A method and apparatus is disclosed for sensing wave flow across a surface wherein at least two pressure levels are sensed and combined to provide a representation of waves within the flow. In the preferred embodiment holes bored through the aircraft surface at an interval of one-half the wavelength of the flow being measured introduce pressure perturbations into a cavity so they may acoustically interfere. The interfering waveform is sensed by at least one microphone disposed in the cavity

Kendall, James M., Jr.

English

NASA Technical Reports Server

Reply to Attn of:
NASA pp., F/.p 
Na,,ona,Aeronaot,csao /N" 3Space Administration
Washington, D.C.
20546 _ ,,/_ 7/7/ _._
Print Figure /
NOTICE
The invention disclosed in this document resulted from research in
aeronautical and space activities performed under programs of the
National Aeronautics and Space Administration. The invention is
owned by NASA and is, therefore, available for licensing in
accordance with the NASA Patent Licensing Regulation (14 Code of
Federal Regulation 1245.2).
To storage commercial utilization of NASA-owned inventions, it is
NASA policy to grant licenses to commercial concerns. Although
NASA encourage nonexclusive licensing to promote competition and
achieve the widest possible utilization, NASA will consider the
granting of a limited exclusive license, pursuant to the NASA
Patent Licensing Regulations, when such a license will provide the
necessary incentive to the licensee to achieve early practical
application of the invention.
Address inquires and all applications for license for this
invention to Offl_ of Patent Count! (LE-LAW)
NASA/Lewis Research Cente_
Cleveland, Ohio 44135
Approved NASA forms for application for nonexclusive or exclusive
license are available from the above address.
(NASA-Case-NPO-17479-I-CU) MEASUREMENT OF
WAVES [N FLn_5 ACROSS A SURFACE Patent
App|ic_fion (NASa) le p CSCI 200
N91-13658
G3/36
https://ntrs.nasa.gov/search.jsp?R=19910004345 2020-03-19T20:56:24+00:00Z
MEASUREMENT OF WAVES IN FLOWS
_CROSS A SURFACE
Inventor:
Contractor:
James M. Kendall, Jr.
Jet Propulsion
Laboratory
JPL Case No. 17479
NASA Case No.
NPO-17479-I-CU
Date: July 16, 1990
AWARDS ABSTRACT
A waveflow sensor for noninvasive measurement of
instability waves and turbulence effects across an
aerodynamic surface (Figs. 1 and 2). The sensor includes
at least two openings 5 spaced along the aerodynamic
surface 2 at an interval equal to one-half of a wavelength
of the flow being measured. The openings lead to two
pressure ducts 3, 4 which open into a cavity 7, allowing
the sensed pressure fluctuations to interfere. The wave
components which were out of phase at the openings 5 are
in phase in the cavity 7 and add. All other components
cancel out when combined in the cavity 7. The interfering
waveform in the cavity is sensed by microphone 8. In a
second embodiment (Fig. 3) the pressure levels are sensed
separately by individual microphones II, 12 and caused to
electrically combine through differential amplifier 15.
\JPL Case No. 17479
NASA Case No. NP0-17479-I-CU
Attorney Docket No. JPLI-Y94
PATENT APPLICATION
MEASUREMENT OF WAVES IN FLOWS
ACROSS A SURFACE
5
Qriqin of the Invention
The invention described herein was made in the
performance of work under a NASA contract, and is subject
to the provisions of Public Law 96-517 (35 U.S.C.
Section 202) in which the Contractor has elected not to
retain title.
i0
Technical Field
This invention relates to the measurement of wave
motion in aerodynamic flows and, more particularly, to a
method and app@ratus for measuring flow by sensing
pressure fluctuations due to wave passage.
15
Backqround Art
The flow of air adjacent to the surface of an
aircraft wing or component, i.e., the so-called boundary-
layer, can be either laminar or turbulent. The onset of
-2- PATENT APPLICATION
5
I0
15
20
turbulence brings about substantial increases in such
aerodynamic parameters as drag and heat transfer. It is
therefore important to study the instability waves which
precede turbulence, and perhaps to try to obtain a
feedback system to reduce the ill effects associated with
turbulence.
Boundary-layer turbulence in general is initiated by
instability waves, which are ever-present in laminar
layers. Due to the effects they produce, such waves are
of considerable importance for both scientific and
engineering reasons.
Aircraft wing design procedures have for several
years included computer programs which predict the growth
of waves, and the resultant onset of turbulence. The
computations are far from perfect, as is the understanding
upon which they are based.
An improved method for instability wave detection and
measurement is needed. Such a method could be applied to
instability wave suppression for turbulence control on
aerodynamic surfaces, whereupon aircraft drag would be
reduced.
The most commonly employed method for wave-detection
in the research laboratory is hot-wire anemometry, but
this technique has several drawbacks. Hot-wire probes are
-3- PATENT APPLICATION
5
invasive and easily broken. When using hot-wire probes it
is necessary to survey across the boundary layer thickness
to find the maximum value of the wave amplitude. Hot-wire
probes and their electronic control units are quite
costly. Finally, the intrusiveness of hot-wire probes
itself causes turbulent waves which preclude observation
at more downwind stations.
I0
15
20
Summarv of the Invention
It is therefore an object of the invention to provide
an apparatus and method for sensing wave-motion in
aerodynamic boundary layer flows.
It is a further object of the invention to provide an
inexpensive, sturdy, and noninvasive probe for the
measurement of boundary layer airflow.
It is yet a further object of the invention to
provide a method for the noninvasive detection of
instability waves and turbulence in boundary layer
airflow.
It is yet a still further object of the invention to
provide a sensing apparatus and method whereby many
noninvasive probes may be permanently installed, and
operated simultaneously for determining the wave
propagation field over a broad spatial area.
-4- PATENT APPLICATION
5
i0
15
2O
These and other objects are achieved according to the
invention by providing a system for the measurement of
pressure fluctuations within a flow over a surface such
that the pressure levels are sensed at a minimum of two
stations and combined to cancel background noise and
produce a representative measurement of pressure
fluctuation across the flow.
In the preferred embodiment, two small holes, or
orifices, pass through an aerodynamic surface leading to
respective pressure ducts. The openings are separated on
the surface by a distance of one-half of a wavelength of
the instability waves being measured.
In the preferred embodiment, the pressure ducts feed
to a cavity wherein the signals acoustically interfere.
Specifically, the ducts lead to opposite sides of a
microphone diaphragm. Unwanted components which are in
phase on the surface cause responses of the diaphragm
which are 180 degrees out of phase. The unwanted
components will subtract and cancel out, and waveflow
components which are out of phase when sensed one-half of
a wavelength apart will add. Thus, the interfering
waveform inside the cavity becomes a representation of the
waveform at the surface, and the onset of turbulence can
be detected before its actual occurrence.
-5- PATENT APPLICATION
A microphone is disposed inside the cavity to produce
an electrical signal representation of the interfering
waveform inside the cavity. This electrical signal
therefore represents the waveform across the surface.
i0
Brief Description of the Drawinas
Fig. 1 is an illustration of an airplane containing
the invention; and
Fig. 2 is a cross-sectional illustration of the
airplane of Fig. 1 showing the preferred embodiment of the
invention disposed in the wing.
Fig. 3 is a cross-sectional illustration of a second
embodiment of the invention.
15
20
Detailed DescriD_ion of _he Preferred Embodiment
The preferred embodiment of the invention is applied
to airflow over a flat plate for research purposes, but
the technique is equally applicable to flows over surfaces
of any figure.
Fig. 1 illustrates how the invention may be disposed
in an airplane wing to sense instability waves and turbu-
lence and help control the ramifications of turbulence.
This figure is not to scale, the separation distance of
-6- PATENT APPLICATION
i0
15
2O
the pressure orifices is greatly exagerated for
illustrative purposes.
In the preferred embodiment, pressure fluctuations
due to wave passage are measured by use of commercially-
available, inexpensive electret microphones (Radio Shack
Cat. No. 276-090) which have excellent sensitivity. Such
an embodiment is illustrated in Fig. 2, which may be
considered a cross-section of the airplane wing of Fig. 1
or a flat plate in a wind tunnel.
In these embodiments, the microphones are affixed to
the reverse, i.e., nonaerodynamic, face of the test model,
and communicate with the flow by means of small orifices
which are open to the flow to be measured. Thus, the
technique is noninvasive into the flow, generally
eliminating turbulence produced by the sensor.
The straightforward use of such sensors would not be
satisfactory in most situations because of the afore-
mentioned interference from other sources. One source is
the boundary layer itself, which undergoes other motions
than those responsible for turbulence, and other sources
arise within wind tunnels where research is conducted.
Thus, it is necessary to exclude unwanted noise and
pressure components from the measurements.
-7- PATENT APPLICATION
5
i0
15
2O
The exclusion of unwanted components is accomplished
by measuring the difference in pressure at two points on
the surface separated by a distance equal to one-half the
wavelength of the waves to be measured. The pressure at
the two points is therefore out of phase. The differ-
encing, i.e., subtraction, doubles the signal due to the
waves, but cancels the background signals because they
have sufficiently long wavelengths to be in phase.
The separation distances, i.e., half-length of the
instability waves, can either be determined theoretically
or measured by use of the technique itself. The optimal
separation distance w_ll, in general, depend upon the
specific location on the aircraft or test model where the
measurement is to be made. The orifice spacing need not
be set accurately, and a substantial error is acceptable.
The preferred embodiment is illustrated in Fig. 2.
The microphone housing 1 is affixed to the reverse side of
the aerodynamic surface 2. In the present implementation
of the preferred embodiment, the housing dimensions are 3
x 1 x 0.4 cm in the flow direction, cross-direction, and
thickness, respectively.
In the preferred embodiment, attachment of the
microphone housing 1 is made by adhesive foam tape, but
attachment may also be made by screws or other connectors.
-8- PATENT APPLICATION
5
10
15
20
Two pressure ducts, a positive pressure duct 3, and a
negative pressure duct 4 feed into the microphone
housing 1 from the aerodynamic surface 2. The openings 5
to these pressure ducts 3, 4 are separated by one-half of
a wavelength of the boundary layer flow being measured.
The boundary layer flow velocity profile is illustrated in
Fig. 1 as 6. The boundary layer flows over the
aerodynamic surface 2 with a change in pressure gradient.
The present invention measures the pressure signal
imposed upon the aerodynamic surface 2 as the waves
(represented in Fig. 1 in the boundary layer flow
profile 6) pass by. The existence of pressure
perturbation accompanying the instability wave motion is
well-known theoretically, but it has not been used for
detecting waves because it has not been considered a
viable approach.
The present invention allows for such a measurement
of wave motion by detecting the change in pressure
perturbation across the aerodynamic surface. The sensing
of pressure due to waves has been recognized by N.S.
Dougherty, Jr. and D.F. Fisher, "Boundary Layer Transition
on a I0 ° Cone: Wind Tunnel/Flight Data Correlation," AIAA
Paper 80-1054, 1980, for supersonic flows, and by
A. Demetriades, "Pressure Fluctuations on Hypersonic
-9- PATENT APPLICATION
10
15
20
Vehicles Due to Boundary Layer Instabilities," AIAA
Journal, 24 (I), p. 188, 1986, for hypersonic flows where
the signals are far stronger than at subsonic speeds.
However, the weak signals due to instability waves in
subsonic flows are often completely obscured by background
interference from other sources.
The preferred embodiment allows for these weak
signals to be obtained by cancelling the noise or unwanted
background. The background interference signals are those
waves other than the pressure signals which are out of
phase at the two sensing points or openings 5 of the
pressure ducts 3, 4 of the preferred embodiment.
The two sensed pressure signals at the openings 5 are
combined and interfered by leading the pressure ducts 3, 4
into a circular cavity 7. The two separate sensed
pressure signals will enter the cavity 7, each imposing
its own level of oscillation on the air within the
cavity 7.
In the preferred.embodiment, the two pressure
ducts 3, 4 lead into the cavity at points 9, i0, respec-
tively. These pressure duct/cavity introductions are
placed to maximize the interference of the positive and
negative pressure signals from the positive and negative
pressure ducts 3, 4, respectively.
-i0- PATENT APPLICATION
5
i0
15
20
The two levels of oscillation interfere, canceling
out unwanted noise signals which reach the cavity 7 out of
phase, while also adding the pressure signals of the wave
flow which are in phase. Thus, a representative waveform
of the wave-flow is produced, and that waveform by the
microphone diaphragm 8 within the cavity 7. The
microphone 8 produces an electrical signal representative
of the waveform produced in the cavity.
In another embodiment of the invention, shown in Fig.
3, individual microphones II, 12 sense pressure levels at
respective separate orifices 13, 14. The microphones
convert the sensed pressure leveles into electrical
signals which are then electrically combined by
differential amplifier 15 to produce representative wave
form 16.
The possibility of annihilating instability waves for
turbulence suppression has been contemplated many times
over the past 20 years or more. What is needed for this
is a means to detect the presence of such waves, and a
means to generate an equal and opposite motion. The
present technique, with modest modification, may have
application to both aspects of this. Piezoplastic sheet
materials, which are readily available (Pennwalt Corp.,
King of Prussia, PA), are well-suited to the mass
-ii- PATENT APPLICATION
i0
fabrication of transducers which would either generate an
electrical signal in response to a pressure perturbation,
or generate a pressure perturbation for cancellation in
response to an appropriate electrical stimulus.
Obviously, a microelectronics system is needed also, but
the transducer system seems quite realizable.
Those skilled in the art will appreciate that various
adaptations and modifications of the just-described
preferred embodiment can be configured without departing
from the scope and spirit of the invention. Therefore, it
is to be understood that, within the scope of the appended
claims, the invention may be practiced other than as
specifically described herein.
PATENT APPLICATION
MEASUREMENTOF WAVESIN FLOWS
ACROSSA SURFACE
i •
\
i0
ABSTRACT _ __ __ .....
A method and apparatus/Zor sensing wave flow across a
%
surface wherein at least two pressure levels are sensed
and combined to provide a representation of waves within
the flow. In the preferred embodiment holes bored through
the aircraft surface at an interval of one-half the
wavelength of the flow being measured introduce pressure
perturbations into a cavity so they may acoustically
interfere. The interfering waveform is sensed by at least
one microphone disposed in the cavity.
FIG. I
"/I
Z
/
,3
) I _o 5 2
F(( (( ./'/////f//._'r<
_,,)",,\\\'< \\\'t N
8/ ) "'4
F/G 2
/3 /4/ /
7/// 'F//////////A" V//////////
MICROPHONE MICROPHONE
"-ff-I =1¢2 - 15
OUTPUT
FIG 3


Measurement of waves in flows across a surface

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server