A system and method are provided for recording density changes in a flow field surrounding a moving object. A mask having an aperture for regulating the passage of images therethrough is placed in front of an image recording medium. An optical system is placed in front of the mask. A transition having a light field-of-view and a dark field-of-view is located beyond the test object. The optical system focuses an image of the transition at the mask such that the aperture causes a band of light to be defined on the image recording medium. The optical system further focuses an image of the object through the aperture of the mask so that the image of the object appears on the image recording medium. Relative motion is minimized between the mask and the transition. Relative motion is also minimized between the image recording medium and the image of the object. In this way, the image of the object and density changes in a flow field surrounding the object are recorded on the image recording medium when the object crosses the transition in front of the optical system

Weinstein, Leonard M.

NASA Technical Reports Server

Schlieren system and method for moving objects

A system and method are provided for recording density changes in a flow field surrounding a moving object. A mask having an aperture for regulating the passage of images is placed in front of an image recording medium. An optical system is placed in front of the mask. A transition having a light field-of-view and a dark field-of-view is located beyond the test object. The optical system focuses an image of the transition at the mask such that the aperture causes a band of light to be defined on the image recording medium. The optical system further focuses an image of the object through the aperture of the mask so that the image of the object appears on the image recording medium. Relative motion is minimized between the mask and the transition. Relative motion is also minimized between the image recording medium and the image of the object. In this way, the image of the object and density changes in a flow field surrounding the object are recorded on the image recording medium when the object crosses the transition in front of the optical system

NASA CASE NO. LAB 15Q53_--
PRINT FIG.
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 patent licensing regulations applicable to U.S.
Government-owned inventions (37 CFR 404.1 et 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 informational requests for this invention to the Technology
Applications Team, NASA Langley Research Center, Code 200, Hampton, Virginia
23681-0001.
Address applications for license to the Patent Counsel Office, Mail Stop 212,
3 Langley Boulevard, Hampton, Virginia 23681-0001.
Serial No." 08/388,652
Filed: 02/14/95 LaRC
(NASA-Case-LAR-15053-!) SCHLIEREN N95-21|91
SYSTEM ANO METHO0 FOR'MOVING
OBJECTS Patent Application (NASA.
Langley Research Center) 14 D Unclas
G3/35 0049678
/ l;/' /'! i
2//
https://ntrs.nasa.gov/search.jsp?R=19950020771 2020-06-17T23:51:38+00:00Z
NASA Case No. LAR - 
SCHLIEREN SYSTEM AND METHOD FOR MOVING OBJECTS 
A system and method are provided for recording density changes in a flow field 
surrounding an aerial vehicle in flight. A mask having an aperture for regulating the 
passage of images therethrough is placed in front of an image recording medium. A 
telescope is placed in front of the mask. A transition between a light field-of-view 
and a dark field-of-view is required to be located beyond the aerial vehicle beihg 
examined. This transition can be constructed of a compostition of components in the 
sky or a composition of components located at some distance away on the earth's 
surface. The telescope focuses an image of the transition at the mask such that the 
aperture causes a band of light to  be defined on the image recording medium. The 
telescope further focuses an image of the aerial vehicle through the aperture of the 
mask so that the image of the aerial vehicle appears on the image recording medium. 
Relative motion is minimized between the mask and the transition, and further is 
minimized between the image recording medium and the image of the aerial vehicle. 
In this way, the image of the aerial vehicle and density changes in a flow field 
surrounding the aerial vehicle are recorded on the image recording medium when the 
aerial vehicle crosses the transition in front of the telescope. 
Inventor: Leonard M. Weinstein 
Address: 13  Burke Avenue 
N e w ~ o r t  News. VA 23601 
55:- 
Employer: NASA LaRC 
Initial Evaluator: William Sellers Ill 
Serial No.: 081388,652 
Filed: February 14, 1995 
LAR 15053-1 -1- PATENT APPLICATION
SCHLIERENSYSTEM AND METHOD FOR MOVING OBJECTS
Origin of the Invention
5 The invention described herein was made by an employee of the United
States Government and may be manufactured and used by or for the
Government for governmental purposes without the payment of any
royalties thereon or therefor.
10 Background of the Invention
1. Field of the Invention
15
This invention relates to flow field analysis. More specifically, the
invention is a schlieren system and method for recording density changes
in a flow field surrounding a moving object.
2. Description of the Related Art
20 Since the early days of flight, scientists have worked to examine the
aerodynamic flow field surrounding an aircraft. Typically, scientists are
interested in the effects of turbulence and shockwaves on aircraft.
However, the invisible nature of air has turned such studies into a branch
of research that is carried out with models of the aircraft in wind tunnel
25 environments.
Initially, wind tunnel testing involved injecting streams of smoke into
the wind tunnel in order to study the flow around models of aircraft.
However, smoke visualization techniques proved inadequate for airplane
speeds reaching into the region of compressible flow where shockwaves
LAR 15053-1
begin to form.
changes in the
-2- PATENT APPLICATION
In order to examine compressible flow (as well as any
optical density of any gas or clean liquid), schlieren
photographic systems were developed in which shockwaves (or any density
changes) appear as dark bands extending from the object or vehicle under
5 examination within a wind tunnel.
A basic schlieren system is shown in the diagrammatic view of FIG.
1 where it is referenced generally by numeral 100. In brief, schlieren
system 100 consists of point light source 102 emitting light represented by
rays 103, two lenses 104 and 106 on either side of test section 108 of a
10 wind tunnel (not shown) housing model 109, image recording film 110, and
mask edge 112 located between lens 106 and film 110 for preventing part
of the light passing through lens 106 from impinging on film 110.
Specifically, if the air flow in test section 108 is left undisturbed, lens 106
simply reconstructs an image of point source 102 and an image of model
15 109 for reproduction on film 110. However, if the density of the air moving
about model 109 is disturbed, then some of light rays 103 passing around
model 109 are bent or refracted. In the example shown, mask edge 112
blocks the light that is bent downward as indicated by dashed line 103a
while light that is bent upward continues on. Thus, the ultimate image
20 appearing on film 110 includes an image of model 109 and light and dark
bands extending from the image of model 109 to indicate density changes
in the air flow about model 109. Such schlieren systems are well known
in the art and are disclosed in greater detail by W. Merzkirch, "Flow
Visualization", Academic Press, New York, 1974, pages 86-102.
25 Although producing useful information, wind tunnels are limited since
not all flow features of a full scale aircraft in flight can be simulated. On the
other hand, in-flight measurements have been limited to using clouds of
smoke or tracer particles to either selectively show the outline of an
aircraft's features or as a detection medium for velocimeters.
LAR 15053-1 -3- PATENT APPLICATION
Unfortunately, these in-flight techniques require complex, on-board
instrumentation and are limited to aircraft flying at moderate speeds, e.g.,
subsonic speeds of less than 743 miles per hour.
5 Summary of the Invention
Accordingly, it is an object of the present invention to provide an
apparatus and method for recording density changes in a flow field
surrounding a moving object.
10 Another object of the present invention is to provide an apparatus
and method for recording changes in a flow field surrounding an aircraft in
flight.
Other objects and advantages of the present invention will become
more obvious hereinafter in the specification and drawings.
15 In accordance with the present invention, a system and method are
provided for recording density changes in a flow field surrounding an aerial
vehicle in flight. A mask having an aperture for regulating the passage of
images therethrough is placed in front of an image recording medium. A
telescope is placed in front of the mask. A transition between a light field-
20 of-view and a dark field-of-view is required to be located beyond the aerial
vehicle being examined. This transition can be constructed of a
compostition of components in the sky or a composition of components
located at some distance away on the earth's surface. The telescope
focuses an image of the transition at the mask such that the aperture
25 causes a band of light to be defined on the image recording medium. The
telescope further focuses an image of the aerial vehicle through the aperture
of the mask so that the image of the aerial vehicle appears on the image
recording medium. Relative motion is minimized between the mask and the
transition, and further is minimized between the image recording medium
LAR 15053-1 -4- PATENT APPLICATION
and the image of the aerial vehicle. In this way, the image of the aerial
vehicle and density changes in a flow field surrounding the aerial vehicle are
recorded on the image recording medium when the aerial vehicle crosses
the transition in front of the telescope.
Brief Description of the Drawings
FIG. 1 is a diagrammatic view of a prior art schlieren photographic
system;
10 FIG. 2 is a diagrammatic view of a schlieren system according to the
present invention used to record density changes in a flow field surrounding
a moving object;
FIG. 3 is a diagrammatic view of the imaged band of light seen by the
image recording medium in the schlieren system of the present invention;
15 FIG. 4 is a diagrammatic view of a schlieren system according to a
preferred embodiment of the present invention configured to record density
changes in the flow field surrounding an aircraft in flight; and
FIG. 5 is a plan view of the mask used to selectively block light from
passing to the image recording medium of the present invention schlieren
20 system as well as the image of the sun focused on the mask.
Detailed Description of the Invention
25
Referring now to the drawings, and more particularly to FIG. 2, the
method of the present invention will be explained with the aid of a
diagrammatic view of schlieren system 200. System 200 includes light-to-
dark (or dark-to-light) transition 202, object 204 moving (as referenced by
arrow 206) in air or any clear liquid medium along a path that passes
between transition 202 and optical system 208, and mask 210 having
LAR 15053-1 -5- PATENT APPLICATION
aperture 212 located between optical system 208 and image recording
medium 214.
Transition 202 is any naturally occurring or man-made transition
between light field-of-view 202a and dark field of view 202b. In general,
5 the greater the optical difference between light field-of-view 202a and dark
field-of-view 202b, the higher the overall sensitivity of system 200. As will
become more apparent hereinbelow, depending on the nature of object 204,
transition 202 can be established by means of, for example, a dark pole
with a white stripe painted thereon, an edge of a building or mountain
10 against a daylight sky, or the edge of a light-emitting celestial body (e.g.,
sun, moon, etc.) against the sky. The optical gradient between light field-
of-view 202a and dark field-of-view 202b is ideally as narrow as possible
and must be within constraints to be defined further below.
Object 204 can be any object or vehicle whose movement through
15 air (or clear liquid) is to be studied. Thus, while the present invention will
be explained in greater detail below with respect to an aircraft in flight, one
of ordinary skill in the art will recognize the potential of utilizing the system
and method of the present invention to record density changes in a flow
field surrounding any moving object, e.g., person, automobile, train, bullet,
20 etc.
Transition 202 and the path traversed by object 204 are ideally
oriented as close to perpendicular as possible for maximum sensitivity to
density changes in the flow field surrounding object 204. As the angle
between transition 202 and the path traversed by object 204 deviates from
25 90 ° , the sensitivity to density changes is reduced in accordance with the
law of cosines.
Optical system 208 can be any optical system, e.g., telescope,
capable of focusing an image of transition 202 at mask 210 while focusing
image 204, of object 204 through aperture 212 to image recording medium
LAR 15053-1 -6- PATENT APPLICATION
214. Aperture 212 is configured to pass a portion of light field-of-view
202a and a portion of dark field-of-view 202b so that image recording
medium 214 effectively sees band of light 216b which is shown
diagrammatically in FIG. 3. In FIG. 3, imaged band of light 216b is that
5 portion of light field-of-view 202a passed by aperture 212. Band of light
216b is bounded on one side by dark portion 216a which is created by
mask 210, and further is bounded on its other side by dark portion 216c
which is essentially that portion of dark field-of-view 202b passed by
aperture 212. The transition between band of light 216b and dark portion
10 216c is the image of transition 202. The width W of band of light 216b is
defined by the overall sensitivity of system 200. The optical gradient
between light field-of-view 202a and dark field-of-view 202b must occur
within a distance on the image of transition 202 that is less than W.
To maintain the stability of band of light 21 6b, it may be necessary
15 to provide an apparatus (not shown in FIG. 2) that minimizes relative motion
between transition 202 and mask 210. Factors determining an acceptable
amount of relative motion include the choice of transition, the sensitivity of
system 200 to changes in width W, etc. The ratio of width W of imaged
band of light 21 6b to the focal length F of optical system 208 defines the
20 sensitivity of system 200. (For example, in the preferred embodiment to
be described further below that records density changes in the flow field
surrounding an aircraft in flight, a satisfactory W/F ratio was approximately
0.0002 radians. This provided for the detection of refractive angles as
small as 4 arc seconds.)
25 Image recording medium 214 can be any imaging medium, e.g.,
photographic film, electronic imaging cameras, etc., capable of recording
both image 204, and band of light 216b bounded by dark portions 21 6a and
216c. Regardless of the type of image recording medium used, relative
movement should be minimized between image recording medium 214 and
LAR 15053-1 -7- PATENT APPLICATION
image 204_. Typically, this means moving image recording medium 214 to
track along with the movement of image 204_. Such movement is
referenced in FIG. 2 by arrow 218. In this way, as object 204 crosses
transition 202, and hence band of light 216b, a schlieren image of the flow
5 field about object 204 is generated at image recording medium 214.
The present invention is ideally suited for generating schlieren images
of an aircraft, e.g., airplane, missile, spacecraft, etc., in flight. A
representative set-up for achieving this is shown diagrammatically in FIG.
4 and is referenced generally by numeral 300. The necessary light field-of-
10 view to dark field-of-view is provided by the edge of a light-emitting celestial
body, e.g., sun 302 (or the moon), against sky 301. Aircraft 304 is guided
along a path to cross substantially perpendicular to the edge of sun 302.
Telescope 308 is typically an astronomical telescope equipped with tracking
system control 310 for controlling movement of telescope 308 in
15 accordance with movement of sun 302. One example of such a telescope
having a tracking system is a model C-8 telescope manufactured by
Celestron International, Torrance, CA, which has a solar rate motor drive to
keep the telescope pointed at the sun. Mask 312 having slit 314 (see FIG.
5), and image recording medium 316, are mounted within enclosed housing
20 318 which is optically coupled to telescope 308. Telescope 308 focuses
an image of sun 302 on mask 312 and also focuses an image of aircraft
304 through slit 314. Focusing of telescope 308 can be checked by means
of, for example, a video camera (not shown) trained on image recording
medium 316.
25 As shown in the plan view of FIG. 5, slit 314 in mask 312 is curved
and is aligned such that telescope 308 focuses the edge of image 302_ of
the sun in slit 314. In this way, the necessary imaged band of light (similar
to imaged band of light 216b described above) used by the present
invention is created by the combination of the portion of sky 301 passing
LAR 15053-1 -8- PATENT APPLICATION
through slit 314, the edge of image 302i, and the portion of mask 312
blocking the remainder of the sun's image. The length of slit 314 should be
sufficient to allow acceptable angular tolerance for the path of aircraft 304.
Image recording medium 316 is, for example, a strip of film moved
5 by motor 320 at a speed that is approximately equal (e.g., within 5-10%)
to the speed of the image of aircraft 304 focused by telescope 308 through
slit 314. Note that if there is a large distance between telescope 308 and
aircraft 304, as is typically the case with any ground mounted telescope,
the band of light generated by mask 312 and the image of aircraft 304
10 (passing through slit 314) may have to be separated. Accordingly,
telescope 308 can include relay optical system 322 between mask 312 and
image recording medium 316 in order to separate the band of light
generated by mask 312 and the image of aircraft 304 passed through slit
314.
15 The advantages of the present invention are numerous. Flow fields
can be examined under "in-flight" conditions. Such information can be used
in, for example, studying far-field sonic boom, prototype aircraft
development, etc.
Although the invention has been described relative to a specific
20 embodiment thereof, there are numerous variations and modifications that
will be readily apparent to those skilled in the art in light of the above
teachings. For example, overlapping fields of view could be obtained by
providing several of the present invention systems spaced apart in a line at
an angle (preferably nearly normal) to the path of the aircraft or other
25 moving object. In this way, a large area of the sky could be covered when
an aircraft's path is not accurately known or controllable, or when the flow
field of interest is larger than that viewable by a single system. In addition,
the present invention could be mounted onboard a movable platform, e.g.,
chase plane, and moved to the optimum position for recording images. Still
LAR 15053-1 -9- PATENT APPLICATION
further, the moving image recording medium could take the form of a
processing and playback imaging system. One such system is an electronic
camera operating under "time delay integration" techniques when a moving
image is effectively tracked by scanning sequential rows of pixels. In this
5 way, the length of the image is only limited by computer limitations. It is
therefore to be understood that, within the scope of the appended claims,
the invention may be practiced other than as specifically described.
What is claimed as new and desired to be secured by Letters Patent
of the United States is:
/O
LAR 15053-1 .--1-6_- PATENT APPLICATION
SCHLIEREN SYSTEM AND METHOD FOR MOVING OBJECTS
Abstract
A system and method are provided for recording density changes in
a flow field surrounding a moving object. A mask having an aperture for
5 regulating the passage of images therethrough is placed in front of an image
recording medium. An optical system is placed in front of the mask. A
transition having a light field-of-view and a dark field-of-view is located
beyond the test object. The optical system focuses an image of the
transition at the mask such that the aperture causes a band of light to be
10 defined on the image recording medium. The optical system further focuses
an image of the object through the aperture of the mask so that the image
of the object appears on the image recording medium. Relative motion is
minimized between the mask and the transition. Relative motion is also
minimized between the image recording medium and the image of the
15 object. In this way, the image of the object and density changes in a flow
field surrounding the object are recorded on the image recording medium
when the object crosses the transition in front of the optical system.

216a
W
216b
Fig.3 216c
301
310
TRACKING
SYSTEM
CONTROL
312
/- 320
MOTOR I
Fig.4
300
312
314 302i
Fig.5


Schlieren system and method for moving objects

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server

NASA Technical Reports Server