# General working of the LBM, flow diagram

Hello,
I’m new at LBM and I have been provided of some code in order to solve a flow with this method.

1. I saw the program in fortran provided, but I cannot print out the values which are supposed to be written, the files: OutputImages and OutoutUx & OutputUy, don’t know why.

2. I have understood the flow diagram, in the sense that after inisialisation of density in the flow domain (let’s say matrix node and n_hlp) and the definition of the vector directions it is possible to calculate the equilibrium function (let’s say matrix node): Redistribute part. Then all the nodes velocities are passed to the next one, this is the streaming part and if it is found an obstacle the velocity direction in the node is changed, so copy node in one position in n_hlp of the next position. Now it is possible to recalculate the equilibrium function and the values are saved in n_equ adopting the n_hlp values. At the last using the relaxation it is possible to calculate the matrix node according to the linearized equation that describes the collision that is: node=n_hlp+omega*(n_equ-n_hlp).

I hope my description is exhaustive, and my questions are:

2a) is the description right?
2b) I cannot understand the streaming part, if we simply move the flow, which kind of iterations are useful to this problem, since it would only be a translation of the inlet (in absence of obstacles and slip conditions).
2c) in the case provided the values of tau is 0.515, which is near to the limit 0.5, could it cause instabilities?
2d) What is collision, substantially?

I hope I didn’t annoy too much and thanks for people who is interested to answer or may find useful my flow diagram, even if I’m not 100% sure!

Bye