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luke84 November 17, 2011 09:37

CFX-PRE monitor point... multi-variable arguments for locator based functions
Is it somehow possible to do as the title of the thread reads, and define a multi-variable argument, locator based function?

A little more info:
My simulation involves a jet of fluid, pointing in the X positive direction. Using CFX_PRE V13
I want to find the spread angle of the jet, i.e angle between the majority of the vectors and the vector (1,0,0) (x-positive direction)

So I start with the equation for an angle between 2 vectors:
\alpha = \text{acos} \left( \frac{\vec{u} \bullet \vec{v}}{\left| \vec{u}\right| \left| \vec{v} \right|} \right)

where u is the vector (1,0,0) and v is the velocity vector at the mesh locator OUTLET.

since the u vector is fully dercribed in only one direction and is the unity vector, the formula can be simplified to:
\alpha = \text{acos} \left( \frac{v_{1} }{ \left| \vec{v} \right|} \right)
*Note; here alpha is actually defining the half spread angle.

This could be set as an expression as follows:
alpha=acos(Velocity u*1 [m s^-1]/(Velocity u^2+Velocity v^2+Velocity w^2))
*Note I multiply with 1 [m s^-1] to keep dimensions consistent.

Now if I set a monitor point as:
I get a error message along the lines of:

"The function 'massFlowAve' referenced in parameter 'Expression Value' in object '/FLOW:Flow Analysis 1/OUTPUT CONTROL/MONITOR OBJECTS/MONITOR POINT:ALPHA_MONITOR' has an invalid argument, 'alpha'. Only arguments that consist of a single recognised variable name are supported by the solver."

As far as I can tell, the problem above basically means that I can only define one variable (e.g Velocity u) and not 2 or 3 variables in the locator based function; massFlowAve().
Note that the exact same method DOES work for me in CFX-POST. Thas is defining the same expression, and using the function calculator.
So does anyone have an idea how to circumvent this problem in CFX-PRE??

Thanks in advance!

stumpy November 17, 2011 16:52

There's an easy way to do this... create an algebraic Additional Variable equal to alpha and then use the AV as the argument to the locator based function.

luke84 November 17, 2011 17:22

You're right! works perfect. Thanks for the help.

D.B November 18, 2011 00:16

I tried to do the same, but I am not getting how to define algebric additional variable. I was able to define AV but the problem is I am not getting an option of writing an expression for it or giveing its type as algebric, i searched CFX help but to no avail.

stumpy November 18, 2011 11:42

After you create the AV, you need to edit your domain to enable the AV in that domain. It's in the domain settings that you say it's algebraic.

D.B November 19, 2011 01:15

Now I am getting it.

jaimesdiegop February 8, 2015 21:51

Hi, I really tried to do the same that you posted. I am trying to set a boundary condition in a multi-phase simulation dependent of (Fluid1. Volume Fraction*Velocity v)=VC= my AV, but when I tried to define the boundary condition I got this messaje

"The Additional Variable 'VC' referenced in parameter 'Normal Speed' in object '/FLOW:Flow Analysis 1/DOMAIN:Default Domain/BOUNDARY:top/BOUNDARY CONDITIONS/MASS AND MOMENTUM' is fluid-dependent and therefore requires a fluid prefix."

How can I set a prefix on my AV?

Thank you for your help

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