Changes between Version 6 and Version 7 of Tutorial/CorrelationImageVelocimetryOptimisation
- Timestamp:
- Jan 27, 2015, 5:05:39 PM (9 years ago)
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Tutorial/CorrelationImageVelocimetryOptimisation
v6 v7 14 14 The PIV operation is conveniently visualised by pressing the button '''[!TestCiv1]''' in the GUI civ. Then the image appear in a new GUI '''view_field''', in which the mouse motion displays the correlation function, which appears in a secondary window, see figure. The resulting vector is shown as a line pointing to the correlation maximum. The corresponding correlation and search boxes are shown in the image. 15 15 16 It is now possible to adjust the search range, using prior knowledge on extremal velocities, see histograms displayed by '''uvmat''' in pixel coordinates. We introduce estimated bounds on each velocity component, [min] to -2 and [max] to 6 for ’u’ and (-3, 3) for ’v’, and press the button '''[Search Range]'''. The optimum search ranges and shifts (for the given correlation box) are now displayed: [33 25] and [2 0] respectively. 16 It is now possible to adjust the search range, using prior knowledge on extremal velocities, see histograms displayed by '''uvmat''' in pixel coordinates. We introduce estimated bounds on each velocity component, [min] to -2 and [max] to 6 for ’u’ and (-3, 3) for ’v’, and press the button '''[Search Range]'''. The optimum search ranges and shifts (for the given correlation box) are now displayed: [33 25] and [2 0] respectively. 17 17 18 Let us run again PIV with '''[!CorrBoxSize]'''=[19 13] and '''[!SearchBoxSize]'''=[27,25] with Shift=0. (limiting ourselves to CIV1), 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 on the edge of the Search box. Then come back to the good parameters.18 Let us run again PIV with '''[!CorrBoxSize]'''=[19 13] and '''[!SearchBoxSize]'''=[27 25] with Shift=0. (limiting ourselves to CIV1), 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 on the edge of the Search box. Then come back to the good parameters. 19 19 20 20 == !CorrSmooth == … … 27 27 28 28 == '''FIX1''': == 29 Select the ’'''FIX1'''’ operation, which eliminates some false vectors using several criteria. Use the default parameters. '''''''''''29 Select the ’'''FIX1'''’ operation, which eliminates some false vectors using several criteria. Use the default parameters. 30 30 31 31 == '''PATCH1''': == 32 Select the ’'''PATCH1'''’ operation, to interpolate the vectors on a regular grid and calculate spatial derivatives. 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.'''''''''''32 Select the ’'''PATCH1'''’ operation, to interpolate the vectors on a regular grid and calculate spatial derivatives. 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.''''''''''' 33 33 34 34 == '''CIV2:''' == … … 36 36 37 37 == FIX2 and PATCH2 == 38 38 Then select ’'''FIX2'''’ and ’'''PATCH2'''’ with the default parameters. 39 39 40 40 == Further Civ iterations == 41 The parameters of a CIV computation are stored in a xml file with extension ..!CivDoc.xml created in the directory containing the velocity files. These parameters can retrieved, opening this xml file with the browser of the GUI '''civ'''. Then the image file itself needs to be opened (the select again the check boxes for the operations beyond civ1 hidden by default). 41 42 42 The parameters of a CIV computation are stored in a xml file with extension ..!CivDoc.xml created in the directory containing the velocity files. These parameters can retrieved, opening this xml file with the browser of the GUI '''civ'''. Then the image file itself needs to be opened (the select again the check boxes for the operations beyond civ1 hidden by default).43 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. 43 44 44 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. 45 46 The following figure shows the final vorticity field, in which the vorticity roll up in the wake of the sphere is clearly visible. A zoom near a vortex shows the vorticity superposed with velocity vectors. 45 The following figure shows the final vorticity field, in which the vorticity roll up in the wake of the sphere is clearly visible. A zoom near a vortex shows the vorticity superposed with velocity vectors. 47 46 48 47 [[Image(vort_civ3-2.jpg)]] [[Image(vort_vel_zoom.jpg)]]
