Boundary Conditions - 2D

I’m undergraduate student (aeronautical eng.) and I started to study Lattice Boltzmann now and I’m simulating a fluid through a channel, but I’m having some problems with the boundary conditions (I think).

The channel have a inlet (size 0.2 m) with velocity fixed (0.02 m/s) and aspect ratio = 25. The grid is 40x1000 and the viscosity in the LB code v = 0.02, Reynolds = 400.
The author suggests that the outlet boundary conditions be:


I always thought it should be done for f6,f3 and f7. I tried but didn’t work.

The moddel of the lattice is:

6 2 5
\ | /
3- 0 -1
/ |
7 4 8

The problem observed is that the speed in the middle of the channel does not stabilize, it keeps accelerating through the channel (until ~ 0.334… in the end of the channel), so i think is wrong.
Someone had one idea about what I can do to solve this problem?