We recently captured shadow graph image of spray, only diameter was post processed, velocities was not post processed. We used lavision to make a shadowgraph for our experiment. We have captured a double frame image of spray with a double frame background image for post processing in the lavision software.
Philippe Leick
You should be able to track velocities provided your time delay (dt) is OK and your images have sufficient contrast. If the optical density is too high, you'll encounter problems.
You'll probably want to:
- use a high pass filter of some kind. Remove either a local or a global background. The choice of a suitable background is critical.
- have some statistics, i.e. repetitions of your experiment
- use normalized, direct cross correlation instead of standard fft-based correlation
- and most important, check whether your images are suitable. If you have plenty of focused, small droplets that you could track manually: good! However, if your structures are large, blurred, have low contrast, or seem difficult to track, then PIV algorithms won't help.
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Dominique Heitz
I you have couple of images you can use optical flow approaches. See our synopsis on the subject in Exp. in Fluids (Heitz etal. 2010).
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