There is considerable interest in detecting objects such as landmines shallowly buried in loose earth or sand. Various techniques involving microwave, acoustic, thermal and magnetic sensors have been used to detect such objects. Acoustic and microwave sensors have shown promise, especially if used together. In most cases, the sensor package is scanned over an area to eventually build up an image or map of anomalies. We are proposing an alternate, acoustic method that directly provides an image of acoustic waves in sand or soil, and their interaction with buried objects. The INEEL Laser Ultrasonic Camera utilizes dynamic holography within photorefractive recording materials. This permits one to image and demodulate acoustic waves on surfaces in real time, without scanning. A video image is produced where intensity is directly and linearly proportional to surface motion. Both specular and diffusely reflecting surfaces can be accommodated and surface motion as small as 0.1 nm can be quantitatively detected. This system was used to directly image acoustic surface waves in sand as well as in solid objects. Waves at frequencies of 16 kHz were generated using modified acoustic speakers. These waves were directed through sand toward partially buried objects. The sand container was not on a vibration isolation table, but sat on the lab floor. Interaction of wavefronts with buried objects showed reflection, diffraction and interference effects that could provide clues to location and characteristics of buried objects. Although results are preliminary, success in this effort suggests that this method could be applied to detection of buried landmines or other near-surface items such as pipes and tanks.

This paper answers some performance and calibration questions about a non-destructive-evaluation (NDE) procedure that uses artificial neural networks to detect structural damage or other changes from sub-sampled characteristic patterns. The method shows increasing sensitivity as the number of sub-samples increases from 108 to 6912. The sensitivity of this robust NDE method is not affected by noisy excitations of the first vibration mode. A calibration procedure is proposed and demonstrated where the output of a trained net can be correlated with the outputs of the point sensors usded for vibration testing. The calibration procedure is based on controlled changes of fastener torques. A heterodyne interferometer is used as a displacement sensor for a demonstration of the challenges to be handled in using standard point sensors for calibration.

Nondestructive Evaluation (NDE) is an important tool for ensuring the inspectability of a structural design and assessing the integrity of the structure during fabrication and service. NDE test results are typically examined by an inspector to determine the location and size of damage. There is significant potential for reducing the human effort involved in this procedure by digitally processing this data to enhance the signatures of flaws and to perform automated identification of suspected flaws. Computational NDE focuses on the development of methods for the simulation of NDE techniques and reduction of NDE data for an assessment of the integrity of the structure. This paper examines a technique that enhances the contrast between damaged and undamaged regions to improve the quality and reliability of flaw identification. An anisotropic diffusion algorithm is applied to the data. Anisotropic diffusion techniques are shown to significantly reduce image noise while maintaining defect contrast and preserving the important features of a flaw. The use of this algorithm is shown to improve detectability levels for thermal NDE data for both standard array imaging infrared cameras as well as the cheaper, more portable microbolometers of interest today. By increasing and automating detectability, significant advances can be made in the use of thermal NDE tools.

A new simplified Holographic Particle Image Velocimetry technique to make simultaneous measurements of 3D velocity vector information throughout a complex fluid flow is presented in this paper. The method uses a variation of Optical Conjugate Reconstruction with complex correlation analysis and dispenses with the need to have a Holographic Optical Element to correct for distortions introduced by non-uniform windows. Subsequent analysis to extract a map of particle velocity is performed digitally using ray tracing techniques to model the effect of the windows. Results are presented for measurements made within a thick windowed diesel engine, showing that flow velocity vectors can be measured to an accuracy of 3% using the technique and, illustrating the ray trace mapping procedure.

Here, the physical and mathematical model is briefly described first, on which the photogrammetric calibration procedure of our Stereoscopic Tracking Velocimetry (STV) system is based. A new hybrid calibration approach is then introduced, which incorporates the use of artificial neural networks. The concept is to improve the performances of conventional calibration techniques of stereoscopic vision. In order to evaluate the quality of the hybrid calibration approach, calibration error is defined for the use of a camera. Our experimental investigation shows that the accuracy in predicting the object frame coordinates has been improved by 30 percents when the hybrid calibration is employed, as compared with the case when only the previous conventional physical and mathematical model is directly applied. It appears that the new idea of using artificial neural networks together with a physical and mathematical model of a system can improve the overall performance of the system. The hybrid method can also be applicable to other general areas in machine vision.

In the past, the use of optical and digital three-dimensional correlation methods have been demonstrated to extract velocity data from the complex amplitude distribution of particle images in holographic particle image velocimetry (HPIV). Recently we have proposed a digital shearing method to extract three-component particle displacement data throughout a complete image field. In contrast to full three-dimensional correlation, it has been shown that all three components of particle image displacement can be retrieved using just four two-dimensional fast Fourier transform (FFT) operations and appropriate coordinate transformations. In this paper we describe three-dimensional correlation and digital shearing methods and compare their performance in terms of computational efficiency and measurement accuracy. The simulated results show that the digital shearing method has comparable accuracy to three-dimensional correlation but is significantly faster.

The present work aims at providing information about the spray structure and its interaction with the tumble motion generated by the intake ducts of a prototype gasoline direct injection (GDI) engine. The investigation was carried out characterizing the fuel evolution within a prototype cylinder, under steady state flow conditions, by the 2-D laser imaging and Particle Image Velocimetry (PIV) techniques. The results offer a database about the spatial and temporal evolution of fuel spray that can validate numerical simulation for prediction of mixture formation and combustion in direct injection gasoline engines. Experiments were taken using a common rail injection system able to work at a maximum pressure of 12 MPa, a swirled type injector with a nozzle diameter of 0.55 mm and a nominal cone angle of 50°. The investigation was conducted by applying the 2-D imaging and the PIV techniques in a flow test rig, designed for capturing the tumble motion in a prototype cylinder. The system included a blower, which supplied the intake flow rate, and a prototype 4-valve direct injection gasoline engine head modified to lay down the swirled-type injector. Tests were taken, on a plane crossing the cylinder and the injector axes, spraying the fuel at P_inj = 5, 8, and 10 MPa for an injection interval of Δt=3 ms. The results provided detailed information on the pre- and main spray evolution. At the first stage of injection, the fuel jet depicted a dense liquid column with a very small cone angle while a transition to a spray hollow-cone structure was observed at later injection time. Images of the interaction of the fuel with the tumble motion displayed, firstly, a fuel jet that traveled as a compact liquid column not affected by the tumble motion within the cylinder because of its high momentum. At later injection time, the fuel was strongly distorted by the tumble motion with the formation of secondary droplets clusters that detached from the main jet and were dispersed within the cylinder. Images highlighted a spray that penetrated with a cone angle smaller than that observed under stagnant conditions. PIV measurements showed a fuel jet having a velocity distribution profile that endorses the liquid column like typical of the pre-spray penetration. When the fuel jet reached the steady cone angle, PIV results depicted an intense momentum exchange with the tumble airflow that becomes the controlling parameter for the jet break-up and the dispersion of droplets within the cylinder.

Concentration profiles of OH, O₂ and NO as well as temperature fields in diffusion flames of a length of approx. 300 mm and 40 mm in diameter used for gas-phase synthesis of fused silica have been determined by Planar Laser Induced Fluorescence (PLIF). The measurements have been carried out using a tunable spectrally narrowed KrF laser, whose wavelengths could be switched pulse-to-pulse. The laser beam was shaped as a light sheet into the flame at a fixed position. The flame area under investigation was monitored by moving the burner mounted on a stepper motor. By adapted synchronization the laser induced fluorescence was continuously recorded over the height of the flame perpendicular to the laser light sheet with an intensified CCD camera (10 fps, 8 bit dynamic range, 768 x 576 pixels). By image processing the spatial offset between images was corrected and superposed images were averaged and analyzed. This method allows to investigate the flame by recording 2D-fluorescence images including an automatic correction of intensity inhomogeneities of the laser light sheet. Based on the excited radical or molecule the fluorescence images were used to determine concentration and temperature distributions to build up a 2D-map of the flame. The PLIF experiment was calibrated with precise determination of the temperature at one coordinate of the flame by Spontaneous Vibrational Raman Scattering (VRS) of N₂. As a result temperatures up to 3200 K could be determined with an accuracy better than 3% and a spatial resolution better than 1 mm. Temperature variations in the flame at different gas flows of fuel and oxidizer could be monitored sensitively. Also, the influence of different carrier gases like N₂, Ar and He on the temperature distribution was investigated. Fluctuations in gas flow caused by turbulence could be monitored as well.

Mixing process of a passive scalar in a heated turbulent jt at Reynolds numbers between 13,000 and 21,000 is studied experientaly using combined two-color Planar Laser-Induced Fluorescence (2λ-PLIF) and Particle-Image Velocimetry (PIV). The PLIF system is based on acetone fluorescence for temperature and concentration measurement. The aim of the present paper is to obtain a reliable reference data set for the validation of numerical simulation of turbulent fluxes. Experiment was carried out on a heated turbulent jet of acetone-seeded air emanating from the 10 mm-diameter nozzle exit of an electric air heater with exit temperature T_i = 500 K. The jet is seeded to approximately 3% acetone by bubbling the air stream through liquid acetone. The mean and fluctuating dynamic and thermal fields are investigated and determined. This tool will allow to determine the temperature-velocity as well as the concentration-velocity cross-correlations in order to characterize the turbulent characteristics of the flow such as the turbulent diffusivity and the turbulent Prandtl number.

There is growing interest in applying “intelligent” technologies to aerospace propulsion systems to reap expected benefits in cost, performance, and environmental compliance. Cost benefits span the engine life cycle from development, operations, and maintenance. Performance gains are anticipated in reduced fuel consumption, increased thrust-to-weight ratios, and operability. Environmental benefits include generating fewer pollutants and less noise. Critical enabling technologies to realize these potential benefits include sensors, actuators, logic, electronics, materials and structures. For propulsion applications, the challenge is to increase the robustness of these technologies so that they can withstand harsh temperatures, vibrations, and grime while providing extremely reliable performance. This paper addresses the role that optical metrology is playing in providing solutions to these challenges. Optics for ground-based testing (development cycle), flight sensing (operations), and inspection (maintenance) are described. Opportunities for future work are presented.

In the paper we discuss some metrological issues connected with measurement and data processing procedures of optical, full-field techniques applied for 3D objects investigation. The methodology presented is focused on determination of final uncertainty of results based on knowledge about uncertainties introduced by each contributing module. It is shown that in complex system the measurement uncertainty should be determined experimentally on the base of comparison of measurement results and certified model data, while the processing uncertainties determination may be based on numerical experiments (comparison with virtual model). The exemplary uncertainty determination paths for true 3D shape measurements of biological and technical objects are presented.

Recent concerns over the possible use of airborne biological particles as weapons of mass destruction have significantly increased the attention that researchers are giving to this threat. The size of these particles, ranging from a fraction of a micrometer to several tens of micrometers, allows them to travel over long distances before settling out of the airstreams carrying these particles. Furthermore, the odd shapes of many of these particles along with uncertainties about their light scattering characteristics make detection and tracking quite a challenge. In the present paper, results are reported on the visualization of airborne biological particles using two-dimensional particle image velocimetry (PIV). These initial results show the utility of PIV in illuminating and tracking airborne biological particles. A compressed air nebulizer is used to aerosolize the biological particles inside a Plexiglas test section. The biological particles prepared for the nebulizer are first inoculated and cultured onto agar media, gypsum board, and acoustic ceiling tile to achieve an abundant growth of spores. A colloidal suspension of biological particles is then made using sterilized, de-ionized water and a mild surfactant to de-agglomerate the biological particles in the suspension. The concentration of biological particles in the colloidal suspension is determined using a hemacytometer. In the visualization experiments, images are captured for polystyrene latex (PSL) test particles, liquid water droplets, and spores of the fungal species Aspergillus versicolor. During the PIV system operation, two successive images are captured with a time delay of 50 μm to develop flow field velocities of the PSL test particles, liquid water droplets, and the A. versicolor spores.

A Planar Doppler Velocimetry (PDV) illumination system has been designed which is able to generate two beams, separated in frequency by about 600 MHz. This allows a common-path imaging head to be constructed, using a single imaging camera instead of the usual camera pair. Both illumination beams can be derived from a single laser, using acousto-optic modulators to effect the frequency shifts. One illumination frequency lies on an absorption line of gaseous iodine, and the other just off the absorption line. The beams sequentially illuminate a plane within a seeded flow and Doppler-shifted scattered light passes through an iodine vapor cell onto the camera. The beam that lies at an optical frequency away from the absorption line is not affected by passage through the cell, and provides a reference image. The other beam, the frequency of which coincides with an absorption line, encodes the velocity information as a variation in transmission dependent upon the Doppler shift. Images of the flow under both illumination frequencies are formed on the same camera, ensuring registration of the reference and signal images. This removes a major problem of a two-camera imaging head, and cost efficiency is also improved by the simplification of the system. The dual illumination technique has been shown to operate successfully with a spinning disc as a test object. The benefits of combining the dual illumination system with a three-component, fiber-linked imaging head developed at Cranfield will be discussed.

The development of a planar Doppler velocimetry is described. The technique is capable of measuring the three, instantaneous components of velocity in two dimensions using a single pair of signal and reference cameras. PDV can be used to measure the instantaneous 3-D velocity of a fluid by using an absorption line filter (ALF) to determine the Doppler shifted frequency of a narrow line pulsed laser (Nd:YAG) that has been scattered off particles seeded into the flow. The velocity of the fluid is determined using the Doppler formula and is dependent on the laser direction and the viewing direction. Hence, only one velocity component of the flow is measured. This component can be measured in two spatial dimensions using an array detector such as a CCD camera. To capture the three components, three such measurement heads have been used viewing from different angles. In the technique presented here the three views are ported from the collection optics to a single imaging plane using flexible fiber imaging bundles. These are made up of a coherent array of single fibers and are combined at one end as the input plane to the measurement head. The paper discusses the issues involved in developing a full three-dimensional velocity measurement system.

Laser-Induced Thermal Acoustics (LITA) has been used to measure the flow field in the slat region of a two-dimensional, high-lift system in the NASA Langley Basic Aerodynamics Research Tunnel (BART). Unlike other point-wise, non-intrusive measurement techniques, LITA does not require the addition of molecular or particulate seed to the flow. This provides an opportunity to obtain additional insight and detailed flow-field information in complex flows where seeding may be insufficient or detection is problematic. Based on the successful use of LITA to measure the flow over a backward-facing step, the goal of this study was to further evaluate the technique by applying it to a more relevant and challenging flow field such as the slat wake on a high-lift system. Streamwise velocities were measured in the slat wake and over the main element at 11.3 degrees angle of attack and a freestream Mach Number of 0.17. The single-component LITA system is described and velocity profiles obtained using LITA are compared to profiles obtained using two-dimensional, Digital Particle Image Velocimetry (DPIV) and a steady, Reynolds-Averaged Navier-Stokes (RANS) flow solver for the same configuration. The normalized data show good agreement where the number of measurement locations had sufficient density to capture the pertinent flow phenomena.

This paper describes a quantitative schlieren technique called Calibrated Color Schlieren (CCS) that is capable of measuring the light deflection angle in both spatial directions simultaneously and hence is able to extract the projected density gradient of a two-dimensional flow. CCS makes use of a graded color filter in combination with a square source of size whose size may be varied to change the sensitivity. A calibration polynomial is used to obtain the deflection angle from color ratios at each pixel. The technique’s performance was assessed in terms of repeatability, sensitivity and accuracy using the Prandtl-Meyer expansion fan at the wedge-plate shoulder in a supersonic flow. From the measured deflection angles the density gradient and the density are computed. The density information agrees well with Prandtl-Meyer theory. The technique is also applied to a more complex wake flow, which required the use of a color correction based on a shadowgraph image.

The properties and behavior of long non-linear strain solitary waves (solitons) in solid wave guides embedded in an external elastic medium are considered. We analyze in theory and experiments how different types of materials used as an external medium affect the soliton parameters. The wave detection and recording are performed by means of pulsed holographic interferometry.

Shearography is a full-field non-contact optical technique usually used for the identification of the location of defects in non-destructive testing. Shearography is sensitive to displacement gradient, a parameter closely related to the surface strain. To fully characterise the surface strain requires the determination of six orthogonal components of displacement gradient and this is achieved in shearography by measuring from three, or more, illumination, or viewing, directions and from a minimum of two directions of applied shear, followed by a coodinate transformation to the orthogonal components. In this paper the authors use illumination from a single direction using dual pulsed injection-seeded Nd:YAG lasers, and view the object from four directions. The images from the four viewing directions are ported to a single interferometer head using a four-leg optical fiber imaging bundle. At the interferometer head the four views are spatially-multiplexed into a single image, pass through the interferometer head and are recorded by a single high resolution dual-framing camera. The direction of applied shear in the interferometer head is adjustable to allow measurements from the two shear directions. Experimental results are presented of displacement gradient from this pulsed laser shearography instrument.

The initial concept and development of a low-cost, adaptable method for the measurement of static and dynamic aeroelastic deformation of aircraft during flight testing is presented. The method is adapted from a proven technique used in wind tunnel testing to measure model deformation, often referred to as the videogrammetric model deformation (or VMD) technique. The requirements for in-flight measurements are compared and contrasted with those for wind tunnel testing. The methodology for the proposed measurements and differences compared with that used for wind tunnel testing is given. Several error sources and their effects are identified. Measurement examples using the new technique, including change in wing twist and deflection as a function of time, from an F/A-18 research aircraft at NASA's Dryden Flight Research Center are presented.

We present a point-diffraction interferometer specially devised for the profile measurement of rough surfaces that are difficult to be measured with conventional two-arm interferometers. The diffraction interferometer comprises multiple two-point-diffraction sources made of a pair of single-mode optical fibers, and performs an absolute profile measurement by projecting multiple fringe patterns on the object surface and then fitting measured phase data into a global geometrical model of multilateration. Test measurement results demonstrate that the proposed point-diffraction interferometer is well suited for the warpage inspection of microelectronics components with excessive height irregularities, such as unpolished backsides of silicon wafers and plastic molds of integrated-circuit chip packages.

A new technique has been developed for sensing both temperature and strain simultaneously by using dual-wavelength fiber-optic Bragg gratings. Two Bragg gratings with different wavelengths were inscribed at the same location in an optical fiber to form a sensor. By measuring the wavelength shifts that resulted from the fiber being subjected to different temperatures and strains, the wavelength-dependent thermo-optic coefficients and photoelastic coefficients of the fiber were determined. This enables the simultaneous measurement of temperature and strain. In this study, measurements were made over the temperature range from room temperature down to about 10 K, addressing much of the low temperature range of cryogenic tanks. A structural transition of the optical fiber was found when the temperature decreased. This transition caused splitting of the waveforms characterizing the Bragg gratings, and the determination of wavelength shifts was consequently complicated. The effectiveness and sensitivities of these measurements in different temperature ranges are also discussed.

A fiber optic based distributed temperature measurement system was implemented in stator windings (straight copper bars) as well as in the end-windings (curved copper bars) of a motor. Usually, in electrical machines such as motors or generators, only a few conventional temperature sensors are used, whereas the distributed temperature system has the potential of providing very detailed temperature distribution by having hundreds of sensors in a single fiber. The sensors were made of Bragg gratings etched onto the fiber itself. For the present study, the spatial resolution of the sensors is 6 mm (nominally at 1/4” apart). The technique uses Optical Frequency Domain Reflectometry (OFDR) to process the back-reflected light signal indicative of the thermal filed. A prototype fiber optic system was implemented in a motor made by GE industrial systems. The sensing length (length of the stator) for the motor was 0.75 m containing approximately 150 sensors thus providing very detailed temperature data. Performance tests were conducted at different heat loads representing different electrical conditions. Continuous tests for the duration of 19 hours were conducted. The temperature of stator windings varied from ambient (~ 23°C) to approximately 85°C. As reference, Resistance Temperature Devices (RTDs) were installed in adjacent slots to the slot where fiber optic sensors were installed. A total of 8 sensors were installed but data were collected on only 3 fibers. Fiber sensor measurements were found to track the temperature trends very well. The fiber data agreed with RTD data within ± 3°C in the entire duration. The RMS value of difference between the fiber and RTD on one side was 0.3°C, and with the RTD on the other side was 0.5°C. The fiber measurements also showed how hotspots could be missed by using few RTDs, as is done in the industry. The fiber measurements also showed the temperature distribution in the endwindings, an area not normally monitored. The maximum temperature was an acceptable 110°C. The feasibility of this technique for measuring stator-winding temperatures is proved. Still some of the problems faced during the installation and experiments are (a) robustness of fiber and sheathing fiber and (b) fiber survivability during manufacturing process and repair.

In the design of optoelectronic devices, size and compatibility with current technology are two significant considerations. A much desired optoelectronic laser design would be one in which the laser formed a very small integrated component of a silicon microchip and was fabricated entirely from CMOS compatible materials. Until recently, the prospect for such a device seemed unlikely, and the future of optoelectronic functionality appeared to lie largely with direct bandgap semiconductors such as GaAs. However, the recent observation of optical gain in silicon nanocrystals has opened an opportunity to develop a nanoscale silicon-based laser. In this paper we report on various designs that could be used to achieve such a breakthrough. The designs are based on photonic crystal architecture and utilize Si nanocrystals embedded in SiO₂ for the lasing media. The designs could be employed for other nanolasers providing specific lasing criteria are met.

Contamination of spacecraft surfaces by deposition and ablation can potentially limit the performance and lifetime of solar panels, optical systems and microsatellite structural elements. However, instrumentation for on-orbit contamination assessment is limited by payload requirements and the experiments which can be conducted are limited by spacecraft geometry and mission lifetime. We present the design of a fiber optics evanescent wave sensor capable of real time detection of the contamination of spacecraft surfaces during flight. While other evanescent wave detection schemes rely on special coatings to selectively and reversibly absorb a target analyte, in the present context, such coatings would themselves be considered undesirable forms of contamination, and are therefore prohibited. The sensor described here is capable of detecting contamination by direct exposure of the evanescent wave to the environment through the use of a reduced-cladding fiber. The sensor can measure contamination from foreign substances, and ablation from the impact of space debris or ion thruster exhaust. In this paper, we briefly discuss the major forms of contamination. We describe the operating principles of the fiber optic evanescent wave sensor we have constructed for monitoring these forms of contamination, and provide preliminary results indicating sensor performance characteristics. We demonstrate the feasibility of the sensor for detecting the deposition of a variety of substances, and for observing the effects of ablation from thruster exhaust. In all cases, we make a qualitative comparison between sensor performance and theory.

In studying hydrodynamic instabilities between two miscible fluid mixtures one often faces the problem of assigning a reliable value to very small diffusion coefficients. As for diffusion between two binary mixtures such as water and glycerin, water and salt, etc., no complete data are available in the literature. In order to measure small diffusion coefficients of miscible fluids, in this paper we propose an improved version of digital projection moire. The system uses a fringe generator realized with a diffractive optical element (DOE). The fringe patterns are projected on the bottom of a ground glass plate. The phase object (diffusion cell) is placed in front of the ground glass (in other words, in front of the fringe pattern), which is imaged by a digital video camera. Grating patterns, during the evolution of diffusion phenomena, are captured by a CCD camera and stored in a computer at different times. With the aid of Fast Fourier Transform (FFT) and signal demodulating techniques, the images are processed to obtain the diffusion coefficients. The theoretical basis of the device is presented. Furthermore, we report some experiments conducted for demonstrating the usefulness of the system.

Photo-thermal effect is in general a well-established technique for remote and nondestructive material and structure evaluation. In particular, the method of transient thermal gratings is typically used for characterization of thermal properties and of low optical absorption in liquids. Conventional experimental configuration implies utilizing two mutually coherent recording beams to produce the thermal grating and one additional probe beam to detect it. We propose to perform recording and simultaneously probing the thermal grating using the same recording beams in configuration of dynamic two-wave-mixing (TWM), which simplifies the detection configuration and, as we hope, increases the sensitivity. In this configuration the sample is irradiated by the interference pattern of two coherent beams, in one of which the periodic phase modulation is additionally introduced. The output signal is detected as an amplitude modulation in one of the transmitted beams using conventional high-sensitivity lock-in amplification technique. The detected output signal is proportional to the amplitude of the thermal grating (but not to the grating diffraction efficiency as in coventional arrangement with additional probe beam), which also potentially increases the sensitivity. While the grating amplitude is evaluated directly from the output signal amplitude detected in this configuration, the photo-thermal grating formation time is obtained from position of the so-called "cut-off" frequency in the signal modulation frequency dependence. Details of experiments with this configuration using slightly dyed acetone sample at the wavelength 633 nm, which resulted in evaluation of the characteristic grating recording time and, finally, of the thermal diffusivity coefficient of the liquid, are presented.

A new laser Doppler velocimeter employing a CO₂ laser has been developed by using its photoacoustic effect. A change of the pressure of a laser discharge tube, induced by mixing of a returned wave with an originally existing wave inside the cavity, is employed to detect the Doppler frequency shift. We found that a Doppler frequency shift as much as 60 kHz was detected, and as well as a good linear relationship between the velocity and the Doppler frequency shift was obtained.

A low-cost and low-maintenance digital focused shadowgraph flow visualization system has been developed to provide fast diagnostics of rapidly changing phenomena in supersonic flows. The system is particularly designed for tracking shock positions in a supersonic inlet, enabling high-speed active shock control. It is based on a low-cost, high-intensity white LED light source, which can be flashed with microsecond pulses enabling freeze-frame imaging of constant illumination quality. The system features three modes of operation: (1) High-resolution digital still frames and sequences (1280 x 1024, 2fps), (2) High-resolution digital frames and sequences showing spatial-temporal variation in flow field (1280 x 1024, 12 fps), (3) Adjustable windowed digital frames at reduced resolution, but at high frame rates (980 fps at 1280 x 8 pixel viewing area). The three modes of operation allow high-speed tracking of flow features such as moving of shock waves (up to 980 Hz) as well as overall instantaneous views of the flow. Furthermore, it allows direct identification of areas where high-speed changes occur. The positional shock data can be transmitted directly to a shock-stabilizing control system. Results are presented of the unsteady flow generated by an aspirated cone-shaped nozzle in a supersonic flow in the supersonic wind tunnel of the MIT Gas Turbine Laboratory.