# Conversion of computed velocities in Lagrangian coordinates to Eulerian coordinates

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 June 3, 2011, 04:37 Conversion of computed velocities in Lagrangian coordinates to Eulerian coordinates #1 New Member   Syed Murtuza Mehdi Join Date: Apr 2011 Posts: 19 Rep Power: 8 Hi, all I am thinking that if we apply Lagrangian-Eulerian model to calculate the position and velocity of a single uniform bubble in a flowing liquid then how one can convert the computed velocities of the bubble at various locations into Eulerian velocity field at that location and time. Any suggestions would be of great help. This actually will be mandatory because the bubble velocities will be calculated in Lagrangian frame of reference but the flow of the continuous phase will be in Eulerian frame of reference and hence to estimate the relative velocity of the two phases at a particular point and time we must express both the velocities in the same frame of reference. Thanks a lot

 June 3, 2011, 10:20 #2 Senior Member   Pauli Join Date: Mar 2009 Posts: 190 Rep Power: 10 Sorry but I don't understand why this is a problem. All velocities are reported in the global coordinate system. So you have both in a common fixed reference frame. Vector subtraction of the bubble velocity from the local fluid velocity gives the relative velocity.

 June 3, 2011, 11:23 #3 New Member   Syed Murtuza Mehdi Join Date: Apr 2011 Posts: 19 Rep Power: 8 Thanks again Pauli for making me understand this point. Actually the answer to my querry lies in the last line of my question it self and as you explained that even though both the frame of reference are different the velocities are always reported in the Global frame of reference so we do not need to convert any of them. However I developed this thought after reading a research paper in which the author explained some thing regarding this kind of an issue but may be I didnt understand exactly what the author reports. You can however go through this paper if you like to. "Numerical investigation of the drag closure for bubbles in bubble swarms" Y.M. Lau, I.Roghair, N.G.Deen, M.van Sint Annaland, J.A.M.Kuipers . Secondly I need to ask one more question can I model the motion of one single bubble in a vertical pipe with continuous fluid flowing in the pipe using a VOF model ?. Do I have to explicitly model the bubble geometry as well or an specification of the void fraction will be enough in this case. I do not want to carry out extensive computation coz my computing resources are limited.

 June 3, 2011, 13:22 #4 Senior Member   Pauli Join Date: Mar 2009 Posts: 190 Rep Power: 10 You can use the spray models with explicit particle injection to model a single liquid drop into a gas. I don't know if it would work with a gas bubble into a liquid background. If it does work, I would be very suspicious of the underlying physics. (To see what I mean, check the Methodology manual section regarding the spray models.) A pure VOF treatment would likely require a rather fine grid. Otherwise the results would be rather coarse. If you want to predict drag on a gas bubble, I'd expect a fine grid is required. Sorry but I've never worked with gas bubbles. I have no good suggestions. Maybe someone else can help.

 June 5, 2011, 09:20 #5 New Member   Syed Murtuza Mehdi Join Date: Apr 2011 Posts: 19 Rep Power: 8 thanks a lot for the opinion

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