How do I solve for fully developed flow with a short length of pipe in CFX-5.7.1?


Fully developed flow implies that the momentum equations are periodic and the level of difficulty of the problem depends on how many other variables are not periodic.

The simplest case is where the pressure is not periodic and you wish to obtain the flow solution for a given pressure drop. In this case you can set up periodic boundaries at the ends of the pipe and create a sub-domain equal to the whole domain and set a general momentum source in the axial flow direction equal to delta_P/delta_x, where delta_P is the expected pressure drop across the pipe and delta_x is the length of the pipe.

If more than one variable is not periodic, then this approach will not work. For example, if you wish to solve for heat transfer at the walls of the pipe then the temperature is not periodic as the solution requires the two ends of the pipe to be at different temperatures. What you can do is solve for the isothermal fully developed flow solution using the momentum source method. Then, in CFX-Post, export a BC profile from the inlet. You can then set-up a new simulation using inlets and outlets and use the exported BC profile as a boundary condition for your inlet.
Have a look at the helpmanual on setting up profile boundary conditions: Help->Master Contents->Solver Modelling->Contents->Boundary Condition Modelling->Profile Boundary Conditions. This should work well provided the fluid properties are not strongly dependent on temperature.

If the fluid properties are strongly dependent on temperature you can manually iterate on the solution. Construct a non periodic simulation with an inlet and outlet and converge this solution with a constant velocity inlet profile. Then export the outlet velocity BC profile and use this as the inlet profile for the next solution. Keep going until the inlet and outlet velocity profiles are sufficiently similar. This process can be automated to some extent by saving a post session file to export the BC data. Keep monitoring thepressure drop across the pipe which may drift slightly depending on how well each solution is converged. You can always scale the inlet velocity profile function by a constant value to obtain the required pressure drop.





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