Changes between Version 28 and Version 29 of Tutorial/ParticleImageVelocimetry


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Timestamp:
Jan 21, 2015, 2:16:26 PM (6 years ago)
Author:
vaillant1p
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  • Tutorial/ParticleImageVelocimetry

    v28 v29  
    4545[[Image(contours.JPG)]]
    4646
    47 To get the vorticity field, 'vort' , and other spatial derivatives, you need to come back to the GUI''' series''' with images as input and action civ_series'''. '''In''' civ_input''', select the check boxes '''[FIX1] ''' and  '''[PATCH1] ''' , validate the input with '''[OK]''', then press '''[RUN]''' in the GUI '''series'''. A question box appears to warn about the existence of the result file, answer '''[OK]''' to refresh it with the new data. This will produce an interpolated velocity field and their spatial derivatives in the same netcdf file.  After this operation vorticity can be visualized in the GUI  '''uvmat''', selecting the option 'vort'''' ''' in the popup  menu '''[FieldName]'''.
     47To get the vorticity field, 'vort' , and other spatial derivatives, you need to come back to the GUI''' series''' with images as input and action civ_series'''. '''In''' civ_input''', select the check boxes '''[FIX1] ''' and  '''[PATCH1] ''' , validate the input with '''[OK]''', then press '''[RUN]''' in the GUI '''series'''. A question box appears to warn about the existence of the result file, answer '''[OK]''' to refresh it with the new data. This will produce an interpolated velocity field and their spatial derivatives in the same netcdf file.  After this operation vorticity can be visualized in the GUI  '''uvmat''', selecting the option 'vort'''' ''' in the popup  menu '''[!FieldName]'''.
    4848
    4949== Superposing image and vectors ==
     
    7373Now repeat the PIV operation to include the time information in the result (in the absence of time information the file index is taken as default value).
    7474
    75 To introduce  the geometric calibration, use the method described in [https://servforge.legi.grenoble-inp.fr/projects/soft-uvmat/search?q=wiki%3ATutorial%2FGeometricCalibration geometric calibration], section 'Simple scaling'. The velocity field is then displayed in terms of physical velocity. To come back to the image coordinates, use the box''' [!!TransformName]''' on the left : select to blank instead of ’phys’.
     75To introduce  the geometric calibration, use the method described in [https://servforge.legi.grenoble-inp.fr/projects/soft-uvmat/search?q=wiki%3ATutorial%2FGeometricCalibration geometric calibration], section 'Simple scaling'. The velocity field is then displayed in terms of physical velocity. To come back to the image coordinates, use the box''' [!TransformName]''' on the left : select to blank instead of ’phys’.
    7676
    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.
     
    8080The 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’.
    8181
    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.
     82Once 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.
    8383
    8484The 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.