Aero-optically induced blur and loss of irradiance (Strehl loss) associated with imaging from an airborne vehicle can significantly limit sensor effectiveness. Increased image size caused by blurring will lengthen the time needed to accurately determine position, and a large enough Strehl loss can reduce image intensity below sensor threshold. These aero-optical effects are caused by density fluctuations that result from high-speed boundary- and shear-layer turbulence over the surface of the vehicle. The distribution of these density fluctuations determines the wavefront distortion (via refractive index fluctuations) and, therefore, determines the type and severity of the image degradation. The degree to which the image is degraded, in turn, depends upon the magnitude of the turbulent velocity fluctuations (i.e., the turbulent intensity) and the energy distribution of the turbulent eddies (i.e., the eddy spectrum).
This paper deals with the influence of aerodynamic disturbances on the quality of images obtained by an airborne optical system. A wind-tunnel experiment has been carried out in order to simulate aero-optical phenomena in transsonic and supersonic flows. The characterization of such effects is obtained by studying the propagation of a laser beam (λ : 0.63 pa) through optical aerodynamic configurations. The influences of turbulent mixing zone and shock waves have been studied. Each aerodynamical configuration has been qualified by both Schlieren visualization and wall pressure probing test. For each configuration, images are recorded and then processed to obtain point spread functions (PSF), optical transfer functions (OTF), modulation transfer function (MTF) and Wiener spectrum of the images. This study should lead to match optical and aerodynamical considerations in designing future airborne imaging systems.
The characteristics of wavefronts propagated through turbulence and the resulting images are discussed. Hot wire anemometry and holographic interferometry techniques are used to measure wavefront distortion. A model is developed to describe the associated images. The effect of viewing wavelength is examined in detail. A short wavelength limit is found in which the image size is aerodynamically limited and independent of wavelength. The distortion weakens with increasing wavelength until long wavelengths are reached where the image size is diffraction limited. The behavior of imaging systems throughout this wavelength region and its dependence on aero-optic parameters is examined.
Unique holographic measurements of viscous interactions in hypersonic air flows are presented. These results are part of a research project conducted in the 96 inch hypersonic shock tunnel at the Calspan-UB Research Center, Buffalo, New York. Flow visualizations of both 2D flat plate and axisymmetric boundary layer separations induced by compression turns, and 2D flat plate boundary layer separation resulting from an external shock impinging on the boundary layer are shown. Holographic interferometric measurements for the turbulent boundary layer flow upstream of the 2D compression-ramp separation case are presented for a free stream flow with a Reynolds number of 35 million per meter, and a Mach number of 11, nominally. The interferometric measurements for the 2D rectilinear fields are influenced by edged effects and refraction which make accurate calculation of the density distributions difficult. Further, the high compression of the flow in the regions of boundary layer reattachment produce a correspondingly strong optical refraction which obliterates definition of the flow and distinction of the wall. Different ways for correcting the interferometric measurements to account for the edge effects are suggested. Means of coping with the edge effects downstream of separation, and means for improving the distinction of the flow near reattachment seem remote for these studies. Thus far, the flow visualizations are the primary contribution of this holographic application, because they show the qualitative details of the shock structures throughout the separation regions with a clarity that surpasses all previous measurements. The few quantitative results presented here are unique, and while they offer new information as well as a potential to obtain density measurements in other types of hypersonic flows, they reveal concerns which need to be resolved before the interferometric data can be claimed to be accurate measurements of these fluid densities.
A high-speed multipass real-time holographic interferometer is being employed to observe flow patterns of cover gas surrounding a gas tungsten arc welding plasma. This cover gas keeps contaminants from interacting with the molten pool. Commercially available gas cups and original gas cup designs were evaluated in obtaining an optimal flow for the cover gas.
Holographic flow field analysis has been applied in a crystal growth experiment conducted on NASA's space shuttle program on Spacelab-3 during April 29 - May 6, 1985. The experimental holograms taken during the crystal growth process are reconstructed into interferograms and later digitized to give refractive-index fields: A comparison has been made with theoretically computed interferograms.
Series-expansion methods for interferometric tomography of continuous flow fields are discussed. The techniques are based on series expansion by orthogonal polynomials and circular harmonics multiplied by an envelope function. These methods employing continuous basis functions are appropriate for reflecting an peculiar characteristics of interferometric tomography of fluid flow fields, namely, continuity, data sparsity, and nonuniform sampling. The high approximating power of the methods allows accurate representation of fields with a small number of series terms. This generates enough redundancy for a given number of data points in setting up a system of linear algebraic equations. The data redundancy thus generated yields accurate reconstruction even under ill-posed conditions including limited view angle, incomplete projections, and high noise level.
A cold flow characterization and simulation of the turbine film cooling flows has been undertaken to assist analytical modeling of these flows for the calculation of heat transfer. Laser sheet lighting of the flow field, in which TiC14 vapor added to the film cooling flow reacts spontaneously with moist air in the channel flow to form TiO2, has been employed in the visualization. Illumination times of ten nano seconds were used the still photographs. The flows have been illuminated in planes parallel, perpendicular, and at 45° to the plane of film injection. The simulated turbine flows range through rho v ratios of .3 to 3.0. A film injection angle of 30° was used. Turbulence has been added to the free stream with a grid. The film flow interaction with two levels of free stream turbulence approaching are examined.
Thermometry of hardware in high temperature combustion environments may be difficult and challenging to perform. Intrusive sensors, such as thermocouples, can significantly modify the local temperature field. Fatigue life of combustor components is a critical function of temperature. Methods based on the temperature-dependent emission properties of certain phosphors show promise in these situations. The temperature of an object, a variable area diffuser centerbody, immersed in an afterburner flame of a jet engine was measured. To our knowledge, this is the first report of a field application of the thermal phosphor technique in this type of environment. The testing, performed at Arnold Engineering Development Center, revealed that useful temperature measurements can be made. The objectives of this work were: (1) to provide a near-term solution to a thermal monitoring problem associated with jet engine testing, and (2) to investigate the phosphor technique for its potential in solving other envisioned thermal mapping problems in combustion and aerodynamic facilities.
High-speed movies of solid propellant deflagration have long provided useful qualitative information on propellant behavior. Consequently, an extension of performance to include quantitative behavior of the surface, particularly the spatial relationships of particles across the surface, the temporal behavior of particles through extended periods of time, and accurate measurements of particle sizes, is highly desirable. Such measurements require the ability to take detailed movies across an extensive surface through the propellant flame for periods longer than the residence time of a given particle. For such experiments, camera optics employing magnification are undesirable, since they severely limit both the field-of-view and the depth-of-field, and hence, the useful duration of a flame sequence. Unfortunately, high resolution without magnification pushes both the diffraction limits and the performance capabilities of standard lenses. At this limit, the modulation transfer function (MTF) of the camera optics and film will greatly affect performace. High-speed movies of propellant deflagration have been made at pressures up to 500 psi. High resolution at unity magnification is achieved by the use of 1.4 mJ of illumination energy per pulse in conjunction with a fine-grain film. This approach has worked well on both aluminized and pure aluminum perchlorate propellants. Since the laser pulses provide enough light to expose tine-grain film at unity magnification, it is possible to encompass an entire 1/4-inch strand surface in our field-of-view. Motion blur at 7 kHz framing rates and unity magnification is negligible (1 μm) due to the 25 ns width of the laser pulses. The short pulse width is also helpful in circumventing flame turbulence. Front-lit high-speed stereo movies have been made of the burning surfaces of solid propellant strands at operating pressures up to 350 psi. Movies were made at viewing angles separated by 90 degrees to achieve a high-depth resolution. These movies, which were recorded through flames across a 1/4 in. field-of-view, have a resolution of 25 gm. The stereo images were simultaneously recorded side by side on the same 16-mm frame by using X2 demagniying optics and a mirror arrangement.
Temporally and spatially resolved two-dimensional imaging of turbulent reacting flows promises to enhance our knowledge of flame chemistry during ignition, propagation, and quenching. There are a number of active research programs in the combustion imaging diagnostics field, for example in high-speed laser shadowgraph recording, and two-dimensional laser-induced fluorescence imaging. The image data from these experiments is typically used to provide quantitative visualization of species concentrations in propaga-ting flames. Up until now, image sequences have always been interpreted with the aid of fairly simple image display or processing techniques, such as pseudocolor enhancement. Relatively little work has been done in the areas of computer recognition, classification, and interpretation of fluid flow. This paper takes an initial step towards this goal. We address the probem of tracking and displaying the flow of local features which occur at fluid interfaces in laser shadowgraph imagery. Image sequences are computer processed to produce two-dimensional maps showing the motion of the interfaces between hot combustion products and cold reactants gases. This is analogous to a simple form of optical flow map, typically used for describing solid object motion in video sequences. The computer processing is applied in the following stages: (1) unwanted background noise is subtracted from each frame in the sequence , (2) frames are thresholded and thinned to produce a sequence of skeletons representing the essential fluid structure, (3) localized correlation is performed between adjacent frames to produce an optical flow map.
Electro-optical position sensing and position control system techniques have been combined to precisely couple the 2-axis motions of a slave traverse system to the 2-axis motions of a master traverse system. This scheme is used to implement a laser Doppler velocimeter (LDV) optical system in the forward-scatter configuration. The slaved traverse approach is useful for LDV applications in large aerodynamic or aeropropulsion testing facilities where a yoke assembly for diametrically positioning the LDV transmitter and receiver may be impractical. The slaved traverse system has, to date, been used successfully for LDV measurements in a large transonic wind tunnel and a large supersonic wind tunnel at AEDC.