Changes between Version 37 and Version 38 of Tutorial/ParticleImageVelocimetry

Jan 27, 2015, 1:09:43 PM (6 years ago)



  • Tutorial/ParticleImageVelocimetry

    v37 v38  
    44Open again the first example of image pair in UVMAT_DEMO01_pair (accessible on
    6 = Starting PIV =
    7 == Visual check ==
     6== Visual check of the image pair ==
    87Particle Image Velocimetry (PIV) measures the displacement of features in a pair of images. Visual evidence of feature displacement between the two images is a prerequisite  for the success of the computation. To observe this motion, write the file indices 1 and 2 in the boxes '''[i1]''' and '''[i2]''' respectively, in the frame '''[File Indices]''' on the left. Then push the red button '''[<-->]''' in the frame '''[Navigate]''', see figure. The image then alternatively switches from 1 to 2. The speed of motion can be adjusted with the slider '''[speed]'''. Press '''[STOP]''' to stop the motion.
    21 = Monitoring PIV results =
    2220== Visualizing  the velocity fields ==
    2321In '''uvmat '''velocity vectors are displayed in the central window, while the histograms of each component are in the lower left windows. The arrow length is automatically set by default. It can be adjusted by the edit box '''[num_!VecScale]''' in the  frame '''[Vectors]''' on the right hand side.
    2927The position (''x,y'') and velocity components (''U,V'') can be displayed in the upper right text display window by moving the mouse over it. The correlation 'C' and warning flag 'F' are also indicated. The warning flag is equal to 0 for good vectors while non-zero values indicate different calculation problems, see below.
    31 == Histograms ==
     29== Histograms of velocity ==
    3230The global histograms of the vector components  are available in the lower left windows. Histograms limited to a sub-region can be extracted by the menu bar tool '''[Projection object]''', selecting either''' [rectangle]''', '''[ellipse]''' or '''[polygon]''' to define the sub-region (see [wiki:Tutorial/ProjectionObjects projection objects])
    34 == Profiles ==
     32== Velocity profiles ==
    3533The velocity profile along a line can be obtained by creating a line with the upper menu bar '''[Projection object/line]''', like for image luminosity (see [wiki:Tutorial/ProjectionObjects projection objects]). Press (and release) the left hand side mouse button, draw the line, and press it again for the end of the line. The transverse and longitudinal velocity components along this line are then plotted in a new  figure '''view_field'''.
    37 == Other vector color representations ==
     35== Various vector color representations ==
    3836Vector color can also  represent another quantity, as chosen in the menu '''[!ColorScalar] ''' in the frame '''[Vectors]'''. For instance the vector length 'norm(U,V)' can be used. Then a color continuous 64 color code is appropriate, as set in the menu '''[!ColorCode]'''. The color code extrema are set by '''[num_!MinVec] ''' and '''[num_MaxVec]''', choose for instance 0 and 5 respectively.'''''''''''
    5250Similarly, the velocity vectors can be superposed to the vorticity field, selecting 'vort'''' ''' in '''[!FieldName_1]''' instead of 'image'''''''.'''
    54 = From pixel displacement to velocity =
     52== From pixel displacement to physical velocity ==
    5553So far all PIV results have been expressed as image displacement expressed in pixels. Conversion to velocity requires timing information and geometric calibration, as described in [wiki:Tutorial/GeometricCalibration geometric calibration]. Both pieces of information must be stored in an xml file named'' 'images.xml''' in the same folder as the folder '''images''' containing the images.'''''
    7775Note that the netcdf file has not been by changed by calibration, whose rescaling is introduced after reading the file. This means that calibration can be provided, and possibly updated, after the PIV processing.
    79 = Masks =
     77== Masks ==
    8078Spurious vectors are observed outside the fluid domain, which particularly disturbing when spatial derivatives are calculated.