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Dipl.-Ing. Dr. Dominik Hofer
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Five, at first glance, very different peer-reviewed journal articles are presented in this doctoral thesis. The topics span from the numerical approximation of the Huygens-Fresnel integral, over the measurement of currents in printed circuit boards or through cast molten steel in a continuous casting plant, to the flow field of small electronics cooling fans. Despite the obvious differences, there is a common factor which ties these publications together: they all rely on image processing and optical methods in instrumentation and measurement.
All points on a wavefront are sources of new spherical waves and their superposition is propagating this wavefront - this most basic principle was solved numerically in `A numerical approximation of the Huygens-Fresnel integral' to simulate laser speckles and interference of monochromatic light in general. In `2D current distributions and their magnetic field' a similar concept - the Biot-Savart law - was employed to estimate currents flowing through the traces of a printed circuit board. An image of the magnetic field strength close up to the copper traces provided the necessary measurement data. Image processing methods in the spatial and frequency domains were used to extract information about the 2D current distribution.
The research interest of an industrial partner, the flow through a cross-section of the submerged entry nozzle of a continuous steel caster, lead to the development of a simulator setup (see Fig. 1). Massive rods of copper represented the flow of molten steel. An electrical current flowing through the metal produced a magnetic field which could be measured and analyzed. `Image processing for calibrating a coordinate measurement set-up' deals with the calibration of the simulator's sensor positioning system by means of optical methods and shows first results of the current measurements.
Fig. 1: Measurement setup for measuring the magnetic field in a plane normal to the vertical electrical conductor.
In `High-Resolution 2.5D Particle Image Velocimetry Measurements of the Flow Fields Generated by Small Fans' a new measurement method, a variant of PIV (see Fig. 2), was developed and tested for the first time. The 3D flow fields of a small axial and a centrifugal fan were measured and analyzed with image processing methods. The jet axis and the velocity distribution of the free jet were analyzed and the rotation of the velocity fields from both fans became apparent. The very same centrifugal fan was also the subject of the more detailed `Particle image velocimetry and constant temperature anemometer measurements of the jet produced by a centrifugal fan,' where the potential of the new measurement procedure was further demonstrated. Beside the time-averaged flow field of the free jet, the volume and momentum flow rates were estimated. Constant temperature hot-wire probe measurements provide additional insights about the turbulence at certain points of interest.
Fig. 2: Schematic of the 2.5D Particle Image Velocimeter.
Free jet emanating from the axial fan. The observed surface moves away from the fan outlet plane.
In all the presented articles the advantages of the methods used become evident: they are non-invasive, return clear (graphic) results, and feature extraordinary precision, due to the quality and sophistication of modern optics, cameras and state-of-the-art algorithms.
Keywords: Huygens-Fresnel Approximation, Magnetic Source Tomography, Velocity Field Measurement, Free Jet from a Fan
