# Periodicity in two directions

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 June 4, 2020, 20:41 Periodicity in two directions #1 New Member   Join Date: Apr 2020 Posts: 22 Rep Power: 2 Background: I am trying to simulate a compressible flow over an infinite plate covered with hemispherical pimples. I reduced my domain to a row of a few hemispherical pimples. Ideally, I would like to apply periodic BC in two directions: sideways (pressure gradient=0) and from inlet to outlet (specified pressure gradient). The top surface is a simple free-slip wall. Problem: FLUENT allows specifying a single pressure gradient value that is applied to all periodic interfaces (sideways and in/out). Question: I am not familiar with UDFs and I am wondering what is the best way to define periodic BC so that I will manage to impose different pressure gradient values in the two directions. Thank you in advance.

 June 5, 2020, 03:53 Compressibility #2 Senior Member     Vinerm Join Date: Jun 2009 Location: Nederland Posts: 2,948 Blog Entries: 1 Rep Power: 31 Compressibility renders the flow aperiodic. You cannot model compressible flow with translational periodic condition. __________________ Regards, Vinerm PM to be used if and only if you do not want something to be shared publicly. PM is considered to be of the least priority.

June 5, 2020, 07:34
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Quote:
 Originally Posted by vinerm Compressibility renders the flow aperiodic. You cannot model compressible flow with translational periodic condition.
First of all, I would like to thank you for your helpfulness.

In a previous post (Link: Flow recirculation - UDF boundary conditions), you said that by applying FLUENT's periodic inlet BC to compressible flows:

1. (rho*V)in = (rho*V)out
2. Temperature follows the scaled temperature treatment
3. rho is calculated through the equation of state
4. V is derived from continuity

My questions are:
1. Can I model a periodic heat transfer problem (thus, compressible) with translational periodicity or I have to develop a user-defined boundary condition for inlet and outlet?
2. In case of compressible flows, is translational periodicity not even applicable to sidewalls?

 June 5, 2020, 07:43 Periodicity #4 Senior Member     Vinerm Join Date: Jun 2009 Location: Nederland Posts: 2,948 Blog Entries: 1 Rep Power: 31 Mass flow is conserved whether periodicity or not. However, one of the limitations of the periodic conditions in Fluent is that the flow should be incompressible. Whether it is limitation of implementation in Fluent or applicable to fluid flow in general would require more thought. __________________ Regards, Vinerm PM to be used if and only if you do not want something to be shared publicly. PM is considered to be of the least priority.

 Tags heat transfer, periodic boundary, periodicity