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How to solve the problem with simulating a pipe with pressure inlet and outflow BC fo

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Old   August 12, 2018, 09:14
Lightbulb How to solve the problem with simulating a pipe with pressure inlet and outflow BC fo
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mostafa
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when we don't know what kind of property is in the outlet, we use "outflow" BC. now I just have inlet pressure and I want to simply see the pressure drop along the pipe.

I set the pressure in the Inlet, but when I want to set outflow boundary condition for outlet, the fluent give warning: "Both outflow and pressure boundaries are present in the domain. This is an incompatibility and the solution cannot proceed until this is fixed."
How can I solve this problem?
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Old   August 12, 2018, 09:31
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I also have this problem in case of open channel flow
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Old   August 13, 2018, 05:06
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In pipe flow you have to either choose velocity inlet and outflow in order to find pressure drop along the pipe or pressure inlet and pressure outlet to find velocity profiles. There is no way for fluent to solve the problem with pressure inlet and outflow because as you said the outlet conditions are unknown.
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Old   August 13, 2018, 08:41
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Quote:
Originally Posted by Biji.Goli View Post
In pipe flow you have to either choose velocity inlet and outflow in order to find pressure drop along the pipe or pressure inlet and pressure outlet to find velocity profiles. There is no way for fluent to solve the problem with pressure inlet and outflow because as you said the outlet conditions are unknown.
So how can we simulate flow in a part of an infinite pipe (The pipe outlet is infinite...like oil pipeline) when we just have pressure inlet?
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Old   August 13, 2018, 12:29
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The problem is simply underdefined when you have a pressure inlet and outflow BC pair because you know nothing about the flowrate. There are infinite possibilities of the velocity (zero being one of them) that satisfies your pressure inlet constraint and outflow constraint. Remember the outflow BC simply means whatever arrives at the outlet flows out. You need something to fix the flowrate/velocity.

You probably want to use a pressure outlet w/ targeted mass flow rate option. This let's you fix the inlet pressure and allows you to set a flowrate and get the pressure drop.
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