Civil Engineering Reference
In-Depth Information
The quantities
R
,
R
s
and
r
are defined by the following
R
Δ
P
s
R
=
,
R
= ⋅
R S
and
r
=
.
(5.67)
s
q
d
v
The quantity
q
v
is volume velocity (m
3
/s) of the airflow through the specimen having an
area
S
, which implies that the mean flow velocity is equal to
q
v
/
S
(m/s). The quantity
d
is
the thickness of the sample in the direction of the flow. The
specific airflow resistance R
s
is the linear airflow resistance of the sample, having the same dimension as the specific
impedance. In the old centimetre-gram-second (CGS) system of units, this quantity had
its own unit, the Rayl in honour of the physicist, Lord Rayleigh. One may find it still in
use but then as mks Rayls reflecting the SI system of units.
100
90
80
70
60
50
40
30
20
R
G
10
9
8
7
6
5
4
3
20
30
40
50 60 70 8090
200
10
100
Density (kg/m
3
)
Figure 5.33
Typical data for airflow resistivity of mineral wool, glass wool (G) and rock wool (R) as a function
of density.
For a homogeneous material we may find the flow resistance per unit length, the
airflow resistivity having the symbol
r
. This is the quantity normally found as product
data for porous materials. Some typical data (Norwegian products) for mineral wool
types of porous material are given in
Figure 5.33
.
Finally, it should be mentioned that in other circumstances an inverse quantity is
used, this in order to characterize the “openness” of a porous material for airflows. This
is the quantity
permeability B
defined by
μ
2
B
=
(m ),
(5.68)
r
