Hi,

it depends, at least, on your image processing technique, typical bubble size, and local flow velocity.

In case of particle image velocimetry (PIV), image pairs are divided into patches. The size of the pixel patch is determined by the magnitude of the expected incremental particle displacement between the two images. Your flow should generate enough texture on the patches, so that PIV is able to track this texture between images. If camera resolution is poor or the images of bubbles or particles seeded in the flow are too small (say, less than 1 or 2 pixel), there might be no characteristic texture (= intensity matrix) for tracking. On the other hand, if bubbles were large or if your flow contains large-scale interfaces (e.g. slug flow), then you might track the interface directly using PIV.

Concerning PIV and two-phase flow, I recommend the following papers:

[1] R. J. Adrian: Particle-Imaging Techniques for Experimental Fluid Mechanics. Annual Review of Fluid Mechanics 23:261-304, 1991

[2] R. J. Adrian: Twenty years of particle image velocimetry. Experiments in Fluids 39:159-169, 2005

[3] R. Lindken, W. Merzkirch: A novel PIV technique for measurements in multiphase flows and its application to two-phase bubbly flows. Experiments in Fluids 33:814-825, 2002

Regards

That sounds like a very useful technique and an interesting problem.

Dear Dr.Aubram,

So, can we say that it has to be two or three or four etc. pixels at least on the bubble for quality analysis?

There is no general answer because you can process images in different ways; see reference [Adrian, 1991] above. The image processing technique should be chosen according to the mean number concentration of bubbles (or scattering particles) per unit volume in relation to the scales of the fluid flow field you want to take into account.

Theoretically, even 1 pixel per bubble might be sufficient as long as a typical patch of pixels contains characteristic texture. For example, one black pixel on an otherwise white patch (= test patch) can be considered characteristic if there is no other black pixel in a small neighborhood of the patch (= search patch).

Regards