Changes between Version 12 and Version 13 of Tutorial/CorrelationImageVelocimetryOptimisation

Timestamp:

Jan 28, 2015, 1:14:31 AM (10 years ago)

Author:

sommeria

Comment:

--

Tutorial/CorrelationImageVelocimetryOptimisation

@@ -7 +7 @@
 The 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’. The choice of image pair is done in [!!ListPairCiv1].
 
-== Parameters for Civ1 ==
+== Civ1 parameters ==
 Once the image pair has been chosen, the next parameter is  the size of the correlation box in both directions ('''[num_!CorrBoxSize]_1]''' and '''_2'''),  expressed in pixels. A smaller box of course improves the spatial resolution but it involves less statistics and false vectors may result from holes in the particle seeding. The particles are densely packed in this example, so we can significantly reduce the size from the default value [25,25] to [19,13]  (in image pixels).  The use of a elongated box along ''x'' allows to optimize  the resolution in the direction ''y'', to deal with the transverse shear.
 
@@ -24 +24 @@
 Finally select the '''Mask option''' like in the previous tutorial.
 
-== '''FIX1''': ==
+== Fix1 parameters ==
 Select the ’'''FIX1'''’ operation, which eliminates some false vectors using several criteria. Use the default parameters.
 
-== '''PATCH1''': ==
-Select 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). 
+== Patch1 parameters ==
+Select 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).
 
-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.  The 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. Projection on a line (as described in tutorial  can be useful to better evaluate the field values on a plot. This is rather small (0.1 px) and erratic, except possibly in the strong shear close to the cylinder''', '''so the smoothing properly reduces the noise without excessive perturbation of the velocity field itself.''' '''
+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. 
 
-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.'
+The 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.''' '''
 
-== '''CIV2:''' ==
+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. 
+
+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. 
+
+**Figure: GUI civ_input + graphe obtenu par !!TestPatch.
+
+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. 
+
+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.
+
+== Civ2 ==
 Select 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.