Changes between Version 36 and Version 37 of Tutorial/ParticleImageVelocimetry


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Timestamp:
Jan 27, 2015, 12:58:39 PM (6 years ago)
Author:
sommeria
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  • Tutorial/ParticleImageVelocimetry

    v36 v37  
    7777Note that the netcdf file has not been by changed by calibration, whose rescaling is introduced after reading the file. This means that calibration can be provided, and possibly updated, after the PIV processing.
    7878
    79 = The main PIV parameters =
    80 The first parameter to adjust is the time interval between images, which should be sufficiently long to provide a displacement of a few pixels. The measurement precision is typically 0.2 pixel, so that a displacement of 4 pixels, as in the example, provides a relative precision of 5 %. A larger displacement would be preferable in terms of precision but may yield to poor image correlation and ’false vectors’.
    81 
    82 Once the image pair has been chosen, the main parameters are the size of the correlation box in both directions ('''[num_!CorrBoxSize]_1]''' and '''_2''') and the size of the search box, ('''[num_!SearchBoxSize_1]''' and '''_2'''), expressed in pixels. For each velocity vector, the correlation box is moved within the search box to optimise the image correlation between the two sub-images inside the correlation box. To allow for a displacement of d pixels, the search box size must exceed the correlation box by at least d+2 on both sides, so 2d+4. A systematic shift '''[num_!SearchBoxShift_1]''', and '''_2''', can be also introduced to minimise the search area in the presence of a mean flow.
    83 
    84 The parameter '''[num_!CorrSmooth]''' is used to fit the correlation data with a smooth function to obtain the maximum with sub-pixel precision. We generally keep the default value 1.
    85 
    86 The parameters '''[num_Dx]''' and '''[num_Dy] '''define the mesh of the measurement grid, in pixels. Reduce them to get more vectors, but keep in mind that the spatial resolution is limited by the size of the correlation box.
    87 
    88 The PIV operation is conveniently visualised by pressing the button '''[!TestCiv1]''' in the GUI civ. Then the image appear in a new GUI '''view_field''', in which the mouse motion displays the correlation function, which appears in a secondary window, see figure. The resulting vector is shown as a line pointing to the correlation maximum. The corresponding correlation and search boxes are shown in the image. Let us run again PIV with '''[!CorrBoxSize]'''=[25 25] and '''[!SearchBoxSize]'''=[33,33], limiting ourselves to CIV1, and visualisae the result with '''uvmat'''. Many black vectors (F=-2) are obtained, showing that the search domain is too small, so that the correlation maximum is constrained by the limited search interval. Repeat the operation with '''[!SearchBoxSize]'''=[35,33], then the arrows are not black anymore.
    89 
    9079= Masks =
    9180Spurious vectors are observed outside the fluid domain, which particularly disturbing when spatial derivatives are calculated.
     
    9988The mask image can be seen as a mask by selecting '''[view_mask]''' at the upper left of the GUI '''uvmat'''. For checking purpose, it can be also opened by the browser of '''uvmat''' like any image. In the GUI '''civ''', the mask is introduced by the selecting the green check box '''[Mask]'''. Vectors under the mask are not calculated in the resulting velocity data.
    10089
    101 Further improvments of the results are discussed in CorrelationImageVelocimetryOptimisation
     90Further improvments of the results are discussed in Tutorial/CorrelationImageVelocimetryOptimisation