Browsing by Author "Nobes, David S."

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    Acousto-optic frequency switching for single-camera planar Doppler velocimetry
    (International Society for Optical Engineering; 1999, 2001-12-31T00:00:00Z) Ford, Helen D.; Nobes, David S.; Tatam, Ralph P.; Mercer, Carolyn R.; Cha, Soyoung S.; Shen, Gongxin X.
    A single-camera Planar Doppler velocimetry system has been demonstrated, in which illumination beams at two closely-spaced optical frequencies are derived from a single Argon-ion laser. The frequency of one beam lies on an absorption line of iodine vapour, and the other just off the absorption line. The beams sequentially illuminate a plane within a seeded flow and the Doppler-shifted scattered light passes through an iodine cell onto a single solid-state camera. Light scattered from the measurement plane from the beam with its wavelength set to be off the absorption line is not affected by its passage through the cell, and provides a reference image, while that from the beam on the absorption line encodes the velocity information as a variation in transmission dependent upon the Doppler shift. The complex imaging system required for conventional PDV is eliminated, since superposition of the reference and signal images is automatic. The two beams are generated by an optical arrangement incorporating an acoustio- optic modulator, and are transmitted to the region of interest by a single mode optical fibre, which ensures both a smooth Gaussian illumination profile and co- linearity of the beams. The system is demonstrated using a spinning disc and a velocity resolution of +/-1 ms-1 is achieved.
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    Determination of view vectors from image warping mapping functions
    (Spie -- the Int Soc for Optical Engineering, 2004-02-28T00:00:00Z) Nobes, David S.; Wieneke, B.; Tatam, Ralph P.
    The measurands of several reported laser-based measurement techniques are sensitive to both the propagation direction of the laser and the viewing direction from the region of interest to the detector. For such imaging techniques, the view vector must be determined uniquely for each pixel in the detector array. The bulk view vector is often physically measured and a simple model used to determine the view vector for each pixel. This, however, has limitations where access is limited, the distances involved are small, or the optical system employed introduces errors. We describe a procedure to determine the unique view vector from a planar region to the detector (CCD camera) for each element in a 2-D array based on a reference target aligned with the planar region of interest. Determination of the view vector is based on the spatial distribution of the mapping function used to dewarp the view. No physical measurement of the view vector is required. Good agreement is achieved when the procedure is compared to a simple pin-hole camera model of the view using a computed test target. (C) 2004 Society of Photo-Optical Instrumentation Engineers.
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    Dual Illumination Planar Doppler Velocimetry using a Single Camera
    (International Society for Optical Engineering; 1999, 2003-12-31T00:00:00Z) Charrett, Thomas O. H.; Ford, Helen D.; Nobes, David S.; Tatam, Ralph P.
    A Planar Doppler Velocimetry (PDV) illumination system has been designed which is able to generate two beams, separated in frequency by about 600 MHz. This allows a common-path imaging head to be constructed, using a single imaging camera instead of the usual camera pair. Both illumination beams can be derived from a single laser, using acousto-optic modulators to effect the frequency shifts. One illumination frequency lies on an absorption line of gaseous iodine, and the other just off the absorption line. The beams sequentially illuminate a plane within a seeded flow and Doppler-shifted scattered light passes through an iodine vapour cell onto the camera. The beam that lies at an optical frequency away from the absorption line is not affected by passage through the cell, and provides a reference image. The other beam, the frequency of which coincides with an absorption line, encodes the velocity information as a variation in transmission dependent upon the Doppler shift. Images of the flow under both illumination frequencies are formed on the same camera, ensuring registration of the reference and signal images. This removes a major problem of a two-camera imaging head, and cost efficiency is also improved by the simplification of the system. The dual illumination technique has been shown to operate successfully with a spinning disc as a test object. The benefits of combining the dual illumination system with a three-component, fibre-linked imaging head developed at Cranfield will be discussed.
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    Instantaneous, three-component planar Doppler velocimetry using imaging fibre bundles
    (Springer Science Business Media, 2004-01-01T00:00:00Z) Nobes, David S.; Ford, Helen D.; Tatam, Ralph P.
    Abstract This paper describes a planar Doppler velocimetry (PDV) technique that is capable of measuring the three, instantaneous and time average components of velocity over two spatial dimensions using a single pair of signal and reference cameras. The three views required to obtain three-component velocity information are guided from the collection optics to a single imaging plane using flexible fibre imaging bundles. These are made up of a coherent array of single fibres and are combined at one end as the input plane to the measurement head. Measurements of the velocity field of a rotating disk are used in the development of the technique and initial results of the instantaneous velocity field of a jet are presented.
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    Investigation into the selection of viewing configurations for three-component planar Doppler velocimetry measurements.
    (Osa Optical Society of America, 2007-07-01T00:00:00Z) Charrett, Thomas O. H.; Nobes, David S.; Tatam, Ralph P.
    A method for the calculation of three orthogonal velocity components in planar Doppler velocimetry (PDV) using four or more measured velocity components (to the three typically used) is presented. The advantages and disadvantages are assessed by use of a Monte Carlo simulation and experimental measurements of the velocity field of a rotating disk. The addition of a fourth velocity component has been shown to lead to reductions in the final errors of up to 25%. The selection of viewing configurations for experiments is discussed by simulation of the level of errors in measured velocity components and investigation of the final level of errors in the orthogonal velocity components. Experimental measurements of the velocity field of a rotating disk are presented, demonstrating the effect of the viewing configuration on the final level of error.
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    Planar Doppler velocimetry measurements of flows using imaging fibre bundles
    (SPIE - International Society for Optics and Photonics, 2003-08-03) Nobes, David S.; Ford, Helen D.; Tatam, Ralph P.
    The development of a planar Doppler velocimetry is described. The technique is capable of measuring the three, instantaneous components of velocity in two dimensions using a single pair of signal and reference cameras. PDV can be used to measure the instantaneous 3-D velocity of a fluid by using an absorption line filter (ALF) to determine the Doppler shifted frequency of a narrow line pulsed laser (Nd:YAG) that has been scattered off particles seeded into the flow. The velocity of the fluid is determined using the Doppler formula and is dependent on the laser direction and the viewing direction. Hence, only one velocity component of the flow is measured. This component can be measured in two spatial dimensions using an array detector such as a CCD camera. To capture the three components, three such measurement heads have been used viewing from different angles. In the technique presented here the three views are ported from the collection optics to a single imaging plane using flexible fibre imaging bundles. These are made up of a coherent array of single fibres and are combined at one end as the input plane to the measurement head. The paper discusses the issues involved in developing a full three-dimensional velocity measurement system.