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HDZ October 10, 2002 02:41

Momentum source
We are interested in using a momentum source to simulate a pump in a time dependent flow system. It looks the information in the manual does not serve our purpose as it is not making any difference to the velocity !

How can a momentum source be implemented? The option in the 1. UDF is one possibility I believe. How involved is this option?

2. Is there another simpler method?



Erwin October 14, 2002 06:37

Re: Momentum source
Go to boundary conditions. Select your fluid zone. Select 'Source terms'. Define your momentum source terms in the different directions. Done.

HDZ October 14, 2002 06:54

Re: Momentum source
I have done that. I have defined a limited volume of a long pipe where the momentum source has been applied but I did not get any difference.

A long pipe with an inlet velocity of say 2 m/s is set. over a certain volume of this pipe a momentum source is set in the direction,as you have suggested but somehow no difference was noticed.

That is why I equired about UDF?

Have you tried the method you suggested? Are we doing something silly there?

Erwin October 14, 2002 08:00

Re: Momentum source
I just tried it, a pipe with 2 m/s water flow and a momentum source in the middle. It worked just fine, I got a nice pressure jump.

Don't forget that the momentum source is in N/m3, so if your zone is small the source term becomes pretty big.

HDZ October 14, 2002 08:49

Re: Momentum source
Thank you very much. I will try it. I was aware of the units.

I assume that you have seen an increase in velocity aftter the momentum source, i.e. more than 2 m/s. Please confirm.

Many thanks


Erwin October 14, 2002 09:35

Re: Momentum source
You would only see a velocity increase if the static pressure decreases a lot. The resulting density reduction would lead to a velocity increase (in compressible flow). However, I 1) used water and 2) I modeled a momentum source (not sink) so the pressure across the zone will rise. Two reasons not to see a velocity increase.

HDZ October 14, 2002 14:54

Re: Momentum source
It all makes sense now.

However it turned out that the problem we are trying to solve is a little different.

Now what would happen if you apply that momentum source to a fluid at rest? Will it give it some velocity similar to a pump?

The problem at hand is simply recirculating flow to a tank.

Erwin October 15, 2002 04:28

Re: Momentum source
I guess so. Flow is a result of pressure gradients, so a local increase in static pressure should result in a velocity in the direction of the lower pressure.

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