[seand]

I am attempting to simulate a bank of cylinders in crossflow, each with the same specified heat flux through the cylinder wall. As a means to increase computational efficiency, my simulation will be 2D, and I am also looking at the possibility of using a periodic boundary conditions to simulate 2 cylinders instead of 10+. However, I noticed in the fluent user manual that "transient simulations for fully-developed fluid flow are not valid with translational periodic flow." Does this mean that if I were to use period boundary conditions, my analysis is limited to steady flows only?

Also, since I'm trying to fully understand this boundary condition, please explain why I can or cannot use this in transient simulations.

[LuckyTran]

I'm guessing you are reading that out of a very old version of the Fluent manual (i.e. v6) because people have been using transient simulations with periodic BC's for more than a decade already. It's really not an issue anymore and you should be able to do transient sims with periodic BC.

It applies to the pressure jump boundary condition, which is technically not valid for periodically fully developed flows exactly because there shouldn't be any pressure jumps in fully developed flows but a constant body force. Fluent has long since gotten rid of the pressure jump formulation for their periodic BC and gone to the correct pressure gradient (which is equivalent to a body force). However, some solvers still use the pressure jump condition, which is technically invalid, but people do it anyway. Star-CCM used to have this formulation (I'm not sure if it still does in the latest version). But I would expect that given enough time, that all solvers would switch to the correct pressure gradient formulation. In general though, the periodic BC is limited in most codes to the segregated / pressure-based solver approach because it is valid only for constant density flows.

On the theory of periodically fully developed flows, see Patankar's paper. Notice the 1977 date.

Patankar, S. V., Liu, C. H., and Sparrow, E. M. "Fully Developed Flow and Heat Transfer in Ducts Having Streamwise-Periodic Variations of Cross-Sectional Area," Journal of Heat Transfer Vol. 99, No. 2, 1977, pp. 180-186. doi: 10.1115/1.3450666