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Old   May 18, 2003, 13:37
Default UDF paraboloid velocity inlet
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Hi, I have to put a velocity profile (fully developed) at the inlet of a 3D case but I never use UDF to define boundaries. The profile is like a paraboloid centered at middle of the pipe. I read the UDF chapter on FLuent doc but there's no 3D case example, only 2D as above (definition of a parabolic profile at inlet):

----------------------------- #include "udf.h"

DEFINE_PROFILE(inlet_x_velocity, thread, position) { real x[ND_ND]; /* this will hold the position vector */ real y; face_t f;

begin_f_loop(f, thread)



y = x[1];

F_PROFILE(f, thread, position) = 20. - y*y/(.0745*.0745)*20.;

} end_f_loop(f, thread) } ----------------------------

I don't know how to change that in 3D . Someone could help me by give the right UDF file format in 3D?

Thx a lot
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Old   May 18, 2003, 17:19
Default Re: UDF paraboloid velocity inlet
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The x vector contains contains x and y position of the cell centroid in 2D, and x,y,z in 3D. So, if you write your paraboloid in the form

z = f(x,y)

your x will be the first position (0) of x vector and your y will be the second one (1). So you have to remove the line

y = x[1];

and change

F_PROFILE(f, thread, position) = 20. - y*y/(.0745*.0745)*20.;


F_PROFILE(f,thread,position) = f(x[0],x[1]);

P.S. I'm supposing your pipe axis is along z axis.

If you want the code, post the equation of the paraboloid


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Old   May 18, 2003, 17:47
Default Re: UDF paraboloid velocity inlet
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Hi ap

The velocity take several values (derived of the mass flow inlet boundary), the diameter of pipe is 10mm , pipe axis is along Z axis. Here is the mesh for the waterblock (a water flow in a copper shape with a maze) to understand situation (inlet in blue and outlet in red color):

Could you put the code for a 1m/s velocity inlet (mean) for example with a typical poiseuille flow shape (paraboloid)?

Thx again ap
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Old   May 18, 2003, 20:42
Default Re: UDF paraboloid velocity inlet
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I write what I did so you can check it (did it in a hurry )

I suppose that the origin of reference system is the center of the inlet. If this is not true...translate your mesh with Gambit

The general paraboloid form is:

z = a*x^2 + b*y^2 + c

But in our case a=b because it's axi-symmetric. The vertex of the paraboloid has z = v_max = 2*v_avg, and it's z = 0 for example in (x=0,y=r).


a = b = -v_max/(r^2)

c = v_max

v_max = 2*v_avg


z = -v_max/(r^2) * (x^2+y^2) + v_max

In the code i put -v_max/(r^2) = coeff.

Here's the code. Remember it requires you have the origin of your reference system in the center of the inlet pipe.

If you need to change diameter and average velocity, change their values in the corresponding #define.

#include "udf.h"

#define PIPE_DIAMETER 10.e-3 // Set here the diameter of your pipe in meters

#define AVG_Z_VELOCITY 1. // Set here the average z velocity at inlet in m/s

DEFINE_PROFILE(paraboloid_velocity, thread, position) { real x[ND_ND];

real coeff,r,v_max;

face_t f;

r = PIPE_DIAMETER/2.; //Calculating radius

v_max = 2.*AVG_Z_VELOCITY; //Calculating paraboloid vertex z (max velocity)

coeff = -v_max/pow(r,2.);

begin_f_loop(f, thread)



F_PROFILE(f, thread, position) = coeff*(pow(x[0],2.) + pow(x[1],2)) + v_max;

} end_f_loop(f, thread) }

Hi and good work

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Old   May 19, 2003, 05:30
Default Re: UDF paraboloid velocity inlet
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Oki, I understand now how to define each velocity on each face at inlet I'll try that soon, I hope it works

Thank you very much ap to help me each time
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Old   September 1, 2011, 02:29
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thank you for detailed discription of udf for paraboloid
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