# relative velocity

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 June 18, 2010, 02:05 #21 Senior Member   Join Date: Jan 2010 Location: Germany Posts: 268 Rep Power: 9 because Water.Velocity U-Air.Velocity U on the inter-phase is almost small because of the shear layer near the interface.. <--------------------------------- <--------------------------- <-----------------------AIR <--------------------- <----------------- <---------- <-- --> ---------------> --------------------> ----------------------->WATER ---------------------------------> ----------> --> The assumption is accordingly to KH-theory: two Fluids flowing over each other with constant velocity: <--------------------------Air-------------- ************************************ ---------------------------WATER--------> Last edited by Zaktatir; June 18, 2010 at 02:22. Reason: :

 June 18, 2010, 06:08 #22 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 11,572 Rep Power: 90 Yes, and what is your point? If the interphase drag is large then the velocity profile looks like your top diagram. If the interphase drag is small then the velocity profile looks like the bottom diagram. Either way the relative velocity between phases is Water.Velocity U-Air.Velocity U.

 June 18, 2010, 07:22 #23 Senior Member   Join Date: Jan 2010 Location: Germany Posts: 268 Rep Power: 9 i have the first case.. and i am trying to model water separation at the crest of the established waves on the interphase.. and its correct.. i only need the average velocity of the air flow and the average velocity of the water flow and then build the difference..

 June 20, 2010, 06:29 #24 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 11,572 Rep Power: 90 Maybe I am dumb but I have no idea what you are talking about. Can you explain what you are trying to do again?

 June 20, 2010, 08:38 #25 Senior Member   Join Date: Jan 2010 Location: Germany Posts: 268 Rep Power: 9 No, you are not dumb i think i could not explain my problem. I have very big drag force between the both phases nevertheless i want to estimate or in other words to create a relative velocity between them as it is the secand case, that means i want theoretically simplify my problem assume that the drag is not too big (assume that there is no velocity boundary layer in the vicinity of the interphase) and get a value that alows me to calculate the wavelength and growth rate of the waves due to the KH. (e.g.: getting the max Air Flow Velocity - the max. Water. Velocity for x= const, y is variable (height of chanel 90mm)) Last edited by Zaktatir; June 20, 2010 at 08:45. Reason: :

 June 20, 2010, 18:45 #26 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 11,572 Rep Power: 90 What you describe sounds easy - draw a line where you want to calculate it and use the function calculator on that line to get the maximum Air.Velocity and maximum Water.Velocity. Doesn't that do what you want?

 June 21, 2010, 02:59 #27 Senior Member   Join Date: Jan 2010 Location: Germany Posts: 268 Rep Power: 9 yes that is it but i need this not in post i need it in pre for the run

 June 21, 2010, 07:54 #28 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 11,572 Rep Power: 90 Firstly - you have not mentioned any of this before. If this is what you are looking for then why didn't you say so at the start? You cannot generate line objects in the solver to my knowledge. The max and min functions should work fine in the solver. You could make little surfaces with a narrow width, or you could make a series of monitor points and take the maximum of them. If you put one monitor point at each node you will have the same resolution as the post-processing approach anyway.

 June 21, 2010, 11:58 #29 Senior Member   Join Date: Jan 2010 Location: Germany Posts: 268 Rep Power: 9 that means defining a variable location in pre is not possible.

 June 21, 2010, 22:13 #30 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 11,572 Rep Power: 90 What do you mean by a variable location?

 June 22, 2010, 02:03 #31 Senior Member   Join Date: Jan 2010 Location: Germany Posts: 268 Rep Power: 9 Something like this: in CFX-Pre i set an Expression which will be calculated for every ZY-plane at X ( e.g. X between 20cm and 40 cm)

 June 22, 2010, 19:52 #32 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 11,572 Rep Power: 90 These functions need to be applied to either a physics region or a meshed region. You would have to have these regions present in the mesh but they don't need to be boundaries, just mesh regions is OK - see the documentation on CEL.

 June 23, 2010, 02:16 #33 Senior Member   Join Date: Jan 2010 Location: Germany Posts: 268 Rep Power: 9 it means that i should define a lot of these planee to solve my problem. how many planes are suitable, and if mesh this planes in ICEM i will face the problem multiple edge e.g...

 June 23, 2010, 08:21 #34 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 11,572 Rep Power: 90 Yes, that's right. It is not a nice solution but you don't have much other choice. THe only other option I can think of is to define monitor points and manually get the max/min values of them. This does not require the mesh to be changed. But if you have lots of monitor points then best set it up using perl or some other text editor.

 June 23, 2010, 09:28 #35 Senior Member   Join Date: Jan 2010 Location: Germany Posts: 268 Rep Power: 9 i´ve already started dealing with the mesh solution meanwhile i am trying to build an entity like the relative velocity based on the Velocity u. Gradient Y or the shear strain rate in the vicinity of the inter phase and multiply this with a hydraulic radius of the channel..

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