CASE
FILE
NATIONAL ADVISORY COMMITTEE
FOR
AERONAUTICS
TECHNICAL
NOTE 3596
ON
THE
PERMEABILITY
OF
POROUS
MATERIALS
By
E.
Carson Yates,
Jr.
Langley
Aeronautical Laboratory
Langley
Field,
Va.
Washington
January
1956
(NASA-TH-79825)
ON THE
PEBHEABHITY
OF
IPOBOUS
HA.TEBIAIS
(National
Advisory
(Committee
for; Aeronaoitics.)
32 p
N78-78604
Dnclas
00/24
32228
NATIONAL ADVISORY COMMITTEE
FOR
AERONAUTICS
TECHNICAL
NOTE
3596
ON
THE
PERMEABILITY
OF
POROUS MATERIALS
By
E.
Carson Yates,
Jr.
SUMMARY
The
effects
on
porous-material permeability characteristics
of the
absolute pressure level
(and
associated scale effects), choking
of the
flow, bending
the
material^
and
other factors have been investigated.
Samples
of
rolled
30-by
250-mesh Dutch weave Monel metal cloth
and
l/l6-inch-thick sintered bronze were calibrated with constant upstream
pressures
of 1
atmosphere
and 2—
atmospheres (varying downstream pressure)
and
with constant downstream pressure
of 1
atmosphere (varying upstream
pressure). Experiments showed permeability characteristics
to be
appre-
ciably
affected
by
absolute pressure level, flow choking,
and
thickness
of
the
material. Moderate bending
of the
material caused
no
noticeable
change
in the
permeability. Simple calculation
and
correlation proce-
dures
are
presented
for
determining permeability characteristics with
reasonable accuracy when experimental data
are
limited.
INTRODUCTION
Accurate
and
comprehensive information
on the
permeability charac-
teristics
of
porous materials
is
essential
to the
proper design
of
area-
suction
installations
on
wings, flaps,
inlets,
and
wind tunnels.
It
was
found
in the
course
of
designing
a
wing using suction through
a
porous
material that available calibration information
was not
sufficient
for
predicting permeability characteristics
for the
range
of
operating
conditions
(absolute pressure levels) anticipated. Since most previous
calibrations (for example, refs.
1 and 2)
were found
to
have been made
with
a
single fixed relation existing between
the
Mach number
and
Reynolds
number
of the
flow,
the
effect
on
permeability
of
independent variation
of
these quantities (caused,
for
example,
by
differences
of
absolute
pressure
level)
could
not be
evaluated.
For
example,
if a
calibration
test
is
made with the. upstream pressure held constant
and the
downstream
pressure
varied,
it can
easily
be
shown that
the
ratio
of
Reynolds num-
ber
to
Mach number (both based
on
upstream conditions)
is a
constant.
In
addition, previous investigations were found
to
include
no
information
regarding choking
of the
flow following
the
occurrence
of
sonic
velocity
within
the
pores, bending
the
material, and,
in the
case
of
wire cloth,
rolling
the
material.
NACA
TN
3596
The
need
for
information
on the
aforementioned factors
is
emphasized
by
the
following considerations.
For a
given pressure difference
across
a
porous material, scale effects associated with absolute pressure
level
were believed
to
influence flow velocities.
If
this influence
is
appre-
ciable,
it
should
be
considered
in the
selection
of a
porous material.
Consideration
of the
limitation
on
inflow velocity
imposed
by
flow
choking
is
important
in
some
installations.
For
example,
in
wing-suction
applications choking
may be
used advantageously
to
limit inflow
over
areas
where large pressure differences exist,
so
that chordwise grading
of
permeability
may be
dispensed with. Bending
the
porous material could
affect permeability
by
changing
the
sizes
of
openings
in the
material
and by
changing
the
flow from
one
dimensional
to two
dimensional.
These
changes could have significant effectSj
for
example,
in
flow near
the
leading
edge
of a
wing.
The
investigation reported herein
was
undertaken
to
determine
the
influence
on the
permeability
of
porous material
of the
aforementioned
factors. Representative samples
of
wire cloth
and
sintered bronze (two
samples
of
each) were calibrated
by
holding
the
upstream pressure con-
stant
at 1
atmosphere
and
varying
the
downstream pressure,
by
holding
the
upstream pressure constant
at 2—
atmospheres
and
varying
the
downstream
pressure,
and by
holding
the
downstream pressure constant
at one
atmos-
phere
and
varying
the
upstream pressure. Testing
in
this manner yielded
independent variation
of
Mach number
and
Reynolds number.
The
analysis
includes comparison
of the
wire-cloth calibrations with
a
calibration
for
the
same material bent
to
form the,leading
edge
of a
wing model which
was
tested
in the
Langley
19-foot
pressure tunnel. Data obtained
in the
Langley
cascade aerodynamics laboratory
for the
wire cloth
rolled
to
var-
ious thicknesses
are
also shown.
The
analysis indicates methods
for
cal-
culating
or
estimating permeability calibration curves,
and
these methods
are
presented herein.
The
possible large effect
on
permeability
of the
presence
of a
com-
ponent
of the
inlet velocity which
is
parallel
to the
porous surface
(as
discussed
in
ref.
3) is not
considered herein. However, since
the
pores
in the
materials investigated
are
very
much
smaller than those
in
the
perforated materials
of
reference
3, it is
believed that
the
influ-
ence
of a
parallel
flow component
in the
present case would
be
consid-
erably
less than that reported
in
reference
3-
SYMBOLS
A .
cross-sectional area
of
venturi throat
D
effective diameter
of
pores,
ft
g
gravitational
acceleration,
ft/sec^
NACA
TN
3596
K
porous-material flow coefficient
M
Mach number
p
static pressure, Ib/sq
in. or
lb/sq
ft
£sp
pressure drop across
the
porous material,
p - p.,
Ib/sq.
in. or in. H
2
0
q
dynamic pressure,
ipV ,
Ib/sq.
ft
R
Reynolds number,
£^S
V-
T
temperature,
°F or °R
t
thickness
of
porous material,
in.
V
velocity
in
test apparatus
(flow
considered
to be
one-
dimensional), ft/sec
a
Darcy flow coefficient
7
ratio
of
specific heat
at
constant pressure
to
specific
heat
at
constant
volume
(i
viscosity coefficient, slugs/ft-sec
p
mass density, slugs/cu
ft
Subscripts:
d
station just downstream
of the
porous material
t
station
at the
venturi throat
u
station just upstream
of the
porous material
DEFINITIONS
;
The
term "porosity"
as
used herein
is
defined
as the
percentage
of
void
present
in the
porous material.
Permeability
is a
qualitative term related
to the
resistance
of the
material
to
fluid flow.
The
greater
the
resistance,
the
less,the
permeability,
and
vice
versa.
NACA
TN
3596
APPARATUS
AMD
TECHNIQUE
APPARATUS
The
apparatus used
in the
tests with constant upstream pressure
is
shown
schematically
in
figure
1. It
consisted
of a
2-inch inside-
diameter pipe with
a
flange, clamp plate,
and
gaskets
at one end to
hold
the
sample
and a
wooden venturi
at the
other end. Downstream
of the
i-inch-diameter-venturi throat
was an
exhaust pipe containing
a
gate
valve
for
regulating
the
downstream pressure
p,.
The
apparatus used
in the
tests with varying upstream pressure
is
shown
in
figure
2.
This
apparatus
was the
same
as
that used
for
con-
stant upstream pressure except that
an
additional length
of
2-inch pipe
was
attached upstream
of the
sample,
and the
downstream pipe
and
valve
were moved upstream
of the
entire setup. Fine wire screens were placed
2
diameters upstream
of the
sample
to
help maintain flow
uniformity.
High
and low
pressure sources used were
the
Langley
19-foot
pressure
tunnel
and the
suction side
of a
centrifugal compressor.
Downstream
flow velocities were determined
by
applying one-
dimensional-flow
relations between stations
d and t
(figs.
1 and 2)
as
follows:
Continuity
and
isdthermal conditions were used
to
relate stations
u
and d.
MATERIAL
Two
flat samples
of
30-by 250-mesh Dutch weave Monel metal wire
cloth were calibrated.
The
wire diameters
for
these samples were
O.OOSO inch
for the
wires that were
30 per
inch
and
0.00^0
inch
for the
wires that were
250 per
inch.
The
samples were rolled
to a
thickness
of
0.017 inch from
an
original thickness
of
0.026
inch.
For
purposes
of
this investigation,
tests
of
wire cloth
of
other mesh dimensions were
not
made because examination
of
data available from
the
Langley
cascade
aerodynamics
laboratory
indicated that other meshes would have generally