Civil Engineering Reference
In-Depth Information
V
p
1
h
p
2
Figure 2.4
A slice of porous material is placed in a pipe. A differential pressure
p
2
−
p
1
induces a steady flow
V
of air per unit area of material.
total volume of porous material
V
T
. Thus,
φ
=
V
a
/V
T
(2.25)
Let
V
b
be the volume occupied by the frame in
V
T
. The quantities
V
a
,
V
b
and
V
T
are
then related by
V
a
+
V
b
=
V
T
(2.26)
Only the volume of air which is not locked within the frame must be considered in
V
a
and thus in the calculation of the porosity. The latter is also known as the open porosity
or the connected porosity. For instance, a closed bubble in a plastic foam is considered
locked within the frame, and its volume therefore belongs to
V
b
. For most of the fibrous
materials and plastic foams, the porosity lies very close to 1. Methods for measuring
porosity are given in Zwikker and Kosten (1949) and Champoux
et al
. (1991).
Flow resistivity
One of the important parameters governing the absorption of a porous material is its
flow resistance. It is defined by the ratio of the pressure differential across a sample of
the material to the normal flow velocity through the material. The flow resistivity
σ
is
the specific (unit area) flow resistance per unit thickness. A sketch of the set-up for the
measurement of the flow resistivity
σ
is shown in Figure 2.4.
The material is placed in a pipe, and a differential pressure induces a steady flow of
air. The flow resistivity
σ
is given by
σ
=
(p
2
−
p
1
)/Vh
(2.27)
In this equation, the quantities
V
and
h
are the mean flow of air per unit area of
material and the thickness of the material, respectively. In MKSA units,
σ
is expressed
in N m
−
4
s. More information about the measurement of flow resistivity can be found
in standards ASTM C-522, ISO 9053 (1991), Bies and Hansen (1980), and Stinson and
Daigle (1988).
It should be pointed out that fibrous materials are generally anisotropic (Attenborough
1971, Burke 1983, Nicolas and Berry 1984, Allard
et al
. 1987, Tarnow 2005). A panel
of fibreglass is represented in Figure 2.5. Fibres in the material generally lie in planes
parallel to the surface of the material. The flow resistivity in the normal direction is
different from that in the planar directions. In the former case, air flows perpendicularly
