Changes between Version 20 and Version 21 of Tutorial/ParticleImageVelocimetry

Jan 15, 2015, 4:18:29 PM (9 years ago)



  • Tutorial/ParticleImageVelocimetry

    v20 v21  
    103103To improve the results, come back to the GUI '''CIV''', and follow these successive steps, corresponding to a sequence of operations.
    104 Optimizing the Civ1 parameters
     105'''- Optimizing the Civ1 parameters'''
    106107Select 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.
    108 '''FIX1'''
     109'''- FIX1'''
    110111Select the ’'''FIX1'''’ operation, which eliminates some false vectors using several criteria. Use the default parameters.
    112 '''PATCH1'''
     113'''- PATCH1'''
    114115Select the ’'''PATCH1'''’ operation, to interpolate the vectors on a regular grid and calculate spatial derivatives. Select a high resolution [nx]=100, [ny]=30, number of grid points in which the velocity will be interpolated. These values correspond to dx=dy=10 pixels for an image 1000x300. Choose the default value 10 for the smoothing parameter ’rho’. You can later try different values, the smoothing effect increasing with rho. Keep the default values for the other parameters.
    116 '''CIV2'''
     117'''- CIV2'''
    118119Select 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 search range is determined automatically using the prior knowledge on velocity, obtained with the civ1 and patch1 operations. Note that ’civ2’ corresponds to a new measurement from the images, the previous civ1 and patch operations being used only as an initial guess for the search of optimal correlations.
    120121Then select ’'''FIX2'''’ and ’'''PATCH2'''’ with the same parameters as ’'''FIX1'''’ and ’'''PATCH1'''’.
    122 '''Running the calculation'''
     123'''- Running the calculation'''
    124125Press '''[RUN]''' to run the calculation. The results are stored in a new subdirectory, CIV_1, so the previous results are not erased (you can also set the name of the subdirectory in the GUI, by the edit boxes '''[SubDirCiv1]''' and '''[SubDirCiv2]'''. The existing subdirectories are listed above.