In imaging methods such as PIV, seeding is of fundamental importance. Seeding particles should be smaller than the smallest turbulence scale if one wants to describe the turbulent scales properly.
My question is thus related with this. Is there an expression/study made that allows one to select the particles size in function of the turbulent scales one needs to analyze?
As far as I know there are several studies that answer this question quite well:
Measurement Science and Technology Volume 8 Number 1406 doi:10.1088/0957-0233/8/12/005
Tracer particles and seeding for particle image velocimetry
A Melling
http://iopscience.iop.org/0957-0233/8/12/005
and references therein and after that.
Classically you use the number of Stokes, which compares the dynamics of the particles (depending of the particle size) with the dynamics of the flow. It gives you a criterion when the particles follow the flow. Just type "PIV" and "Stokes number" in publication search engines and you will find a large variety of articles on this subject.
I think that the article of Melling (and others, e.g. Mei 1992) addressed precisely this point - Stokes number is not sufficient to describe the limitations the particles might experience while "tracking" the turbulent flows.
Thank you Alex, and Marek for your answers.
In Melling's (1997) paper he relates the characteristic frequency of the seeding particles with the maximum turbulent frequency of the flow. Then he presents values for the seeding particles diameter limit. In a certain sense it answers to my question.
My first line of thought, was thinking in a sort of a "Nyquist" criterion to describe spatial scales of the flow. If the flow's Kolmogorov length scale is k, and if the tracer dimension is d=k, then I wonder if this particle size is enough to describe that scale, since it is in the same order of magnitude, in the same way that it is not convenient to sample a 50Hz signal with a 50 Hz sampling frequency.
In this sense, I was wondering if this wouldn't impose also a limit for the particles size, meaning that the particles size would have to be at least half of the length scales one needs to analyze, or in an equivalent way, that the largest spatial scale one can describe is at least 2d, where d is the tracer diameter.
Dear R. Aleixo ,
I am looking for an answer for the same question you asked. Have you been successful to find an answer for that inquiry through the past years?
If yes, I will be very glad to hear from you.
Thank you
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