Pressure drop in translationally periodic flow?

dfonseka · April 26, 2018, 23:59

So I was simulating laminar flow in a 2-D wavy channel but I get 0 pressure drop between the translational boundary conditions. I calculated the pressure at the inlet and the outlet and found the difference and divided it by the length but I get meaningless results which I suspect to be incorrect. In addition I computed it alternatively by using the wall shear stress but I get meaningless results too. What should I do?

Does this also mean that my pressure data is invalid? Can I instead calculate the pressure a distance after the inlet and a distance before the outlet to calculate the pressure drop?

LuckyTran · April 27, 2018, 05:36

Are you using the target mass flow rate option? If not, the pressure gradient is an input, not an output.

dfonseka · April 27, 2018, 23:28

Quote:

Originally Posted by LuckyTran

Are you using the target mass flow rate option? If not, the pressure gradient is an input, not an output.

Yes.. I am..

https://www.sharcnet.ca/Software/Flu...ug/node390.htm

I found this source which says it can be used as an output as well. However when I go to the surface integrals calculator and find dP-dx at the inlet it does give me a negative result which makes sense. Do you think it makes sense to use this value or is it an artefact of the simulation?

does dP-dx at the inlet represent the entire flow?

LuckyTran · April 28, 2018, 00:33

The pressure gradient you find in the periodic conditions box is the one you want, which is the body force term & represent the force driving the flow through your periodic section. I.e. this one is the one that you use to calculate the friction factor.

The dp-dx at the inlet is literally the pressure gradient only at the inlet, which tells you the pressure forces at that plane and not much else. Heck in 1D thermo analysis, you often assume this dp-dx to be 0.

dfonseka · April 28, 2018, 19:51

Thank you for your reply LuckyTran. I still 0 on the periodic box even after update.. What am I doing wrong do you think?

LuckyTran · April 30, 2018, 14:56

So when you click update, it stays 0? If its always 0, then it's bugged.

Are you using v18.2? I just recently learned that in a particular version of Fluent, this is a known bug. But only in that one version. See post #8 in that thread. Strangely I have not yet encountered the same bug in 18.2.

Unfortunately if it is bugged then you can't get your pressure drop and need to do it manually using probes of some sort or re-run it in a different version.

April 26, 2018, 23:59	Pressure drop in translationally periodic flow?	#1
dfonseka New Member Dev Fons Join Date: Apr 2018 Posts: 6 Rep Power: 8	So I was simulating laminar flow in a 2-D wavy channel but I get 0 pressure drop between the translational boundary conditions. I calculated the pressure at the inlet and the outlet and found the difference and divided it by the length but I get meaningless results which I suspect to be incorrect. In addition I computed it alternatively by using the wall shear stress but I get meaningless results too. What should I do? Does this also mean that my pressure data is invalid? Can I instead calculate the pressure a distance after the inlet and a distance before the outlet to calculate the pressure drop?

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April 27, 2018, 05:36		#2
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,761 Rep Power: 66	Are you using the target mass flow rate option? If not, the pressure gradient is an input, not an output.

April 28, 2018, 00:33		#4
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,761 Rep Power: 66	The pressure gradient you find in the periodic conditions box is the one you want, which is the body force term & represent the force driving the flow through your periodic section. I.e. this one is the one that you use to calculate the friction factor. The dp-dx at the inlet is literally the pressure gradient only at the inlet, which tells you the pressure forces at that plane and not much else. Heck in 1D thermo analysis, you often assume this dp-dx to be 0.

April 30, 2018, 14:56		#6
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,761 Rep Power: 66	So when you click update, it stays 0? If its always 0, then it's bugged. Are you using v18.2? I just recently learned that in a particular version of Fluent, this is a known bug. But only in that one version. See post #8 in that thread. Strangely I have not yet encountered the same bug in 18.2. Unfortunately if it is bugged then you can't get your pressure drop and need to do it manually using probes of some sort or re-run it in a different version.