Digital inline holography has been proven to provide three-dimensional droplet position, size, and velocity distributions with a single camera. These data are crucial for understanding multiphase flows. In this work, we examine the performance of this diagnostic in the limit of very small particles, on the order of a pixel in diameter and smaller, and propose a postprocessing method to improve them: Lanczos interpolation. The Lanczos interpolation kernel is the digital implementation of the Whittaker sinc filter and strikes a compromise between maintaining the spatial frequency ceiling of the original digital image and computational cost of the interpolation. Without Lanczos interpolation, or supersampling, the ultimate detectable particle size floor is on the order of four pixel widths. We show in this work that this limit can be reduced by 50% or more with supersampling, depending upon the desired diameter accuracy, and examine the effect of supersampling on the resulting accuracy of the extracted size and position of spherical particles. Extending this resolution limit increases the overall detection efficiency of the diagnostic. Since this increases the spatial dynamic range of the diagnostic, it can also allow a larger field of view to be captured with the same particle size floor.
Trajectories of unique particles were tracked using spatially and temporally interlaced single-shot images from multiple views. Synthetic data were investigated to verify the ability of the technique to track particles in three-dimensions and time. The synthetic data was composed of four images from unique perspectives at four instances in time. The analysis presented verifies that under certain circumstances particle trajectories can be mapped in three dimensions from a minimal amount of information, i.e. one image per viewing angle. These results can enable four-dimensional measurements where they may otherwise prove unfeasible.
With the growth of light field imaging as an emerging diagnostic tool for the measurement of 3D particle fields, various algorithms for 3D particle measurements have been developed. These methods have exploited both the computational refocusing and perspective-shift capabilities of plenoptic imaging. This work continues the development of a 3D particle location method based on perspective-shifted plenoptic images. Specific focus is placed on adaptations that provide increased robustness for variations in and measurement of size and shape characteristics, thus allowing measurements of fragment fields. An experimental data set of non-spherical fragment simulants is studied to examine the dependency of the uncertainty of this perspective-shift based processing method on particle shape and the uncertainty of size measurements of fragments. Synthetic data sets are examined to provide metrics of the relationship between measurement uncertainty that can be achieved using this method, particle density, and processing time requirements.
Knowledge of soot particle sizes is important for understanding soot formation and heat transfer in combustion environments. Soot primary particle sizes can be estimated by measuring the decay of time-resolved laser-induced incandescence (TiRe-LII) signals. Existing methods for making planar TiRe-LII measurements require either multiple cameras or time-gate sweeping with multiple laser pulses, making these techniques difficult to apply in turbulent or unsteady combustion environments. Here, we report a technique for planar soot particle sizing using a single high-sensitivity, ultra-high-speed 10 MHz camera with a 50 ns gate and no intensifier. With this method, we demonstrate measurements of background flame luminosity, prompt LII, and TiRe-LII decay signals for particle sizing in a single laser shot. The particle sizing technique is first validated in a laminar non-premixed ethylene flame. Then, the method is applied to measurements in a turbulent ethylene jet flame.
Liquid metal breakup processes are important for understanding a variety of physical phenomena including metal powder formation, thermal spray coatings, fragmentation in explosive detonations and metalized propellant combustion. Since the breakup behaviors of liquid metals are not well studied, we experimentally investigate the roles of higher density and fast elastic surface oxide formation on breakup morphology and droplet characteristics. This work compares the column breakup of water with Galinstan, a room-temperature eutectic liquid metal alloy of gallium, indium and tin. A shock tube is used to generate a step change in convective velocity and back-lit imaging is used to classify morphologies for Weber numbers up to 250. Digital in-line holography (DIH) is then used to quantitatively capture droplet size, velocity and three-dimensional position information. Differences in geometry between canonical spherical drops and the liquid columns utilized in this paper are likely responsible for observations of earlier transition Weber numbers and uni-modal droplet volume distributions. Scaling laws indicate that Galinstan and water share similar droplet size-velocity trends and root-normal volume probability distributions. However, measurements indicate that Galinstan breakup occurs earlier in non-dimensional time and produces more non-spherical droplets due to fast oxide formation.
Imaging diagnostics that utilize coherent light, such as digital in-line holography, are important for object sizing and tracking applications. However, in explosive, supersonic, or hypersonic environments, gas-phase shocks impart imaging distortions that obscure internal objects. To circumvent this problem, some research groups have conducted experiments in vacuum, which inherently alters the physical behavior. Other groups have utilized single-shot flash x-ray or high-speed synchrotron x-ray sources to image through shock-waves. In this work, we combine digital in-line holography with a phase conjugate mirror to reduce the phase distortions caused by shock-waves. The technique operates by first passing coherent light through the shock-wave phase-distortion and then a phase-conjugate mirror. The phase-conjugate mirror is generated by a four-wave mixing process to produce a return beam that has the exact opposite phase-delay as the forward beam. Therefore, by passing the return beam back through the phase-distortion, the phase delays picked up during the initial pass are canceled, thereby producing improved coherent imaging. In this work, we implement phase conjugate digital in-line holography (PCDIH) for the first time with a nanosecond pulse-burst laser and ultra-high-speed cameras. This technique enables accurate measurement of the three-dimensional position and velocity of objects through shock-wave distortions at video rates up to 5 MHz. This technology is applied to improve three-dimensional imaging in a variety of environments from imaging supersonic shock-waves through turbulence, sizing objects through laser-spark plasma-generated shock-waves, and tracking explosively generated hypersonic fragments. Theoretical foundations and additional capabilities of this technique are also discussed.
The volumetric calibration of a plenoptic camera is explored to correct for inaccuracies due to real-world lens distortions and thin-lens assumptions in current processing methods. Two methods of volumetric calibration based on a polynomial mapping function that does not require knowledge of specific lens parameters are presented and compared to a calibration based on thin-lens assumptions. The first method, volumetric dewarping, is executed by creation of a volumetric representation of a scene using the thin-lens assumptions, which is then corrected in post-processing using a polynomial mapping function. The second method, direct light-field calibration, uses the polynomial mapping in creation of the initial volumetric representation to relate locations in object space directly to image sensor locations. The accuracy and feasibility of these methods is examined experimentally by capturing images of a known dot card at a variety of depths. Results suggest that use of a 3D polynomial mapping function provides a significant increase in reconstruction accuracy and that the achievable accuracy is similar using either polynomial-mapping-based method. Additionally, direct light-field calibration provides significant computational benefits by eliminating some intermediate processing steps found in other methods. Finally, the flexibility of this method is shown for a nonplanar calibration.