[James_mCFD]

I'm wondering if anyone can help me. I'm currently writing a UDF to impose a parabolic velocity profile for fluid flow through a rectangular pipe. I have created a parabolic function for the z-direction (vertical plane) and the y-direction (normal to the direction of fluid flow).

I've used the UDF tutorial as a starting point, and have seen one other similar question, but my problem is different in that the width of the pipe is greater than the height of the pipe.

I have defined height as h and width as w, and maximum velocity as u_max.

Velocity profile in the y-direction is defined as:

u_y = -0.01.*(x[1]*x[1]-w*x[1]+0.25*w*w)+u_max

Velocity profile in the z-direction is defined as:

u_z = -0.062.*(x[2]*x[2]-h*x[2]+0.25*h*h)+u_max

I got to the stage where I had to write the F_PROFILE and realised I was mistaken in my technique as I was defining y as u_y and z as u_z. Please see the code below:

#include "udf.h"

DEFINE_PROFILE(inlet_x_velocity, thread, position)
{

real x[ND_ND]; /* this holds a position vector */
real y, w;
real z, h;
real u, u_max;
face_t f;

h = 2; /* inlet height in mm */
w = 5; /* inlet width in mm */
u_max = 0.062; /* maximum velocity at centre of the pipe in m/s*/

begin_f_loop(f, thread)
{
F_CENTROID(x, f, thread);
y = x[1];
z = x[2];
F_PROFILE(f, thread, position) = ...
}
end_f_loop(f, thread)
}

What I think is required is that I need to create a vector using the F_PROFILE that uses a vector [w,h], and [x[1],x[2]], as well as [0.01, 0.062] and u_max.

My issues are:

1. I'm not sure how to implement this in C and also check the velocity profile quickly using ansys FLUENT.

2. The coefficients in the expressions for the velocity profiles in the two directions are rounded to two decimal places and do not give exactly 0 at the walls; can I use a rounding function within the code that is compatible with the ANSYS compiler and solver?

[AlexanderZ]

to define profile in several directions you need several define-profile macros

DEFINE_PROFILE(inlet_x_velocity)
DEFINE_PROFILE(inlet_y_velocity)
u_y = -0.01.*(x[1]*x[1]-w*x[1]+0.25*w*w)+u_max
DEFINE_PROFILE(inlet_z_velocity)
u_z = -0.062.*(x[2]*x[2]-h*x[2]+0.25*h*h)+u_max

[James_mCFD]

Hi Alexander. Thank you for your suggestions.

Is it necessary to create a profile for each component of the velocity? I have found a similar problem here that used a circular pipe, and for their solution it was suggested that only one DEFINE_PROFILE is necessary.

In case my original question was unclear; what I am after is the velocity in the x-direction, but this velocity changes with dy and dz as described by the functions I derived for my geometry. Velocity in the x-direction should be a component of the velocity in the y and z directions. What makes my problem (seemingly) more complicated is that y>z, so d_u/d_y ≠ d_u/d_z.

Thank you

[pakk]

Originally Posted by James_mCFD

In case my original question was unclear; what I am after is the velocity in the x-direction, but this velocity changes with dy and dz as described by the functions I derived for my geometry. Velocity in the x-direction should be a component of the velocity in the y and z directions. What makes my problem (seemingly) more complicated is that y>z, so d_u/d_y ≠ d_u/d_z.

Thank you

You are wrong about that: your velocity in x-direction is not what you have in these two equations. These two equations are for 1D inlets, your inlet is 2D. You need to make an equation for a 2D inlet.

That y>z is really not relevant, if y=z then your approach would also not work. You need to find out how to make a "parabolic" profile on a rectangle.

[James_mCFD]

Originally Posted by pakk

your velocity in x-direction is not what you have in these two equations. These two equations are for 1D inlets, your inlet is 2D. You need to make an equation for a 2D inlet.

I think you misunderstood what I wrote; I know these do not represent velocity in the x-direction; I never said that they did! These are velocity profiles in the y and z-plane. If these functions are plotted with a surface plot using Matlab, it would produce a parabola that represents the velocity front at a single point in time.

[pakk]

Your terminology is not standard, that makes it difficult...

You talk about "Velocity profile in the y-direction", where you mean "velocity profile in the x-direction at z=0". Agreed?

"I know these do not represent velocity in the x-direction; I never said that they did!"
Well, you should have said so, because they are in the x-direction and they are velocity profiles. It's just that they are 1d, where you need 2d. You have to make an equation that combines the 1d profiles into a 2d. You, not Fluent.

"If these functions are plotted with a surface plot using Matlab, it would produce a parabola..."
The functions are 1d, a surface plot is 2d,and a parabola is 1d. I don't understand how you see this. Maybe you can show the surface plot that you talk about?

"... that represents the velocity front at a single point in time. "
In time? Is your profile transient? Everything until now suggested a steady-state situation... Do you really mean it like that? If so, you also need to tell us how your profile should change over time.