Learning Forum

[naibaf]

 

Dear Community,

 

I have a question related to the UDF Density function for compressible Liquids from the Fluent Customization Manual in Chapter 2.3.27.7. The UDF is as defined as follows:

 

/********************************************************************

   Density and speed of sound UDFs.

 *********************************************************************/

#include "udf.h"

 

#define BMODULUS 2.2e9

#define rho_ref 1000.0

#define p_ref 101325

 

DEFINE_PROPERTY(superfluid_density, c, t)

{

    real rho;

    real p, dp;

    p = C_P(c,t) + op_pres;

    dp = p-p_ref;

    rho = rho_ref/(1.0-dp/BMODULUS);

    return rho;

}

On top of the page, it is mentioned, "Compressible liquid density UDFs can be used in the pressure-based solver and for single phase, multiphase mixture and cavitation models, only. See the example below for details."

 

Now my question. Could anyone explain why it cannot be used with the VOF model regarding two-phase flow? I cannot find any explanation for this. I am wondering since one define the UDF for primary phase and for example a constant density for the second phase, the UDF is calculating density for the primary phase. The VOF model than averages the density for the mixture based on volume fraction coefficients. So I don't understand why the function should not work with the VOF model. Maybe my understanding of the VOF model is not correct?

Thank you for your time and your help! 

[DrAmine]

You can use it with all multiphase models. Only thing is that if there is strong pressure dependency better to rely on udrgm.
Perhaps you just require to rely on the already available trait equation for compressible liquids.

[naibaf]

Thank you very much for your answer. This helps.

 

Regards 

Fabian

[DrAmine]

Welcome!