Large error in Pressure Drop in pipe

ajhunte · July 3, 2013, 18:39

I have a single pipe with a triangular input and output:

Picture of the geometry (basically a pipe filled with water shaped like a triangle)
https://plus.google.com/u/0/photos/1...28021247361055

When i do a hand calculation based on the width and height of the flow path and the velocity at the inlet, I get about 10 psi pressure drop that I should be getting from inlet to outlet. When I run Ansys, I get approximately 20 psi.

I have made sure that I have an inflation for the boundary layer, I have made the mesh ridiculously fine, and that I run it to 1e-5 convergence criteria. I have also confirmed with my professor that I should be getting around 10 psi (meaning that I did not make a math error when doing the hand calculation).

Any ideas why this is occurring?

ghorrocks · July 3, 2013, 20:14

What is the hand calculation based on? Is the hand calculation based on fully developed flow? Did you model fully developed flow in CFX? If you just modelled a constant velocity across the entire inlet the flow would develop in the simulation and have a much higher pressure drop - his could explain your error.

The mesh you show is horribly coarse. I trust that is not the mesh you used.

Is the triangle cut out of the cylinder (ie the flow is a cylinder pipe with a triangle in the middle ) or does the flow go through the triangle bit?

Also, have you looked at the FAQ: http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F

July 3, 2013, 18:39	Large error in Pressure Drop in pipe	#1
ajhunte New Member AJ Hunter Join Date: Feb 2013 Posts: 29 Rep Power: 13	I have a single pipe with a triangular input and output: Picture of the geometry (basically a pipe filled with water shaped like a triangle) https://plus.google.com/u/0/photos/1...28021247361055 When i do a hand calculation based on the width and height of the flow path and the velocity at the inlet, I get about 10 psi pressure drop that I should be getting from inlet to outlet. When I run Ansys, I get approximately 20 psi. I have made sure that I have an inflation for the boundary layer, I have made the mesh ridiculously fine, and that I run it to 1e-5 convergence criteria. I have also confirmed with my professor that I should be getting around 10 psi (meaning that I did not make a math error when doing the hand calculation). Any ideas why this is occurring?

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Pressure drop in a pipe	mark	FLUENT	15	December 4, 2019 08:17
a problem in calculating pressure drop in Fluent?	yu chun	FLUENT	1	May 18, 2004 04:40
Terrible Mistake In Fluid Dynamics History	Abhi	Main CFD Forum	12	July 8, 2002 10:11
what the result is negatif pressure at inlet	chong chee nan	FLUENT	0	December 29, 2001 06:13
Hydrostatic pressure in 2-phase flow modeling (long)	DS & HB	Main CFD Forum	0	January 8, 2000 16:00

July 3, 2013, 20:14		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	What is the hand calculation based on? Is the hand calculation based on fully developed flow? Did you model fully developed flow in CFX? If you just modelled a constant velocity across the entire inlet the flow would develop in the simulation and have a much higher pressure drop - his could explain your error. The mesh you show is horribly coarse. I trust that is not the mesh you used. Is the triangle cut out of the cylinder (ie the flow is a cylinder pipe with a triangle in the middle ) or does the flow go through the triangle bit? Also, have you looked at the FAQ: http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F