LES convergence problem in ANSYS/FLUENT 14.0
 

Dear all,

I want to solve flow field in a complex geometry(respiratory system) Using LES method. I used an LRN k-epsilon converged solution as an initialization for LES. the pressure-velocity coupling is PISO algorithm and the 2nd order implicit method was used for discritizing unsteadiness. however, the solution diverged at the first time step after about 50 iterations. I'm using ANSYS/FLUENT 14.0. I have solved the flow field by fluent 6.3.26 without any problem but since I must include inlet perturbation for my pressure-inlet boundary condition and FLUENT 6.3.26 doesn't have inlet perturbation for pressure-inlet condition, I should use ANSYS/FLUENT 14.0. Does anybody know why ANSYS/FLUENT 14.0 cannot give a converged solution while FLUENT 6.3.26 can do so?
Moreover can you help me in solving my problem?

helps will be appreciated.
Regards,
Ebrahim