Inlet Velocity BC:: What does checking on "Fully developed Flow" imply?

nikesh · August 18, 2014, 02:29

Hi FloEFD users,
When applying "Normal to Face" Inlet Velocity BC, there is an option to check on "Fully Developed Flow".
By checking on it, do I get fully developed flow right at that plane? In my case (flow through a pipe and valve, incompressible flow), if I checked on it does it mean that there is no need for me to have sufficiently long pipe (~20D) upstream of the valve since the flow is already fully developed? And if so, I can actually reduce the upstream length to just 1D or 2D from the valve position. Did I get this right?
What difference can I get if I don't check on it?
Any insights highly appreciated!
Thanks
Nikesh.

Boris_M · August 18, 2014, 03:15

That it correct. The fully developed flow will add the parabolic flow profile you would get from a longer flow through a pipe but it only work for circular and I think square shaped opening. So any weird shaped inlets will require a longer pipe to be able to develop the correct flow profile as there is not a known profile for every weird shape one can think of.
The 1D or 2D is probably a little short as there might be some disturbance upstream from the partial blockage of the valve geometry itself similar to the vortex crosses pressure opening problem you will get if you place the outlet directly behin the valve and there is a vortex.
The difference would be that your flow profile is not developed fully when entering the valve and you might get different pressure drops also. How much that would change the results in general is hard to tell as it depends on various factors such as fluid, flow rate, pressure ect.
I usually use it if there is some piping before the simulated geometry.

Regards,
Boris

nikesh · August 18, 2014, 19:50

Thanks Boris,
Yes in my case, I need to make sure that the flow is fully developed when it reaches the valve position. So, by enabling the "Fully Developed Flow" option, I don't need to make the piping (circular pipe) as long as 20D upstream of the valve (there is an expression for this length, known as Entrance length for the incompressible turbulent flow to be fully developed in a pipe). But what I do need to make sure is that it is long enough such that the disturbed flow does not reach out and affect the inflow BC.
This explanation is for the "Normal to Face" Inlet Velocity BC.

But when applying "Vector" Inlet Velocity BC the option of "Fully Developed Flow" is not present. This case is similar to the first case while not enabling "Fully Developed Flow", in that, flows in both these cases require to travel certain distance for it to be fully developed. Did I get it right here?

Boris_M · August 19, 2014, 01:34

Yes, that's true, it doesn't work for vector definitions of the inlet as a flow profile such as the fully developed flow not straight into a pipe wouldn't make sense in 99% of all cases. Maybe there are some strange cases where one would want that but then it is a parabolic profile which can be defined as a function or as a table over the radius and I think that is still possible with the dependency option.

Boris

August 18, 2014, 02:29	Inlet Velocity BC:: What does checking on "Fully developed Flow" imply?	#1
nikesh Member Nikesh Bhattarai Join Date: Nov 2011 Location: Sacheon, South Korea Posts: 82 Rep Power: 13	Hi FloEFD users, When applying "Normal to Face" Inlet Velocity BC, there is an option to check on "Fully Developed Flow". By checking on it, do I get fully developed flow right at that plane? In my case (flow through a pipe and valve, incompressible flow), if I checked on it does it mean that there is no need for me to have sufficiently long pipe (~20D) upstream of the valve since the flow is already fully developed? And if so, I can actually reduce the upstream length to just 1D or 2D from the valve position. Did I get this right? What difference can I get if I don't check on it? Any insights highly appreciated! Thanks Nikesh.

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August 18, 2014, 03:15		#2
Boris_M Disabled Join Date: Jul 2009 Posts: 616 Rep Power: 23	That it correct. The fully developed flow will add the parabolic flow profile you would get from a longer flow through a pipe but it only work for circular and I think square shaped opening. So any weird shaped inlets will require a longer pipe to be able to develop the correct flow profile as there is not a known profile for every weird shape one can think of. The 1D or 2D is probably a little short as there might be some disturbance upstream from the partial blockage of the valve geometry itself similar to the vortex crosses pressure opening problem you will get if you place the outlet directly behin the valve and there is a vortex. The difference would be that your flow profile is not developed fully when entering the valve and you might get different pressure drops also. How much that would change the results in general is hard to tell as it depends on various factors such as fluid, flow rate, pressure ect. I usually use it if there is some piping before the simulated geometry. Regards, Boris

August 18, 2014, 19:50		#3
nikesh Member Nikesh Bhattarai Join Date: Nov 2011 Location: Sacheon, South Korea Posts: 82 Rep Power: 13	Thanks Boris, Yes in my case, I need to make sure that the flow is fully developed when it reaches the valve position. So, by enabling the "Fully Developed Flow" option, I don't need to make the piping (circular pipe) as long as 20D upstream of the valve (there is an expression for this length, known as Entrance length for the incompressible turbulent flow to be fully developed in a pipe). But what I do need to make sure is that it is long enough such that the disturbed flow does not reach out and affect the inflow BC. This explanation is for the "Normal to Face" Inlet Velocity BC. But when applying "Vector" Inlet Velocity BC the option of "Fully Developed Flow" is not present. This case is similar to the first case while not enabling "Fully Developed Flow", in that, flows in both these cases require to travel certain distance for it to be fully developed. Did I get it right here?

August 19, 2014, 01:34		#4
Boris_M Disabled Join Date: Jul 2009 Posts: 616 Rep Power: 23	Yes, that's true, it doesn't work for vector definitions of the inlet as a flow profile such as the fully developed flow not straight into a pipe wouldn't make sense in 99% of all cases. Maybe there are some strange cases where one would want that but then it is a parabolic profile which can be defined as a function or as a table over the radius and I think that is still possible with the dependency option. Boris