Split-screen single-camera stereoscopic PIV application to a turbulent confined swirling layer with free surface

An annular liquid wall jet, or vortex tube, generated by helical injection inside a tube is studied experimentally as a possible means of fusion reactor shielding. The hollow confined vortex/swirling layer exhibits simultaneously all the complexities of swirling turbulence, free surface, droplet formation, bubble entrapment; all posing challenging diagnostic issues. The construction of flow apparatus and the choice of working liquid and seeding particles facilitate unimpeded optical access to the flow field. A split-screen, single-camera stereoscopic particle image velocimetry (SPIV) scheme is employed for flow field characterization. Image calibration and free surface identification issues are discussed. The interference in measurements of laser beam reflection at the interface are identified and discussed. Selected velocity measurements and turbulence statistics are presented at Re_λ = 70 (Re = 3500 based on mean layer thickness)

Defocusing digital particle image velocimetry and the three-dimensional characterization of two-phase flows

Defocusing digital particle image velocimetry (DDPIV) is the natural extension of planar PIV techniques to the third spatial dimension. In this paper we give details of the defocusing optical concept by which scalar and vector information can be retrieved within large volumes. The optical model and computational procedures are presented with the specific purpose of mapping the number density, the size distribution, the associated local void fraction and the velocity of bubbles or particles in two-phase flows. Every particle or bubble is characterized in terms of size and of spatial coordinates, used to compute a true three-component velocity field by spatial three-dimensional cross-correlation. The spatial resolution and uncertainty limits are established through numerical simulations. The performance of the DDPIV technique is established in terms of number density and void fraction. Finally, the velocity evaluation methodology, using the spatial cross-correlation technique, is described and discussed in terms of velocity accuracy

Ground-based detectors in very-high-energy gamma-ray astronomy

Following the discovery of the cosmic rays by Victor Hess in 1912, more than 70 years and numerous technological developments were needed before an unambiguous detection of the first very-high-energy gamma-ray source in 1989 was made. Since this discovery the field on very-high-energy gamma-ray astronomy experienced a true revolution: A second, then a third generation of instruments were built, observing the atmospheric cascades from the ground, either through the atmospheric Cherenkov light they comprise, or via the direct detection of the charged particles they carry. Present arrays, 100 times more sensitive than the pioneering experiments, have detected a large number of astrophysical sources of various types, thus opening a new window on the non-thermal Universe. New, even more sensitive instruments are currently being built; these will allow us to explore further this fascinating domain. In this article we describe the detection techniques, the history of the field and the prospects for the future of ground-based very-high-energy gamma-ray astronomy.Comment: 21 pages, 13 figure

Camera positioning for 3D panoramic image rendering

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University London.Virtual camera realisation and the proposition of trapezoidal camera architecture are the two broad contributions of this thesis. Firstly, multiple camera and their arrangement constitute a critical component which affect the integrity of visual content acquisition for multi-view video. Currently, linear, convergence, and divergence arrays are the prominent camera topologies adopted. However, the large number of cameras required and their synchronisation are two of prominent challenges usually encountered. The use of virtual cameras can significantly reduce the number of physical cameras used with respect to any of the known camera structures, hence adequately reducing some of the other implementation issues. This thesis explores to use image-based rendering with and without geometry in the implementations leading to the realisation of virtual cameras. The virtual camera implementation was carried out from the perspective of depth map (geometry) and use of multiple image samples (no geometry). Prior to the virtual camera realisation, the generation of depth map was investigated using region match measures widely known for solving image point correspondence problem. The constructed depth maps have been compare with the ones generated using the dynamic programming approach. In both the geometry and no geometry approaches, the virtual cameras lead to the rendering of views from a textured depth map, construction of 3D panoramic image of a scene by stitching multiple image samples and performing superposition on them, and computation of virtual scene from a stereo pair of panoramic images. The quality of these rendered images were assessed through the use of either objective or subjective analysis in Imatest software. Further more, metric reconstruction of a scene was performed by re-projection of the pixel points from multiple image samples with a single centre of projection. This was done using sparse bundle adjustment algorithm. The statistical summary obtained after the application of this algorithm provides a gauge for the efficiency of the optimisation step. The optimised data was then visualised in Meshlab software environment, hence providing the reconstructed scene. Secondly, with any of the well-established camera arrangements, all cameras are usually constrained to the same horizontal plane. Therefore, occlusion becomes an extremely challenging problem, and a robust camera set-up is required in order to resolve strongly the hidden part of any scene objects. To adequately meet the visibility condition for scene objects and given that occlusion of the same scene objects can occur, a multi-plane camera structure is highly desirable. Therefore, this thesis also explore trapezoidal camera structure for image acquisition. The approach here is to assess the feasibility and potential of several physical cameras of the same model being sparsely arranged on the edge of an efficient trapezoid graph. This is implemented both Matlab and Maya. The quality of the depth maps rendered in Matlab are better in Quality

Pressure forces on sediment particles in turbulent open-channel flow : a laboratory study

Acknowledgements This research was sponsored by EPSRC grant EP/G056404/1 and their financial support is greatly appreciated. We also acknowledge Dr S. Cameron, who developed the PIV system and its algorithms. The design and construction of pressure sensors was carried out at the workshop and the experiments were conducted in the fluids laboratory at the University of Aberdeen. We therefore express our gratitude to the workshop and laboratory technicians and also to Mr M. Witz and Mr S. Gretland for their assistance in carrying out these experiments. The authors would also like to thank Professor J. Frohlich, Professor M. Uhlmann, Dr C.-B. Clemens and Mr B. Vowinckel for their useful suggestions and discussions throughout the course of this project. The Associate Editor Professor I. Marusic and four anonymous reviewers provided many useful and insightful comments and suggestions that have been gratefully incorporated into the final version.Peer reviewedPublisher PD

A low-cost defocus blur module for video rate quantified 3D imaging

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2004.Includes bibliographical references.Existing three-dimensional surface imaging systems are expensive, difficult to use, time consuming, and do not always provide the best accuracy or resolution. By using an offset aperture on a rotating disc, the 3D Monocular Imaging System provides a fast, portable, accurate, and cheap method of 3D surface imaging by relating the differences in images generated by the aperture at different positions to the depth of the features on the target surface. A cheaper and simpler alternative to the Monocular System was designed such that two offset fixed apertures would replace the rotating aperture. Rhombic prisms and a light-blocking mask ensured that the images would be generated properly on the camera's imaging surface. The new prototype was never completed, but the purchasing of the parts suggested that the cost of production would not drop enough to consider the module a popular purchase for home electronics usage. In addition, the requirement of precision-machined parts increased the time, effort, and cost to produce the module. However, the design for this new system is a viable alternative to the original 3D Monocular System since it is smaller, simpler, and cheaper.by Leeway Ho.S.B

Holographic Particle Image Velocimetry of Ink Jet Streams

Ink jet technology is a rapidly growing and diverse field of research. Ink jets are used to deliver very precise and small (picolitre) volumes of fluid to a surface. Recent advances in ink jet technology demand a better understanding of the dynamics of the fluid during jetting. The aim of this project was to design a method capable of measuring the flow velocities inside ink jet streams. This objective has been achieved by the use of digital holographic particle image velocimetry. The difficulty with measuring flows inside tightly curved samples is that the refractive index change over the boundary leads to an optical distortion and therefore particles cannot be viewed or tracked reliably. Optical distortion is compensated for by taking advantage of the ability to replay a holographically recorded wave. The light scattered by particles is propagated numerically back through the sample’s surface, to form a three-dimensional image in which all refractions at the interface have been accounted for. Three dimensional particle fields are then analysed using custom particle detection and correlation code to extract the displacement of individual particles between exposures, which facilitates the construction of full flow profiles. Holograms were recorded with a simple off-axis holographic microscope, comprising two point sources of divergent light, formed from the same objective lens, acting as the source of illumination and reference light, respectively. Experiments were conducted on continuous ink jet streams of water issuing from a nozzle with 100 µm diameter. For a few millimetres after the nozzle exit, the jet is cylindrical, it then starts to form swells and necks; the swells continue to grow at the expense of the necks until the jet breaks up into a stream of droplets. Measurements of the stream wise component of velocity have been successful in the cylindrical parts of the jet, in swells and in necks greater than 20 µm in diameter. To my knowledge measurements of particle velocities on fluid jets at this scale have not been accomplished previously

Analytical and experimental investigations of dual-plane PIV

In its 'classical' form particle image velocimetry (PIV) extracts two components of the flow velocity vector by measuring the displacement of tracer particles within a double-pulsed laser light sheet. The method described in this paper is based on the additional recording of a third exposure of the tracer particles in a parallel light sheet, which is slightly displaced with respect to the first one. The particle images resulting from these three exposures are stored on separate frames. The locations of the correlation peaks, as obtained by cross-correlation methods, are used to determine the projections of the velocity vectors onto the plane between both light sheets. In the manner described below, the amplitudes of these peaks are used to obtain information about the velocity component perpendicular to the light sheet planes. The mathematical background of this method is described in the paper. Numerical simulations show the influence of the main parameters (e.g. light sheet thickness, light sheet displacement and out-of-plane component) on the resolution and reliability of the new method. Two different recording procedures and their results will be shown to demonstrate the ease of operation when applying this technique to liquid flows

Tri-dimensional optical inspection based on flexible image guide: first step toward 3D industrial endoscopy

International audience3D optical endoscopy is now a major challenge to allow the high resolution inspection of industrial equipments. The proposed instrument is based on a flexible image guide (70 000 fibres) and a Digital Micro mirror Device (DMD, 1024 x 768 "on-off" micro mirrors). The optical design is as follows: the light emitted by a 532 nm laser diode is dynamically structured by the DMD chip as a fringes pattern which is phase-shifted due to the active control of the DMD chip and projected onto an object on a circular field of 6 mm in diameter. Due to a telecentric and binocular arrangement that creates a stereoscopic angle, it is possible to get a depth of field of 2 mm along the optical axis without keystone distortions and few disturbances created by defocus and coma aberrations. Then, images are captured by a 1024 x 768 digital camera (not yet moved away by fibres) at 15 fps and directly used in the reconstruction algorithm to access the tri-dimensional shape of the unpainted object. The results are compared to incoherent white light results obtained with white painted mechanical objects. The lateral resolution is 31.3 µm and the RMS axial resolution is 10 µm for the laser-based design after speckle attenuation