Changes between Version 6 and Version 7 of Tutorial/CorrelationImageVelocimetryOptimisation


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Timestamp:
Jan 27, 2015, 5:05:39 PM (6 years ago)
Author:
vaillant1p
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  • Tutorial/CorrelationImageVelocimetryOptimisation

    v6 v7  
    1414The 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.
    1515
    16 It 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. 
     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.
    1717
    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.
     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.
    1919
    2020== !CorrSmooth ==
     
    2727
    2828== '''FIX1''': ==
    29 Select the ’'''FIX1'''’ operation, which eliminates some false vectors using several criteria. Use the default parameters.'''''''''''
     29Select the ’'''FIX1'''’ operation, which eliminates some false vectors using several criteria. Use the default parameters.
    3030
    3131== '''PATCH1''': ==
    32 Select the ’'''PATCH1'''’ operation, to interpolate the vectors on a regular grid and calculate spatial derivatives. Choose the default value 10 for the smoothing parameter '''[!FieldSmooth]'''. You can later try different values, the smoothing effect increasing with '''!FieldSmooth'''. Keep the default values for the other parameters.'''''''''''
     32Select the ’'''PATCH1'''’ operation, to interpolate the vectors on a regular grid and calculate spatial derivatives. Choose the default value 10 for the smoothing parameter '''[!FieldSmooth]'''. You can later try different values, the smoothing effect increasing with '''[!FieldSmooth]'''. Keep the default values for the other parameters.'''''''''''
    3333
    3434== '''CIV2:''' ==
     
    3636
    3737== FIX2 and PATCH2 ==
    38   Then select ’'''FIX2'''’ and ’'''PATCH2'''’ with the default parameters.
     38Then select ’'''FIX2'''’ and ’'''PATCH2'''’ with the default parameters.
    3939
    4040== Further Civ iterations ==
     41The parameters of a CIV computation are stored in a xml file with extension ..!CivDoc.xml created in the directory containing the velocity files. These parameters can retrieved, opening this xml file with the browser of the GUI '''civ'''. Then the image file itself needs to be opened (the select again the check boxes for the operations beyond civ1 hidden by default).
    4142
    42   The parameters of a CIV computation are stored in a xml file with extension ..!CivDoc.xml created in the directory containing the velocity files. These parameters can retrieved, opening this xml file with the browser of the GUI '''civ'''. Then the image file itself needs to be opened (the select again the check boxes for the operations beyond civ1 hidden by default).
     43The result can be improved again by performing a third civ iteration, civ3. For that purpose, select only the ’'''civ2'''’, ’'''fix2'''’ and ’'''patch2'''’ operations with the same parameters as previously. The previous result is now considered as ’'''civ1'''’, so set CIV as the subdirectory in the edit window '''[SubDirCiv1]'''. Select a new subdirectory name, for instance ’CIV3’ in the edit window '''[SubDirCiv2]'''. Further iterations could be similarly performed, but the improvement becomes negligible.
    4344
    44   The result can be improved again by performing a third civ iteration, civ3. For that purpose, select only the ’'''civ2'''’, ’'''fix2'''’ and ’'''patch2'''’ operations with the same parameters as previously. The previous result is now considered as ’'''civ1'''’, so set CIV as the subdirectory in the edit window '''[SubDirCiv1]'''. Select a new subdirectory name, for instance ’CIV3’ in the edit window '''[SubDirCiv2]'''. Further iterations could be similarly performed, but the improvement becomes negligible.
    45 
    46   The following figure shows the final vorticity field, in which the vorticity roll up in the wake of the sphere is clearly visible. A zoom near a vortex shows the vorticity superposed with velocity vectors.
     45The following figure shows the final vorticity field, in which the vorticity roll up in the wake of the sphere is clearly visible. A zoom near a vortex shows the vorticity superposed with velocity vectors.
    4746
    4847  [[Image(vort_civ3-2.jpg)]] [[Image(vort_vel_zoom.jpg)]]