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-   -   relative velocity (https://www.cfd-online.com/Forums/cfx/77022-relative-velocity.html)

 Zaktatir June 10, 2010 11:34

relative velocity

hello,
how can i calculate the relative velocity between air (continuous) flowing over water (continuous) in CFX..i need a mean value that means a difference between the velocity in the air core flow and in the water layer because the velocity (inhomogeneous calculation) at the Void_Level=0.5 is near zero (interphase)

 Zaktatir June 10, 2010 11:42

or how can i calculate the drag force (mixture model) i need this relative velocity

 ghorrocks June 10, 2010 18:34

Why do you need it? The variable slip velocity is available for post processing.

The relative velocity is already built into the drag force calculation in CFX, you don't need to calculate it again. You just call for the drag law you want.

 Zaktatir June 11, 2010 02:42

i need it for further calculation in cfx pre.. i need this velocity to calculate the most instabil wave and the wavenumber and the wavegrowth... i need it not in post

 ghorrocks June 14, 2010 18:26

Have you tried to use it in the solver? You can access a lot of variable in the solver, but not all of them.

 Zaktatir June 15, 2010 02:34

what do you mean with in he solver...
with two continuous phases there is no slip velocity variable?
any ideaaa

 ghorrocks June 15, 2010 07:01

Most variables available in CFD-Post are also available in the solver during a simulation. I am not sure if the relative velocity variable is available in the solver so have you checked?

 Zaktatir June 15, 2010 07:32

There is no variable called relative velocity in post or in pre.

That's why was my question at the beginning.

Because the relative velocity is at the free surface equal to zero (theory since there is jump between the velocity profile) want to estimate or to create a velocity difference not directly at the free surface but in the vicinity (boarders of the two phase layer)

 ghorrocks June 15, 2010 07:57

The variable Slip Velocity is available in CFD-Post, it is in the variables list. I am not sure it is in the solver though. It probably is.

In a inhomogeneous flow there will be a slip velocity between the phases so the slip velocity will exist, even at VF=0.5.

 Zaktatir June 15, 2010 08:55

i don't find this slip velocity variable: i think this variable is only to find in case of Euler/Lagrange or continuous phase/dispersed phase

 ghorrocks June 15, 2010 18:23

OK, but as the variable you talk about is fundamental to a inhomogeneous model it will be in there somewhere, it is just a matter of finding its name. Look in the CEL reference guide in the documentation for the list of variables and hopefully you will find it.

 Zaktatir June 16, 2010 02:58

such a kind of variable doesn´t exist for Euler-Euler (with my two continuous phases ).. i hink you mean the slip velocity variable related to particles or 1 continuous+1 disperse:

Slip Velocity
slipvel
[ ]
1
C, M, R, TS
Velocity of an algebraic slip component relative to the continuous component.

i need a method to estimate this velocity

 Zaktatir June 16, 2010 04:57

can i change the variables list file ?

 ghorrocks June 16, 2010 08:27

No. But somewhere in the list will be the variable you are looking for. You just need to find it.

 Zaktatir June 16, 2010 08:31

no ...
Drag=Cd*A*rho_light phase*Urel^2 in the mixture mode
this relative velocity doesn´t exist in the variable list...
slip magnitude exists only in other models not in inhomogeneous two continuous phase model

 ghorrocks June 16, 2010 18:36

Have a look in the <CFX_ROOT>/etc/VARIABLES and RULES files. I can find the following variables, not sure exactly which one you need but hopefully it is one of them:
Slip Velocity Magnitude
Slip Reynolds Number
Slip Velocity
Drift Velocity

 Zaktatir June 17, 2010 02:04

this variable are usable only when there is one continuous phase+dispersed phase ....

in my case i have at least 2 continuous phase... thats why was asking for idea how to estimate it.... and which Urel takes then CFX to calculation for the Drag force

 ghorrocks June 17, 2010 08:14

OK. Have you tried something like U Velocity.phase1-U Velocity.phase 2?

 Zaktatir June 17, 2010 09:34

yeah i tried that, but my Water.Velocity U Field is the same like the velocity field of Air although the calculation is inhomogeneous, and if use the superficial ones, it leads to value near zero (off course due to the volume fractions) thats why i wanted to estimte a relative velocity under the assumption that i have two constant Velocities (under and above the interface)... Onlx the cells at the interface have both entities (water and air) the other not.. and i am not sure if i can while calculating take the average value of Air.Velocity Field and the average value of Water.Velocity Field every time step and for every X location and then build the difference...

i hope u can follow it..

 ghorrocks June 17, 2010 19:43

I don't follow what you are saying, and I don't understand why Water.Velocity U-Air.Velocity U does not give what you want.

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