Changes between Version 13 and Version 14 of Tutorial/CorrelationImageVelocimetryOptimisation


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Timestamp:
Jan 28, 2015, 9:27:58 AM (9 years ago)
Author:
sommeria
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  • Tutorial/CorrelationImageVelocimetryOptimisation

    v13 v14  
    3030Select the ’'''PATCH1'''’ operation, to interpolate the vectors and calculate spatial derivatives. First choose the default parameters, press OK, run the caluclation and visualise with uvmat. We observe that a few erratic vectors have been flagged as false (painted in magenta).
    3131
    32 Two fields can be visualised, as selected by the menu '''[!!VelType]''' in the upper part of '''uvmat''': the initial field 'civ1' and the smoothed one 'filter1', obtained by the spline interpolation/smoothing of PATCH1. Select the option 'blank' in the menu''' [! TransformName] '''(on the left side of uvmat), to observe fields as displacement in pixel units (not physical coordinates), which is the appropriate option to analyse PIV features. 
     32Two fields can be visualised, as selected by the menu '''[!!VelType]''' in the upper part of '''uvmat''': the initial field 'civ1' and the smoothed one 'filter1', obtained by the spline interpolation/smoothing of PATCH1. Select the option 'blank' in the menu''' [! TransformName] '''(on the left side of uvmat), to observe fields as displacement in pixel units (not physical coordinates), which is the appropriate option to analyse PIV features.
    3333
    3434The difference between the two fields can be directly visualized by selecting 'civ1' in the menu  '''[!!VelType]''' and 'filter1' in the menu  '''[! VelType_1] '''just below'''.''' Adjust the scale [num_!!VecScale] (value 10 for instance) to better see the difference.  This is rather small (0.1 px) and erratic, except in the strong shear close to the cylinder, where it reaches a value about 0.3, so the smoothing properly reduces the noise without excessive perturbation of the velocity field itself. You can also use the scalar representation, selecting the field 'U' for both 'civ1' and 'filter1'. Projection on a line (as described in tutorial  2) is also useful to get field values on a plot.''' '''
    3535
    36 Repeat the operations by choosing the value 100 for '''[!FieldSmooth]''' instead of the default value 10. Now the smoothing effect is quite clear, widening the shear region at the edge of the cylinder. 
     36Repeat the operations by choosing the value 100 for '''[!FieldSmooth]''' instead of the default value 10. Now the smoothing effect is quite clear, widening the shear region at the edge of the cylinder.
    3737
    38 Now come back to the default value 10, and press the button [wiki:TestPach [!TestPatch]1]. This will perform patch calculations with a range of values for the smoothing parameters, and provide the rms difference between the filtered velocity field and the initial Civ1 field. This ranges from 0.12, [wiki:FieldSmooth !FieldSmooth]=1, to more than 0.2 for [https://servforge.legi.grenoble-inp.fr/projects/soft-uvmat/search?q=wiki%3AFieldSmooth !FieldSmooth]=100. The value 0.15 for [https://servforge.legi.grenoble-inp.fr/projects/soft-uvmat/search?q=wiki%3AFieldSmooth !FieldSmooth]=10 is less that the expected error on the PIV, about 0.2 pixel. 
     38Now come back to the default value 10, and press the button [wiki:TestPach [!TestPatch]1]. This will perform patch calculations with a range of values for the smoothing parameters, and provide the rms difference between the filtered velocity field and the initial Civ1 field. This ranges from 0.12, [wiki:FieldSmooth !FieldSmooth]=1, to more than 0.2 for [https://servforge.legi.grenoble-inp.fr/projects/soft-uvmat/search?q=wiki%3AFieldSmooth !FieldSmooth]=100. The value 0.15 for [https://servforge.legi.grenoble-inp.fr/projects/soft-uvmat/search?q=wiki%3AFieldSmooth !FieldSmooth]=10 is less that the expected error on the PIV, about 0.2 pixel.
    3939
    4040**Figure: GUI civ_input + graphe obtenu par !!TestPatch.
    4141
    42 This test also provides the proportion of excluded vectors (marqued as false) by the criterion of the excessive difference between the Civ1 value and the filtered one, which is attributed to false vectors. The threshold (expressed in pixels) is given by the box '''[num_!!MaxDiff]'''. The result obtained  with the default value 1.5 is about 2 %, so that most vectors are preserved. Repeating the test  with a higher value, for instance 10, logically reduces the number of rejected vectors, but significantly increases the rms difference: the interpolation is perturbed by a few erratic vectors. 
     42This test also provides the proportion of excluded vectors (marqued as false) by the criterion of the excessive difference between the Civ1 value and the filtered one, which is attributed to false vectors. The threshold (expressed in pixels) is given by the box '''[num_!!MaxDiff]'''. The result obtained  with the default value 1.5 is about 2 %, so that most vectors are preserved. Repeating the test  with a higher value, for instance 10, logically reduces the number of rejected vectors, but significantly increases the rms difference: the interpolation is perturbed by a few erratic vectors.
    4343
    44 The last parameter for Patch1 is [num_! SubDomainSize] which corresponds to a partition in subdomains for the spline calculation, in order to avoid computer memory overflow in  the spline calculation. In this case the default choice 100 leads to a single domain.
     44The last parameter for Patch1 is [num_! SubDomainSize] which corresponds to a partition in subdomains for the spline calculation, in order to avoid computer memory overflow in  the spline calculation. In this case the default choice 100 leads to a single domain.
    4545
    46 == Civ2 ==
     46== Civ2, Fix2 and Patch2 ==
     47The Civ2 operation repeats the Civ1, but it uses the result of Patch1 as a prior estimate. Therefore while Civ1 is purely local, Civ2 restricts the research to a correlation maximum which is close to the values obtained for neighborhing vectors.
     48
     49The parameter  '''[num_SearchBoxShift] '''therefore does not appear in the Civ2 panel, as it is given at each point by the result Patch1. The other parameters have the same meaning as for Civ1. The search box must be small enough to effectively reduce the research to the prior estimate. Take !CorrBoxSize +6 in each direction. Since it is the final result, you can optimise the grid by taking Dy=5. 
     50
     51The parameter '''[deformation]''' (check box) improves the prior estimate by deforming the subimage taking into account the velocity gradients, so it can improve the processing in zones of strong shear or strong rotation, like vortex cores. It involves an interpolation of the sub-images to perform the deformation. 
     52
     53Fix2 and Patch2 act on the Civ2 results like Fix1 and Patch1 on the Civ1 results. Choose a smaller smoothing parameter [wiki:FieldSmooth !FieldSmooth]=2, to limitate systematic smoothing effects in the final result.
     54
    4755Select the ’'''CIV2'''’ operation to improve the correlation results, using the information on local image deformation, provided by the previous knowledge on velocity spatial derivatives (calculated in patch1). Use a finer grid dx= dy=5 than for civ1. The spatial resolution can be slightly improved by decreasing the correlation box, using for instance Bx,By=(15,11). The shift of the search range is here given at each point by the prior estimate from Civ1, so that the search range can be optimized: choose [21,17] which provides a margin of 3 pixels on each side of the correlation box. Note that ’civ2’ corresponds to a new measurement from the images, the previous civ1 and patch1 operations being used only as an initial guess for the search of optimal correlations.
    4856