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//
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 

New~ort 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 field20 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-todark (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 fieldof-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-of10 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