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-   -   Mass source, Momentum source theory. (http://www.cfd-online.com/Forums/fluent/67602-mass-source-momentum-source-theory.html)

diffo August 20, 2009 07:07

Mass source, Momentum source theory.
 
Intending to model an inlet (nozzle) that is smaller than a cell in a domain, i decided to define a mass source in the cell where my inlet is supposed to be, and to add to that mass source a momentum source.
Hence I expect to rebuild the given nozzle mass flow (m_dot).

Mass source UDF: m_dot/Volume _Cell
Momentum source UDF: m_dot*Exit_Velocity/Volume_Cell

m_dot: known from the details of the nozzle
Exit_Velocity: calculated by dividing m_dot with Air_density*Exit_Area_Nozzle

THE PROBLEM:
on a velocity Plot of a surface crossing the chosen Cell i see a velocity distribution, where the maximum velocity is though a the chosen cell, but the amount is not near that what i had implemented (implemented Velocity was "Exit_Velocity").


THE HINK in the logic:
according to the Fluent-user-guide (page 7-225) it is possible to create a mass source term (m_dot) without giving it any impuls, then create an impuls(Momentum) source term that orientates the mass.
But i u look carefully at the formula of the mass source, there is already a velocity in there (logically). Then i give in Momentum source a second velocity... from there on , I'm lost.

To make things easy, i chose a cell with Faces, that all have an area of 1mē.


Can someone please give me a clue how to manage to have a plotted maximal velocity that corresponds to implemented one....

jason_t August 20, 2009 11:25

Are you smoothing the contour plot values by plotting nodes? Try deselecting node values when you plot and see if that helps.

diffo August 21, 2009 04:32

Hi jason, thanks for the answer.
I tried it and i have 2 elements to report:
- the picture i get is unsharp and looks like an histogramm
- the maximal velocity i get is: 27,5m/s

Now back to the case-file i've implemented.
i wanted to pull the air mass 0,01kg/s with the Impuls (velocity) 90m/s out of a chosen Cell.
i get a max velocity (node based) of 11,3 m/s.

But i'm expecting a max velocity of 90m/s since i've implemented it in my Momentum-udf.

I'm confused... and clueless...
I followed the advices of the user guide (read my first post).

Any other idea..?

coglione August 21, 2009 05:05

Hello diffo

"But i'm expecting a max velocity of 90m/s since i've implemented it in my Momentum-udf" : Not so simple, actually you define a momentum source but you do not fix the velocity in this cell to a given value. As you stated in your first post, your nozzle is smaller than your cell spacing, thus your fluid coming out of the nozzle has to accelerate the fluid which means that the average velocity in this cell must be smaller than at the physical nozzle exit.
To see 90 m/s you either have to refine your grid or to artificially increase your momentum source or define this cell as seperate fluid zone and fix the respective velocity component to this value .

cheers

diffo August 21, 2009 10:26

Hello Coglione,
Thanks for pointing that out.

How can i define the cell as separate fluid?

Any UDF function for that?

Thanks


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