That error message most probably tells you that C_WALL_DIST(c,t) has not been assigned a value (so that you receive an access violation when you try to retrieve it). I haven't worked with this macro for many years now, but I would seem to recall that it is mainly used in conjunction with near-wall turbulence modelling, so perhaps the Viscous model of choice may influence whether it is used by the solver or not. You might also want to try to plot the cell wall distance and/or the cell coordinates in a graphical window to make sure that the appropriate loops have been performed.

After a search in the UDF manual it seems that C_WALL_DIST doesn't exist anymore. I'd be curious to know how we can retrieve the distance between a cell centroid and the nearest wall.

I don't think that these macros were ever covered by the manual. I just had a look in the source files for Fluent 15.0.7 and all of the following are there still:
C_WALL_DIST(c,t)
C_WALL_DIST_G(c,t)
C_WALL_DIST_RG(c,t)
C_WALL_NORMAL(c,t)
However, depending on your situation it might be "safer" to construct your own macro. All you need then is a UDM and to loop once and use the standard (and well documented) macros for cell and face centroid locations, plus the aforementioned boolean macro to judge if the cell is next to a boundary.
Hope this helps!

Thank you very much HenrikS

Define_wall_function
 

:confused:

Udf DEFINE_WALL_FUNCTION
 

:confused:
dear HenrikS,
i need the turbulece production at the near wall cell, but i don't know which UDF macro to use.
pls help me.

kind regards.

Turbulent kinetic energy: C_K(c,t) and turbulent kinetic energy dissipation rate: C_D(c,t).

There's an undocumented macro called that might be useful (see below). However, I wouldn't recommend using something like this unless you know what you're doing. The safest bet is probably to define your own macro, then you will know exactly what you are calculating :). Good luck!

Get_Wall_k_Prod(face_t f, Thread *t, cell_t c0, Thread *t0,
real ks, real rkcon, real yp, real uStar, real up,
real rho, real mu,
real alpha, real beta, real gamma, real vwPlus,
int ictyp);

Hi,
How do you find out the closest wall in a 2D model?

Dear Henrik,

By using C_WALL_DISTANCE(c,t); as a macro, FLUENT fails to initialize the solution. I do not understand what is the error in this.

I have the same problem too.

Has your code worked (interpreted without errors and solution initialized without errors)?
If yes, Do you mind sharing the wall distance part of it. I am struggling to find the wall distance from a cell (centroid).

Unfortunately, the macros
C_WALL_DIST(c,t)
C_WALL_DIST_G(c,t)
C_WALL_DIST_RG(c,t)
C_WALL_NORMAL(c,t)
did not worked in my code so that, I have used a constant cell wall distance when generating the grid by using the inflation technique in the grid generation near the wall.
I wish I could help you

Mohamed,

This is because fluent recently has removed C_WALL_DIST from its macros and for that reason we cannot use it anymore.
Do you by any chance have found a solution for this? I am having the same problem and I cannot find an alternate solution.
Any kind of help is much appreciated

Dear Ali,

I suggest using a constant height of an inflation layer near the wall, so that you knows the height of the first cell centroid and it is also constant. you can now use it in your application.


Another solution in to know the thread ID and use the macro
C_CENTROID(c,f,t);
y = x[1]; to get the y-distance for each cell

Thank you Mohamed,

You mean in the equations for F1 and F2 as you see in this picture, you used a constant value for y? and that constant value is the thickness of the first layer?

the value in the equations are the first cell centroid distance from the wall, isn't it?

So, you can use a constant value or the second method to get every distance for the wall adjacent cells for non-uniform grids near the wall


Hope it will help

I am not sure. Are you sure about this?

Mohamed,

I recently got in touch with a person who is an expert in Fluent UDF and he told me that C_WALL_DIST(c,t) is still available. It just has to be filled. He sent me this code and said that it contains one way that this macro could be filled.
I do not understand what this code is doing and how I can fill it and then use it.
Do you mind taking a closer look at this code?

#include "udf.h"
#include "prox.h"

static cxboolean wall_dist_set = FALSE;

DEFINE_ON_DEMAND(set_wall_dist_udm0)
{
#if !RP_HOST

Domain *domain;
Thread *t;
cell_t c;

if (! wall_dist_set)
{
domain = Get_Domain(ROOT_DOMAIN_ID);

Alloc_Storage_Vars(domain, SV_RTMP_0, SV_NULL);

Calc_Cell_Wall_Distance_New(domain, SV_RTMP_0);

thread_loop_c(t,domain)
{
begin_c_loop(c,t)
{
C_UDMI(c,t,0) = C_TMP0(c,t);
}
end_c_loop(c,t)
}

wall_dist_set = TRUE;
}
#endif /* !RP_HOST */
}

DEFINE_ON_DEMAND(reset_udm0)
{
#if !RP_HOST

Domain *domain;
Thread *t;
cell_t c;

domain = Get_Domain(ROOT_DOMAIN_ID);

thread_loop_c(t,domain)
{
begin_c_loop(c,t)
{
C_UDMI(c,t,0) = 0.0;
}
end_c_loop(c,t)
}

#endif /* !RP_HOST */

wall_dist_set = FALSE;
}

Hi,

The code in question will introduce two Define-on-Demand functions:
set_wall_dist_udm0
reset_udm0
that use the user-defined memory location 0 to store the wall distances.

The second one will initialize UDM0 to zero everywhere, the first one will use an undocumented built-in function
Calc_Cell_Wall_Distance_New
to fill it with the wall distance.

I don't know whether this works in practice, but it looks fine. Don't forget to activate the user-defined memory or the code will crash for you :)

/Henrik