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Old   October 8, 2012, 09:01
Default pressure drop - pipe flow
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Hi all,

I want simulate a pipe flow in fluent... the inlet and outlet are linked by periodic boundary conditions... In the pressure gradient (periodic boundary condition) I have a value, but in the static pressure profile I have negative values. I want compare the pressure drop with an experimental value. My question is: what value should I use?
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Old   October 8, 2012, 10:36
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First of all: If you specify your periodic interface with a pressure drop, then there is no point in comparing the pressure drop obtained in the simulation with experimental data.
You could compare mass flow this way.
If you want to compare pressure drop, define a mass flow.

Then again, the result of your simulation with a highly non-linear pressure drop (even an increase at the inlet) doesn't look trustworthy. There must be something wrong with your setup.

The issue with negative static pressure is just a question of normalization. The pressure derivative (pressure drop) is unaffected by this issue.
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Old   October 9, 2012, 15:03
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In Fluent, the periodic pressure condition is imposed by decomposing the pressure gradient into a constant part and a varying part, i.e. \nabla p =\nabla p_{const}+\nabla p_{varying}. The constant part is imposed as a body force and the varying part is imposed as a surface force. The constant part is iterated to satisfy your mass flow rate and the varying part is solved for. For a fully-developed pipe flow, the constant part is your pressure gradient along the pipe and the varying part is zero (since the pressure profile is linear). When you plot a contour, Fluent plots the varying part of the pressure. As you can see, the magnitude of your pressures are less than one Pascal which is negligible (numerical error). Ignore this and look at the constant part. The first image you posted shows your answer for the constant pressure gradient along the pipe's axis.
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Old   November 17, 2012, 10:55
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Thanks for the elaborate explanation. I am having problems postprocessing periodic flows, for example how do you get the pressure drop to use in the calculation of the friction factors? Do you use the varying part or the constant part? How do you get these to parts in post processing?
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Old   November 19, 2012, 10:05
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Dear Chris DeGroot,

Thank you for your explanation
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Old   November 19, 2012, 10:12
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Quote:
Originally Posted by cdegroot View Post
In Fluent, the periodic pressure condition is imposed by decomposing the pressure gradient into a constant part and a varying part, i.e. \nabla p =\nabla p_{const}+\nabla p_{varying}. The constant part is imposed as a body force and the varying part is imposed as a surface force. The constant part is iterated to satisfy your mass flow rate and the varying part is solved for. For a fully-developed pipe flow, the constant part is your pressure gradient along the pipe and the varying part is zero (since the pressure profile is linear). When you plot a contour, Fluent plots the varying part of the pressure. As you can see, the magnitude of your pressures are less than one Pascal which is negligible (numerical error). Ignore this and look at the constant part. The first image you posted shows your answer for the constant pressure gradient along the pipe's axis.
Thanks Chris, i now get it. Is there any way of getting the pressure gradient as an output parameter into workbench for optimization studies??
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Old   November 19, 2012, 11:02
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Quote:
Originally Posted by Kamu View Post
Thanks Chris, i now get it. Is there any way of getting the pressure gradient as an output parameter into workbench for optimization studies??
You are welcome. I am sure that there is, but I don't know it. I'm not a big workbench user.
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