Problem with periodic boundary layers

Phil

I try to investigate a flow (water) in a duct steady state, laminar with exchange of heat. I have set periodic boundary conditions for the in and outflow so I try to simulate fully developed flow. To reach faster convergence I try to solve the problem by a step to step solution process. I am using the segregated solver so I turned of turbulence (k-epsilon RNG) and energy. After convergence of the laminar flow I turned the laminar equations off and solved only the turbulent equation to get a good initial guess for k and epsilon.

**Here I have the first question, why must I not make an initial guess for k and epsilon. When I simulate without periodic boundary layer I have to give at the inlet and outlet (in the boundary condition panel) an initial guess for k and epsilon, or alternatively the hydraulic diameter and the turbulence intensity.

Anyway I solved the turbulence equations and turned on after a while the flow field. After a while I had convergence and the results seemed to be in agreement with what I have expected. Then I turned of the flow field and the turbulence, and turned on the energy. I'm using constant wall temperatures and made a guess for the bulk temperature in the periodic boundary condition panel. But with the default values for the relaxations factors for the energy field the calculation didn't converge and the temperatures were limited in nearly all cell volumes to a value about 1 Kelvin, which seems to be a little bit too cold. ;-) When I decrease my relaxationfactor for energy from 1 to 0,1 the value of cells where the Temperature is limited is less and the iteration seemed to converge but the results weren't impressing. Perhaps I have to wait longer because I was far away from the convergence (Residual about 1e-3) but it seems to be a general problem.

Anybody has got an idea?????

Thank you Phil