# Momentum source

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 October 10, 2002, 02:41 Momentum source #1 HDZ Guest   Posts: n/a 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? Thanks HDZ

 October 14, 2002, 06:37 Re: Momentum source #2 Erwin Guest   Posts: n/a Go to boundary conditions. Select your fluid zone. Select 'Source terms'. Define your momentum source terms in the different directions. Done.

 October 14, 2002, 06:54 Re: Momentum source #3 HDZ Guest   Posts: n/a 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?

 October 14, 2002, 08:00 Re: Momentum source #4 Erwin Guest   Posts: n/a 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.

 October 14, 2002, 08:49 Re: Momentum source #5 HDZ Guest   Posts: n/a 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 HDZ

 October 14, 2002, 09:35 Re: Momentum source #6 Erwin Guest   Posts: n/a 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.

 October 14, 2002, 14:54 Re: Momentum source #7 HDZ Guest   Posts: n/a 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.

 October 15, 2002, 04:28 Re: Momentum source #8 Erwin Guest   Posts: n/a 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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