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Nigel Banks March 9, 2005 13:23

3D UDF - Profile dependant on ground height
Hi there,

I am modelling the flow around a group of buildings situated on undulating ground. My initial model used a flat ground and I sucessfully used the UDF below to model the atmospheric boundary layer velocity inlet conditions (velocity increases with distance above ground).

However, I have now included the local hills and so have different ground levels along the inlet. Is there any way to reference the coordinates of lower edge (which would give me the ground height at that point) in order to get the correct velocity profile???

Any help is very appreciated!!!

Many thanks, Nigel

#include "udf.h"

DEFINE_PROFILE(ABL_profile, thread, index)


real x[ND_ND];

real y;

face_t f;

begin_f_loop(f, thread)



y = x[1];

F_PROFILE(f, thread, index) = 1.465*log((y+0.01)/0.01);


end_f_loop(f, thread)


SteveGoat September 5, 2012 06:50

Dear Nigel,
have you solved this issue?
I'm facing the same problem and it would be very useful to know your opinion about it

Many thanks

Fabio88 July 14, 2017 06:51

Dear Nigel, Dear Stefano,

did you solve this issue?
I should do something similar and I am not sure how to approach the problem.

Some people suggested to use C_WALL_DIST in Fluent UDF. I did not try but from what I read it was not really successful (

the guy of the post found an alternative way "first write out a profile file at the inlet boundary, then obtain a profile file, which contains vectors of x,y,z,cell wall distance and velocity magnitude, then I read the file with excel and replace the velocity magnitude by a function of cell wall distance (which is the logarithmic function for velocity profile). Then i save the altered data and read this back to fluent as profile. Finally, then I used these profiles as boundary conditions for the inlet."

Do you think is possible to do so directly from fluent UDF?


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