Kyle P. Lynch

A high-order time-accurate interrogation method for time-resolved PIV

A novel method is introduced for increasing the accuracy and extending the dynamic range of time-resolved particle image velocimetry (PIV). The approach extends the concept of particle tracking velocimetry by multiple frames to the pattern tracking by cross-correlation analysis as employed in PIV. The working principle is based on tracking the patterned fluid element, within a chosen interrogation window, along its individual trajectory throughout an image sequence. In contrast to image-pair interrogation methods, the fluid trajectory correlation concept deals with variable velocity along curved trajectories and non-zero tangential acceleration during the observed time interval. As a result, the velocity magnitude and its direction are allowed to evolve in a nonlinear fashion along the fluid element trajectory. The continuum deformation (namely spatial derivatives of the velocity vector) is accounted for by adopting local image deformation. The principle offers important reductions of the measurement error based on three main points: by enlarging the temporal measurement interval, the relative error becomes reduced; secondly, the random and peak-locking errors are reduced by the use of least-squares polynomial fits to individual trajectories; finally, the introduction of high-order (nonlinear) fitting functions provides the basis for reducing the truncation error. Lastly, the instantaneous velocity is evaluated as the temporal derivative of the polynomial representation of the fluid parcel position in time. The principal features of this algorithm are compared with a single-pair iterative image deformation method. Synthetic image sequences are considered with steady flow (translation, shear and rotation) illustrating the increase of measurement precision. An experimental data set obtained by time-resolved PIV measurements of a circular jet is used to verify the robustness of the method on image sequences affected by camera noise and three-dimensional motions. In both cases, it is demonstrated that the measurement time interval can be significantly extended without compromising the correlation signal-to-noise ratio and with no increase of the truncation error. The increase of velocity dynamic range scales more than linearly with the number of frames included for the analysis, which supersedes by one order of magnitude the pair correlation by window deformation. The main factors influencing the performance of the method are discussed, namely the number of images composing the sequence and the polynomial order chosen to represent the motion throughout the trajectory.

Volumetric particle image velocimetry with a single plenoptic camera

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.

Third-generation megahertz-rate pulse burst laser system

The design and performance of a third-generation megahertz-rate pulse burst laser system is described. The third-generation system incorporates two distinct design changes that distinguish it from earlier-generation systems. The first is that pulse slicing is now achieved by using an economical acousto-optic modulator (AOM), and the second is the use of a variable pulse duration flashlamp driver that provides relatively uniform gain over a ~700 mus window. The use of an AOM for pulse slicing permits flexible operation such as pulse-on-demand operation with variable pulse durations ranging from 10 ns to DC. The laser described here is capable of producing a burst of laser pulses at repetition rates as high as 50 MHz and peak powers of 10 kW. Second-harmonic conversion efficiency using a type II KTP crystal is also demonstrated.

Three-Dimensional Particle Image Velocimetry Using a Plenoptic Camera

A novel 3-D, 3-C PIV technique is described, based on volume illumination and a plenoptic camera to measure a velocity field. The technique is based on plenoptic photography, which uses a dense microlens array mounted near a camera sensor to sample the spatial and angular distribution of light entering the camera. Various algorithms 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 plenoptic photography, and describes the algorithms used to reconstruct the measurement volume. A comparison is made between the use of a combined computational refocusing and thresholding approach versus a direct tomographic reconstruction approach. This discussion lays the groundwork for a more detailed study of reconstruction accuracy, achieveable particle number density, reconstruction ambiguities (e.g., ghost particles), and other factors in a following study. Additionally, the construction of a prototype camera based on a 16-megapixel interline CCD sensor is described and preliminary experimental renderings are given.

Development of a High-Speed Three-Dimensional Flow Visualization Technique

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.

Density Measurements of a Turbulent Wake Using Acetone Planar Laser-Induced Fluorescence

The application of a planar density measurement technique for compressible flowfields based on acetone planar laser-induced fluorescence is presented. An error analysis indicates a minimum inherent uncertainty of ∼2.5% in density measurements due to uncertainty in local pressure and a total experimental uncertainty of 8%, primarily driven by shot noise due to low-signal levels. The technique is demonstrated through the visualization of the separated shear layer and turbulent wake of a wall-mounted hemisphere at a freestream Mach number of 0.78 and a Reynolds number of approximately 900,000. The flow is marked by a large-scale flapping motion of the wake and low-density vortex cores, where density drops of up to 50% of the freestream density are detected. In addition, closeup images of the shear layer near the separation point reveal the formation of lambda shocks. The density fields are used to perform aerooptic distortion calculations through spatial integration of the density field. A correlation is fou...

Preliminary Development of a 3-D, 3-C PIV Technique using Light Field Imaging

A novel 3-D, 3-C PIV technique is described, based on volume illumination and a plenoptic camera to measure a velocity eld. The technique is based on plenoptic photography, which uses a dense microlens array mounted near a camera sensor to sample the spatial and angular distribution of light entering the camera. This combination of spatial and angular information is termed the light eld, and is measured in this work with a plenoptic camera. Computational algorithms are then used to reconstruct a volumetric intensity eld after the image is taken. This paper provides an introduction to the concepts of light eld photography and describes the algorithms used to render the intensity eld. The algorithms are tested using simulated data to evaluate their reconstruction accuracy and their eectiveness with correlation algorithms. Additionally, the construction of a prototype camera based on a 16-megapixel sensor is described. This work demonstrates the ability to make 3-D, 3-C PIV measurements using simulated data of uniform and vortex ow, and provides the basis for further development using experimental data and more advanced reconstruction algorithms.

Three-Dimensional Flow Visualization Using a Pulse Burst Laser System

Further development of a high-speed three-dimensional flow visualization system is explored. The technique is based on the high-speed scanning and imaging of a laser sheet produced by a pulse burst laser capable of operating in excess of 1 MHz. The focus of this study is on the further development of the pulse burst laser, synchronization between the multiple system components, and data processing to yield 3-D images and high-quality 2-D images. Multiple 3-D visualizations (220 x 220 x 68 voxel resolution) of an acousticallyexcited jet are presented, each taken at “full-speed,” with the scanning mirror directing the laser through the flow, and the high-speed camera operating at 500,000 fps. In this configuration, each 3-D image took 136 μs to acquire. The presented visualizations demonstrate the ability of the technique to visualize complex, three-dimensional flow structures, such as ring vortices. Future work will concentrate on increasing laser intensity at shorter pulse durations to increase resolution and allow the technique to be used for supersonic flows.

3-D POD Analysis of a Naturally Excited 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

Density Measurements of a High-Speed, Compressible Flow Field Using Acetone Planar Laser Induced Fluorescence (PLIF)

Progress in the development of a density measurement technique using acetone planar laser induced uorescence (PLIF) is presented. PLIF data has been acquired in the separated shear layer and wake downstream of a wall-mounted hemisphere at transonic conditions (M1 0:78, ReD 9 10). An established acetone uorescence signal model was slightly modi ed and implemented to convert the raw images to density eld data. The uncertainty of the density measurement was determined to be approximately 8% due to uncertainty in local pressure and measurement noise. The density eld is marked by large scale structures and distinct vortex cores in the shear layer and wake region where density drops of up to 50% of the freestream density were observed. In addition, images of the shear layer reveal the formation of shock waves and lambda shocks at or near the separation point. This density data is used to calculate the aero-optical distortion for a planar wavefront passing through the ow eld. These results indicate that acetone PLIF is a viable technique for measuring density elds and studying aero-optical distortion in high-speed, compressible ows.