Due to the particular features that appear in the vector maps delivered by the PIV method, there are postprocessing steps that can substantially enhance its performance. These steps include: detection of false vectors, correction of these vectors and the calculation of derived flow magnitudes. Many derived magnitudes can be of interest but this work focuses on the calculation of the first spatial derivative, component of flow divergence or vorticity, on a two-dimensional flow configuration. New algorithms, developed for each step, are described, with the aim of jointly applying them in a coherent way. Where applicable, an analytical tool for filter analysis and design is explained. Application to synthetic and real PIV data is presented as well as a performance contrast with other conventional algorithms, in terms of accuracy, frequency response and error propagation, among others.
A digital-based image analysis system has been developed for the
experimental determination of size, spatial distribution and two components of
velocity for particles suspended in any clear fluid flow. A volumetric
measurement directly provides number density and consequently particle flow. A
peculiarity of the developed system is its suitability for relatively large
particles (some mm in diameter), which can hardly be measured with other
systems. Double exposed particle images, in the same frame, provide the
starting point for image processing. The system includes a procedure for the
classification of the particles in the image, the separation of the partially
overlapped particles and the selection of those that are found in the control
volume. These parameters serve as a basis for the velocity determination
through a particle-tracking algorithm (PTV), which is based on iteratively
estimating the match probability as a measure of the likelihood of pairing.
The depth of the control volume is obtained with a new technique, based on the
loss of focus of the particles in the image, combined by diffuse illumination,
provided by twin short pulse argon spark discharge lamps. The system has been
extensively checked using calibrated micro-balls and sprays that simulate
rain.
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