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CF Design Initial and Boundary Conditions

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Old   August 26, 2010, 12:03
Default CF Design Initial and Boundary Conditions
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quick question about CF Design. why in all the examples that you encounter about setting up CFD simulation do they set the outlet as boundary layer with a zero pressure condition? How can you know what your pressure is at the outlet until you run the simulation? Is that not the job for the software? I understand setting your boundary of your inlet with temperature and possibly flowrate but your outlet is kind of an unknown isn't it?? I feel that if I set my outlet to zero pressure boundary that the pressure results will be all out of wack. There is an option in CFD to set the outlet with an INITIAL condition, which makes more sense setting an initial condition at zero pressure. Does this make sense? I guess what I am trying to do is set up a simulation with an inlet temperature and flow velocity and let it run.
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Old   August 26, 2010, 14:27
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Hi Rob.. For flow problems, a pressure = 0 is a common boundary condition in most CFD packages. It simply is a Static Pressure condition. You can also set the "reference pressure", which often defaults to STP.

The above assume incompressible flow/steady state, typically.

Typical BCs would be a pressure drop , P1 at the inlet and P2 at the outlet and CFD will compute the flow rate.

Or as you are seeing Flow Rate at an inlet and a P=0 at the outlet and the Pressure computed at the inlet is the pressure drop.

Happy to help guide you in the right direction.

Initial Conditions are used for transient problems.

Let me know if this isn't clear?
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Old   August 26, 2010, 18:29
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hmm does it not make more sense then to set your inlet boundary at 0 and then let it calculate your drop across a water block between the inlet and outlet for example, or not have any pressure variable at all since I don't really know it? Or do you HAVE TO set some boundary condition on the oulet? because really all I know is the temperature of fluid going in roughly, the inlet mass flow velocity and that is about it. Thanks for your response earlier
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Old   September 3, 2010, 12:07
Default Boundary conditions
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Hi Rob,

Some boundary condition is needed for both the inlet and outlet. Typically the mass flow going in is specified, along with a uniform static pressure condition at the outlet. Note that the inflow condition does not need to be a uniform velocity. The closer to reality you can make the profile the better, which is exactly the point about boundary conditions. They are treated as the *knowns* which drive the solution.

The pressure difference between the inlet and outlet is not necessarily the irrecoverable pressure drop, as there may be a difference in the kinetic energy of the fluid at these points, for example if the cross-sectional areas are different.
Dr John Parry, CEng CITP
Electronics Industy Manager
Mecahnical Analysis Division
Mentor Graphics Corporation
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