wiki:Tutorial/AdvancedParticleImagingVelocimetry2

Version 12 (modified by sommeria, 8 years ago) (diff)

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This is an example using more advanced tools than the simple example .... and .... Image pre_processing and merging of mutiple fields is used to optimise PIV in the case of a narrow parietal jet (produced by convection in a cavity). Open the example UVMAT_DEMO6_PIVconvection (accessible on http://servforge.legi.grenoble-inp.fr/pub/soft-uvmat/).

calibration

mark the four box corners with the mouse. We find .... The corresponding physical coordinates are known to be (0,0),(58.8,0),(58.8,55.1),(0,55.1), with an origin (0,0) taken at the lower left. Introduce those in the table ![ListCoord?]. This can be conveniently done by copy paste of the Matlab vector x=[0 58.8 58.8 0] in the upper line of the x column, and y=[0 0 55.1 55.1] in the y column (use carriage return to validate the input). Then apply rescale. we observe that the figre is slightly rotated. We then apply linear which allows for rotation. The precison is improved.

mask

Open the original image with uvmat, selecting the transform option 'phys'. withCreate a mask polygon by the menu bar command Projection object/mask_polygon. Set the option 'mask_outside' and introduce the coordinates of the four corners in Coord (like for geometry calib). Plot the polygon. Then create the corresponding mask by Tools/make mask.

sub_background

We observe parasitic light rays on the images which correspond to fixed features, leading possibly to spurious velocity vectors equal to 0. To eliminate those we use sub_bacground. Open the image with 'series' and select the program 'sub_background'. This function is not provided in the default menu, so you need to use the last menu option 'more...', and select the function in the sub-folder 'series/' of the package uvmat. This option is then preserved in the menu for later use. Then run sub_background over the whole index range in i and j, using the default parameters. Answer Yes to the quaestion 'apply levels', which will conveniently rescale the image brightness after background removal.

first PIV

making two masks

The parietal jet requires a very good resolution, particularly among x. To limitate PIV to this jet, let us create a specific mask. Open the previously created contour polygon contour_mask.xml in uvmat by the menu bar Tools/Projection object/browse.... Check the box edit (tag [CheckEditObject]) in the frame Object of uvmat (right hand side) to allow editing of the polygon. then replace the lower x bound 0 by 52. Then create the corresponding mask by Tools/make mask, save it with name Dalsa1.mask/mask_plume.png. Similarly create a mask for the bulk Dalsa1.mask/mask_bulk.png, with bounds in x [0 55].

PIV on the parietal plume

Choose the following parameters:

  • pair j=1-2 for Civ1 and Civ2 : it minimises the time interval which is needed to capture the large velocity in the plume.
  • CorrBox x,y=(5 31) which optimizes the resolution in x (5 pixels)
  • Search x, y=(25 55) which allows for a larger velocity component along y .
  • auto-grid Dx=2, Dy=10, consistent with CoorBox (about half the box size).
  • Mask selected, with name .../Dalsa1.mask/mask_plume.png .
  • For Civ2, same parameters CorrBox, auto-grid and Mask.
  • For patch and Fix, default parameters.

PIV on the interior

Select the pair j=1-3 to deal with the small velocities (considering that the parietal plume has been masked). Then use the default parameters.

merging data on a common grid

create a projection grid in phys coordinates. For that open a velocity field with uvmat, displayed in phys coordinates. Use the upper bar menu option Projection object/plane. Then in the GUI set_object, choose the option ProjMode=interp_lin. Choose a mesh 0.2 cm in each direction, ranging from 0 to 58.8 in x and 0 to 55 in y. Press REFRESH to see the result of projection in the GUI view_field. Check the option nb_vec/4 to reduce the number of vectors displayed on the plot. Now in series open the PIV file Dalsa1.sback.civ_bulk as input. Then append the second file Dalsa1.sback.civ_plume using the menu bar selection Open/Browse? append.... Set FieldTransform? to 'phys', and selection Projection Object. The plane is then incorporated in series.

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