2.4.1 DEFINE_CAVITATION_RATE

Description

You can use DEFINE_CAVITATION_RATE to model the cavitation source terms $R_e$ and $R_c$ in the vapor mass fraction transport equation used in the Singhal et. al model (see this equation in the separate Theory Guide ). Assuming $m_{dot}$ denotes the mass-transfer rate between liquid and vapor phases, we have

where $f_1$ and $f_v$ are the mass-fraction of the liquid and vapor phase, respectively.

DEFINE_CAVITATION_RATE is used to calculate $m_{dot}$ only. The values of $R_e$ and $R_c$ are computed by the solver, accordingly.

Usage

DEFINE_CAVITATION_RATE( name, c, t, p, rhoV, rhoL, mafV, p_v, cigma, f_gas, m_dot)

Argument Type	Description
symbol name	UDF name.
cell_t c	Cell index.
Thread *t	Pointer to the mixture-level thread.
real *p[c]	Pointer to shared pressure.
real *rhoV[c]	Pointer to vapor density.
real *rhoL[c]	Pointer to liquid density.
real *mafV[c]	Pointer to vapor mass fraction.
real *p_v	Pointer to vaporization pressure.
real *cigma	Pointer to liquid surface tension coefficient.
real *f_gas	Pointer to the prescribed mass fraction of
	non condensable gases.
real *m_dot	Pointer to cavitation mass transfer rate.
Function returns
void

There are eleven arguments to DEFINE_CAVITATION_RATE: name, c, t, p, rhoV, rhoL, mafV, p_v, cigma, f_gas, and m_dot. You supply name, the name of the UDF. c, t, p, rhoV, rhoL, mafV, p_v, cigma, f_gas, and m_dot are variables that are passed by the ANSYS FLUENT solver to your UDF. Your UDF will need to set the value referenced by the real pointer m_dot to the cavitation rate.

Example

The following UDF named c_rate, is an example of a cavitation model for a multiphase mixture that is different from the default model in ANSYS FLUENT. This cavitation model calculates the cavitation mass transfer rates between the liquid and vapor phase depending on fluid pressure ( *p), turbulence kinetic energy ( C_K(c,t)), and the liquid vaporization pressure ( *p_v).

In general, the existence of turbulence enhances cavitation. In this example, the turbulence effect is taken into account by increasing the cavitation pressure by 0.195* C_R(c,t) * C_K(c,t). The pressure p_vapor that determines whether cavitation occurs increases from p_v to

p_v + 0.195 * C_R(c,t) * C_K(c,t)

When the absolute fluid pressure ( ABS_P) is lower than p_vapor, then liquid evaporates to vapor ( $R_e$). When it is greater than p_vapor, vapor condenses to liquid ( $R_c$).

The evaporation rate is calculated by

  If ABS_P < p_vapor, then
    c_evap * rhoV[c] * sqrt(2.0/3.0*rhoL[c]) * ABS(p_vapor - ABS_P(p[c]))

The condensation rate is

  If ABS_P > p_vapor, then
    -c_con*rhoL[c] * sqrt(2.0/3.0*rhoL[c]) * ABS(p_vapor - ABS_P(p[c]))

where c_evap and c_con are model coefficients.

/***********************************************************************
   UDF that is an example of a cavitation model different from default.
   Can be interpreted or compiled.   
************************************************************************/

#include "udf.h"

#define c_evap 1.0
#define c_con 0.1

DEFINE_CAVITATION_RATE(c_rate,c,t,p,rhoV,rhoL,mafV,p_v,cigma,f_gas, m_dot) 
{
    real p_vapor = *p_v;
    real dp, dp0, source;
    p_vapor  += MIN(0.195*C_R(c,t)*C_K(c,t), 5.0*p_vapor);
    dp = p_vapor - ABS_P(p[c], op_pres);
    dp0 = MAX(0.1, ABS(dp));  
    source = sqrt(2.0/3.0*rhoL[c])*dp0;

    if(dp > 0.0)
      *m_dot = c_evap*rhoV[c]*source; 
    else
      *m_dot = -c_con*rhoL[c]*source; 
}

Hooking a Cavitation Rate UDF to ANSYS FLUENT

After the UDF that you have defined using DEFINE_CAVITATION_RATE is interpreted (Chapter  4) or compiled (Chapter  5), the name of the argument that you supplied as the first DEFINE macro argument (e.g., c_rate) will become visible and selectable in the User-Defined Function Hooks dialog box in ANSYS FLUENT. See Section  6.3.1 for details.