Changes between Version 2 and Version 3 of Tutorial/CorrelationImageVelocimetryOptimisation
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 Jan 27, 2015, 2:47:59 PM (9 years ago)
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Tutorial/CorrelationImageVelocimetryOptimisation
v2 v3 2 2 3 3 = [wiki:Tutorial] / Correlation Image Velocimetry: optimisation of parameters = 4 To improve the results, come back to the GUI '''CIV''', and follow these successive steps, corresponding to a sequence of operations. 4 To improve the results, come back to the GUI '''CIV''', and follow these successive steps, corresponding to a sequence of operations. 5 5 6 = T he main PIV parameters=7 The first parameter to adjust is the time interval between images, which should be sufficiently long to provide a displacement of a few pixels. The measurement precision is typically 0.2 pixel, so that a displacement of 4 pixels, as in the example, provides a relative precision of 5 %. A larger displacement would be preferable in terms of precision but may yield to poor image correlation and ’false vectors’. 6 = Time interval = 7 The first parameter to adjust is the time interval between images, which should be sufficiently long to provide a displacement of a few pixels. The measurement precision is typically 0.2 pixel, so that a displacement of 4 pixels, as in the example, provides a relative precision of 5 %. A larger displacement would be preferable in terms of precision but may yield to poor image correlation and ’false vectors’. The choice of image pair is done in [!!ListPairCiv1]. 8 8 9 == Correlation box, shift and search box == 9 10 Once the image pair has been chosen, the main parameters are the size of the correlation box in both directions ('''[num_!CorrBoxSize]_1]''' and '''_2''') and the size of the search box, ('''[num_!SearchBoxSize_1]''' and '''_2'''), expressed in pixels. For each velocity vector, the correlation box is moved within the search box to optimise the image correlation between the two subimages inside the correlation box. To allow for a displacement of d pixels, the search box size must exceed the correlation box by at least d+2 on both sides, so 2d+4. A systematic shift '''[num_!SearchBoxShift_1]''', and '''_2''', can be also introduced to minimise the search area in the presence of a mean flow. 10 11 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. 13 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. 15 16 == !CorrSmooth == 11 17 The parameter '''[num_!CorrSmooth]''' is used to fit the correlation data with a smooth function to obtain the maximum with subpixel precision. We generally keep the default value 1. 12 18 19 == Measurement grid and mask == 13 20 The parameters '''[num_Dx]''' and '''[num_Dy] '''define the mesh of the measurement grid, in pixels. Reduce them to get more vectors, but keep in mind that the spatial resolution is limited by the size of the correlation box. 14 21 15 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. 22 == '''FIX1''': == 23 Select the ’'''FIX1'''’ operation, which eliminates some false vectors using several criteria. Use the default parameters.'''''' 16 24 25 == '''PATCH1''': == 26 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.'''''' 17 27 18 '''* Optimizing the Civ1 parameters''': 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. 28 == '''CIV2:''' == 29 Select the ’'''CIV2'''’ operation to improve the correlation results, using the information on local image deformation, provided by the previous knowledge on velocity spatial derivatives (calculated in patch1). Use a finer grid dx= dy=5 than for civ1. The spatial resolution can be slightly improved by decreasing the correlation box, using for instance Bx,By=(15,11). The shift of the search range is here given at each point by the prior estimate from Civ1, so that the search range can be optimized: choose [21,17] which provides a margin of 3 pixels on each side of the correlation box. Note that ’civ2’ corresponds to a new measurement from the images, the previous civ1 and patch1 operations being used only as an initial guess for the search of optimal correlations. 19 30 20 '''* FIX1''': Select the ’'''FIX1'''’ operation, which eliminates some false vectors using several criteria. Use the default parameters. 21 22 '''* PATCH1''': 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. 23 24 '''* CIV2''': Select the ’'''CIV2'''’ operation to improve the correlation results, using the information on local image deformation, provided by the previous knowledge on velocity spatial derivatives (calculated in patch1). Use a finer grid dx= dy=5 than for civ1. The spatial resolution can be slightly improved by decreasing the correlation box, using for instance Bx,By=(15,11). The shift of the search range is here given at each point by the prior estimate from Civ1, so that the search range can be optimized: choose [21,17] which provides a margin of 3 pixels on each side of the correlation box. Note that ’civ2’ corresponds to a new measurement from the images, the previous civ1 and patch1 operations being used only as an initial guess for the search of optimal correlations. 25 31 == FIX2 and PATCH2 == 26 32 Then select ’'''FIX2'''’ and ’'''PATCH2'''’ with the default parameters. 27 33