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-   -   Validating Brownian motion in OpenFOAM (https://www.cfd-online.com/Forums/openfoam-verification-validation/215127-validating-brownian-motion-openfoam.html)

raptorishungry February 22, 2019 20:05

Validating Brownian motion in OpenFOAM
 
Hello Foamers!

I am trying to validate OpenFOAM's Brownian motion force. For this purpose, I am following the process highlighted in this link .
HTML Code:

https://bugs.openfoam.org/view.php?id=2153
I understand that the source code is updated by henry. But I am having trouble validating it, I tried all possible combinations (changing particle density, carrier fluid, mean free path, temperature of the fluid/particle) but unfortunately not able to pull it off. Also I do not understand what henry means in the last comment of this post
HTML Code:

https://bugs.openfoam.org/view.php?id=2153
where he says "RMS of the magnitude of the displacement in the spherical distribution.", as of now I think solving that ambiguity might help me get closer to validating it.

Could someone please shed some light on this?

Thanks,
adithya

raptorishungry March 6, 2019 13:51

Hey all, I found out the solution to this after performing quite a lot of computational runs and varying constants here and there to narrow down on the discrepancy. The results will not match if you follow this thread
HTML Code:

https://bugs.openfoam.org/view.php?id=2153
This is because the sphereDrag in OpenFOAM needs to be corrected by the Cunningham slip correction factor. I modified OpenFOAM's sphereDrag to a new force following the formulation in ANSYS for Stokes-Cunningham drag
HTML Code:

https://www.sharcnet.ca/Software/Ansys/16.2.3/en-us/help/flu_th/x1-62400016.7.3.html
I also modified the BrownianMotionFoce in OpenFOAM according to Omid et al.
HTML Code:

https://www.tandfonline.com/doi/pdf/10.1080/02786820802587888
Making these changes validated Brownian motion force in OpenFOAM.

jafarbakhshi1374 September 4, 2019 10:25

j have problem in validating brownian force openfoam
 
hello dear
Adithya Gurumurthy
i have the same problem and my results have about 50 precent deviation by exact answere (sqrt(2Dt)) so would you please send the changes y have done to validate this force?

xuegy October 18, 2019 14:45

Hello Adithya

Could you please share your modification? I also have problem validating the Brownian motion force. Seems like the code from OpenFOAM is always smaller by sqrt(2).

raptorishungry October 18, 2019 18:00

5 Attachment(s)
Hello jafarbakshi and xuegy, sorry for the late reply. I have attached my files below.

xuegy October 21, 2019 13:09

Quote:

Originally Posted by raptorishungry (Post 747473)
Hello jafarbakshi and xuegy, sorry for the late reply. I have attached my files below.

I just tried your code. The mathematics equation are basically the same as OpenFOAM cubic distribution(comment in the source code). It has the same problem verifying the Brownian motion. It's always diffusing sqrt(2) times slower than the continuum model.

raptorishungry October 21, 2019 17:27

It worked for me. What are you validating your model with? could you post your case set-up/plots?

xuegy October 22, 2019 14:28

1 Attachment(s)
Quote:

Originally Posted by raptorishungry (Post 747603)
It worked for me. What are you validating your model with? could you post your case set-up/plots?

I'm trying the easiest 1D case. A 2um*2um*20um box(1*1*100 mesh) is filled with water. Half of the box has 100nm spherical particles initially (dimensionless concentration "2") while another half is empty (0). The continuum model solves the diffusion equation while the discrete model simulates Brownian motion and converts the Lagrangian field to the void fraction.

My results shows that the discrete model is always slower by sqrt(2), e.g. the result of discrete model at t=11.3 s converges to the result of continuum model at t=16 s.

Attachment 72909

Please correct me if I make any mistake!


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