 
| 
Co 
ik 
1 
i 
oO 
| 
(a) 
\ 
oO 
GY 
cE 
i 
ratte 
\ 
i 
ie 
Hs 
wi 
Mn 
I 
HM 
WN 
HN 
oo... oe 
v2. é. Wi = 
SSS asa ——— 
Et 
5 
5 
sq 
= 
aT 
ffl 
Pp 
all) 
LT | 
: 
4 
[iT 
Ha 
ELT 
I 
== 2S Se 
2 ae — ae nt 
=" a 
=a SS ae 
a2) ae eee 
= Sass Sa 
== SS SSS Seas ateoen 
p 
1 
} 
q 
i 
) 
y 
1) S| 23 5° Se. 0- 1 12 13 14——15= 
4 : f = FREQUENCY IN KILOCYCLES PER SECOND] =! E = =s 
Fig. 14.6. Narrow-band spectrum analysis of flow noise. 
So 
The noise generated in a frequency band around 24 kc by the surface rough- 
nesses has been studied with the aid of a rotating cylindex of a height of 42°), in. 
and a diameter of 17.2 in. Such a cylinder has a very thick boundary layer, and 
consequently generates little high-frequency boundary-layer noise. A rotating 
cylinder is, therefore, particularly suited to study the effect of roughnesses. * 
Figure 14.7 shows a number of curves that have been obtained for various 
types of roughnesses on the surface of the cylinder with a 2.5-in.-diameter hy - 
drophone. The noise level now exceeds that of the ambient noise at a much slower 
speed than in the previous case. Forthe same velocities, the flow noise produced 
by the rough surface is 20 to 50 db greater than that produced by the smooth, 
painted surface. Since nothing else has been changed, this greater noise must be 
attributed to the effect of the surface roughnesses. There is thus no doubt that 
roughnesses generate flow noise. For Grit No. 180, 80% of the particles have a 
height of about 5- 107° in. The critical speed [Eq. (26), where the roughnesses 
become effective] is therefore 2 knots or 1.08 m/sec. Ifwe extrapolate the curve 
for this kind of grit down to a speed of 2 knots (from whence the roughnesses 
*Rotating cylinder measurements have also been performed by L.N. Wilson [17]. 
