Outlet pressure boundary for pipe low Re

kingjewel1 · June 30, 2016, 12:10

I'd like to know whether a 0Pa pressure outlet boundary condition is correct for the following:

I'm modelling a flow through a sudden contraction: A rectangular box followed by a cylinder. Dimensions of the box are 1.7mx1.7x1.7 and the diameter of the cylinder is 0.165m and length 0.1m.

Reynolds' number, based on the inlet flow rate (0.4m^3/s) and outlet diameter is about 1000.

My turbulence model is K-omega with low Re correction. y+ <5

Problem: I'm finding flow does not develop properly in the pipe as it's so short. What should I do about the outlet condition?

Far · July 1, 2016, 03:57

increase the length of pipe

kingjewel1 · July 1, 2016, 04:40

I'm comparing against experiment and there, the pipe is of that length.

What do you think?

Far · July 1, 2016, 05:03

no problem. only compare the portion of geometry that is available in experiments but extended the pipe.

kingjewel1 · July 1, 2016, 05:18

Ok, maybe I can change the definition of my problem. So the end of the pipe is venting into the atmosphere and I'm interested in looking at how the length of the pipe affects the pressure inside the box. Do you see why I'm stuck with the specific length of pipe.

Perhaps I need to model a large box on the end of the pipe to represent the room in which I did the experiment?

June 30, 2016, 12:10	Outlet pressure boundary for pipe low Re	#1
kingjewel1 Senior Member Join Date: Jul 2009 Posts: 260 Rep Power: 18	I'd like to know whether a 0Pa pressure outlet boundary condition is correct for the following: I'm modelling a flow through a sudden contraction: A rectangular box followed by a cylinder. Dimensions of the box are 1.7mx1.7x1.7 and the diameter of the cylinder is 0.165m and length 0.1m. Reynolds' number, based on the inlet flow rate (0.4m^3/s) and outlet diameter is about 1000. My turbulence model is K-omega with low Re correction. y+ <5 Problem: I'm finding flow does not develop properly in the pipe as it's so short. What should I do about the outlet condition?

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July 1, 2016, 03:57		#2
Far Senior Member Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,553 Blog Entries: 6 Rep Power: 54	increase the length of pipe

July 1, 2016, 04:40		#3
kingjewel1 Senior Member Join Date: Jul 2009 Posts: 260 Rep Power: 18	I'm comparing against experiment and there, the pipe is of that length. What do you think?

July 1, 2016, 05:03		#4
Far Senior Member Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,553 Blog Entries: 6 Rep Power: 54	no problem. only compare the portion of geometry that is available in experiments but extended the pipe.

July 1, 2016, 05:18		#5
kingjewel1 Senior Member Join Date: Jul 2009 Posts: 260 Rep Power: 18	Ok, maybe I can change the definition of my problem. So the end of the pipe is venting into the atmosphere and I'm interested in looking at how the length of the pipe affects the pressure inside the box. Do you see why I'm stuck with the specific length of pipe. Perhaps I need to model a large box on the end of the pipe to represent the room in which I did the experiment?