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?

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July 3, 2013, 20:14		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,871 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