Lattice Boltzmann failing at reversible flow... Why?

Lattice Boltzmann Method LBM: Theory
#1

I have a 100X100 mesh and i have problems with the boundary conditions

Bb equals Bounceback
Uin is fixed value ( i have 0.05)

Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb
Uin 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 dU=0
Uin 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 dU=0
Uin 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 dU=0
Uin 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 dU=0
Uin 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 dU=0
Uin 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 dU=0
Uin 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 dU=0
Uin 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 dU=0
Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb

When I run the simulation …after 1 time step i get

Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb
Uin -0.01 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 dU=0
Uin -0.01 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 dU=0
Uin -0.01 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 dU=0
Uin -0.01 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 dU=0
Uin -0.01 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 dU=0
Uin -0.01 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 dU=0
Uin -0.01 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 dU=0
Uin -0.01 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 dU=0
Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb

However, if i reverse it…after one time step i get (remember Uin is 0.05)

Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb
dU=0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0.039 0.05
dU=0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0.039 0.05
dU=0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0.039 0.05
dU=0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0.039 0.05
dU=0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0.039 0.05
dU=0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0.039 0.05
dU=0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0.039 0.05
dU=0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0.039 0.05
dU=0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0.039 0.05
Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb

Which is correct, if i increase timesteps the value progresses properly

Also, i tried (uin=-0.05)

Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb
dU=0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0- 0.044 -0.05
dU=0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0- 0.044 -0.05
dU=0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0- 0.044 -0.05
dU=0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0- 0.044 -0.05
dU=0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0- 0.044 -0.05
dU=0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0- 0.044 -0.05
dU=0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0- 0.044 -0.05
dU=0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0- 0.044 -0.05
dU=0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0- 0.044 -0.05
Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb

Which is also correct…

Can anyone explain why Left to right flow fails…

I can give more details (not mistakes)about rho, fEq if someone takes interest in my problem

Please help

#2

On using Neumann conditions

if i go for Left is Inlet
rho becomes less than 1 if i have positive Uin

if i go for right as inlet
rho becomes more than 1 if i have negative Uin (inlet)

I think the difference is causing arbitrary values in my code

Can someone elaborate please

#3

Hello,

you should first try to debug your code. Please make sure that you have coded the equations correctly. It is nasty work but necessary.

Good luck,
Timm

#4

It was indeed an error…thanks for the advice and sorry for posting this