A device for measuring turbulence in high-speed flows is provided which includes a micro-sensor thin-film probe. The probe is formed from a single crystal of aluminum oxide having a 14 deg half-wedge shaped portion. The tip of the half-wedge is rounded and has a thin-film sensor attached along the stagnation line. The bottom surface of the half-wedge is tilted upward to relieve shock induced disturbances created by the curved tip of the half-wedge. The sensor is applied using a microphotolithography technique

Cruz, Vincent B.

Hopson, Purnell, Jr.

Mcginley, Catherine B.

Sheplak, Mark

Spina, Eric F.

Stephens, Ralph M.

English

NASA Technical Reports Server

NASA CASE NO.
PRINT FIG. _IA_
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LAR 15112-1-CU
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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 patent licensing regulations applicable to U.S.
Government-owned inventions (37 CFR 404.1 at seq.).
To encourage commercial utilization of NASA-owned inventions, it is NASA
policy to grant licenses to commercial concerns. Although NASA encourages
nonexclusive licensing to promote competition and achieve the widest possible
utilization, NASA will provide the necessary incentive to the licensee to achieve early
practical application of the invention.
Address inquiries and all applications for license(s) for this invention to the
Technology Applications Team, NASA Langley Research Center, Code 200, Hampton,
Virginia 23681-0001.
Serial No.: 08/361,601
Filed: 11/24/94 LaRC
(NASA-Case-LAR-15112-1-CU)
MICRO-SENSOR THIN-FILM ANEMOMETER
Patent App|ication (NASA. Langley
Research Center) 14 p
N95-22906
Unclas
G3/34 0042565
https://ntrs.nasa.gov/search.jsp?R=19950016489 2020-06-17T23:54:01+00:00Z
NASA Case No. 151 12-1-CU 
MICRO-SENSOR THIN-FILM ANEMOMETER 
AWARDS ABSTRACT 
Turbulence measurements in high-speed flows have historically been obtained by hot-wire 
anemometry. However, high stagnation temperatures, high dynamic pressures and flow contaminants 
severely limit the life of hot-wire elements in hypersonic flow. An alternative to conventional hot-wire 
anemometry is hot-film memometry. However, the frequency response of existing hot-film probes is 
inadequate to resolve the full turbulent spectrum for hypersonic flows. 
An anemometer is provided having a micro-sensor thin-film probe. The probe is a half-wedge formed 
from a single crystal of sapphire and is configured to prevent generation of a deta&ed shoclc through 
much of the hypersonic boundary layer. The tip of the probe is rounded to minimize flow disturbance 
and contains an iridium sensor formed on the rounded tip of the probe along the stagnation line. The 
sensor is deposited using a unique microphotolithography technique. 
Novel aspects of the present invention include the configuration of the probe, the use and formation 
of the iridium sensor and the placement of the sensor along the stagnation line of the rounded tip of 
the probe. 
Inventors: Marlc Sheplalc Catherine B. McGinley 
Address: 7 1 86 White Plains Way 1 108 North Green Drive 
, FL 32656 Newport News, VA 23602 
ss: 
Employer: Student NASA LaRC 
Inventors: Eric F. Spina Ralph M. Stephens 
Address: 206 Edmount Drive 530 Pembrolce Avenue 
Syracuse, NY 13203 Norfollc, VA 23507 
SS : ss: 
Employer: Syracuse Univ. NASA LaRC 
Inventors: Pumell Hopson, Jr. Vincent B. Cruz 
407 Cheadle Loop Road Route 4, Box 1365 
Seaford, VA 23696 Haves, VA 23072 
ss: 
Employer: NASA LaRC 
1niGal Evaluator: Michael J. Walsh 
ss: 
NASA LaRC 
SerialNo.: 08/361,701 
Filed: November 2 1, 1 994 
LAR 15112-1 -1- PATENT APPLICATION
MICRO-SENSOR THIN-FILM ANEMOMETER
Origin of the Invention
5 The invention described herein was made jointly in the performance of
work under NASA Grant No. NAG-l-1400 with Syracuse University and a
graduate student, and employees of the United States Government. In
accordance with 35 U.S.C. 202, the grantee elected not to retain title.
]0 Back,qround of the Invention
1. Field of the Invention
]5
The present invention relates generally to a device and method for
measuring turbulence in high-speed flows, and more particularly to a micro-
sensor thin-film probe capable of measuring turbulence in hypersonic flows.
2. Description of the Related Art
20 Turbulence measurements in high-speed flows have historically been
obtained by hot-wire anemometry. However, high stagnation temperatures, high
dynamic pressures, and flow contaminants severely limit the life of hot-wire
elements in hypersonic flow. Nonintrusive measurement techniques such as
laser-Doppler velocimetry and particle-image velocimetry also are limited when
25 applied to hypersonic flow. In particular, data-rate limitations and difficulties in
flow seeding present the most significant obstacles to their accurate application
in high-speed flows. An alternative to conventional hot-wire anemometry is
hot-film anemometry, with a thin metallic film deposited along the stagnation line
of a rigid, dielectric substrate, thus increasing mechanical strength.
30
LAR 15112-1 -2- PATENT APPLICATION
Hot-film probes incorporating various combinations of materials and
construction techniques have displayed excellent durability and moderate
frequency response characteristics in the few high-speed and high temperature
flows in which they have been tested.
5 Ling and Hubbard introduced the thin-film probe as a
resistance-temperature transducer to measure turbulent fluctuations in flowfields
in which hot-wires could not survive (Ling, S.C. and Hubbard, P.G. 1956 "The
Hot-Film Anemometer: A New Device for Fluid Mechanics Research", J.
Aeronaut. Sci. 23, 890). This probe consisted of a thin layer of platinum fused
10 to a glass substrate. The main body of the probe consisted of a 4.0 mm
diameter Pyrex rod with two 32-gauge platinum lead wires (2.0 mm apart)
embedded within the core. The rod was ground down into a 8 ° wedge, tipped
by a 30 ° wedge. Fused on the wedge tip was a thin platinum film sensor (1.0
mm x 0.2 mm) with a nominal cold resistance of 200. The ends of the sensor
]5 were attached to the exposed lead wires by thick platinum overplatings. It was
tested at high temperatures (1100°F) without detectable changes in
thermoelectric properties. Experiments indicated a frequency response of 100
kHz at a flow velocity of 1000 ft/sec.
Later, Seiner used commercial hot-film probes to investigate high-speed,
20 cold jet flows (Mach 0.5-2.0), (Seiner, J.M. 1983 "The Wedge Hot-Film
Anemometer in Supersonic Flow", NASA Technical Paper 2134). The probe
consisted of a thin film of nickel sputtered on a 4 ° semivetrex wedge of quartz
substrate. A protective coating of quartz (0.5-2.0 IJm) was sputtered over the
nickel (1.0 mm x 0.2 mm) for electrical isolation and protection from particles.
25 A maximum frequency response of 130 kHz for the 1:1 balanced CTA bridge
was realized via the square-wave injection test, and was found to be inversely
proportional to the thickness of the protective coating.
More recently, Demetriades and Anders presented a report on the
ongoing development of a constant-current probe for use in
30 high-temperature/hypervelocity flows (Demetriades, A. and Anders, S. G. 1990
LAR 15112-1 -3- PATENT APPLICATION
"Characteristics of Hot-Film Anemometers for Use in Hypersonic Flows", AIAA
Jouma128,2003). The design consists of a platinum sensor (0.5mm x 1.8mm)
painted along the stagnation line of a wedge-shaped glaze bead positioned at
the tip of a twin-bore alumina tube (10.0 cm x 0.25 cm). Results from
5 temperature endurance testing (temperature cycling to 1400°F) demonstrated
the excellentelectrical stability characteristics of the probe. These probes were
also run for hours in high-temperature, high dynamic-pressure environments
(Mach8.0, 800°F and 20.7 kPa)without failure, further confirming the durability
characteristics. Presently, there is no experimental data available from
]0 Demetriadesdocumentingthe frequency response characteristics of this probe.
However, painted sensors contain cross-sectional non-uniformities which lead
to "hot-spots" and sensor failures. In addition, the large sensor surface area
used by Seiner and Demetriades limits the spatial resolution of the
measurement technique.
15 The frequency responsecharacteristicsof the existing hot-film probes are
inadequate to resolve the full turbulent spectrum for hypersonic flows. The "dual
swept-surface" wedge designs used by Seiner are a poor approximation of
stagnation-line heat-transfer, since the sensor extends 0.1 mm away from the
tip on both sides of the wedge. In addition to this fundamental problem, Seiner
20 found that the probe displayed poor directional behavior and was Mach-number
sensitive. Seiner hypothesized that these problems were associated with the
40 ° wedge geometry and recommended examination of a larger semi-vertex
angle or a wedge fitted with a rounded nose. Finally, this design is
unacceptable for measurements in wallbounded shear flows, as shocks
25 emanating from the bottom of the probe would disturb the flow.
LAR 15112-1
Summary of the Invention
-4- PATENT APPLICATION
It is a primary object of the present invention to provide a device and
method for measuring turbulence in high-speed flows such as hypersonic
5 flows.
A further object of the present invention is to measure turbulence in
high-speed flows with a device and method having fast response and
durability in the severe environment of hypersonic airflows.
The foregoing objects are achieved by providing an anemometer
]0 having a micro-sensor thin-film probe. The probe is a half-wedge formed
from a single crystal of aluminum oxide (i.e., sapphire) and is configured to
prevent the wedge from generating a detached shock through much of a
hypersonic boundary layer. The tip of the sapphire probe is rounded to
minimize flow disturbance and contains an iridium sensor formed on the
[5 rounded tip of the probe along the stagnation line.
The iridium sensor is formed by first depositing a layer of copper over
the sapphire substrate in the area where the sensor is to be located. A
layer of photoresist is then deposited over the copper and dried. A contact
print of the sensor shape is then made into the photoresist by exposing the
20 photoresist to ultraviolet light. The photoresist is then developed leaving an
opening to the copper layer corresponding to the sensor shape. The
exposed copper is etched to produce an opening through the copper to the
sapphire substrate corresponding to the sensor shape. The photoresist is
removed and niobium is deposited on the exposed sapphire substrate by
25 electron beam vapor deposition. Without breaking the vacuum, iridium is
deposited by electron beam vapor deposition onto the niobium layer. The
copper is then removed with an etchant and the probe containing the sensor
is annealed in a hard vacuum at approximately 1000°C to stabilize the
resistance of the sensor. Once the sensors are formed, they are connected
LAR 15112-1 -5-
to leads which connect to a coaxial cable.
the driving circuit of the anemometer.
Sensors formed according to this
PATENT APPLICATION
The coaxial cable connects to
method show a significant
improvement in frequency response due to the lower thermal inertia of the
sensor compared to conventional 5.0 #m diameter hot wire and existing hot
films.
B[ief Description of the Drawings
]0
FIG.
FIG. 1A.
FIG.
FIG. 1A is a side view of the thin-film probe of the present invention.
1B is a side view of the tip of the thin-film probe depicted in
1C is an illustration of the angle of a portion of the probe
depicted in FIGS. 1A and lB.
]5 FIG. 2 is a drawing of the sensor region of the thin-film probe of the
present invention.
FIG. 3A is a drawing of the sensor pattern formed through the
photoresist and copper layers.
FIG. 3B is a drawing of the niobium and iridium layers of the sensor
20 deposited through the copper layer.
FIG. 3C is a drawing of the sensor formed on the sapphire substrate.
Description of the Preferred Embodiments
25 Referring to FIG. 1A, the probe 10 geometry consists of a straight
portion leading to an approximately 14 ° half-wedge, 1/2 inch long x 1/8
inch wide, diamond tooled out of a single crystal aluminum oxide (i.e.,
sapphire). The use of sapphire for the substrate material enables the probe
to withstand severe temperatures without mechanical failure or a significant
LAR 15112-1 -6- PATENT APPLICATION
change in dielectric properties. The capability of sapphire to be machined
and polished to a finish less than one micro-inch (r.m.s.) coupled with the
microphotolithographic technique allows for a significant reduction in sensor
size over existing hot-films, thus improving the spatial resolution of the
5 probe. In addition, the substrate material was selected because it has a
low thermal conductivity and a coefficient of thermal expansion that closely
matches the iridium and niobium layers, thus reducing the risk of sensor
detachment during thermal expansion. The 14 ° half-angle represents e=i t
for an air flow with a freestream Mach number slightly below 1.6, thus
]0 preventing the wedge from generating a detached shock through much of
a hypersonic boundary layer. This angle can be modified for application to
specific airflow conditions. The half-wedge geometry is preferred in order
to minimize flow disturbance and allow for near-wall measurements in
boundary layers. Referring to FIG. 1B, the bottom portion of the wedge is
]5 angled upward at 3 ° in an effort to relieve shock induced disturbances
created by the finite curvature of the nose. This will lessen any
boundary-layer disturbance that can propagate upstream, making for more
accurate measurements. This angle of tilt can be modified to suit the
application and different wedge tip radii. The wedge tip 20 is rounded to
20 a radius of curvature of approximately 0.2 ram, and is then diamond
polished to a r.m.s, surface finish of 1 microinch or less to prepare the
surface for deposition of the sensor 30. The radius of curvature of the
wedge tip 20 should be as small as possible to minimize flow disturbance
while still accommodating the sensor. The rounded-nose, wedge tip
25 geometry offers benefits over the "dual swept-surface" probes. Since the
sensor is very narrow, "true" stagnation point heat-transfer will occur:
existing heat transfer data for heated cylinders in turbulent cross-flow
shows a negligible decrease in local Nusselt number for angles less than 5 °
from the stagnation line. It is thus possible to approximate the rounded
LAR 15112-1 -7- PATENT APPLICATION
nose as a cylinder in cross-flow, for which a considerable amount of data
exists in all Mach number regimes. The onset of Mach number
independence may also be reduced, as a normal shock will occur locally in
front of the stagnation line, rather than a weaker oblique shock in front of
5 a wedge.
A "dog-bone" shaped micro-sensor (approx. 2000 A x 12.5 pm x
0.25 mm) 30 of iridium is deposited along the stagnation line 40 of the
substrate using a microphotolithography technique, as described below.
The "dog-bone" shape of the sensor results in reducing failures at the
]0 junction of the lead wire with the sensor because the "dog-bone" is wider
at the ends so the actual junction doesn't become too hot. Additional
turbulence measurements may be made from additional sensors located off
the stagnation line.
To form the sensor 30, an approximately 4000-5000 A thick layer of
]5 copper 50 is deposited by sputtering or vapor deposition onto the sensor
area of the substrate 60. Although copper is preferred, other materials such
as Cr, Ag and Ni can be used which can be selectively etched. A 2.0 pm
thick layer of positive-phase photoresist 70 is then spun onto the copper
layer 50 and baked. A negative of the "dog-bone" shaped sensor pattern
20 is contact printed into the photoresist layer 70 by exposing the photoresist
to ultraviolet light, preferably a highly collimated mercury-vapor light source.
The photoresist is developed, leaving an opening 80 through to the copper
layer 50 with a shape corresponding to the shape of the sensor. The
copper is then etched with a suitable etchant such as ammonium
25 phersulphate to produce an opening in the copper layer shaped like the
sensor pattern. This opening extends to the sapphire substrate 60. The
photoresist 70 is removed and a 150-200 A thick layer of niobium 90 is then
sputter deposited or vapor deposited onto the sapphire substrate 60 through
the opening in the copper layer 50. Although niobium is preferred, other
LAR 15112-1 -8- PATENT APPLICATION
materials such as chromium can be used as long as the material selected is
stable at high temperatures and is essentially nonreactive with adjacent
layers. Deposition by electron beam vapor deposition is preferred because
the vapor flux tends to be directional. The thickness of the niobium layer
5 90 can vary, but must be thick enough to provide an adhesive base and to
ensure that the niobium is contiguous throughout the layer. Without
breaking vacuum, an approximately 2000 A layer of iridium 100 is
deposited on the niobium layer 90 by sputtering or vapor deposition,
although electron bean vapor deposition is preferred. The thickness of the
]0 iridium can be varied to produce a sensor having a desired resistance.
Although iridium is preferred, other materials such as platinum can be used
as long as the material selected is stable at high temperatures, has a
reasonable thermal coefficient of resistance in the temperature range of
interest, and is essentially nonreactive with the underlying layer. The
]5 copper layer 50 is removed with an etchant such as ammonium
phersulphate and the substrate 60 containing the sensor 30 is annealed in
a hard vacuum at approximately 1000°C to stabilize the resistance of the
sensor. The hard vacuum is required because iridium slowly forms volatile
oxides at temperatures above approximately 900°C.
20 The sensor 30 is then connected at each end to a corresponding lead
110 which is electrically connected to the driving circuit of the anemometer
(not shown). Any suitable leads can be used as long as the leads do not
protrude above the surface of the sapphire substrate enough to affect the
airflow. For example, organometallic leads could be painted on, or the leads
25 could be deposited by sputtering or vapor deposition in channels formed in
the substrate. The leads can be any electrically conductive material having
suitable resistance and stability at the operating temperature of the
anemometer.
]0
LAR 15112-1 -9- PATENT APPLICATION
The microphotolithographic technique allows for a significant
reduction in sensor size over previous designs, thus improving the spatial
resolution of the probe. The thermal inertia of the sensor is two orders of
magnitude smaller than that of a conventional 5.0 pm diameter hot wire and
existing hot films. Therefore, a significant improvement in frequency
response is expected. Preliminary results indicate a frequency response of
800 kHz via square-wave injection.
Although the present invention has been described in detail with
respect to certain preferred embodiments thereof, it is understood by those
of skill in the art that variations and modifications in this detail may be made
without any departure from the spirit and scope of the present invention, as
defined in the hereto-appended claims.
]5
What is claimed is:
/0
LAR 15112-1 -1_-- PATENT APPLICATION
MICRO-SENSOR THIN-FILM ANEMOMETER
Abstract
5 A device for measuring turbulence in high-speed flows is provided
which includes a micro-sensor thin-film probe. The probe is formed from a
single crystal of aluminum oxide having a 14 ° half-wedge shaped portion.
The tip of the half-wedge is rounded and has a thin-film sensor attached
along the stagnation line. The bottom surface of the half-wedge is tilted
]0 upward to relieve shock induced disturbances created by the curved tip of
the half-wedge. The sensor is applied using a microphotolithography
technique.
10
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Micro-sensor thin-film anemometer

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