| 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 **, 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. '''''' |
| | 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 **, 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. ''''''''''' |
| 63 | | = Other Example: = |
| 64 | | == Further Civ iterations == |
| 65 | | 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. |
| | 63 | == Another Example == |
| | 64 | With the GUI uvmat open an image in ''/UVMAT_DEMO04_PIVchallenge_2001A/images'' taken from PIV challenge (ref web site **). A short description of the experimental conditions is given in the file ''readme.txt'' in the folder of the images. It is said that the field of view is 170 x140 mm, so that, since the image size is 1280x1024 px, the scale is 7.4 px/cm. but no time Dt **. The flow is the vortex behind a wing with poor particle seeding in the core. |
| | 65 | |
| | 66 | From uvmat select Run/PIV so the GUI civ_input pops up. With the option''' [wiki:ImportParam [!ImportParam]]''', import the parameters from the file ''Images.ref.civ/0_XML/A_1_1.xml'' and run the civ calculation. The result should look like the reference result ''Image.ref.civ_2/A_1_1-2.nc''. As expected form the dark spot at the vortex centre, the core is not properly resolved in civ1. The result is improved in civ2. |
| | 67 | |
| | 68 | Let us improve again the result by a third civ iteration. This is launched in the GUI '''series/civ_series''', opening the netcdf file result of the previous civ2 calculation instead of an image. The source image then automatically appears in the second line of the input table of '''series'''. '''' |
