Browsing by Author "Xu, Wenjiang"

Now showing 1 - 2 of 2
  • Thumbnail Image
    Development and validation of a reconstruction algorithm for three-dimensional nonlinear tomography problems
    Lei, Qingchun; Wu, Yue; Xu, Wenjiang; Ma, Lin (Optical Society of America, 2016-07-06)
    This work reports the development and experimental validation of a reconstruction algorithm for three-dimensional (3D) nonlinear tomography problems. Many optical tomography problems encountered in practice are nonlinear, for example, due to significant absorption, multiple-scattering, or radiation trapping. Past research efforts have predominately focused on reconstruction algorithms for linear problems, and these algorithms are not readily extendable to nonlinear problems due to several challenges. These challenges include the computational cost caused by the nonlinearity (which was compounded by the large scale of the problems when they are 3D), the limited view angles available in many practical applications, and the measurement uncertainty. A new algorithm was therefore developed to overcome these challenges. The algorithm was validated both numerically and experimentally, and was demonstrated to be able to solve a range of nonlinear tomography problems with significantly enhanced efficiency and accuracy compared to existing algorithms.
  • Thumbnail Image
    Three Dimensional Laser Diagnostics for Turbulent Flows and Flames
    Xu, Wenjiang (Virginia Tech, 2017-11-01)
    Due to their scientific significance and practical applications, turbulent flows and flames have been under extensive and intensive research for a long time. Turbulent flows and flames of interests to practice inherently have three-dimensional (3D) spatial structures, and therefore diagnostic techniques that can instantaneously resolve their 3D spatial features have long been desired and probably are needed to ultimately answer some of the open research questions. The goal of this dissertation thus is to investigate such diagnostics and demonstrate their capability and limitations in a range of turbulent flows/flames. To accomplish this goal, this dissertation developed and evaluated the following three diagnostic methods: tomographic chemiluminescence (TC), volumetric laser induced fluorescence (VLIF), and super-resolution planar laser induced fluorescence (SR-PLIF). First, 3D flame topography of well-controlled laboratory flames was measured with TC method and validated by a simultaneous 2D Mie scattering measurement. The results showed that the flame topography obtained from TC and the Mie scattering agreed qualitatively, but quantitative difference on the order of millimeter was observed between these two methods. Such difference was caused by the limitations of the TC method. The first limitation involves TC's reliance on chemiluminescence of nascent radicals (mainly CH*) in reacting flows, causing ambiguity in the definition of flame front and limiting its applications to certain types of reactive flow only. The second limitation involves TC's inability to study an isolated region of interest because the chemiluminescence is emitted everywhere in the flame. Based on the above understanding of the TC technique, the second part of this dissertation studied a VLIF method to overcome the above limitations of the TC technique. Compared with the TC technique, the VLIF method can be used in either reacting or non-reacting flow and on any particular region of interest. In the VLIF technique, the fluorescence signal was generated by exciting a target species with a laser slab of certain thickness. The signal was recorded by cameras from different perspectives, and then a VLIF tomographic algorithm was applied to resolve the spatial distribution of the concentration of the target species. An innovative 3D VLIF algorithm was proposed and validated by well-designed experiment. This model enables analysis of VLIF performance in terms of signal level, size of the field of view in 3D, and accuracy. However, due to the limited number of views and the tomographic reconstruction itself, the spatial resolution of VLIF methods is limited. Hence, the third part of this dissertation investigated a SR-PLIF method to provide a strategy to improve the spatial resolution in two spatial directions, and also to extend the measurement range of scanning 3D imaging strategies. The SR-PLIF method used planar images captured simultaneously from two (or more) orientations to reconstruct a final image with resolution enhanced or blurring removed. Both the development of SR algorithm, and the experimental demonstration of the SR-PLIF method were reported.