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calculate distance from wall to neighbourig cell centroid

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Old   January 8, 2015, 05:22
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Henrik Ström
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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.
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Old   January 8, 2015, 15:52
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François Grégoire
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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.
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Old   January 9, 2015, 03:56
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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!
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Old   January 9, 2015, 13:24
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Mohamed Refaat
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Thank you very much HenrikS
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Old   March 19, 2015, 07:28
Default Define_wall_function
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Quote:
Originally Posted by Mohamed Refaat View Post
Thank you very much HenrikS
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Old   March 19, 2015, 07:30
Default Udf DEFINE_WALL_FUNCTION
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Quote:
Originally Posted by HenrikS View Post
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!

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.
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Old   March 19, 2015, 15:47
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Turbulent kinetic energy: C_K(c,t) and turbulent kinetic energy dissipation rate: C_D(c,t).
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Old   March 20, 2015, 03:38
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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);

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Old   April 11, 2016, 11:26
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Quote:
Originally Posted by rr123 View Post
dear all,

i need to calculate the distance between the wall and the centroid of the cell touching this wall. my system is 2D.

this is the main code snippet:

begin_c_loop (c,t)
{
c_face_loop(c,t,n)
{
tf = C_FACE_THREAD(c,t,n);
if(THREAD_TYPE(tf) == THREAD_F_WALL)
{
f = C_FACE(c,t,n);
F_CENTROID(x,f,t);
wf = x[1]; /* for 2D */
C_CENTROID(cell,c,t);
nc = cell[1];
dist = FABS(wf-nc);
}
}
}
end_c_loop (c,t)


am i right ? is it correct ordering of the loop structure ?

hopign for a reply.

best regards.
Hi,
How do you find out the closest wall in a 2D model?
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Old   April 13, 2016, 16:40
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Quote:
Originally Posted by HenrikS View Post
In your code, you calculate "dist" for every cell next to a wall boundary. My guess is that you want to use this value for some purpose elsewhere in your code. Now, there is a macro called C_WALL_DIST(c,t) which I believe (although I have not verified that) will give you this distance directly.

In your case, your snippet would become:

begin_c_loop (c,t)
{
dist = C_WALL_DISTANCE(c,t);
}
end_c_loop (c,t)

This is probably not only shorter but also quicker. Anyway, I suggest you validate this approach before relying on it. You can store "dist" in a UDM and compare to the cell wall distance in a contour plot in Fluent for example.

/Henrik
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.
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Old   April 14, 2016, 18:25
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I have the same problem too.
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Old   April 18, 2016, 10:36
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Quote:
Originally Posted by rr123 View Post
thanks for helping me henrik and max !
i gratefully acknowledge your support.
i keep you informed of this.
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).
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Old   April 18, 2016, 17:21
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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
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Old   December 19, 2016, 17:34
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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
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Old   December 21, 2016, 09:44
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Quote:
Originally Posted by alinik View Post
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
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Old   December 21, 2016, 11:36
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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?
Attached Images
File Type: png Screen Shot 2016-12-21 at 10.32.33 AM.png (39.1 KB, 46 views)
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Old   December 22, 2016, 12:52
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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
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Old   December 23, 2016, 02:53
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Quote:
Originally Posted by Mohamed Refaat View Post
the value in the equations are the first cell centroid distance from the wall, isn't it?
I am not sure. Are you sure about this?
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Old   March 13, 2017, 13:34
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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;
}
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Old   March 14, 2017, 01:41
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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
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