Changes between Version 20 and Version 21 of Tutorial/CorrelationImageVelocimetryOptimisation


Ignore:
Timestamp:
Jan 28, 2015, 5:00:21 PM (6 years ago)
Author:
vaillant1p
Comment:

--

Legend:

Unmodified
Added
Removed
Modified
  • Tutorial/CorrelationImageVelocimetryOptimisation

    v20 v21  
    3838Now come back to the default value 10, and press the button '''[!TestPatch1]'''. 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, '''[!FieldSmooth]'''=1, to more than 0.2 for '''[!FieldSmooth]'''=100. The value 0.15 for '''[!FieldSmooth]'''=10 is less that the expected error on the PIV, about 0.2 pixel.
    3939
    40 
    41 
    42 **Figure: GUI civ_input + graphe obtenu par !!TestPatch.
     40[[Image(41.png)]]
    4341
    4442This 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.
     
    5755The final vorticity field can be observed in the following figure, in which the  vorticity roll up in the wake of the cylinder is clearly visible. A zoom  near a vortex shows the vorticity superposed with velocity vectors.
    5856
    59 A cut of the velocity along a transverse line x=250, y from 0 to 300 (in pixel coordinates), provides a good representation of the strong velocity shear in the wake of the cylinder. This can be done by displaying the velocity field filter2, open set_object/line, choose '''[!ProjMode]'''=inter_tps, '''Mesh'''=2 to get the profile with spline interpolation from filter2. Then select hold on on the GUI view_field and repeat the same cut with the field 'civ2', '''!!ProjMode'''='projection'. We can then compare the civ2 measurement points to the interpolation, showing some fluctuations are smoothed out but without widening of the strong shear zone. The result has been exported in figure **, using the menu bar tool '''[Export/extract figure] '''in '''view_field'''.  The typical precision can be estimated from the scattering of the points as +-0.1 px, with typically 5-10 pixels in spatial resolution.
     57A cut of the velocity along a transverse line x=250, y from 0 to 300 (in pixel coordinates), provides a good representation of the strong velocity shear in the wake of the cylinder. This can be done by displaying the velocity field filter2, open set_object/line, choose '''[!ProjMode]'''=inter_tps, '''Mesh'''=2 to get the profile with spline interpolation from filter2. Then select hold on on the GUI view_field and repeat the same cut with the field 'civ2', '''!!ProjMode'''='projection'. We can then compare the civ2 measurement points to the interpolation, showing some fluctuations are smoothed out but without widening of the strong shear zone. The result has been exported in figure, using the menu bar tool '''[Export/extract figure] '''in '''view_field'''.  The typical precision can be estimated from the scattering of the points as +-0.1 px, with typically 5-10 pixels in spatial resolution.
    6058
    6159[[Image(RMS Patch1-Civ1.png)]]