A method and apparatus 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, Jr., James M.

NASA Technical Reports Server

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US005136881A 
United States Patent (191 (111 Patent Number: 5,136,881 
Kendall, Jr. (451 Date of Patent: Aug. 11, 1992 
MEASUREhlEhT OF WAVES IN FLOWS 
ACROSS A SURFACE 
Inventor: James M. Kendall, Jr., Pasadena, 
Calif. 
Assignee: The United States of America as 
represented by the Administrator of 
the Kational Aeronautics and Space 
Administration, Washington, D.C. 
Appl. No.: 568,127 
Filed: Aug. 16, 1990 
Int. (3.5 .............................................. GOlM 9/00 
US. (3. ................................... 73/147; 73/178 R; 
73/180 
Field of Search ..................... 73/147, 180, 178 R, 
73/178 H, 178 T 
References Cited 
U.S. PATENT DOCUMENTS 
2,551,526 5/1951 Campbell .............................. 73/147 
2.871.698 2/1959 Bagby ........ 
3,452.707 7/1969 Warren ...... 
3,620.069 1 ]/I971 Cole. Jr. ................................ 73/587 
4,188,823 2/1980 Hood .............................. 
4,372.157 2/1983 Caruthers et a]. ............. 
4,459,851 7/1984 Crostack ............................... 73/587 
4.463.453 7/1984 a h e n  et al 73/646 
4,524.620 6/1985 Wright et al. ......................... 73/587 
4,738,137 4/1988 Sugg et al. ............................ 73/587 
OTHER PUBLICATIONS 
J. M. Kendall “Experimental Study of Disturbance 
produced in a pretransitional laminar boundary layer by 
weak freestream turbulence” AIAA-pp. 1-10. 
W. Nitsche et al. “Investigations on flow instabilities on 
airfoils by means of piezofoil-arrays (wind tunnel and 
flight tests)”-Institut fur Luft und Taumfahrt Technis- 
che Unicersitat Berlin-pp. 1-7. 
N. S. Dougherty, Jr. et al.-“Boundary-layer transi- 
tion on a IO-DEG Cone: wind tunnel/flight correla- 
tion”-AIAA-pp. 1-6. 
A. Demetriades-“Pressure fluctuations on hypersonic 
vehicles due to boundary-layer instabilities”-Montana 
State University, Bozeman, Mont. pp. 188-190. 
J. M. Kendall “Boundary Layer Receptivity to Frees- 
tream Turbulence” AIAA-pp. 1-7-Jun. 18-20, 1990. 
Primory Examiner-Donald 0. Woodiel 
Attorney, Agenr, or Firm-Thomas H. Jones; John R. 
Manning; Guy M. Miller 
1571 ABSTRACT 
A method and apparatus 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 intro- 
duce pressure perturbations into a cavity so they may 
acoustically interfere. The interfering waveform is 
sensed by at least one microphone disposed in the cav- 
ity. 
2 Claims, 2 Drawing Sheets 
i‘ 
I 1 
‘ Y / / / / / / / / / / / S  V////.////) 
I I / I /  I I 
# 2  
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I 16 c 
https://ntrs.nasa.gov/search.jsp?R=20080004320 2019-08-30T02:18:54+00:00Z
U.S. Patent Aug. 11, 1992 Sheet 1 of 2 5,136,881 
US. Patent Aug. 11, 1992 Sheet 2 of 2 5,136,881 
5,136,881 
2 
operated simultaneously for determining the wave 
propagation field over a broad spatial area. 
These and other objects are achieved according to 
the invention by providing a system for the measure- 
ment of pressure fluctuations within a flow over a sur- 
face such that the pressure levels are sensed at a mini- 
mum of two stations and combined to cancel back- 
ground noise and produce a representative measure- 
ment 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 micro- 
phone 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 interfer- 
ing waveform inside the cavity becomes a representa- 
tion of the waveform at the surface, and the onset of 
turbulence can be detected before its actual occurrence. 
A microphone is disposed inside the cavity to pro- 
duce an electrical signal representation of the interfer- 
ing waveform inside the cavity. This electrical signal 
therefore represents the waveform across the surface. 
BRIEF DESCRIPTION OF THE DRAWINGS 
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. 
DETAILED DESCRIPTION OF T H E  
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 the 
pressure orifices is greatly exagerated for illustrative 
PUrPorn. 
In the preferred embodiment, pressure fluctuations 
due to wave passage are measured by use of commer- 
cially-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 mea- 
sured. Thus, the technique is noninvasive into the flow, 
generally eliminating turbulence produced by the sen- 
sor. 
The straightforward use of such sensors would not be 
satisfactory in most situations because of the aforemen- 
MEASUREMEST OF WAVES IN FLOWS ACROSS 
A SURFACE 
ORIGIN OF T H E  INVENTION 5 
The invention described herein was made in the per- 
formance of work under a NASA contract, and is sub- 
ject to the provisions of Public Law 96-517 (35 U.S.C. 
Section 202) in which the Contractor has elected not to 
retain title. 10 
TECHNICAL FIELD 
This invention relates to the mmurement of wave 
motion in aerodynamic flows and, more particularly, to 
a method and apparatus for measuring flow by sensing 
pressure fluctuations due to wave passage. 
BACKGROUND ART 
1s 
precede-turbulence, and perhaps to try to obtain a feed- 
back 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 engi- 
neering 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 under- 
standing 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 cocimonly employed method for wave- 
detection in the research laboratory is hot-wire ane- 
mometry, but this technique has several drawbacks. 
Hot-wire probes are 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 elec- 
tronic control units are quite costly. Finally, the intru- 
siveness of hot-wire probes itself causes turbulent waves 
which preclude observation at more downwind sta- 
tions. 
SUMMARY OF T H E  INVENTION 
It is therefore an object of the invention to provide an 
apparatus and method for sensing wave-motion in aero- 
dynamic boundary layer flows. 
It is a further object of the invention to provide an 
inexpensive, sturdy, and noninvasive probe for the mea- 
surement 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 pro- 
vide a sensing apparatus and method whereby many 
noninvasive probes may be permanently installed, and 
The flow of air adjacent to the surface of an aircraft 2o 
wing or component, Le., the so-called boundary-layer, 
can be either laminar or turbulent. The onset of turbu- 
lence brings about substantial increases in such aerody- 
namic parameters as drag and heat transfer. It is there- 
fore important to study the instability waves which 2s 
30 
35 
40 
45 
50 
5s 
60 
65 
5.136.88 1 
3 
tioned interference from other sources. One source is 
the boundary layer itself, which undergoes other mo- 
tions 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. 
The exclusion of unwanted components is accom- 
plished 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 differencing, 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 insta- 
bility waves, can either be determined theoretically or 
measured by use of the technique itself. The optimal 
separation distance will, 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 accept- 
able. 
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 implemen- 
tation of the preferred embodiment, the housing dimen- 
sions are 3X 1 X0.4 cm in the flow direction, cross- 
direction, and thickness, respectively. 
In the preferred embodiment, attachment of the mi- 
crophone housing l is made by adhesive foam tape, but 
attachment may also be made by screws o other connec- 
tors. 
Two pressure ducts, a positive pressure duct 3, and a 
negative pressure duct 4 feed into the microphone hous- 
ing 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 aero- 
dynamic surface 2 with a change in pressure gradient. 
The present invention measures the pressure signal 
imposed upon the aerodynamic surface 2 ;s the waves 
(represented in FIG. 1 in the boundary Ia;ir flow pro- 
file 6) pass by. The existence of pressure perturbation 
accompanying the instability wave motion is well- 
known theoretically, but it has not been used for detect- 
ing 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 sens- 
ing of pressure due to waves has been recognized by N. 
S. Dougherty, Jr. and D. F. Fisher, “Boundary Layer 
Transition on a lo’ Cone: Wind Tunnelmight Data 
Correlation,” AIAA Paper 80-1054, 1980, for super- 
sonic flows, and by A. Demetriades, “Pressure Fluctua- 
tions on Hypersonic 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 un- 
wanted background. The background interference sig- 
nals are those waves other than the pressure signals 
4 
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 
5 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 o the air within the cavity 7. 
In the preferred embodiment, the two pressure ducts 
10 3, 4 lead into the cavity at points 9, 10, respectively. 
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. 
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 
2o waveform by the microphone diaphragm 8 within the 
cavity 7. The microphone 8 produces an electrical sig- 
nal representative of the waveform produced in the 
cavity. 
In another embodiment of the invention, shown in 
25 FIG. 3, individual microphones 11, 12 sense pressure 
levels at respective separate orifices 13, 14. The micro- 
phones convert the sensed pressure leveles into electri- 
cal signals which are then electrically combined by 
3o 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 
35 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., 
4o King of Prussia, Pa.), are well-suited to the mass fabri- 
cation 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. Obvi- 
45 ously, 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 pre- 
ferred embodiment can be configured without depart- 
50 ing from the scope and spirit of the invention. There- 
fore, it is to  be understood that, within the scope of the 
appended claims, the invention may be practiced other 
than as specifically described herein. 
* S  
I claim: 
1. A method for measuring wave motion in flows 
(a) sensing a first pressure level at a first point on the 
(b) sensing a second pressure level at a second point 
(c) combining said first and second sensed pressure 
levels such that interference is canceled and a rep- 
resentative signal of a pressure fluctuation across 
the flow is produced, and wherein said pressure 
levels are sensed at surface openings separated by 
one-half of a wavelength of the flow. 
2. An apparatus for the measurement of wave motion 
55 
across a surface, comprising the steps of: 
surface; 
60 on the surface; 
65 
in a flow across a surface, comprising: 
5 
5,136,881 
6 
(a) a first sensing means for detecting a first pressure 
(b) a second sensing means for detecting a second 
(c) a combining means for combining said first and 5 
ence is canceled and a representative signal of a 
pressure fluctuation across the flow is produced, 
and wherein said first and second pressure levels 
are sensed at surface openings separated by one- 
half of a wavelength of the flow. 
level in said flow; 
pressure level in said flow; and 
second sensed pressure levels such that interfer- * * * * *  
20 
25 
30 
35 
45 
50 
55 
60 
65 


Measurement of waves in flows across a surface

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