[miguwhynot]

Hi, I'm currently working on a model to define the pressure and temperature of the atmospheric air, in an altitude dependant model(see attached file).

I'm reading a lot of different guides, but I'm not familiar with the define types and every example I can find involves different profiles(as shown in the udf guide). It should be pretty easy to make my case, I hope any of you can give me a hand or give me any tips so I can start to understand the coding.

Thanks in advance!

PS: I will change the h in the ecuations, since my model has a 4 m/s ascend velocity it will be h=4*time

[`e`]

Use the DEFINE_PROFILE macro, loop through all faces on the boundary and find the height with F_CENTROID (where x[1] would correspond to the y-direction in the Cartesian coordinate system) and then use conditional if statements to apply the relevant equation.

The examples in the UDF manual provide starting codes for this task. Example 2 contains one conditional statement, and you would then extend to include three sections:

if ( (h > 0.) && (h <= 11.e3) ) {
	// Eq. 2a
} else if (h <= 20.e3) {
	// Eq. 2b
} else if (h <= 32.e3) {
	// Eq. 2c
} else {
	// Capture h values outside the allowed range:
	// print a message to the console and stop.
	Message("%d: Error: h=%e not allowed.\n",myid,h);
	return;
}

[miguwhynot]

Thank you for your fast reply, it has been really helpfull. I've checked the example and it was indeed pretty similar to my needs.

Based on my model, I now have a pressure defined only by time dependant ecuations as follows:

For times between 0s and 2750s
Eq 1

For times between 2750 and 5000s

Eq 2

For times between 5000 and 8000s

Eq 3

I want to use this equations to set the value of my inlet flow pressure.
This is what I have so far (thanks for the coding from your previous reply):

#include "udf.h"
DEFINE_PROFILE(pressure_inlet, thread, position)
{
face_t f;
real t = CURRENT_TIME;
begin_f_loop(f, thread)
{
if (t <= 2750) ) {
F_PROFILE(f, thread, position) = 101325.0*((288.15-0.0065*4*t)/288.15)^5.25577;
} else if (t <= 5000) {
F_PROFILE(f, thread, position) = 22632^(-(4*t-1000)/6341.62);
} else if (h <= 8000) {
F_PROFILE(f, thread, position) = 5474.87*((216.65+0.010*(4*t-20000))/216.65)^-34.163;
} else {
// Capture h values outside the allowed range:
// print a message to the console and stop.
Message("%d: Error: h=%e not allowed.\n",myid,h);
return;
}
end_

[`e`]

Originally Posted by miguwhynot

F_PROFILE(f, thread, position) = 101325.0*((288.15-0.0065*4*t)/288.15)^5.25577;

Use the pow function instead of the ^ symbol:

101325.0*pow((288.15-0.0065*4*t)/288.15,5.25577)

and you have variable h instead of t on lines 12, 15 and 17.

[miguwhynot]

Thanks again, for your fast reply.

Final code:

#include "udf.h"
DEFINE_PROFILE(pressure_inlet, thread, position)
{
face_t f;
real t = CURRENT_TIME;
begin_f_loop(f, thread)
{
if (t <= 2750) ) {
F_PROFILE(f, thread, position) = 101325.0*pow((288.15-0.0065*4*t)/288.15,5.25577)
} else if (t <= 5000) {
F_PROFILE(f, thread, position) = pow(22632,(-(4*t-1000)/6341.62));
} else if (t <= 8000) {
F_PROFILE(f, thread, position) = 5474.87*pow(((216.65+0.010*(4*t-20000))/216.65),-34.163);
} else {
// Capture t values outside the allowed range:
// print a message to the console and stop.
Message("%d: Error: t=%e not allowed.\n",myid,t);
return;
}
end_


Would this be a correct answer?

Thank you again for your time and efforts.

[`e`]

You're missing the end_f_loop and a few brackets are not matching. Carefully check each opening bracket has a closing bracket and is where it should be, and then try compiling the UDF (the compilation notifications are usually helpful to identify errors in your code).

[miguwhynot]

Compilation notifications were really useful.

I think I know have it:


#include "udf.h"
DEFINE_PROFILE(pressure_inlet, thread, position)
{
face_t f;
real t = CURRENT_TIME;
begin_f_loop(f, thread)
{
if (t <= 2750)
{
F_PROFILE(f, thread, position) = 101325.0*pow((288.15-0.0065*4*t)/288.15,5.25577);
}
else if (t <= 5000)
{
F_PROFILE(f, thread, position) = pow(22632,(-(4*t-1000)/6341.62));
}
else if (t <= 8000) {
F_PROFILE(f, thread, position) = 5474.87*pow(((216.65+0.010*(4*t-20000))/216.65),-34.163);
}
else {
// Capture t values outside the allowed range:
// print a message to the console and stop.
Message("%d: Error: t=%e not allowed.\n",myid,t);
return;
}
end_f_loop(f,thread)

}
}

Compilation seemed to work just fine. Is it just correct?

Thank you again for your time and sorry for my many questions.

[`e`]

Sounds good, try running the simulations and see if the results are as expected. You could monitor the inlet conditions with a surface monitor to double check that the correct values have been applied.