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-   -   Why is Multiple Reference Frame not applicable to micrometric models? (http://www.cfd-online.com/Forums/fluent/97177-why-multiple-reference-frame-not-applicable-micrometric-models.html)

 dixylo February 10, 2012 13:09

Why is Multiple Reference Frame not applicable to micrometric models?

I used the Multiple Reference Frame (MRF) to calculate the fluid field of a stirrer with a size of tens of microns, but I got an very odd result, as shown in the first figure below, that there seems to be no interation on the interface between the swirl zone (inner) and the stationary zone (outer).
What has been confusing me is that when I calculated the model 1000 times bigger (a millimetric one), I could get an acceptable result, as shown in the second figure.
Then, I calculated models 10 times and 100 times bigger, respectively, the results are shown as figure 3 and 4.
The only difference among the four models is the scale factor. I don't know why MRF becomes gradually inapplicable as the scale factor decreases.
I'm doing some work on microfluidic calculation, but I'm unable to get a reasonable result of a micrometric model, no matter i use MRF, sliding mesh, or single reference frame. please help me. any help would be much appreciated.

http://ww3.sinaimg.cn/mw600/64d22f47jw1dpx4xj0pcaj.jpg
figure 1, vertical cross section (y-z plane) of the velocity field of a micrometric model

http://ww2.sinaimg.cn/mw600/64d22f47jw1dpx4xn6g7fj.jpg
figure 2, vertical cross section (y-z plane) of the velocity field of a millimetric model

http://ww4.sinaimg.cn/mw600/64d22f47jw1dpx4xk6f0zj.jpg
figure 3, vertical cross section (y-z plane) of the velocity field of a model 1/10 smaller than the above one

http://ww2.sinaimg.cn/mw600/64d22f47jw1dpx4xl2h9tj.jpg
figure 4, vertical cross section (y-z plane) of the velocity field of a model 1/10 smaller than the above one

http://ww4.sinaimg.cn/mw600/64d22f47jw1dpx4xlv7k8j.jpg
figure 5, the stirrer model, which is 5μm high, 20μm in diameter, and rotates in a cylinder wall. The fluid field is divided into two volumes, which are the swirl zone (inner) and the stationary zone (outer), respectively. The interface contains two sets of superposed cylindrical faces that belong to the swirl zone and the stationary zone, respectively.

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