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The three-dimensional (3-D) shock-wave/boundary-layer interaction (SBLI) between a fin-generated shock and a turbulent boundary layer is examined using plenoptic particle image velocimetry (PIV): a...

Rotating 3D velocimetry (R3DV) is a single-camera PIV technique designed to track the evolution of flow over a rotor in the rotating reference frame. A high-speed (stationary) plenoptic camera capable of 3D imaging captures the motion of particles within the volume of interest through a revolving mirror from the central hub of a hydrodynamic rotor facility, a by-product being an undesired image rotation. R3DV employs a calibration method adapted for rotation such that during MART reconstruction, voxels are mapped to pixel coordinates based on the mirror’s instantaneous azimuthal position. Interpolation of calibration polynomial coefficients using a fitted Fourier series is performed to bypass the need to physically calibrate volumes corresponding to each fine azimuth angle. Reprojection error associated with calibration is calculated on average to be less than 0.6 of a pixel. Experimental uncertainty of cross-correlated 3D/3C vector fields is quantified by comparing vectors obtained from imaging quiescent flow via a rotating mirror to an idealized model based purely on rotational kinematics. The uncertainty shows no dependency on azimuth angle while amounting to approximately less than 0.21 voxels per timestep in the in-plane directions and correspondingly 1.7 voxels in the radial direction, both comparable to previously established uncertainty estimations for single-camera plenoptic PIV.

Time of Flight (ToF) cameras are a type of range-imaging camera that provides three-dimensional scene information from a single camera. This paper assesses the ability of ToF technology to be used for threedimensional particle tracking velocimetry (3D-PTV). Using a commercially available ToF camera various aspects of 3D-PTV are considered, including: minimum resolvable particle size, environmental factors (reflections and refractive index changes) and time resolution. Although it is found that an off-the-shelf ToF camera is not a viable alternative to traditional 3D-PTV measurement systems, basic 3D-PTV measurements are shown with large (6mm) particles in both air and water to demonstrate future potential use as this technology develops. A summary of necessary technological advances is also discussed.

This paper investigates the effect of smoothing operation in 3D reconstruction using a plenoptic camera. A plenoptic camera - also known as light field camera - features a commercial off the shelf camera with added microlens array (MLA) behind the imaging lens, directly in front of the sensor. The main lens focuses the light to the MLA plane, where each microlens then re-directs the light to small regions of pixels behind, each pixel corresponding to different angle of incident (T. Fahringer (2015)) (Adelson and Wang (1992)). Thus, MLA encodes angular information of incident light rays into the recorded image that assist to acquire 4D information (u,v,s,t) of light-field including both position and angular information of light rays captured by the camera (Ng et al. (2005)) (Adelson and Wang (1992)).

Flow evolution over helicopter rotors, wind turbine blades, and insect wings are unsteady, three-dimensional (3D), and influenced by phenomena unique to the rotating frame of reference (FoR), e.g., Coriolis and centrifugal forces. Conventional 3D-PIV techniques are unable to fully characterize these rotating FoR physics, since the measurements are limited to a fixed FoR of a relatively small volume through which the rotor blade or wing traverses intermittently. In this paper, a new “Rotating Three-Dimensional Velocimetry (R3DV)” technique is proposed to address these gaps. R3DV consists of 3D measurements made with a single stationary plenoptic camera in combination with a hub-mounted mirror that aligns the camera’s field of view with a rotating wing. In post-processing R3DV data, a rotational volumetric calibration method is developed to account for image acquisition through a rotating mirror. Rotating FoR volumes are then reconstructed using the Multiplicative Algebraic Reconstruction Technique (MART) algorithm with the adapted calibration scheme and subsequently cross-correlated to derive a 3D velocity field. R3DV was experimentally demonstrated in a study of 3D unsteady flow over an impulsively rotated flat-plate wing. Prominent flow features like the formation and shedding of the primary and secondary leading-edge vortices (LEVs) were observed, which corroborate well with the existing literature on rotating wings. The time-resolved variation of LEV velocity profiles and circulation with azimuthal angle exhibited expected trends. The ability to quantify 3D and time-resolved velocity fields in the rotating FoR demonstrates the feasibility of adopting R3DV as a technique to investigate rotating flows.

Vortex topology is analyzed from measurements of flow over a flat, rectangular plate with an aspect ratio of 2 which was articulated in pitch and roll, individually and simultaneously. The plate was immersed into a $$\text {Re} = 10,000$$ flow (based on chord length). Measurements were made using a 3D–3C plenoptic PIV system to allow for the study of complete vortex topology of the entire wing. The prominent focus is the early development of the leading-edge vortex (LEV) and resulting topology. The effect of the wing kinematics on the topology was explored through a parameter space involving multiple values of pitch rate and roll rate at pitch and roll angles up to $${50}^{\circ }$$ . Characterization and comparisons across the expansive data set are made possible through the use of a newly defined dimensionless parameter, $${\textit{k}_{\text {Rg}}}$$ . Termed the effective reduced pitch rate, $${\textit{k}_{\text {Rg}}}$$ , is a measure of the pitch rate that takes into account the relative rolling motion of the wing in addition to the pitching motion and freestream velocity. This study has found that for a purely pitching wing, increasing the reduced pitch rate $${\textit{k}}$$ delays the vortex evolution with respect to $$\alpha _\mathrm {eff}$$ . For a purely rolling wing, as the advance coefficient $${\textit{J}}$$ is increased, the vortex evolution is advanced with respect to nondimensionalized time and the bifurcation point of the LEV shifts inboard. For a pitching and rolling wing, the addition of roll stabilizes and delays the evolution of the LEV in both nondimensionalized time and effective angle of attack.

Plenoptic background oriented schlieren imaging has recently been introduced as a single-camera technique used to observe three-dimensional density gradients in a flow field. With the ability to generate focused BOS images, the signature of density gradients produced at different depth locations can be distinguished from one another. Two experiments demonstrate the capabilities of this technique. The first experiment visualized the rising plumes produced from two simple flames placed at different depths in a low magnification configuration. The second experiment used a high magnification configuration with long working distance to visualize shock waves in a 6.35 millimeter diameter underexpanded jet. These experiments demonstrate plenoptic BOS as a simple and convenient three-dimensional visualization technique that can be applied in facilities with limited optical access.

The motivation of this work is to compare results from plenoptic and conventional background oriented schlieren (BOS) systems through two experimental configurations using a buoyant thermal plume to produce the inhomogeneous density field. The first set of experiments fixed the nominal focal plane and varied the plume position along the optical axis. The second set of experiments fixed the position of the plume while varying the nominal focal plane. These experiments were used to observe how measurement performance was influenced by (1) spatial resolution, (2) plume position, (3) focal plane position, (4) depth of field, (5) exposure time, and (6) aperture size. It was determined that both imaging systems follow the trend that the signal is dependent on the distance between the background and the plume but relatively independent of the nominal focal plane position. Conventional BOS images result in high-resolution, single line-of-sight measurements with a high signal-to-noise ratio. These images typically require a small aperture with a longer exposure time in order to achieve an appropriate depth of field. Focused BOS images are a result of lower resolution, multiple line-of-sight measurements with lower signal-to-noise ratios compared to that of the conventional system. Images from this system use a larger aperture to more efficiently collect light while still maintaining an extended depth of field.

A three-color pyrometer has been developed based on plenoptic imaging technology. Three bandpass filters placed in front of a camera lens allow separate 2D images to be obtained on a single image sensor at three different and adjustable wavelengths selected by the user. Images were obtained of different black- or grey-bodies including a calibration furnace, a radiation heater, and a luminous sulfur match flame. The images obtained of the calibration furnace and radiation heater were processed to determine 2D temperature distributions. Calibration results in the furnace showed that the instrument can measure temperature with an accuracy and precision of 10 Kelvins between 1100 and 1350 K. Time-resolved 2D temperature measurements of the radiation heater are shown.

A recently developed 3-D flow visualization system is used to capture instantaneous 3-D images in the near-field of an axisymmetric jet at Reynolds number 6,700 and 10,200. The 3-D images, which are acquired through the scanning of a pulsed MHz-rate laser sheet, have spatial resolution of up to 312 × 260 × 100 voxels and time resolution of 100 μs, producing effectively instantaneous, high-resolution 3-D images of the jet. These images represent the first truly volumetric flow visualization images to be captured in the near-field of a jet over this range of Reynolds numbers. The images show the formation of large-scale axisymmetric vortex rings along with the formation of counter-rotating streamwise vortex pairs that surround the periphery of the jet in a periodic manner and co-exist with the ring vortices. The azimuthal frequency of the streamwise vortices increases with Reynolds number from 10 to 14 structures around the circumference at Re = 6,700 to 18–22 structures at Re = 10,200. In some instances, the streamwise vortices are observed to maintain their phase and span across several vortex rings in the streamwise direction. In other instances, however, the streamwise vortices display abrupt changes in the downstream direction. The images also display a complex interaction between the vortex rings and streamwise structures in the transition region near the end of the potential core illustrating the importance of the streamwise vortices on the evolution of the jet flow.

A high-speed three-dimensional flow visualization system has been developed and is described. The technique is based on the high-speed scanning and subsequent imaging of a two-dimensional laser sheet through the flowfield. A three-dimensional image is then reconstructed from the stack of two-dimensional slices. The technique achieves high speeds using a home-built megahertz-rate pulse-burst laser system, a galvanometric scanning mirror, and a high-speed intensified charge-coupled-device camera capable of 500,000 frames per second. These components allow for the acquisition of three-dimensional image data with a resolution of 220 x 220 x 68 volumetric elements (voxels) in 136 μs. The speed of the technique is limited by the available camera speed and can be increased substantially using a higher-speed camera. The technique is demonstrated through three-dimensional visualization a turbulent round jet (Re = 6700) seeded with small water droplets for light scattering. Three-dimensional flow visualization images display numerous three-dimensional features of the jet, including ring vortices, azimuthal modes and counterrotating streamwise vortex pairs. Future work will focus on the development of a high-speed three-dimensional laser-induced fluorescence technique.

This research effort addresses the use of numerical simulations to predict the flow physics over an airborne turret, represented by a hemisphere. Computational fluid dynamics codes, Loci-Chem, Kestrel and OVERFLOW were used in this study with different high fidelity turbulence models, including DES, DDES, D-MS and hybrid RANS/LES, to determine their capabilities of predicting these complex flow features. These numerical simulations were compared against the experiment conducted at Auburn University, in terms of wall-pressure data, instantaneous density contour images, and optical path difference (OPD). Among the three codes, Loci-Chem using the hybrid RANS/LES turbulence model was able to capture the unsteady flow structures, such as shear-layer fluctuations, and the rolling motions of the vortex shedding. Both Kestrel using DDES+SST and OVERFLOW using the DES, DDES, and D-MS turbulence models were able to predict certain levels of shear-layer oscillations; however the presence of the vortex motions behind the wake region were not fully captured. Therefore, the results from Kestrel and OVERFLOW predicted limited OPD values as compared to the experimental data. Since the solution from Loci-Chem was able to produce the vortex shedding, the OPD analysis was able to follow the most of the trends, except that under-predicted the vortex motion shown in the experiment.

A pulse burst laser and either one or two high-speed charge-coupled-device cameras were used to perform onecomponent time-resolved planar-Doppler-velocimetry (PDV) measurements in a rectangular Mach 2.0 jet. The measurements were carried out on a streamwise plane passing through the jet centerline and covering approximately 6‐12 jet heights downstream of the jet exit. The pulse burst laser operated at 0.532-µ mw avelength and produced 28 pulses at 250 kHz with approximately 9 mJ/pulse energy. Velocity image sequences consisting of 28 frames showed dynamics of the velocity field over a time span of 108 µs (approximately 4.5 convective timescales). A typical sequence of images is presented, which demonstrates the process of entrainment of low-speed fluid into the high-speed region of the jet. Mean and standard deviation statistics of the velocity calculations produced expected trends and showed good agreement between the single- and two-camera experiments. An error analysis revealed speckle as the predominant source of noise, as in a conventional PDV technique. At a transmission ratio of 0.5, the estimated total error is 13 m/s for the single-camera experiment and 15 m/s for the two-camera experiment. I. Introduction P LANAR Doppler velocimetry is a powerful optical diagnostic technique that can be used to measure all three components of instantaneous velocity over a two-dimensional plane within a flowfield with high spatial resolution. This is accomplished by using an atomic or molecular vapor filter to measure the frequency shift of light as it is scattered by particles contained in the flowfield. The �

A newly developed MHz rate imaging system that provides real-time flow visualization is described. The technique utilizes a custom-built Nd:YAG pulse burst laser and an ultra high-speed digital camera and is capable of capturing 17 images over 150 microseconds. The system was used to visualize a Mach 1.3 (M c =0.6) axisymmetric jet. Sample results indicate the potential of the technique to provide detailed information on the dynamic characteristics of large-scale structures. A two-dimensional cross-correlation technique was used to calculate the convective velocity of large-scale structures. Present results generally agree with the findings of earlier investigations that indicate a significant deviation of the convective velocity from theoretical predictions.

As with any flow diagnostic imaging system, plenoptic PIV must be able to capture images under a wide variety of imaging conditions. One common problem is the change in magnification caused by a change in the index of refraction between a flow volume and the camera, often seen in water tunnel imaging. This work seeks to analytically derive the effect of refraction on the imaging equations and to find a correction factor for the depth of particles in a tomographic reconstruction of the volume. This solution was tested by a series of simulated images that were generated under three different conditions: no refractive interfaces, a flat refractive interface between the volume and the camera, and a flat refractive interface between the main lens and the microlens array. Results demonstrated that the depth of a particle is shifted with regard to the focal point of the system by a factor of the index of refraction. For an internal interface, this scaling remains true for all particles focused behind the interface, but there is significant deviation in particle depth for particles focused in front of this plane.

Simultaneous particle image velocimetry (PIV) and flow visualization measurements were performed in a turbulent boundary layer in an effort to better quantify the relationship between the velocity field and the image intensity typically observed in a classical flow visualization experiment. The freestream flow was lightly seeded with smoke particles to facilitate PIV measurements, whereas the boundary layer was densely seeded with smoke through an upstream slit in the wall to facilitate both PIV and classical flow visualization measurements at Reynolds numbers, Re θ , ranging from 2,100 to 8,600. Measurements were taken with and without the slit covered as well as with and without smoke injection. The addition of a narrow slit in the wall produces a minor modification of the nominal turbulent boundary layer profile whose effect is reduced with downstream distance. The presence of dense smoke in the boundary layer had a minimal effect on the observed velocity field and the associated proper orthogonal decomposition (POD) modes. Analysis of instantaneous images shows that the edge of the turbulent boundary layer identified from flow visualization images generally matches the edge of the boundary layer determined from velocity and vorticity. The correlation between velocity deficit and smoke intensity was determined to be positive and relatively large (>0.7) indicating a moderate-to-strong relationship between the two. This notion was extended further through the use of a direct correlation approach and a complementary POD/linear stochastic estimation (LSE) approach to estimate the velocity field directly from flow visualization images. This exercise showed that, in many cases, velocity fields estimated from smoke intensity were similar to the actual velocity fields. The complementary POD/LSE approach proved better for these estimations, but not enough to suggest using this technique to approximate velocity measurements from a smoke intensity image. Instead, the correlations further validate the use of flow visualization techniques for determining the edge and large-scale shape of a turbulent boundary layer, specifically when quantitative velocity measurements, such as PIV, are not possible in a given experiment.

The particle reconstruction capabilities of a novel 3-D, 3-C PIV technique, based on volume illumination and a plenoptic camera to measure a velocity field, are tested. This technique is based on light-field photography, which uses a dense microlens array mounted near a camera sensor to sample the spatial and angular distribution of light entering the camera. Tomographic algorithms (MART) are then used to reconstruct a volumetric intensity field after the image is taken, and cross-correlation algorithms extract the velocity field from the reconstructed volume. This paper provides an introduction to the concepts of light fields and describes the tomographic algorithms used to reconstruct the measurement volume. A preliminary test on the accuracy of single particle reconstructions is presented, showing that laterally we can expect errors to be less than a voxel for most cases. A test on the reconstruction quality is presented for a volume of particles as a function of particle density and is shown to vary little as the particle density is increased. Finally a simulated Gaussian ring vortex is presented showing a full simulation as well as the velocity accuracy.

This paper describes current calibration methods for plenoptic cameras and introduces a new method of calibration that seeks to estimate the position and orientation of the microlens array based on the camera geometry and on a calibration image. A geometrical model was formulated to relate the position of a microlens to the location on the image sensor where the lens was focused. The location of the focal points were determined by stopping down the aperture of the main camera lens such that only a small beam of light was incident on each microlens. The position and orientation of the microlens array was assumed, and the predicted focal points were compared with the known points determined from the calibration data. This process was repeated until the root-mean-square difference between the expected and predicted results was minimal. The geometrical method was shown to provide a reasonable estimate for the orientation of the microlens array, as each translational parameter converged to less than one pixel and each rotational parameter converged to within 0.00034 radians. Preliminary results show that the best estimate for the image distance is obtained from a measurement of the magnification by imaging a ruler.

Digital in-line holography (DIH) and plenoptic photography are two techniques for single-shot, volumetric measurement of 3D particle fields. Here we present a comparison of the two methods by applying plenoptic imaging to experimental configurations that have been previously investigated with DIH. These experiments include the tracking of secondary droplets from the impact of a water drop on a thin film of water and tracking of pellets from a shotgun. Both plenoptic imaging and DIH successfully quantify the 3D nature of these particle fields. This includes measurement of the 3D particle position, individual particle sizes, and three-component velocity vectors. For the initial processing methods presented here, both techniques give out-of-plane positional accuracy of approximately 1-2 particle diameters. For a fixed image sensor, digital holography achieves higher effective in-plane spatial resolutions. However, collimated and coherent illumination makes holography susceptible to image distortion through index of refraction gradients, as demonstrated in the shotgun experiments. In contrast, plenoptic imaging allows for a simpler experimental configuration and, due to the use of diffuse, white-light illumination, plenoptic imaging is less susceptible to image distortion in the shotgun experiments.

An experimental investigation was conducted on the ow elds behind various hemisphereon-cylinder turret geometries in subsonic ow. The e ects that geometric modi cations had on the baseline ow eld were characterized to determine which designs, if any, demonstrate potential as passive ow control mechanisms. Qualitative results were obtained for each turret model using a pulse burst laser to perform three-dimensional ow visualization. Additional, quantitative measurements along the wake centerline for each design were made via planar particle image velocimetry (PIV). The results of each modi ed ow eld are presented with the baseline case for comparison. Suggested future work is also given for those designs showing the most promise as passive ow control devices.

A density measurement technique based on the planar laser induced fluorescence (PLIF) of acetone vapor was developed and shown to be practical for 2-D density measurements in a high speed wind transonic wind tunnel. The random uncertainty in density measurements was estimated to be on the order of 8% without pixel binning and as low as 3% with pixel binning, thus showing that acetone PLIF is a viable technique for measurement of density in compressible flow fields. The technique was applied to visualize the shock induced separation and turbulent wake of a hemisphere mounted in a Mach 0.78 free stream. Density images clearly showed the presence of shock waves, a flapping wake structure and significant density drops within the cores of vortices formed in the separating free shear layer. The density drops could reach magnitudes up to 50% from the free stream value confirming that compressibility effects on density fluctuations within a weakly compressible shear layer are significant and cannot be explained through mixing models alone.

The fundamental principles of Background Oriented Schlieren (BOS) imaging are conducive for the measurement of optical wavefront distortions imposed by turbulent flows. This work explores the initial development of a wavefront sensor based on BOS. The advantages of a BOS based wavefront sensor over a competing device, such as a Shack-Hartmann wavefront sensor, is the ability to measure large aperture wavefronts with potentially high spatial resolution in an economical fashion. An analytical analysis which incorporates the imaging function requirement of the sensor illustrates the ability to measure local wavefront tilts with accuracies on the order of 20 microradians. The analysis finds that under imaging conditions, sensitivity is mainly a function of the lens f-number, which allows the background to be positioned further away from the test section, with the lens focal length only playing a minor role. These ideas were tested using experiments conducted on a cone mounted in a Mach 2.0 wind tunnel. Wavefront measurements agreed qualitatively with the distortion expected from an analytical model of the conical shock’s density field. Turbulent boundary layers and Mach wave radiation from the tunnel walls were also detected. Future experiments will calibrate the measurement with a known distortion source. Overall, the concept of a BOS based wavefront sensor is shown to be a valid and viable option for wavefront measurements, particularly for aero-optics studies in medium to large-scale environments.

A three-dimensional (3-D) modal analysis of an excited circular jet with Reynolds number of 9500 is conducted using proper orthogonal decomposition (POD) and a highspeed 3-D flow visualization system. Large sets of 3-D data are acquired in the near field before the end of the potential core and the transition region of the jet. POD is used to objectively characterize and classify the 3-D images and elucidate the fundamental structure of the flow. As expected, the near field of the jet is dominated by the formation and growth of ring vortices, which is also reflected in the shape of the POD modes. The onset of azimuthal instabilities is also clear in the instantaneous 3-D images, which show long streamwise fingers, or side jets, of fluid surrounding the vortex rings. The results presented here show the strength of 3-D modal analysis for understanding the most dominant structures and instabilities in the jet.

A high-speed 3-D flow visualization system is used to capture large sets of instantaneous 3-D images of a naturally excited jet with Reynolds number of 6700. Images were acquired in both the near-field before the end of the potential core and the far-field of the jet just beyond the end of the potential core. Proper orthogonal decomposition (POD) is used to objectively characterize and classify the 3-D images and elucidate the fundamental structure of the flow. Preliminary results from this analysis are presented. As expected, the near field of the jet is dominated by the formation and growth of ring vortices, which is also reflected in the shape of the POD modes. The onset of azimuthal instabilities is also clear in the instantaneous 3-D images, which show long streamwise fingers of fluid surrounding the vortex rings. In the far-field, 3-D images continue to show the presence of unmixed jet core fluid; however, the shape of this region is highly convoluted with a different structure than in the near-field. In some images the core is seen to follow a helical type path downstream while in other images the core appears as a vortex puff type structure. Both types of structures are captured in the POD analysis along with some other unique 3-D flow structure. The results presented here are preliminary and further analysis is necessary to fully appreciate the information contained in the 3-D images. Future work will look at additional locations in the flow as well as the influence of axial excitation on the fundamental structure of the jet both in the near field and the far field. I. Introduction xcited jets are an excellent platform for the study of fluid dynamics and turbulence as they display numerous phenomena that are encountered throughout the field of fluid dynamics. Topics represented in an excited jet include instability, receptivity, vortex dynamics, transition, coherent structures and fully developed turbulence. As these features present themselves and develop with increasing distance from the jet nozzle, a jet flow field is convenient for the in-depth study of any of these topics by making measurements at the appropriate location downstream of the nozzle exit. In this paper, we present our preliminary efforts towards using a high-speed 3-D flow visualization technique and proper orthogonal decomposition to study the physics of transition to fully developed turbulent flow in an excited jet. Jets have received considerable attention over the last several decades [see Refs. 1-20 for a small sampling of the literature available on the subject]. Briefly, a jet’s flow field can be summarized as follows. The flow at the exit of the nozzle is uniform at the jet centerline with a region of shear near the wall. Upon exiting the nozzle, the shear layer with thickness, θ, is susceptible to the Kelvin-Helmholtz, or shear-layer, instability where small disturbances, typically characterized by their Strouhal number (Stθ = fθ/U), are amplified and eventually roll-up into organized and quasi-periodic sets of vortices. Further shear layer growth is dominated by the dynamics of these vortices and described by events such as vortex pairing. As the vortices grow and move towards the end of the potential core, the dominant jet instability mode becomes that of the preferred mode, or jet column mode, which is characterized by the Strouhal number based on the nozzle diameter (StD = fD/U). Near the end of the potential core and beyond, the

Advances in high-repetition rate laser and camera technology present a renewed opportunity to develop high-speed three-dimensional flow diagnostics based on the rapid scanning of a laser sheet through a flow field. Recent progress towards developing a high-speed 3-D flow visualization system is presented. The system is based on a MHz rate pulse burst laser, a galvanometric scanning mirror and an ultra-high-speed camera. Once development is complete, the system will be able to acquire a 3-D image in tens of microseconds, making it suitable for a broad range of high-speed flow fields. In this paper, a low-speed version of the system was used to image the 3-D structure of vortex rings emitted from a small smoke chamber. The rich nature of the 3-D images demonstrates the inherent potential of the technique. Future work includes incorporation of the pulse burst laser into the 3-D system.

A novel three-dimensional (3D), three-component (3C) particle image velocimetry (PIV) technique based on volume illumination and light field imaging with a single plenoptic camera is described. A plenoptic camera uses a densely packed microlens array mounted near a high resolution image sensor to sample the spatial and angular distribution of light collected by the camera. The multiplicative algebraic reconstruction technique (MART) computed tomography algorithm is used to reconstruct a volumetric intensity field from individual snapshots and a cross-correlation algorithm is used to estimate the velocity field from a pair of reconstructed particle volumes. This work provides an introduction to the basic concepts of light field imaging with a plenoptic camera and describes the unique implementation of MART in the context of plenoptic image data for 3D/3C PIV measurements. Simulations of a plenoptic camera using geometric optics are used to generate synthetic plenoptic particle images, which are subsequently used to estimate the quality of particle volume reconstructions at various particle number densities. 3D reconstructions using this method produce reconstructed particles that are elongated by a factor of approximately 4 along the optical axis of the camera. A simulated 3D Gaussian vortex is used to test the capability of single camera plenoptic PIV to produce a 3D/3C vector field, where it was found that lateral displacements could be measured to approximately 0.2 voxel accuracy in the lateral direction and 1 voxel in the depth direction over a voxel volume. The feasibility of the technique is demonstrated experimentally using a home-built plenoptic camera based on a 16-megapixel interline CCD camera and a array of microlenses and a pulsed Nd:YAG laser. 3D/3C measurements were performed in the wake of a low Reynolds number circular cylinder and compared with measurements made using a conventional 2D/2C PIV system. Overall, single camera plenoptic PIV is shown to be a viable 3D/3C velocimetry technique.

st change is that the pulse slicing is achieved using an acousto-optic modulator (AOM) rather than a pair of electro-optic Pockels cells. The AOM is less expensive (by ~90%) and is able to produce flexible bursts of pulses with variable pulse durations (as short as 10 nsec) at repetition rates ranging from DC to greater than 5 MHz. The energy of the pulses prior to amplification is estimated to be from 0.5 to 3.3 nJ for 10 to 50 nsec long pulses, respectively. The contrast ratio of the pulses produced by the AOM is ~800:1 before spatial filtering and ~15,000:1 after spatial filtering. The 2 nd major change in the design of the laser system is the temporal shape of the gain curve created by the power amplifier chain. The design of the flashlamp controller has been specified to produce approximately uniform gain over a 1 msec long period of time. This is contrast to previous designs, which had 100-200 μsec long gain curves with Gaussian shaped profiles. Preliminary measurements show that the total gain for the amplifier chain is approximately 10 7 resulting in individual pulse energies on the order of 10 mJ/pulse. This level of gain is expected to drop by only 10% over the 1 msec duration of gain envelope.

The convective velocity of large-scale structures in a Mach 2.0 compressible axisymmetric jet is explored using both qualitative flow visualization and quantitative velocity data acquired using MHz rate planar Doppler velocimetry. Space-time correlations based on qualitative flow visualization image sequences appear to show two modes of convective velocity, a fast and a slow mode, that deviate significantly from the theoretically expected value. Space-time correlations based on velocity data, however, appear to show only a single mode of convective velocity with a magnitude close the theoretical value. It is found that the use of space-time correlations on inherently qualitative flow visualization images can produce misleading results. The implications of these findings are significant with further work necessary before broader conclusions can be drawn.

Recent progress in the development of MHz frame flow diagnostics are presented, based on enhancements to a "burst" mode Nd:YAG laser. In particular, a PDV system that can measure velocity over a plane at rates up to 1 MHz is demonstrated, as well initial results from a custom-built Optical Parametric Oscillator, designed specifically for use with the burst mode pump. Production of broadly wavelength tunable pulse trains, with order mJ individual pulse energy and order 1 cm/sup -1/ spectral linewidth is described.

Temporally resolved PDV is further developed by extending the technique from one component to two components. This is achieved by using a pulse burst laser with MHz rate repetition capabilities and two ultra-high speed framing rate CCD cameras. Each camera is used in a conventional splitter/recombiner PDV system to measure one component of velocity. These two measurements are then combined to resolve the u and w components of velocity. Very preliminary results on a newly constructed planar shear layer facility show the strong potential of this technique and identify a few issues that must be addressed in the next iteration of experiments.

The dynamics of large-scale turbulence structures within a high Reynolds number, ideally expanded Mach 1.3 jet were investigated during both the periods of production of strong acoustic radiation and extended periods of relative quiet that lacked such acoustic radiation. These results were acquired through a unique experiment where the sources of large amplitude sound waves were estimated with a three-dimensional microphone array and the flow field was simultaneously visualized on two orthogonal planes. The images from one of the planes were taken at a 167 kHz rate. Proper Orthogonal Decomposition (POD) was employed to create a basis of the size and distribution of large-scale structures within the two planes. These POD modes were then used to objectively determine the differences in the jet structure during noise generation and periods lacking significant noise generation. The results show that the flow during the periods of relative quiet cases is dominated by the lower order POD modes that consist of relatively large turbulence structures while it is dominated by higher order POD modes that capture the dynamic interplay of the large-scale structures during noise generation periods. For approximately one convective time scale prior to the moment of noise emission, a series of large-scale structures forms and disintegrates within the mixing layer and in the process a large amount of ambient fluid is entrained into the core of the jet. For the first time, these results show how the dynamic interplay of large-scale turbulence structures generates acoustic radiation within a high Reynolds number jet.

The dynamics of large-scale turbulence structures within a high Reynolds number, ideally expanded Mach 1.3 jet were investigated during both the periods of production of strong acoustic radiation and extended periods of relative quiet that lacked such acoustic radiation. These results were acquired through a unique experiment where the sources of large amplitude sound waves were estimated with a three-dimensional microphone array and the flow field was simultaneously visualized on two orthogonal planes. The images from one of the planes were taken at a 167 kHz rate. Proper Orthogonal Decomposition (POD) was employed to create a basis of the size and distribution of large-scale structures within the two planes. These POD modes were then used to objectively determine the differences in the jet structure during noise generation and periods lacking significant noise generation. The results show that the flow during the periods of relative quiet cases is dominated by the lower order POD modes that consist of relatively large turbulence structures while it is dominated by higher order POD modes that capture the dynamic interplay of the large-scale structures during noise generation periods. For approximately one convective time scale prior to the moment of noise emission, a series of large-scale structures forms and disintegrates within the mixing layer and in the process a large amount of ambient fluid is entrained into the core of the jet. For the first time, these results show how the dynamic interplay of large-scale turbulence structures generates acoustic radiation within a high Reynolds number jet.

A pulse burst laser and two high-speed CCD cameras were used to perform one-component MHz Rate Planar Doppler Velocimetry (PDV) measurements in an ideally expanded Mach 2.0 jet. The pulse burst laser produced 28 0.532 micron pulses at 250 kHz with ~9 mJ/pulse. Both two-camera and a single-camera experimental set-ups were used to measure a single component of velocity on a streamwise plane passing through the jet centerline and covering ~6 – 12 jet heights downstream of the jet exit. Velocity image sequences consisting of 28 frames showed dynamics of the velocity field over a time span of 108 microseconds (~4.5 non-dimensional time scales). A typical sequence of images is presented, which demonstrates the process of entrainment as low-speed fluid rolls up into the high-speed portion of the jet. Mean and standard deviation statistics of the velocity calculations produced expected trends and showed good agreement between the single- and two-camera experiments. An error analysis revealed speckle as the predominant source of noise, as in a conventional PDV technique. Accuracy is estimated to be 16 m/s for the single-camera system and 24 m/s for the two-camera system.

Simultaneous high spatial-resolution flow visualization and wavefront sensing are used to investigate the optical aberrations that occur due to a compressible shear layer. A preliminary model is developed to relate flow visualization images with wavefronts measured using a Shack-Hartmann wavefront sensor. Initial results are quite encouraging as a comparison between the Shack-Hartmann measured wavefronts and wavefronts produced by applying the model to flow visualization images produces correlation levels well above 0.7. Future work will incorporate a more realistic geometry to further develop the model and investigate the effects of individual large-scale structures on wavefront distortion with more detail.

Newly developed optical diagnostic techniques are used to examine the temporal evolution of wavefront aberrations imposed upon an optical beam as it passes through a rectangular compressible turbulent jet. A recently developed MHz rate system based on a pulse burst laser and ultra highspeed CCD camera is used for flow visualization. A newly developed two-dimensional MHz rate Shack-Hartman wavefront sensor is used to measure the wavefront distortion. The wavefront sensor consists of a HeNe laser, a micro-lens array and an ultra high-speed camera. The MHz rate flow visualization and wavefront sensing are used simultaneously to investigate the aero-optic effects of an ideally expanded, high Reynolds number, Mach 1.3 rectangular jet. Preliminary results are presented that show the potential of the technique to acquire time-resolved two-dimensional wavefront data with detailed aero-optic effects. A number of improvements to the technique that will be used in future work are also discussed.