12 | | Select the '''[CIV1]''' check box so the corresponding parameters show up. Improve the spatial resolution by selecting smaller correlation boxes in the '''civ1''' menu, for instance [Bx],[By] to 19 and 13 (image pixels). This is possible because of the good image quality and high particle density. The use of a smaller box in y allows to optimize the resolution in this direction, to deal with the strong vertical shear. It is now possible to adjust the search range, using knowledge on extremal velocities, see histograms displayed by '''uvmat''' in phys 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 are now displayed. In consistency with higher resolution, we set the grid mesh to [Dx] and [Dy] to 10. Finally select the '''Mask option''' : the mask name should be displayed in the corresponding edit box, else a browser leads to the appropriate mask file. |
| 12 | Select the '''[CIV1]''' check box so the corresponding parameters show up. Improve the spatial resolution by selecting smaller correlation boxes in the '''civ1''' menu, for instance '''[num_!CorrBoxSize_1, 2]''',to 19 and 13 (image pixels). This is possible because of the high density particl seeding. The use of a elongated box along ''x'' allows to optimize the resolution in the direction ''y'', to deal with the strong vertical shear. |
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. Let us run again PIV with '''[!CorrBoxSize]'''=[25 25] and '''[!SearchBoxSize]'''=[33,33], limiting ourselves to CIV1, and visualisae 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. Repeat the operation with '''[!SearchBoxSize]'''=[35,33], then the arrows are not black anymore. |
| 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 | |
| 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 | |
| 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. |