[gearman]

Dear everyone!
I used fluent to calculate the pressure drop inside a thin circular tube. My doubt is how the pressure drop is calculated? How do I know if the calculation is accurate and is there a theoretical formula to verify it?Thank you inadvance!
gearman

[LoGaL]

I would look to use the usual moody chart, see https://en.wikipedia.org/wiki/Moody_chart. It works

[gearman]

Originally Posted by LoGaL

I would look to use the usual moody chart, see https://en.wikipedia.org/wiki/Moody_chart. It works

Dear LoGaL！Thank you for your reply!What is the relationship between Roughness Height and Darcy-Weisbach friction factor fD in Fluent? Or if I know the Darcy-Weisbach friction factor fD=0.023, how should I set the Roughness Height of the wall?

[LoGaL]

just do the case with zero roughness.. It's the smooth pipe curve in the moody diagram

[gearman]

Originally Posted by LoGaL

just do the case with zero roughness.. It's the smooth pipe curve in the moody diagram

Dear LoGal!Thank you for your reply!You mean to do the case with zero roughness to compare the result with the smooth pipes on the Moody diagram?But if the pipe I'm simulating is rough, how do I set the roughness in the fluent and compared with the rough tube in the moody diagram?

[LuckyTran]

There is no general rule for setting the roughness height. There isn't even a definition for what the roughness height even is. Roughness functions are developed by blatantly assuming that Nikuradse's law of the wall holds via circular logic. You find an equivalent roughness function by fitting the pressure drop and not the other way around. You can't predict the pressure drop unless you already know the roughness height and you can't know the roughness height unless you've measured the pressure drop.

[evcelica]

Originally Posted by LuckyTran

There is no general rule for setting the roughness height. There isn't even a definition for what the roughness height even is. Roughness functions are developed by blatantly assuming that Nikuradse's law of the wall holds via circular logic. You find an equivalent roughness function by fitting the pressure drop and not the other way around. You can't predict the pressure drop unless you already know the roughness height and you can't know the roughness height unless you've measured the pressure drop.

This is exactly correct. The absolute roughness value is self-defining, and can only be measured via pressure drop. I like your "circular logic" explanation. If it helps, I have found that in CFX, the sand grain roughness value seems to be a decent approximation to this absolute roughness value. I just use published absolute roughness values for sand grain roughness inputs, as it is the best estimate I have.