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.

| |
| |
| |
| |
| | 0.05 m
| |
| |
| |
|__________________________________________ |

The grid is 50x 200

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

Thanks

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 .

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