Tests were conducted in the Langley 24-inch highspeed tunnel to ascertain the static-pressure and total-pressure losses through screens ranging in mesh from 3 to 12 wires per inch and in wire diameter from 0.023 to 0.041 inch. Data were obtained from a Mach number of approximately 0.20 up to the maximum (choking) Mach number obtainable for each screen. The results of this investigation indicate that the pressure losses increase with increasing Mach number until the choking Mach number, which can be computed, is reached. Since choking imposes a restriction on the mass rate of flow and maximum losses are incurred at this condition, great care must be taken in selecting the screen mesh and wire dimmeter for an installation so that the choking Mach number i

Adler, Alfred A

English

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U.S. DEPARTMENT OF COMMERCE
National Technical Information Service
NACA WRL 23
VARIATION WITH MACH NUMBER OF STATIC AND TOTAL
PRESSURES THROUGH VARIOUS SCREENS
NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS
WASHINGTON, DC
FEB 1946
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NATIONAL ADVISORY COMMITTEE F'O-R AERONAUTICS
  TI RTIMI, REP()RT
ORIGINALLY ISSUED
February 1946 as
Confidential Bulletin L_F28
VARIATION WITH MACH NUMBER OF STATIC AND TOTAL
PRESSIIRES THRO_H VARIOUS SCREENS
By Alfred A. Adler
Langley Memorial Aeronautical Laboratory
Langley Field, Va.
REPRODUCED BY
U.S. DEPARTMENT OF COMMERCE
NATIONAL TECHNICAL
INFORMATION SERVICE
SPRINGFIELD, VA 22161
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WASHINGTON
NACA WARTL_E REPORTS are reprint s of papers originally issued to provide rapid distribution of
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: ! INFORMATION SERVICE L_ii__
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NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS
V_RIATION WITH MACH .}_A_!BER OF STATIC AND TOTAL
PRESST/RES THROUGH VARIOUS SCREENS
By Alfred A. Adler
S U_'_iARY
Tests were conducted in the Langley 24-inch high-
speed tunnel to ascertain the static-pressure and total-
pressure losses through screens ranging in mesh from
3 to 12 wires per inch and in wire diameter from 0.023
to 0.041 inch. Data were obtained from a Mach number
of approximately 0.20 up to the maximum (choking) Mach
number obtsinable for each screen.
The results of this investigation indicate that
the pressure losses increase with increasing Mach number
until the choking Mach number, which can be computed, is
reached. Since choking imposes a restriction on the
mass rate of flow and maximum losses are incurred at
this condition, great care must be taken in selecting
the screen mesh and wire dimmeter for an installation
so that the choking Mach number is not approached at
the maximum operating velocity.
INTRODUCTION
In the design of ducts for aircraft it has become
necessary to know the losses in static and total pressure
through screens installed within these ducts. Data on
the losses through screens at low speeds have long been
available, but for current aircraft ducting problems
data on the losses through screens at high speeds are
necessary. Tests were thereforeconducted in the
Langley 2_-inch high-speed tunnel to ascertain the
static-pressure and total-pressure losses t_rough
screens ranging in mesh from _ to 12 wires per inch and
in wire diameter from 0.023 to 0.0_l inch, at Mach
numbers from approximately 0.20 to the maximum (choking)
Mach number obtainable for each screen.
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NACA CB No. L5F28
Mach number ahsad of screen
choking Mach number, also known as limiting
Mach number, the Mach number ahead of
screen at which a Mach number of 1 is
obtained completely across openings in
screen
total-pressure loss through screen
static-pressure drop through screen
dynamic pressure ahead of screen
section drag coefficient _Drsg/Unit are_
• . q
mesh, wires per inch
wire diameter, inch
screen solidity, ratio of blocked area to
original free area _2md - m2d 2)
density ahead of screen
velocity ahead of screen
velocity downstresm from screen
@
APPARATUS AND ._TKODS
The Langley 24-inch high-speed tunnel, in which
these tests were run, is a nonreturn, induction-type
tunnel with the induction nozzle placed downstream from
the test section (reference 1). The total pressure
throughout the test section is therefore equal to atmos-
pheric pressure, except for• a negligible loss through the
entrance screens.
Each screen was supported for tests within a
16-inch length of seamless steel tubing having an inside
1 inches (fig. 1), which was mounteddiameter of 53
"" .....!" i'.
:W .....NASA CB N : 3
longitudinally in the test section of the tunnel by means
1
of _ inch-diameter steel cables.
The Mach number was determined from the static
pressure ahead of the screen, which was measured by means
of two diametrically opposed static-pressure orifices
located in the walls of the supporting tube 3 inches
_uead of the screen. The static pressure behind the
screen was measured by means of two additional static-
pressure orifices located at a distance of 4 inches
behind the screen. The total-pressure loss through the
screen was taken as the difference between atmospheric
pressure and the total pressure as indicated by two
tubes placed behind the screen with the noses in the
seine plane as the static-pressure orifices. The tube-
entrance losses were assumed to be negligible.
RESULTS AND DISCUSSION
The data for all the screens tested are presented
in figures 2 and 3, which show the static-pressure drop
and total-pressure loss, respectively, plotted against
Nach number. The data show, mainly, that both static-
pressure drop and total-pressure loss, in percent of
dynamic pressure, increase with increasing Nach n_:_ber
until the highest obtainable test Nach number is reached.
This Nach number is known to correspond to the choked
condition, since further increase in tunnel speed did
not increase the Mach number ahead of the screen.
The data of figures 2 and 3, plotted in terms of
screen solidity rather than mesh and wire dimneter, are
presented in figures 4 and 5. From these figures it
can be seen that the pressure losses are dependent only
on screen solidity and Msch number within the range of
this investigation.
Figure 6 shows dra_ coefficient cd plotted against
Mach number. These values of drag coefficient were
obtained from the equation
Ap PlU! (u2- Ul)
cd -- _ .q q
This equation was d_Ived on the asstumption that con-
ditions were uniform across the wake. As was expected,
the data show that the drag coefficient increases with
increase in Mach nurabe_,
Since choking of tha i_iow through a tube occurs
when a Mach number of 1 is reached at the cross section
of minimum area, choking of the flow would be expected
to occur when a Mach number of 1 is obtained completely
across the openings in the screen. This choking fixes
a limit to the Mach number that can be obtained in the
flow ahead of the screen. Thero al_e now two possi-
bilities. If the total• pressure ahead of the screen is
maintained constant, no further increase in mass rate of
flow is possible. Any decreases in back pressure will
only increase the losses through the screen. If, how-
ever, the total pressure _ead of the screen is
increased - as is possible in flight - the mass rate of
flow can be increased. This increase in mass rate of
flow will be accompanied bF large losses _ud will pro-
gress at a far lesser rate than for speeds below
choking. It is highly desirable therefore that care be
taken in the selection of screen mesh and wire diameter
so that the choking Mach number is not approached at
the maximum operating velocity. The following equation,
derived from the continuity eqaation, the adiabatic law,
and Bernoulll,s equation for compressible flow, relates
the speed at which this choking occurs to the screen
vsriables :
1 - 2md + m2d2 =
0.579
Mchoke
C1 + 0.2Mchoke2) 3
This relation, presented graphically in figure 7 interms of the screen variables and in figu, e 8 in terms
of screen solidity, shows good agreement with the
experimentally determined choking Mach numbers.
C0NCL_Di NG RENARKS
Tests in the Langley2_-Inch high-speed tunnel to
ascertain the ststlc- and total-pressure losses through
screens indicate that the losses increase with increasing
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NACA CB No. L5F28 5
Nach number until the choking Math number, which can be
comouted, is reached. Because choking imposes s restriction
on the mass rate of flow and because maximum losses
are incurred at this condition, great care must be taken
in selecting the screen mesh and wire diameter for an
installation so that the choking _Zach number is not
approached at the maximu_m operating velocity.
Langley Memorial Aeronautical Laboratory
National Advisory Co_._mittee for Aeronautics
Langley Field, Va.
REFERENCE
Stack, John, Lindsey, W. F., and Littell, Robert E.:
The Compressibility Burble and the Effect of Com-
pressibility on Pressures and Forces Acting on an
Airfoil. NACA Rep. No. 646, 1938.
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Variation with Mach Number of Static and Total Pressures Through Various Screens

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