Changes between Version 5 and Version 6 of Tutorial/CorrelationImageVelocimetryOptimisation


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Timestamp:
Jan 27, 2015, 3:38:36 PM (6 years ago)
Author:
vaillant1p
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  • Tutorial/CorrelationImageVelocimetryOptimisation

    v5 v6  
    1010Once 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.
    1111
    12 Select the '''[CIV1]''' check box so the corresponding parameters show  up. Improve the spatial resolution by selecting smaller correlation  boxes in the '''civ1''' menu, for instance [Bx],[By] to 19 and 13  (image pixels). This is possible because of the good image quality and  high particle density. The use of a smaller box in y allows to optimize  the resolution in this direction, to deal with the strong vertical  shear. It is now possible to adjust the search range, using knowledge on  extremal velocities, see histograms displayed by '''uvmat''' in phys  coordinates. We introduce estimated bounds on each velocity component,  [min] to -2 and [max] to 6 for ’u’ and (-3, 3) for ’v’, and press the  button '''[Search Range]'''. The optimum search ranges and shifts are  now displayed. In consistency with higher resolution, we set the grid  mesh to [Dx] and [Dy] to 10. Finally select the '''Mask option''' : the mask name should be displayed in the corresponding edit box, else a browser leads to the appropriate mask file.
     12Select the '''[CIV1]''' check box so the corresponding parameters show  up. Improve the spatial resolution by selecting smaller correlation  boxes in the '''civ1''' menu, for instance '''[num_!CorrBoxSize_1, 2]''',to 19 and 13  (image pixels). This is possible because of the high density particl seeding. The use of a elongated box along ''x'' allows to optimize  the resolution in the direction ''y'', to deal with the strong vertical  shear.
    1313
    14 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.
     14The 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.
     15
     16It is now possible to adjust the search range, using prior knowledge on extremal velocities, see histograms displayed by '''uvmat''' in pixel  coordinates. We introduce estimated bounds on each velocity component,  [min] to -2 and [max] to 6 for ’u’ and (-3, 3) for ’v’, and press the  button '''[Search Range]'''. The optimum search ranges and shifts (for the given correlation box) are  now displayed: [33 25] and [2 0] respectively.
     17
     18 Let us run again  PIV with '''[!CorrBoxSize]'''=[19 13] and '''[!SearchBoxSize]'''=[27,25] with Shift=0. (limiting ourselves to CIV1), and visualise 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. Using  '''[!TestCiv1]''' , it can be seen that the correlation maximum is indeed on the edge of the Search box. Then come back to the good parameters.
    1519
    1620== !CorrSmooth ==
     
    1822
    1923== Measurement grid and mask ==
    20 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.
     24The 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. If the mesh is below half the correlation box size, the added velocity vectors become redondant as they correspond to overlapping sub-images.
     25
     26Finally select the '''Mask option''' like in the previous tutorial.
    2127
    2228== '''FIX1''': ==