2.3.28 DEFINE_WALL_FUNCTIONS

Description

You can use DEFINE_WALL_FUNCTIONS to provide custom wall functions for applications when you want to replace the standard wall functions in ANSYS FLUENT. Note that this is available only for use with the $k$- $\epsilon$ turbulence models.

Usage

DEFINE_WALL_FUNCTIONS( name, f, t, c0, t0, wf_ret, yPlus, Emod)

Argument Type	Description
symbol name	UDF name.
face_t f	face index.
Thread *t	pointer to cell thread
cell_t c0	cell index.
Thread *t0	pointer to face thread.
int wf_ret	wall function index
real yPlus	y+ value
real Emod	wall function E constant
Function returns
real

There are eight arguments to DEFINE_WALL_FUNCTIONS: name, f, t, c0, t0, wf_ret, yPlus, and Emod. You supply name, the name of the UDF. f, t, c0, t0, wf_ret, yPlus, and Emod are variables that are passed by the ANSYS FLUENT solver to your UDF. Your UDF will need to compute the real value of the wall functions U+, dU+, and dY+ for laminar and turbulent regions and return them to the solver.

Example

The following UDF, named user_log_law, computes U+ and dU+, and dY+ for laminar and turbulent regions using DEFINE_WALL_FUNCTIONS. The source code can be interpreted or compiled in ANSYS FLUENT.

/****************************************************************** 
User-defined wall functions: separated into turbulent and 
laminar regimes        
/*****************************************************************/
#include "udf.h"

DEFINE_WALL_FUNCTIONS(user_log_law, f, t, c0, t0, wf_ret, yPlus, Emod)
{
  real wf_value;

  switch (wf_ret)
    {
    case UPLUS_LAM:
      wf_value = yPlus;
      break;
    case UPLUS_TRB:
      wf_value = log(Emod*yPlus)/KAPPA;
      break;
    case DUPLUS_LAM:
      wf_value = 1.0;
      break;
    case DUPLUS_TRB:
      wf_value = 1./(KAPPA*yPlus);
      break;
    case D2UPLUS_TRB:
      wf_value = -1./(KAPPA*yPlus*yPlus);
      break;
    default:
	printf("Wall function return value unavailable\n");
    }  
  return wf_value;
}

Hooking a Wall Function UDF to ANSYS FLUENT

After the UDF that you have defined using DEFINE_WALL_FUNCTIONS is interpreted (Chapter  4) or compiled (Chapter  5), the name of the argument that you supplied as the first DEFINE macro argument (e.g., user_log_law) will become visible and selectable in the Viscous Model dialog box in ANSYS FLUENT. See Section  6.2.28 for details.