Results of a feasibility study to apply laser Rayleigh scattering to nonintrusively measure flow properties in a small supersonic wind tunnel are presented. The technique uses an injection seeded, frequency doubled Nd:YAG laser tuned to an absorption band of iodine. The molecular Rayleigh scattered light is filtered with an iodine cell to block light at the laser frequency. The Doppler-shifted Rayleigh scattered light that passed through the iodine cell is analyzed with a planar mirror Fabry-Perot interferometer used in a static imaging mode. An intensified CCD camera is used to record the images. The images are analyzed at several subregions, where the flow velocity is determined. Each image is obtained with a single laser pulse, giving instantaneous measurements.

A nonintrusive Moire interferometry system has been designed to acquire the instantaneous deformation of models during wind tunnel testing. The resulting interferograms are evaluated without manual intervention using a technique based on the Fourier Transform. The deformation of a large- scale model wing (generic model of a transport aircraft with a full span of 3.40 m) has been measured in the 8 m X 6 m test section of the subsonic Deutsch-Niederlandischer-Windkanal (DNW). At 400 individual locations along the span, the bending and twist deformations of the wing have been measured with an average accuracy of +/- 0.1 mm and +/- 0.03 degree(s), respectively. In a second experiment, the bending angle of a flap of a hypersonic vehicle was measured in order to determine the hinge moment due to aerodynamic loads. The experiment was carried out in the transonic wind tunnel Gottingen (TWG). To obtain data for comparison, the hinges were equipped with strain gauges. The results of both techniques show a maximum deviation of 0.02 degree(s).

Measurement of an instantaneous flow field by interferometric tomography, that is, reconstruction of a 3D refractive-index field from multidirectional projection data, has ben conducted. In order to simulate the expected experimental arrangement at a wind tunnel, reconstructions are made from a restricted view angle less than 40 degrees and incomplete projections. In addition, appreciable ambient air and experimental setup disturbances are present. A new phase-stepping technique, based on a generalized phase-stepping approach of a four- bucket model, is applied for expeditious and accurate phase information extraction from projection interferograms under the harsh environments. Phase errors caused by the various disturbances, which can include ambient refractive-index change, optical component disturbance, hologram repositioning error, etc., are partially compensated with a linear corrective model. A new computational tomographic technique based on a series expansion approach was also utilized to efficiently deal with arbitrary boundary shapes and the continuous flow fields in reconstruction. The results of the preliminary investigation are encouraging; however, the technique needs to be further developed in the future through refinement of the approaches reported here and through hybridization with previously developed techniques.

Particle Image Velocimetry (PIV) has been used successfully for measuring instantaneous two dimensional velocity fields. Analyzing PIV images involves matching particle images captured sequentially. In the usual practice, correlation (auto- or cross-correlation) is used to find the displacement (hence velocity) of the particles within a large number of small 'interrogation areas' in the field of view. An image correlation within an interrogation area is inherently sensitive to the rotation and distortion of the fluid flow within the area, and is also sensitive to the brightness of particles. It can only find the mean spatial offset of particles weighted by their brightness. Rotation, dilation and distortion of fluid flow within the interrogation region and intensity changes of particles introduced correlation errors or bias. As a consequence, the size of the interrogation area and the time interval between images must be kept small to avoid these problems. A feature-recognition method is proposed here for analyzing PIV images. It first extracts structural features of the particle pattern after their locations have been isolated from images. A preliminary process is to replace the particle images by the Cartesian coordinates of particle centers. In this way the brightness of particle images plays no further part, and the point positions are used to establish structural features: topological relations between each point and its neighbors. The interrogation area is defined by a limited number of neighboring points. The size and shape of each interrogation area varies with the distribution of neighbors. A fit to motion, rotation and distortion among the neighbors is then carried out in the space of topological relations of successive images. In this way changes of structural features define fluid spatial translation, rotation, and deformations within each interrogation region. Measurement of feature space in two successive images demands knowledge of the locations of corresponding points derived from individual particles in the two images. Classification of point correspondences, despite confusingly discordant displacements from one image to the next, can be made by taking advantage of physical limitations on the possible movement of particles between the two images. It is found that feature space search and correlation is a much more efficient procedure than correlation operations in the two dimensional image domain.

Particle Image Velocimetry provides a means of measuring the instantaneous 2-component velocity field across the planar region of a seeded flow field. In this work only two camera, single exposure images are considered where both cameras have the same view of the illumination plane. Two competing techniques which yield unambiguous velocity vector direction information have been widely used for reducing the single exposure. multiple image data: cross-correlation and particle tracking. Correlation techniques yiedl averaged velocity estimates over subregions of the flow, whereas particle tracking technique give individual particle velocity estimates. The correlation technique requires identification of the correlation peak on the correlation plane corresponding to the average displacement of particles across the subregion. Noise on the images and particle dropout contribute to spurious peaks on the correlation plane, leading to misidentification of the true correlation peak. The subsequent velocity vector maps contain spurious vectors where the displacement peaks have been improperly identified. Typically these spurious vectors are replaced by a weighted average of the neighboring vectors, thereby decreasing the independence of the measurements. In this work fuzzy logic techniques are used to determine the true correlation displacement peak even when it is not the maximum peak on the correlation plane, hence maximizing the information recovery from the correlation operation, maintaining the number of independent measurements and minimizing the number of spurious velocity vectors. correlation peaks are correctly identified in both high and low seed density cases. The correlation velocity vector map can then be used as a guide for the particle tracking operation. Again fuzzy logic techniques are used, this time to identify the correct particle image pairings between exposures to determine particle displacements, and thus velocity. The advantage of this technique is the improved spatial resolution which is available from the particle tracking operation. Particle tracking alone may not be possible in the high seed density images typically required for achieving good results from the correlation technique. This two staged approach offers a velocimetric technique capable of measuring particle velocities with high spatial resolution over a broad range of seeding densities.

Streaked particle imaging velocimetry and sizing (SPIVS), an image based system for planar instantaneous measurement of two-component droplet velocity and size is demonstrated in the spray of a simplex atomizer. The SPIVS technique upgrades traditional double light pulse particle imaging techniques by overlaying particle streakings on the particle image pairs. The advantages of this system are the enhanced reliability of the droplet pairing process, the identification of the time sequence between images of a droplet image pair, and the ability to measure the size of individual droplets from the recorded scattered light energy. Measurements of droplet size and velocity in the solid cone spray of a simplex atomizer were conducted with both SPIVS and phase Doppler particle analyzer (PDPA) techniques. The results generated by these two systems are presented and discussed.

In its 'classical' form particle image velocimetry (PIV) extracts two components of the flow velocity vector by measuring the displacement of tracer particles within a double-pulsed laser light sheet. The method described in this paper is based on the additional recording of a third exposure of the tracer particles in a parallel light sheet, which is slightly displaced with respect to the first one. The particle images resulting from these three exposures are stored on separate frames. The locations of the correlation peaks, as obtained by cross-correlation methods, are used to determine the projections of the velocity vectors onto the plane between both light sheets. In the manner described below, the amplitudes of these peaks are used to obtain information about the velocity component perpendicular to the light sheet planes. The mathematical background of this method is described in the paper. Numerical simulations show the influence of the main parameters (e.g. light sheet thickness, light sheet displacement and out-of-plane component) on the resolution and reliability of the new method. Two different recording procedures and their results will be shown to demonstrate the ease of operation when applying this technique to liquid flows.

Using experimental data from Particle Image Velocimetry (PIV) measurements, coherent structures of a transitional, spatially developing boundary layer are determined. The coherent structures are calculated utilizing the Proper Orthogonal Decomposition (POD), which is based on an expansion of the flow field variables into so-called Karhunen-Loeve eigenfunctions. To get reproducible flow conditions we excite the flow with controlled disturbances by means of periodic velocity fluctuations. We are able to introduce two- and three- dimensional traveling waves. Therefore it is possible to investigate different transition scenarios. This paper focuses on the oblique transition. Phase locked signals allows to record the flow field at certain instants of time. We can show that PIV is a suitable technique to provide experimental data for POD. The POD shows that already a small number of modes cover most of the energy.

A number of PIV (Particle Image Velocity) measurements have been made at transonic speeds. The initial objective of the work has been to explore if such measurements could be made remotely and processed in an accurate and automatic fashion. Subsequently PIV measurements have been made remotely at optical stand-off distance of up to 1 m. PIV results are presented, made at ARA Bedford on a 1/12 th scale model of an Airbus wing, where a separation induced shock has been measured in three dimensions. The paper also presents results achieve using PIV at DRA Pyestock in mapping the flow within a full size annular turbine cascade with a velocity measurement accuracy of 1%. Measurements are presented showing the wake measured downstream of a annular turbine stator row. The measurements clearly show the presence of a vortex street which has a width of approximately 3 mm.

We employ Forward Scattering Particle Image Velocimetry (FSPIV) to measure all three components of the velocity of a buoyant polystyrene particle in oil. Unlike conventional particle image velocimetry (PIV) techniques, FSPIV employs coherent or partially coherent back illumination and collects the forward scattered wavefront; additionally, our field-of-view is microscopic. Using FSPIV, it is possible to easily identify the particle's centroid and to simultaneously obtain the fluid velocity in different planes perpendicular to the viewing direction without changing the collection or imaging optics. We have trained a neural network to identify the scattering pattern as function of displacement along the optical axis (axial defocus) and determine the transverse velocity by tracking the centroid as function of time. We present preliminary results from Mie theory calculations which include the effect of the imaging system. To our knowledge, this is the first work of this kind; preliminary results are encouraging.

A high-speed digital camera based on video technology for application of particle image velocimetry in wind tunnels is described. The camera contains two independently triggerable interline CCD sensors which are mounted on two faces of a cube beam splitter permitting the use of a single lens. Each of the sensors has a minimal exposure time of 0.8 microsecond(s) with a trigger response time of less than 1 microsecond(s) . The asynchronous reset capability permits the camera to trigger directly off a pulsed laser with a repetition rate differing from the standard 25 Hz CCIR video frame rate. Captured images are digitized within and stored in RAM the camera which can be read through the parallel port of a computer. The camera is software configurable with the settings being non-volatile. Technical aspect such as sensor alignment and calibration through software are described. Close-up PIV measurements on a free jet illustrated that, in the future, the camera can be successfully utilized at imaging high-speed flows over a small field of view covering several cm², such as the flow between turbine blades. Further, the electronic shutter permits its use in luminous environments such as illuminated laboratories, wind tunnels or flames.

In the cooling of surfaces jet impingement arrays have been found to provide effective surface heat transfer. Considerable work has been done in identifying the optimal jet array geometry, including jet diameter, spacing and relative distance to the surface to be cooled. Most all of these studies rely on surface averaged heat transfer results. However, there are applications where the local distribution of the impingement heat transfer is important. The magnitude of the local variations may cause serious problems in terms of surface temperature gradients. Thermochromic liquid crystals provide a means to directly measure the surface temperature which can be used to study the local heat transfer coefficient distribution. Both steady state and transient methods have been identified. The steady state method is a direct application of Newton's Law of Cooling. The transient method establishes a step change in the surface boundary condition and solves the conduction problem in the surface substrate. This method can have advantages of lower experimental uncertainty. However, there are practical issues of time response that need to be addressed to determine actual local heat transfer coefficient. This paper addresses the issues associated with the transient method and provides results of impingement cooling. Of primary concern is the transient response and how that is related to the actual instantaneous convective condition at the surface. Results show a nonsteady convective coefficient which must be corrected based on the experimental design parameters.

Forward-scattering degenerate four-wave mixing is presented as a sensitive nonlinear laser-based absorbance detection method for room- temperature condensed-phase analytes using compact low-power lasers. In the liquid phase, the signal is generated mainly by the formation of spatial gratings due to thermally-induced refractive index change, resulting from constructive interference between the input beams. This nonlinear laser method offers convenient and efficient optical signal detection since the signal is a coherent beam and it can be collected and measured virtually against a dark background. Since only two input beams are used, the optical alignment is simple compared to other multiphoton methods. The use of a single lens for all the input beams provides tighter focusing and higher wave-mixing efficiency and maximizes photon density available at the sample cell. Hence, laser power requirements are unusually low (<10 mW), allowing the use of portable, low-cost lasers such as He-Ne lasers and diode lasers. Since only a single laser is required, the overall optical setup in this one- color one-laser method can fit in a simple compact package with minimum laser and optics requirements. The detection sensitivity approaches those of laser fluorescence methods, yet this compact nonlinear absorbance detector can detect both fluorescing and nonfluorescing analytes.

The utilization of liquid crystal thermography to study heterogeneous boiling phenomena has gained popularity in recent years. In order not to disturb the nucleation process, which occurs in the microstructure of the heating surface, the crystals are applied to the backside of a thin heater. This work critically examines the ability of liquid crystal thermography to quantitatively capture the thermal field on the boiling surface. The thermal field identified experimentally through liquid crystal thermography is compared against that computed in the vicinity of a growing vapor bubble using a simulation which considers the simultaneous heat transfer between three phases: the solid heater, the liquid microlayer, and the growing vapor bubble. The temperature history beneath a growing vapor bubble elucidates the high frequency response required to capture the transient thermal fields commonly encountered in boiling experiments. Examination of the governing equations and numerical results reveal that due to the heater thermal inertia, the temperature variation on the bottom of the heater is significantly different than that on the boiling surface. In addition, the crystals themselves have a finite spatial resolution and frequency response which filter out much of the microscale phenomenon associated with boiling heat transfer. Analysis of existing pool and flow boiling liquid crystal thermographs indicate that the typical spacial resolution is on the order of 0.25 mm and the response time is on the order of 5 ms which are insufficient to resolve the fine spacial and temporal details of the hating surface thermal field. Thus the data obtained from liquid crystal thermography applied to boiling heat transfer must be cautiously interpreted.

The initialization of a flow field with distinct and spatially segregated scalar components represents a significant experimental difficulty. Many theoretical modeling efforts in turbulent mixing, however, seek to describe the temporal evolution of a scalar concentration field that begins with this type of idealized initial conditions experimentally. This technique uses photoactivatable (caged) fluorescence dyes dissolved in the flow medium. Caged fluorescent dyes differ from tradition dyes in that excitation and subsequent emission will not occur until a bond within the caged dye molecule is broken with an ultraviolet photon. The flow field is then tagged by activating or 'uncaging' the appropriate regions with an excimer laser. Mixing between the tagged and untagged regions is quantified by illuminating the points to be studied with an argon ion laser, and measuring the subsequent emission intensity using standard laser induced fluorescence techniques. The intensity of the emission is proportional to the concentration of the uncaged dye. High sensitivity photodetectors allow very low intensity fluctuation measurements to be made. This method is currently being used to study mixing in a turbulent pipe flow, and shows potential to be used in a large number of other flow situations.

A Ronchi system has been designed and built into the STDCE-2 flight hardware to measure deformations of an oil free surface in microgravity under the influence of localized heating. The system design conforms to packaging and shuttle constraints and satisfies all Science requirements. During End-to-End testing, the system demonstrated its value in providing valuable science data about the physics of the experiment.

According to the mechanism of plasma shielding laser energy, the plasma shielding has been studied experimentally by using optical shadowing synchronous photography. The development of the vapor plasma produced by a Q-Switched Nd:YAG laser upon Aluminum target has been investigated, and the optical thickness of the plasma has been measured with light intensity delay-time comparing method.

A lateral shearing interferometer is used to measure the slope of perturbed wavefronts after propagating through turbulent shear flows. This provides a two-dimensional flow visualization technique which is nonintrusive. The slope measurements are used to reconstruct the phase of the turbulence-corrupted wave front. Experiments were performed on a plane shear mixing layer of helium and nitrogen gas at fixed velocities, for five locations in the flow development. The two gases, having a density ratio of approximately seven, provide an effective means of simulating compressible shear layers. Statistical autocorrelation functions and structure functions are computed on the reconstructed phase maps. The autocorrelation function results indicate that the turbulence-induced phase fluctuations are not wide-sense stationary. The structure functions exhibit statistical homogeneity, indicating the phase fluctuation are stationary in first increments. However, the turbulence-corrupted phase is not isotropic. A five-thirds power law is shown to fit one-dimensional, orthogonal slices of the structure function, with scaling coefficients related to the location in the flow.

This paper investigates the validity of applying the simplified (under the assumptions of isotropic and homogeneous turbulence) aero-optic linking equation to a flowfield that is known to consist of anisotropic and nonhomogeneous turbulence. The investigation is performed in the near nozzle-region of a heated two-dimensional jet, and the study makes use of a conditional sampling experiment to acquire a spatio-temporal temperature field data base for the heated jet flowfield. After compensating for the bandwidth limitations of constant-current-wire temperature measurements, the temperature field data base is applied to the computation of optical degradation through both direct methods and indirect methods relying on the aero-optic linking equation. The simplified version of the linking equation was found to provide very good agreement with direct calculations provided that the length scale of the density fluctuations was interpreted as being the integral scale, with the limits of the integration being the two first zero crossings of the covariance coefficient function.

A bifurcated shock tube is used to create two synchronized waves of equal strength. Essentially a single shock wave is split symmetrically in two, the two waves then are later brought back together at a trailing edge of a wedge to interact, the plane of symmetry acting as an ideal rigid wall. The normal method of studying Mach reflections is to allow a plane shock wave to impinge on a wedge, however the boundary layer growth on the wedge surface effectively ensures that the flow direction behind the Mach stem does not have to satisfy the boundary condition of being parallel to the surface of the wedge. Thus the transition from regular to Mach reflection occurs at higher angles of incidence than theory allows. The present experiment was initiated to generate data on the 'ideal' case of reflection off a plane wall. The advantage of the new system is that like classical theory and computational solutions of the inviscid Euler equations, the boundary layer no slip condition is not imposed at the plane of reflection. Optical methods ar used to investigate the post-shock flows, as well as to help explain the complex interactions which occur when the two shock waves are not synchronized. These interactions show many very interesting features and clearly indicate the need for higher resolution measurements such as are obtained using holographic interferometry, and also to extend the work to different wedge angles and Mach numbers.

A set of polynomials is derived to represent surface deformations and experimentally observed Ronchigrams for the Surface Tension Driven Convection Experiment-2 (STDCE-2). The observed deformations are steady state and oscillatory. Steady state deformations are described by radially symmetric polynomials. Oscillatory deformations are described by a modified cylindrical Fourier series. Both satisfy specific experimental boundary conditions.

A computational tomographic technique, termed the variable grid method (VGM), has been developed for improving interferometric reconstruction of flow fields under ill-posed data conditions of restricted scanning and incomplete projection. The technique is based on natural pixel decomposition, that is, division of a field into variable grid elements. The performances of two algorithms, that is, original and revised versions, are compared to investigate the effects of the data redundancy criteria and seed element forming schemes. Tests of the VGMs are conducted through computer simulation of experiments and reconstruction of fields with a limited view angel of 90 degree(s). The temperature fields at two horizontal sections of a thermal plume of two interacting isothermal cubes, produced by a finite numerical code, are analyzed as test fields. The computer simulation demonstrates the superiority of the revised VGM to either the conventional fixed grid method or the original VGM. Both the maximum and average reconstruction errors are reduced appreciably. The reconstruction shows substantial improvement in the regions with dense scanning by probing rays. These regions are usually of interest in engineering applications.

Flow velocities measured in the near-wake of bluff bodies contain information of mean flow, turbulent fluctuations and vortex motions. Flow parameters like RMS values of velocity fluctuations, Reynolds shear stresses, and auto- / spatial correlation of velocity fluctuations, derived by direct averaging of measured data, will include the total contributions of the periodic vortex motions and the random turbulent fluctuations. In the case of near-wake flows behind a cylinder at sub- critical Reynolds numbers, a vortex-street is formed in the wake and the vortex-shedding frequency is well defined. This offers a possibility for simplifying the decomposition or regular vortex motions and the random turbulent fluctuations. Velocity profiles and cross-stream spatial correlation of streamwise velocity were measured by positioning a TSI's two-component LDA system in the region 2d to 4d downstream of a circular cylinder at Re equals 35,000. The randomly sampled data were first interpolated linearly, and then resampled with a pertinent sampling frequency. The optimal FIR filters, designed using the Remez exchange algorithm, were applied to reject the digital signals in a narrow band around the vortex-shedding frequency. Various flow parameters associated with random turbulence were computed. The integration of the auto- and spatial correlation, obtained based on the filtered data gave a physically adequate estimation of the integral time and length scales of the turbulent fluctuations. Errors due to linear interpolation and filter were discussed.

Signal processing in Laser Doppler Anemometry (LDA) and Phase Anemometry (PDA) is carried out in both, the time domain, for instance by counter processors, and the frequency domain by processors based on the Fast Fourier Transform (FFT). FFT based signal processing allows a frequency and phase determination even for rather low signal to noise ratios. Nevertheless, the time-varying amplitude, frequency, and phase difference of Doppler bursts contain relevant information which is generally not utilized by conventional signal analysis. By determining the bust envelopes of PDA bursts it is possible to improve the resolution of particle sizing which is restricted to a corresponding phase difference from 0 to 360 degrees when conventional FFT processing is used. Time-resolved phase difference and frequency determination allow an improved signal validation since effects like trajectory effects, particle coincidences within the measuring volume, droplet oscillations or non-spherical particle characteristics can be detected. In this contribution signal processing methods for determining such effects are introduced. Thee methods will offer improved signal processing facilities with respect to the above mentioned topics. Practical applications are demonstrated by simulation and experimental results.

This paper deals with error analysis of experimental data of a laser speckle photography (LSP) application which measures a temperature field of natural convection around a heated cylindrical tube. A method for error corrections is proposed and presented in detail. Experimental and theoretical investigations have shown errors in the measurements are induced due to four causes. These error sources are discussed and suggestions to avoid the errors are given. Due to the error analysis and the introduced methods for their correction the temperature distribution, respectively the temperature gradient in a thermal boundary layer can be obtained more accurately.

The use of holography for viewing inside irregular media, ranging from highly scattering, or translucent, to aberrated systems, was proposed 30 years ago. Since then, an enormous amount of research has been done for this application. The holographic techniques for viewing objects, moving or stationary, in such irregular media.

The study of an underexpanded compressible jet using pulsed, phase- shifted interferometry in conjunction with a 9 beam tomographic illumination system is described. A plane wave holographic interferometer using a pulsed ruby laser has been adapted to provide multiple illumination directions of a volume that is approximately 4 centimeters on a side. This set-up is being used to study the transient behavior of compressible jets and may be operated using double-exposure holographic interferometry to study the instantaneous behavior of the flow; alternatively, the system may be operated in a double-pulse mode to study the fluctuations in the flow. The tomographic reconstructions are made using a Fourier-Bassel expansion. To illustrate the performance of the system, an underexpanded nozzle flow producing a series of compression-rarefaction 'diamonds' was studied. The images show sharp reconstructions of the compression-rarefaction diamond pattern as well as some interesting secondary flow structures.

A two-dimensional nonlinear regression method for accurate analysis of single-frame interferograms has been developed and tested. Similar to a simple algorithm for one-dimensional regression, analytical expressions of individual terms in a nonlinear intensity model are estimated through an iterative procedure. Computer simulation of experiments and real interferogram analysis show stable convergence and accurate phase extraction of the method. The method also works well under relatively high-level noise and broken fringe interferograms.

Application of phase-shift holographic interferometry to nonintrusive temperature measurement of a laminar, axisymmetric, propane diffuse flame is investigated in this work. In our previous work the effect of composition changes on the holographic temperature measurement of reacting flows was experimentally studied for the cases of the axisymmetric, propane, laminar premixed lean (PLF) and partially premixed flame (PPF). To further examine this effect, in this work the effect for the case of diffuse flame (DF) is investigated. It has been shown that for the DF, the composition distribution is most complicated and the composition effect is severest comparing to the PLF and PPF. In order to obtain accurate and satisfactory holographic results, it is necessary to measure the species concentration at lower flame heights, and use linear interpolation of the molar refractivity at higher test sections.

The optical method of imaging fluid surface gradient is extended to measure multiple fluid interfaces of refractive index changes. The density of a fluid is a function of the fluid's refractive index. This method may be applied to the study of such flows that are associated with density changes caused by either compressibility, thermal effects, or mixing of fluids of different densities. A lens is used to transform the rays of an optical light source into a series of colored parallel light beams by passing the light through a slide of 2D color patterns at the focal planes of the lens. This system of parallel light beams is used to illuminate through the test section of a fluid volume. The out- going rays of refracted light of different directions are instantaneous recorded by multiple cameras located far away from the test section. The different camera captures different light rays. To observed colors of rays provide measurements of the total volume refraction of the rays. According to ray tracing equations, the images of total fluid volume refraction are then used to estimate the fluid isopycnal surfaces. The color tomography proposed here inverts fluid isopycnal surfaces through the measurements of integrated volume refraction rather than the traditional radiological tomography which studies the body properties through measuring the absorption in the ray-path, or seismic and ocean acoustic tomography which inverts the properties of earth or ocean water body by detecting the travel times of rays between sources and receivers.

The schlieren technique involves the manipulation of knife edges to limit the field of view in the image formation process. Practical implementation of this technique is not easy due to difficulties in positioning the knife edge in the optical system. A related problem concerns the reproducibility of an event to generate a series of schlieren images for different knife edge positions. A particularly successful method to overcome this problem is the use of the computer to generate such images from single pictures of the event. Computer generation of schlieren images involves the inverse Fourier transformation of the modified complex-valued diffraction pattern (magnitude and phase) of the event. Recording media in general respond only to light intensity and no difficulty is encountered in recording the intensity, and therefore the magnitude. The phase is either unobservable directly or cannot be determined anywhere nearly as accurately as the intensity. The Gerchberg and Saxton interative algorithm is used to recover the phase from records of intensity (magnitude) taken from the image and Fourier domains of the optical system. The knowledge of magnitude and phase in the Fourier domain (diffraction pattern) will enable us to modify it through a computer knife edge and generate the corresponding schlieren images.

The spatial profile of fluid flow velocity in transparent glass conduits is measured using optical Doppler tomography (ODT). The flow velocity at any spatial location in the conduits is determined by measuring the Doppler shift of backscattered light from moving microspheres in the fluid. ODT is an accurate and inexpensive method for high resolution characterization of fluid flow velocity profile.

Optical diagnostics are extremely useful in fluid mechanics because they generally have high inherent bandwidth, and are nonintrusive. However, since optical probe measurements inherently integrate all information along the optical path, it is often difficult to isolate out-of-plane components in 3D flow events. It is also hard to make independent measurements of internal flow structure. Using an arrangement of 1D wavefront sensor, we have developed a system that uses tomographic reconstruction to make 2D measurements in arbitrary flow. These measurements provide complete information in a plane normal to the flow. We have applied this system to the subsonic free jet because of the wide range of flow scales available. These measurements rely on the development of a series of 1D wavefront sensors that are used to measure line-integral density variations in the flow of interest. These sensors have been constructed using linear CCD cameras and binary optics lenslet arrays. In designing these arrays, we have considered the coherent coupling between adjacent lenses and have made comparisons between theory and experimental noise measurements. This paper will present examples of the wavefront sensor development, line-integral measurements as a function of various experimental parameters, and sample tomographic reconstructions.

The hybrid operation of digital image processing and a knowledge-based AI system has been recognized as a desirable approach of the automated evaluation of noise-ridden interferogram. Early noise/data reduction before phase is extracted is essential for the success of the knowledge- based processing. In this paper, new concepts of effective, interactive low-level processing operators: that is, a background-matched filter and a directional-smoothing filter, are developed and tested with transonic aerodynamic interferograms. The results indicate that these new operators have promising advantages in noise/data reduction over the conventional ones, leading success of the high-level, intelligent phase extraction.

Simultaneous quantitative measurements of both the temperature and velocity fields for 2D transient natural convection in an enclosure are made using calibrated multichannel electronic interferometry and digital particle image velocimetry. Calibrated multichannel electronic interferometry, a technique for quantitative flow visualization of transient phenomena, is discussed. This technique uses an interferometer combined with diffraction gratings to generate the three phase shifted interferograms simultaneously. The optical system is calibrated using standard piezoelectric phase shifting. The alignment is determined using spatial crosscorrelation. The calibration and alignment routines account for errors due to the separation of the phase shifted interferograms. Digital particle image velocimetry is implemented simultaneously with the interferometric measurements using a separate video channel. The system is used to examine the development of transient natural convection in enclosures at three angles of inclination.

A new approach to the interferometric and other optical techniques in combustion research is under discussion. It relies on the direct employment of the property of integrality of the techniques. This approach makes it possible to assess characteristics of the burning wave which describe the gaseous phase of burning (or combustion process as a whole) taken in its entirety, and those averaged over the gaseous phase mass, its volume or sections of the gaseous phase flow. They are evaluated without solving inverse problems of optics, which have to be solved with a number of current optical methods applied in a conventional manner to the quantitative studies of physical processes. As far as interferometry is concerned, the evolution of the approach has eventually enabled the solving of problems in a wide variety of tasks of the combustion, gas dynamic and heat transfer research, which are otherwise difficult to solve experimentally. Also the paper deals with a new approach to the traditional problem of quantitative interferometry, i.e. to the definition of flame temperature and density fields. In this approach any change in the flame gas mixture composition is taken into consideration for a variety of gaseous and condensed systems based on general principles.

While the classical methods of shadowgraph and schlieren do yield a shadow in the neighborhood of a shock, they often suffer from low power densities and the need for relatively long distances. Scanning methods may help in solving these problems. The paper describes various scanning techniques, presents experimental data obtained by mechanical scanning, and identifies conditions at which the data were taken.

STDCE-2 is scheduled to generate approximately 12 hours of steady-state and oscillatory free-surface deformation Ronchigram data. During the mission, this data will be recorded to video tape for subsequent analysis. Because of the massive amount of data, the analysis software must be highly automated. This paper reviews the data analysis requirements and process implementation.

A time division multiplexed laser Doppler anemometer is reported, using a single high frequency pulsed laser diode as a source. Time division multiplexing requires a single detector channel, removing the need for multiple detectors and wavelength separation optics found in conventional CW laser Doppler systems. By incorporating optical fibers into the system to distribute the pulses into each channel and impose a delay between channels, the electronic requirements of such an instrument are reduced. Results for a 2D system are presented, measured on a water-seeded air jet. Individual velocity components of up to 16 ms^-1 and overall velocities of up to 20 ^-1 have been detected, but the potential range of the instrument is very much greater.

The expansion of optical sensors and the interest of the sol-gel process for the working out of new materials, have led us to develop a sensor applied to cryogenic temperature measurement in harsh environments. The measurement technique uses the decay-time of the luminescence emitted by doped crystals, in response to a short duration excitation pulse. The principle of the measurement has already been demonstrated by a prototype with crystals under bulk shape, and we present here a non contact version of the sensor, using fluorescent layers deposited on mechanical parts. These layers are composed of photoluminescent crystals reduced to powder and mixed to a binder. The originality of this work is that the binder is a silica-based hybrid gel. Such a gel has a good adherence to metallic substrates in cryogenic mediums and it can be used in chemically oxidizing environments. The fluorescence decay-time technique associated with the sol-gel process can then provide an interesting alternative in the development of new noncontact optical fiber sensors, working in hostile conditions.

A compact reference beam laser Doppler velocimeter, constructed using a semiconductor laser diode, optical fiber components and semiconductor detectors, is reported. The device has been designed to overcome many of the problems commonly associated with reference beam configurations. The anemometer may be operated with the laser diode operation in cw and pulsed modes, demonstrating its applicability to wavelength and time division multiplexing schemes for 3D laser Doppler velocimetry. The probe is used to measure the velocity of a spinning disk in the range -20 m/s to +20 m/s.

For control and optimization of atomization processes measurement techniques are employed being able to determine the diameter and the velocity of the generated droplets. The so-called 'Phase-Doppler- Anemometry' (PDA) measuring simultaneously both distributions has been successfully applied to sprays of homogeneous liquids like water, molten metals etc. In a lot of other technical processed liquids contain optical inhomogeneities consisting of suspended superfine particles. In general the measured droplet size distributions of such process fluids appear to be broadened compared to the 'real-one'. In the following an approximate method for finding the 'real-size' distributions is introduced. A blurring function is obtained by performing PDA measurements on single-sized droplets. An interative constrained inversion algorithm is applied evaluating the real-size distribution. The precision of the algorithm is checked by comparing the calculated distributions with undistorted measuring results of an atomized optically homogeneous liquid.

Laser anemometry - as it is commonly applied - requires particles. However, in certain types of high-speed flows and plasma flows no particles are present. Collective light scattering may then be applied to measure fluid or plasma velocity. A new hybrid scheme has been proposed; the scheme allows for better axial resolution than what has previously been demonstrated. The effects of propagating and nonpropagating fluctuations on the expected crosscorrelation functions are investigated. The correlation function will in general be asymmetric and will consist of three identifiable peaks; two caused by (counter) propagating fluctuations and one caused by nonpropagating fluctuations. Thermal diffusivity cases the maximum of the 'nonpropagating peak' to be displace relative to the case of no diffusivity.

Doppler Global Velocimetry (DGV) is a full-field optical technique for the measurement of fluid flow velocities. The flow is illuminated using a light sheet, and the Doppler shift imposed on light scattered from moving particles within the sheet is imaged through a cell containing iodine vapor onto a solid-state array camera, thereby converting the Doppler frequency shifts into intensity variations in the image. In this paper, a DGV system is presented based around an argon-ion laser source and a fast digital image-processing system, which allows the DGV velocity map to be updated at camera frame rate. Interpretation of DGV images is complicated by errors which arise at positions some way out in the field of view due to the modified illumination and viewing vectors corresponding to these positions. Typical magnitudes of such errors are calculated. Significant errors can arise for points more than about 5 degree(s) out from the center of the field of view, and for divergence angles of the illumination beam exceeding about 10 degree(s) at a distance of 5 cm from the beam axis. Other considerations affecting system accuracy are also discussed.

An optically accessible flame tube combustor is described which has high temperature, pressure, and air flow capabilities. The windows in the combustor measure 3.8 cm axially by 5.1 cm radially, providing 67% optical access to the square cross section flow chamber. The instrumentation allows one to examine combusting flows and combustor subcomponents, such as fuel infectors and air swirlers. These internal combustor subcomponents have previously been studied only with physical probes, such as temperature and species rakes. Planar laser-induced fluorescence (PLIF) images of OH have been obtained from this lean burning combustor burning Jet-A fuel. These images were obtained using various laser excitation lines of the OH A$IMPX (1,0) band for two fuel injector configurations with pressures ranging form 1013 kPa (10 atm) to 1419 kPa (14 atm), and equivalence ratios from 0.41 to 0.59. Nonuniformities in the combusting flow, attributed to differences in fuel injector configuration, are revealed by these images.

H₂O (v) is an important species in combustion and hypersonic flow measurements because it is a primary combustion product. Measurements of water vapor can be used to determine performance parameters, such as extent and efficiency of combustion in propulsion and aerodynamics facilities. Water vapor concentration measurement in these high- temperature hypervelocity combustion conditions requires very high sensitivity and fast time response. A three-beam diode laser H₂O (v) measurement system for nonintrusive combustion diagnostics has been developed at NASA Langley Research Center and successfully tested and installed at GASL NASA HYPULSE facility for routine operation. The system was built using both direct laser absorption spectroscopy and frequency modulation laser spectroscopy. The output beam from a distributed feedback (DFB) InGaAsP diode laser (emitting around 1.39 micrometers ) is split into three equal-powered equal-distanced parallel beams with separation of 9 mm. With three beams, we are able to obtain water vapor number densities at three locations. Frequency modulation spectroscopy technique is used to achieve high detection sensitivity. The diode laser is modulated at radio frequency (RF), while the wavelength of the diode laser is tuned to scan over a strong waver vapor absorption line. The detected RF signal is then demodulated at the fundamental frequency of the modulation (one-F demodulation). A working model and a computer software code have been developed for data process and data analysis. Water vapor number density measurements are achieved with consideration of temperature dependency. Experimental results and data analysis will be presented.

A two dimensional optical diagnostic technique based on light extinction was improved and demonstrated in an investigation of diesel spray characteristics at high injection pressures. Traditional light extinction methods require the spray image to be perpendicular to the light path. In the improved light extinction scheme, a tilted spray image which has an angle with the light path is still capable of being processed. This technique utilizes high speed photography and digital image analysis to obtain qualitative and quantitative information of the spray characteristics. The injection system used was an electronically controlled common rail unit injector system with injection pressures up to 100 MPa. The nozzle of the injector was a mini-sac type with six holes on the nozzle tip. Two different injection angle nozzles, 125 degree(s) and 140 degree(s), producing an in-plane tilted spray and an out of plane tilted spray were investigated. The experiments were conducted on a constant volume spray chamber with the injector mounted tilted at an angle of 62.5 degree(s)$. Only one spray plume was viewed, and other sprays were free to inject to the chamber. The spray chamber was pressurized with argon and air under room temperature to match the combustion chamber density at the start of the injection. The experimental results show that the difference in the spray tip penetration length, spray angle, and overall average Sauter mean diameter is small between the in- plane tilted spray and the out of plane tilted spray. The results also show that in-plane tilted spray has a slightly larger axial cross- section Sauter mean diameter than the out of plane tilted spray.

An experiment is described for the direct measurement of laminar burning velocity within an optically accessed cylindrical combustion chamber. The laminar burning velocity was determined directly as the difference between the flame propagation speed and the unburned gas velocity immediately ahead of the flame front. Particle Image Velocimetry (PIV) has been applied to measure the unburned gas velocity field. The local flame speed and flame front position were determined from a pair of ionization probes in conjunction with the simultaneous PIV measurement. The laminar burning velocity of propane-air mixtures initially at atmospheric condition for different equivalence ratios from 0.7-1.4 are presented. Close agreement with other measurements and predicted results was found.

We report new models for the absorption, excitation, and fluorescence spectra for excitations within both the A $IMP X (0,0) band of NO and the B $IMP X bands of O₂ together with supporting measurements obtained from the burned regions of flames operated in pressure from 1 to 10 atm. As developed through these models, a strategy appropriate for imaging planar laser-induced fluorescence is proposed that, for lean combustion environments, would afford sensitive, single-shot measurements of NO concentration despite the photolytic interference from O₂ that intensifies with increasing pressure and temperature.

We have recorded laser-induced grating signals from mixtures of NO₂ and air over a pressure range extending from less that 100 kPa (1 atm) to 10 MPa (100 atm). Signals generated from concentrations of NO₂ at the part-per-million level have been successfully detected with high signal-to-noise rations. The measurements were made using the technique of laser-induced thermal acoustics (LITA). Analysis of the acquired data was made using a comprehensive theory which includes the hydrodynamic response of the fluid and finite beam-size effects. The observed pressure dependence of the peak amplitude signals is consistent with the theory. Additionally, least squares fits between the theory and the temporally resolved signal yield accurate values of the local sound speed and thermal diffusivity. Determination of the local sound speed provides a measurement of the local temperature.

The increasing need to mitigate pollutants produced by liquid fired combustion systems is leading to critical assessment of current systems in an effort to sufficiently refine them. These systems typically feature complex aerodynamic flows which serve to stabilize the reaction and are often the result of years of trail and error testing. In order to advance current systems suitable for future environmentally sensitive applications, basic understanding of the processes occurring is required. Recent developments in optical diagnostic techniques are providing the opportunity to attain such understanding. The present work applies a relatively well established technique (phase Doppler interferometry - PDI) to study the complex behavior of two-phase flows. PDI provides a wealth of information about the droplet size and velocity distributions as well as droplet concentration at discrete points within the flow. In addition, a relatively new technique (planar liquid laser induced fluorescence - PLLIF) is applied in order to extend the understanding of the structure of the flow. The PLLIF technique provides, in principle, the spatial distribution of liquid mass within a plane through the spray. This information is important because, in combustion systems, the homogeneity of the spatial distribution of fuel can play a significant role in the formation of pollutants. The results demonstrate the utility and potential limitations of the PLLIF technique. In addition, basic understanding of the complex flow structure is provided by combining the results from the two techniques.

Studies of direct injection, compression ignition engines demand a particle sizing technique which can reliably retrieve droplet sizes in transient fuel sprays. Many of the existing sizing techniques cannot analyze the dense, transient fuel sprays or provide information which is awkward to interpret. This work compares the performances of a diffraction based particle sizing technique and a light extinction sizing technique on a series of Diesel sprays. Both techniques were effective in acquiring useful data from the sprays. Under equivalent conditions, the light extinction technique provides somewhat smaller sizes than the diffraction based method.

Recent requirements for better characterization of the trace species in the exhausts of advanced aircraft engines have resulted in the development, under NASA sponsorship, of a tunable infrared diode laser system capable of making both in situ and extractive sampling measurements. In this paper, we describe the first application of this instrument to nonintrusive, cross-flow measurements in the exhaust of an aircraft engine operating in an altitude test cell. Simultaneous absorption measurements of NO, NO₂ and H₂O using the combined beams from two lasers were made at a variety of engine and flight conditions. The utility of measuring concentrations of major species such as H₂O to convert line of sight observations of trace species into mass fluxes is discussed, as are uncertainties in the measurement resulting from fluctuations in the absorption spectra and from conversion from integrated column density to concentration profiles.

Holographic particle image velocimetry (HPIV) encounters problems such as intrinsic speckle noise, external noise and aberration, and practical restrictions in test flow volume or media. The novel technique, In-line recording off-axis viewing (IROV), uses a single laser beam to record a hologram which is then viewed off axis during reconstruction. It combines advantages of both in-line and off-axis holography, achieving high spatial information capacity, low noise, and practicality. Instantaneous velocity vector field in the 3D flow of an unstable vortex ring in water has been measured by IROV. The vorticity distribution and circulation as a function of radius from the vortex core center are extracted from the 3D velocity field.

We consider dilute suspensions of optical and size polydisperse spherical particles for which the Rayleigh-Gans-Debye (RGD) approximation is valid. To develop the theory a concentric core-shell hard sphere model is adopted, in which particles possess a continuous variation in the core sizes but have constant shell thickness, thus giving rise to a distribution in the particle refractive indices. For two-component system, we theoretical analyze the dependence on scattering vector q of the average scattered intensity I(q) and the effective diffusion coefficient D_e(q) obtained from the first cumulant measured by dynamic light scattering in the case of the refractive index of the solvent and the refractive index of the shell are matched, i.e., n_m equals n_s. Under favorable conditions it should be possible to measure small polydispersities.

Some peculiarities of holographing optically soft microparticles are analyzed in this paper. We propose a technique to calculate light intensity distribution in the plane of a hologram and in the plane of a holographic image of a particle of an arbitrary shape at an arbitrary distance from the latter plane. Optical arrangement with the image transfer is taken as the basic one through the calculational scheme can readily be adapted to other optical arrangements of holographing. A technique for measuring refractive index of transparent particles is proposed as well. The results obtained in the experiments with water drops are presented in the paper to illustrate the feasibility of the technique.

Laser light scattering interferometry and laser Fraunhofer diffraction are used for the diagnostic studies of aerosols produced by a simple, high-efficiency pneumatic nebulizer. The new device, utilized in plasma spectrochemical analysis, operates more efficiently at low solution uptake rate (down to 10 (mu) L/min) compared to the conventional nebulizers consuming 1.2 mL/min. The efficacy of the cited techniques for assessing droplet-size information is contrasted for test aerosols prior introduction into high-temperature plasmas. Both the primary and tertiary aerosols are probed at uptake rate ranging from 10 to 1200 (mu) L/min. The relevance of these measurements and data in elemental analysis is discussed.

Diffraction effects produced by internally liquid cooled silicon windows have been characterized. Experimental point-spread function data for three window designs are presented. Measurements were made using a single wavelength (3.39 micrometers ) laser as well as a broadband blackbody source operating at various temperatures. From these data, angular distribution of energy and the ratio between the contained energy of the central and higher order lobes were derived. Comparisons to analytically generated curves show excellent agreement.