How to convert this simple example to lattice units? Please help

Hi everybody,
I read the article of Dr Jonas latt about lattice units but i still can’t understand the conversion.
Have a 2-D channel flow that is 0.05 m wide and 0.2 m long. I’m simulating a flow past a cylinder ( diameter= 0.025 m) , model D2Q9, BGK.

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The grid is 50x 200

The physical viscosity :
nu_{Phys} = 10^ - 6 m ^2/s
u _{Phys} = 0.002 m/s (at the inlet)

Idon’t understand how to convert to lattice units. Please help me even with simple hints.


first of all you should find reynolds number in physicsl which is the same in boltzman you can take velocity 0.1-0.2 in lattice scale and with the known grid numbers the kinetic viscosity can find this is one of the way u can use

Suppose that Reynolds number found equal 200 for example. How can i choose the lattice velocity? why it must be 0.1 or 0.2 ? Is it arbitrary? .
Then the number of nodes in length direction for example am i free to choose 100 nodes, 300 nodes …? A lot of questions in my mind .
Please help … and thank you very much

Dear dhouha,

the reason for the small velocities is that lattice Boltzmann is only valid for small Ma-numbers. Since the lattice speed of sound c_s is fixed, the only way to satisfy this condition is to ensure that the lattice velocities are small. As stated above, usually values <=0.1 are picked.

Concerning the unit conversion, I think the approach presented in

might be easier accessable. There, it is described in detail and some examples are shown.

You might also want to check out some older threads that deal with the problem of unit conversion,4891,4894#msg-4894,4958,5409#msg-5409,3685,4828#msg-4828

These are just a few, there are more.



Precious resources , thanks a lot !!