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All sourcesNASA NTRSDOE OSTI
NASA NTRSDate not supplied

Recent Development Work with Self-Aligned Focusing Schlieren

A novel compact focusing schlieren instrument has recently been developed that eliminates the need for a separate source and cutoff grid. The technique works by projecting an image of a grid onto a retroreflective surface and then reimaging the projection back onto the original grid. By manipulating the polarization state of the light passing through the instrument, a single Rochon polarizing prism can be used to adjust the schlieren sensitivity by translating it along the instrument’s optical axis, which changes the in-plane offset between the reimaged projection and the original grid. This arrangement makes the instrument inherently self-aligned. Relative to conventional focusing schlieren instruments, which require alignment of a separate source grid and cutoff grid to achieve sensitivity, the setup time required in order to obtain high quality focusing schlieren images is significantly reduced. Further, the self-aligned nature of this instrument makes it insensitive to vibrations and allows for on-the-fly adjustments to field-of-view. In this paper we discuss recent work that has been performed to expand the capabilities of this self-aligned focusing schlieren instrument including: schlieren visualization at extreme angles for optically-restricted test facilities, 3D schlieren visualization using a light field camera, acquisition of high quality focusing schlieren images from facilities with low quality windows, and obtaining focusing schlieren images in full-scale ground test facilities.

Brett Bathel↗
NASA NTRS2000

Reflective Focused Schlieren System Improved for Use in 10- by 10-Foot Supersonic Wind Tunnel

The reflective focused schlieren system that was developed for use in the 10- by 10-Foot Supersonic Wind Tunnel (10x10 SWT) at the NASA Glenn Research Center at Lewis Field as part of the Unstart Test Program was improved this past year. In April 1999, the development and use of the system was presented at the Supersonic Tunnel Association International in Bedford, England. A focused schlieren system is similar to a standard schlieren system in that shock waves coming from an object in supersonic flow can be seen using a standard video camera. Unlike the standard schlieren system, which produces a two-dimensional schlieren image, a focused schlieren system can produce a threedimensional image. The preceding drawing shows the components of the reflective focused schlieren system being developed for use in the 10x10 SWT. Although the system worked well for the Unstart Test Program, it was not sensitive enough to be classified as a facility capability. Therefore, a program was implemented to improve the sensitivity of the reflective focused schlieren system so that it could be a facility capability for Glenn's 10x10 SWT. Several techniques were implemented to increase the sensitivity and to improve the overall operation of the system. These included refinement of the source grid, improvement in the cutoff grid production, improvement of the source grid and cutoff grid alignment, installation of an improved light source, and incorporation of an image-enhancing system. These changes are being implemented with the system set up in the laboratory. A checkout test of the system is planned in the 10x10 SWT in March 2000. Of these techniques, the most developed is the refinement of the source grid. The original system had a pattern of -in. by -in. squares on -in. centers (distance between the center points of adjacent squares). This gave a ratio of light to dark (reflective to nonreflective area) of 3:1. The recommended ratio is 1:1. In order to accomplish this, a pattern of -in.-diameter dots on 3/8-in. centers was developed. Preliminary tests with this pattern showed an increase in system sensitivity and image clarity. Further testing and refinement are scheduled. The former and improved dot patterns are shown. The use of glass plates to produce the cutoff grid improved the cutoff grid's quality and helped to align it with the source grid. The cutoff grid, which is the negative of the source grid, is made by exposing photography film (or glass plates) to the proper light while it is installed in the focused schlieren system. The exposed film or plate is removed from the system and then developed as a photograph negative. Because the glass plates are rigid, they eliminate the risk of distortion when the negative is reinstalled in the system and realigned with the source grid.

Williamson, Gary Scott↗
NASA NTRS2021

Compact, Self-Aligned Focusing Schlieren System

A novel compact, self-aligned focusing Schlieren system is presented that eliminates the need for a separate source grid and cutoff grid. A single grid element serves to both generate a projected source grid onto a retroreflective background, and to act as the cutoff grid for the reflected light. This is made possible by manipulating the polarization of the light through the system. The use of only a single grid element eliminates the need to create a cutoff grid that is perfectly matched and scaled to the source grid, and removes the need to align the source and cutoff grids to each other. The sensitivity to density objects is adjustable with the use of a polarizing prism. Images obtained with this system show similar operation to existing focusing Schlieren systems, but with much reduced complexity and setup time. Images taken with acrylic windows placed nor-mal to the optical axis further demonstrate the system’s utility for wind tunnel measurements.

Focusing Schlieren↗
NASA NTRS1992

A wind tunnel application of large-field focusing schlieren

A large-field focusing schlieren apparatus was installed in the NASA Lewis Research Center 9 by 15 foot wind tunnel in an attempt to determine the density gradient flow field of a free jet issuing from a supersonic nozzle configuration. The nozzle exit geometry was designed to reduce acoustic emissions from the jet by enhancing plume mixing. Thus, the flow exhibited a complex three-dimensional structure which warranted utilizing the sharp focusing capability of this type of schlieren method. Design considerations concerning tunnel limitations, high-speed photography, and video tape recording are presented in the paper.

Ponton, Michael K.↗
NASA NTRSDate not supplied

Development of a Self-Aligned Compact Focusing Schlieren System for NASA Test Facilities

A novel compact, inherently self-aligned focusing schlieren system is presented for use in experiments that require flow visualization. Unlike existing focusing schlieren systems that require a separate, and precisely aligned, source grid and cutoff grid to achieve sensitivity to density gradients, this system projects an image of a physical grid element onto a background and then reimages this projection back onto the physical grid element thereby achieving self-alignment. Further, unlike existing focusing schlieren systems where the image of the source grid is physically translated relative to the cutoff grid to achieve sensitivity to density gradients, a polarization prism in the system acts on the polarization state of the light of the projected grid element to provide sensitivity adjustment. By adjusting the position of the prism along the optical axis of the instrument, changes to the sensitivity can be realized. Since the outgoing and returning light used to project an image of the grid element are orthogonally polarized to one another, reflections from windows that may be present between the lens and background of the system can be eliminated. The system can also be implemented in test facilities where optical access is limited and/or exists on only one side. A discussion on different aspects of the development of this technology is provided, as are sample images from the system at the different stages of development. Images that demonstrate the ability to make on-the-fly adjustments to the instrument's field-of-view and working distance are also presented.

Brett F Bathel↗
NASA NTRS1996

Retroreflection Focusing Schlieren System

A retroreflective type focusing schlieren system which permits the light source to be positioned on the optic side of the system is introduced. The system includes an extended light source, as opposed to a point source, located adjacent to a beam splitter which projects light through the flow field onto a reflecting grating in the form of a grid which generates sheets of light that are directed back through the flow field and the beam splitter onto a primary lens behind which is located a cut-off grid having a grid pattern which corresponds to the grid pattern of the reflecting grating. The cut-off grid is adjustably positioned behind the primary lens and an image plane for imaging the turbulence is adjustably located behind the cut-off grid.

Heineck, James T.↗
NASA NTRSDate not supplied

Recent Progress and Development of Self-Aligned Focusing Schlieren

A broad range of developments and applications of self-aligned focusing schlieren (SAFS)systems is presented. The replacement of the Ronchi ruling (RR) with a𝜇LCD was demonstrated with digital SAFS, which provides the ability to modulate the pattern and orientation without physical intervention. Additionally, replacement of the traditional RR with spectral-spatial filters was demonstrated to yield the ability to simultaneously image horizontal and vertical index of refraction gradients using two separate wavelengths. The application of the Scheimpflug principle to SAFS systems enables off-axis imaging and allowed for extended fields-of-view to be captured using a two-camera system. The addition of a plenoptic camera to a baseline SAFS system enables refocusing capabilities, which is demonstrated on two jets offset along the optical axis. Furthermore, application of an event-based camera to SAFS was shown to enable acquisition of sparse, spatio-temporal data. Beyond the listed developments, SAFS was applied to difficult and constraining facilities, such as the National Full-Scale Aerodynamics Complex(NFAC) 80- by 120-ft Wind Tunnel and the Thermal Acoustic Engines (TAE) rig at NASA Ames Research Center and NASA Glenn Research Center, respectively. Behavior of Ronchi rulings, Rochon prisms, and the use of quarter-wave plates versus quarter-wave films are evaluated to provide insight for future experimental design of SAFS-systems.

Self-Aligned Focusing Schlieren↗
NASA NTRS1993

Quantitative results from the focusing schlieren technique

An iterative theoretical approach to obtain quantitative density data from the focusing schlieren technique is proposed. The approach is based on an approximate modeling of the focusing action in a focusing schlieren system, and an estimation of an appropriate focal plane thickness. The theoretical approach is incorporated in a computer program, and results obtained from a supersonic wind tunnel experiment evaluated by comparison with CFD data. The density distributions compared favorably with CFD predictions. However, improvements to the system are required in order to reduce noise in the data, to improve specifications of a depth of focus, and to refine the modeling of the focusing action.

Cook, S. P.↗
NASA NTRS1991

An improved large-field focusing schlieren system

The analysis and performance of a high-brightness large-field focusing schlieren system is described. The system can be used to examine complex two- and three-dimensional flows. Techniques are described to obtain focusing schlieren through distorting optical elements, to use multiple colors in a time multiplexing technique, and to use diffuse screen holography for three-dimensional photographs.

Weinstein, Leonard M.↗
NASA NTRS1994

Aerodynamic Investigation with focusing schlieren in a cryogenic wind tunnel

A flow visualization study was performed using a focusing schlieren system in the 0.3m Transonic Cryogenic Tunnel at NASA Langley Research Center. The system proved to be a useful flow visualization tool for flows as low as M = 0.4. This study marked the first verification of the focusing schlieren technique in a major subsonic/transonic wind tunnel and the first time that high-quality, detailed pictures of high-Reynolds-numbers flows were obtained in a cryogenic wind tunnel. This test was part of a development program to implement instrumentation techniques in cryogenic wind tunnels, with the ultimate aim to use them in the National Transonic Facility (NTF).

Gartenberg, Ehud↗
NASA NTRS1993

Aerodynamic investigation with focusing schlieren in a cryogenic wind tunnel

A flow visualization study was performed using a focusing schlieren system in the 0.3m Transonic Cryogenic Tunnel at NASA Langley Research Center. The design employed proved to be a useful flow visualization tool for flows as low as M = 0.4. This study marked the first verification of the focusing schlieren technique in a major subsonic/transonic wind tunnel, and the first time that high quality, detailed pictures of high-Reynolds number flows were obtained in a cryogenic wind tunnel. This test was part of a development program to implement instrumentation techniques in cryogenic wind tunnels, with the ultimate aim to use them in the National Transonic Facility (NTF).

Gartenberg, Ehud↗
NASA NTRS1993

A sharp-focusing schlieren optical deflectometer

A new instrument capable of localized, nonintrusive turbulence measurements is developed by combining a focusing schlieren system with an optical deflectometer. This instrument records the fluctuating light intensity at a point in the focused schlieren image. Its capability is verified by making benchmark measurements of Kelvin-Helmholtz vortices produced in a low-speed axisymmetric mixing layer. The sharp-focusing effect is demonstrated both visually and quantitatively. The results show that the instrument is capable of optical turbulence measurements within a 4 mm depth-of-field.

Alvi, F. S.↗
NASA NTRS1995

Application of Bacteriorhodopsin Films in an Adaptive-Focusing Schlieren System

The photochromic property of bacteriorhodopsin films is exploited in the application of a focusing schlieren optical system for the visualization of optical phase information. By encoding an image on the film with light of one wavelength and reading out with a different wavelength, the readout beam can effectively see the photographic negative of the original image. The potential advantage of this system over previous focusing schlieren systems is that the updatable nature of the bacteriorhodopsin film allows system adaptation. I discuss two image encoding and readout techniques for the bacteriorhodopsin and use film transmission characteristics to choose the more appropriate method. I demonstrate the system principle with experimental results using argon-ion and He-Cd lasers as the two light sources of different wavelengths, and I discuss current limitations to implementation with a white-light source.

Downie, John D.↗
NASA NTRS1995

Application of Bacteriorhodopsin Films in an Adaptive-Focusing Schlieren System

The photochromic property of bacteriorhodopsin films is exploited in the application of a focusing schlieren optical system for the visualization of optical phase information. By encoding an image on the film with light of one wavelength and reading out with a different wavelength, the readout beam can effectively see the photographic negative of the original image. The potential advantage of this system over previous focusing schlieren systems is that the updatable nature of the bacteriorhodopsin film allows system adaptation. I discuss two image encoding and readout techniques for the bacteriorhodopsin and use film transmission characteristics to choose the more appropriate method. I demonstrate the system principle with experimental results using argon-ion and He-Cd lasers as the two light sources of different wavelengths, and I discuss current limitations to implementation with a white-light source.

Downie, John D.↗
NASA NTRS1991

Preliminary demonstrations of a focusing schlieren system for flow field diagnostics - Research in progress

A preliminary study to demonstrate the potential of the focusing schlieren technique to provide quantitative measurements of a three-dimensional flow field is described. The light intensity distribution in a focusing schlieren plane of view is shown to be related to the density gradients both in the focus plane and in the out of focus planes. The potential to implement digital filtering techniques to compensate for the out of focus features and to enhance the in focus features is discussed.

Doggett, G. P.↗
NASA NTRS1993

Improved Large-Field Focusing Schlieren System

System used to examine complicated two- and three-dimensional flows. High-brightness large-field focusing schlieren system incorporates Fresnel lens instead of glass diffuser. In system with large field of view, image may also be very large. Relay optical subsystem minifies large image while retaining all of light. Facilities candidates for use of focusing schlieren include low-speed wind and water tunnels. Heated or cooled flow tracers or injected low- or high-density tracers used to make flows visible for photographic recording.

Weinstein, Leonard M.↗
NASA NTRSDate not supplied

Recent Progress and Development of Self-Aligned Focusing Schlieren

A broad range of developments and applications of self-aligned focusing schlieren (SAFS)systems is presented. The replacement of the Ronchi ruling (RR) with a𝜇LCD was demonstrated with digital SAFS, which provides the ability to modulate the pattern and orientation without physical intervention. Additionally, replacement of the traditional RR with spectral-spatial filters was demonstrated to yield the ability to simultaneously image horizontal and vertical index of refraction gradients using two separate wavelengths. The application of the Scheimpflug principle to SAFS systems enables off-axis imaging and allowed for extended fields-of-view to be captured using a two-camera system. The addition of a plenoptic camera to a baseline SAFS system enables refocusing capabilities, which is demonstrated on two jets offset along the optical axis. Furthermore, application of an event-based camera to SAFS was shown to enable acquisition of sparse, spatio-temporal data. Beyond the listed developments, SAFS was applied to difficult and constraining facilities, such as the National Full-Scale Aerodynamics Complex(NFAC) 80- by 120-ft Wind Tunnel and the Thermal Acoustic Engines (TAE) rig at NASA Ames Research Center and NASA Glenn Research Center, respectively. Behavior of Ronchi rulings, Rochon prisms, and the use of quarter-wave plates versus quarter-wave films are evaluated to provide insight for future experimental design of SAFS-systems.

Self-Aligned Focusing Schlieren↗