[jchan033]

Hi everyone,

I'm new to writing UDFs, and my current problem is (refer to image)

I want to have a square wave, where at a specific edge to have an increment of temperature (or addition of heat flux) for 10 seconds, and the following 10 seconds to have no heat addition. Cycle to repeat.

My thoughts are wrapping around a sinusoidal equation.

This is what I've tried

/************************************************** *********************
UDF for specifying an varying temp with time profile boundary profile
************************************************** **********************/

#include "udf.h"
#define A 1000

DEFINE_PROFILE(temp, thread, position)
{
face_t f;
real t = CURRENT_TIME;

begin_f_loop(f, thread)
{
F_PROFILE(f, thread, position) = Asin(0.05*t);
}
end_f_loop(f, thread)
}

I know there should be more edits to the sine, perhaps 'absoluting' it in Excel terms, but I'm unsure of the conventions in ANSYS UDF. And a square wave is not just a simple sine as I have written.

Thanks for reading, and looking forward to any input!

Simon

[KevinZ09]

I'm not completely getting your question but if you're asking how to generate a square wave I'd suggest using the modulus operator. You can do for example:

#include "udf.h"
#define A 1000

DEFINE_PROFILE(temp, thread, position)
{
face_t f;
real t = CURRENT_TIME;
real per = 20.0; /* period of your oscillation */

begin_f_loop(f, thread)
{
if ( (t % p)/p < 0.5)
{
F_PROFILE(f,thread,position) = A; /* on for the first 10 seconds of your period */
}
else
{
F_PROFILE (f,thread,position) = 0; /* off for the 2nd half */
}
}
end_f_loop(f, thread)
}

I'm not totally sure you can use modulus on reals; in some languages you can and some later versions allow you to use it too. If it doesn't just write your own version of it, which is quite straightforward.

[jchan033]

Hi Kevin,

Thank you for replying to my question!

Though, may I know what does "if ( (t % p)/p < 0.5)" mean?

I understand t is the current time, but what is p? and the significance of '%' in this line?

[KevinZ09]

Oh, sorry, on my paper I scrabbled it as p, but in the code I posted I used "per" (short for period). But yeah, it should read if ( (t % per)/per < 0.5).

This if-statement basically checks if you are in the first 10 seconds of your 20 seconds period (10 seconds flux on, then 10 seconds flux off) or in the second half of it.
- "t % per" means modulus of t with per, which calculates the remainder of the division of t/p. For example, if t = 5 and p = 2, the remainder is 1 (and the quotient is 2). Google for remainder and quotient if you're not familiar with it.
- Knowing the remainder, you can know if you're in the 1st or 2nd half of your period by dividing by the period itself.

You can generalize the code in case your period isn't half on then half off, simply by changing the 0.5 to your desired fraction of the period you want the heat flux to be on (or off).

Hope this is more clear?

[jchan033]

Hi Kevin,

Thank you very much! You have been very informative, I really appreciate your time and effort!

Cheers!

[jchan033]

Originally Posted by KevinZ09

Oh, sorry, on my paper I scrabbled it as p, but in the code I posted I used "per" (short for period). But yeah, it should read if ( (t % per)/per < 0.5).

This if-statement basically checks if you are in the first 10 seconds of your 20 seconds period (10 seconds flux on, then 10 seconds flux off) or in the second half of it.
- "t % per" means modulus of t with per, which calculates the remainder of the division of t/p. For example, if t = 5 and p = 2, the remainder is 1 (and the quotient is 2). Google for remainder and quotient if you're not familiar with it.
- Knowing the remainder, you can know if you're in the 1st or 2nd half of your period by dividing by the period itself.

You can generalize the code in case your period isn't half on then half off, simply by changing the 0.5 to your desired fraction of the period you want the heat flux to be on (or off).

Hope this is more clear?

Hi Kevin,

I have encountered an issue.

Error: C:\\Users\\hallo\\Desktop\\Stuff\\NTU\\FYP\\Stage 2\\temp&time.c: line 16: invalid type for integral binary expression: double % double.

Can you assist?

Thanks!

[KevinZ09]

That's because the modulus needs integers as operands. There's two alternatives you can try:

1: Use C's real version of %, fmod. Then "if ( (t % per)/per < 0.5)" becomes "if ( (fmod(t,per))/per < 0.5 )"
2: Or write your own version of the modulus operator: "if ( (t % per)/per < 0.5)" becomes "if ( (t - per*int(t/per))/per) < 0.5 )"

The latter one should definitely work. The first one depends again on your compiler.