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)

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

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.

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

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

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

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?

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)

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.

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

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.

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

can i change the variables list file ?

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

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

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

this variable are usable only when there is one continuous phase+dispersed phase ....
i saw already this variables


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

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

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..

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.

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-------->

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.

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..

Maybe I am dumb but I have no idea what you are talking about. Can you explain what you are trying to do again?

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))

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?

yes that is it but i need this not in post i need it in pre for the run

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.

that means defining a variable location in pre is not possible.

What do you mean by a variable location?

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)

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.

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...

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.

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..