Problems when using D3Q27


I have tried to simulate flow through a nozzle defined by an stl file using Palabos v0.6 and 0.7. The area ratio between the inlet and the end of the nozzle is 9:1. However the increase in centreline velocity I observe is more than ninefold. As I increase the density the velocity increase becomes smaller, but when the outlet of the nozzle is 20 lattice units in diameter the centreline velocity still increases to more then nine times the maximum inlet velocity. D3Q19 does not show this problem and agrees well with experimental data when simulating the nozzle at similar density. Does anyone know why the D3Q27 lattice is behaving this way?

Also I have found that for D3Q27 only the results obtained in some situations can be very different when run as a single block (as in fullDomainFromSTL) rather than as a multi block (as in sparseDomainFromSTL). Does anyone know why this might be? I can send examples if necessary.

Alex White