Changes between Version 27 and Version 28 of Tutorial/CorrelationImageVelocimetryOptimisation


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Timestamp:
Feb 2, 2015, 9:26:02 AM (6 years ago)
Author:
vaillant1p
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  • Tutorial/CorrelationImageVelocimetryOptimisation

    v27 v28  
    1818To observe the influence of the search box, come back to the GUI '''civ_input''', set '''[!CorrBoxSize]'''=[19 13] and '''[!SearchBoxSize]'''=[27 25] with '''[Shift]'''=0, 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 at the edge of the Search box in the main flow with u$\simeq$4 (while a gap of 2 pixels is required to properly determine the maximum without edge effect).
    1919
    20 The parameter '''[num_!CorrSmooth]''' is used to fit the correlation data with a Gaussian function to obtain the maximum with sub-pixel precision. We generally keep the default value 1, while the value 2 should be more appropriate for larger particles (with wider correlation maximum). The quality of this feature can be tested by taking the image autocorrelation, selecting the option 'displacement' instead of 'PIV' in the menu '''[!ListCompareMode]''' of '''civ_input'''. Then run the '''[CIV1]''' computation with '''series'''. Visualise the velocity field with uvmat: it is very close to 0 as expected but the histogram of the error can be estimated with the Tool/rectangle.  The curve exported from '''view_field''' is shown in figure below**, comparing '''[!CorrSmooth]'''=1 and 2'''. '''We see that the histogram is somewhat more narrow for '''[!CorrSmooth]'''=1, corresponding to a slightly better result, but the typical error of the order of 0.1 px in both cases.
     20The parameter '''[num_!CorrSmooth]''' is used to fit the correlation data with a Gaussian function to obtain the maximum with sub-pixel precision. We generally keep the default value 1, while the value 2 should be more appropriate for larger particles (with wider correlation maximum). The quality of this feature can be tested by taking the image autocorrelation, selecting the option 'displacement' instead of 'PIV' in the menu '''[!ListCompareMode]''' of '''civ_input'''. Then run the '''[CIV1]''' computation with '''series'''. Visualise the velocity field with uvmat: it is very close to 0 as expected but the histogram of the error can be estimated with the Tool/rectangle.  The curve exported from '''view_field''' is shown in figure below, comparing '''[!CorrSmooth]'''=1 and 2'''. '''We see that the histogram is somewhat more narrow for '''[!CorrSmooth]'''=1, corresponding to a slightly better result, but the typical error of the order of 0.1 px in both cases.
     21
     22[[Image(Tutorial6 - Corr1vs2.png)]]
    2123
    2224The 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 anyway limited by the size of the correlation box, so that velocity vectors become redondant when the sub-images highly overlap those of the neighboring vector. Then the choice '''Dx'''='''Dy'''=10, about half the correlation box, provides a good optimum.