# Reply To: Bounce Back Boundary conditions and lattice nodes locations

OpenLB – Open Source Lattice Boltzmann Code › Forums › on OpenLB › General Topics › Bounce Back Boundary conditions and lattice nodes locations › Reply To: Bounce Back Boundary conditions and lattice nodes locations

Julius,

Thank you very much for the discussion. To finish understanding your point I made a simple sketch which I attach. https://ibb.co/gmv52zY

In the figure, the orange thick line represents a geometry indicator for the boundary. We can say that the red nodes which coincide with the orange line are at the wall boundary. The blue nodes are fluid nodes and the black nodes are virtual nodes.

That said, for the Temperature lattice, after collision and streaming, the perpendicular population entering the fluid domain is set accordingly to impose the Dirichlet temperature there. However for the velocity, the halfway or the fullway bounce back would set the velocity to zero, deltax/2 in the upwards direction of the figure.

What I can say though, is that looking at the results of some examples in OpenLB, at those boundary nodes, the velocity is zero and the temperature is set as it is supposed to (rayleighBernard2d example). I am just having a struggle figuring out how it is done.

Thanks,

Ramiro