# Help in Examples

Dear Friends

I know that the best way to learn LB is using Examples, But I have some problem in using OpenLB examples.
in Poiseuille2d:

in tmp folder , there is a LOG file :

``````                                                    [center]Velocity in lattice units: u=0.02
Reynolds number:           Re=200
Lattice resolution:           N=50
Extent of the system:      lx=2
Extent of the system:      ly=1
Extent of the system:      lz=0
Grid spacing deltaX:      dx=0.02
Time step deltaT:            dt=0.0004
``````

[/center]
1- I think dx= lx/N (2/50= .04) but in log file dx=0.02. N=50 is for X or Y? I think we must greater length in dt calculation.
I am confused !
Our computational domain is 50100 ? or 51101 (Zou B.C)?

2- dx is in SI system (dx = 2 cm)? or it is dimensionless?
I want to calculate Pressure drop, thus use saveAsciiData (that explained in OpenLB docs) to extract pressure data.
These values are physical or LB?
Should i use Delta_P = Delta_P [sub]LB[/sub] * dx[sup]2[/sup]/dt[sup]2[/sup] *Phro[sub]phys[/sub] ?
(thank you Timm for this!)

Thanks a lot

Best Wishes

Hi,

By definition, the grid spacing is dx = 1/N. To know the computational domain, use the methods lattice.getNx() and lattice.getNy() in the simulations.

The parameters are computed relative to a dimensionless system. You need an additional unit conversion to get physical units.