Learning Forum

[iitdhn.a]

hi, I want to apply convention boundary condion on a wall as a functin of varing ambient temperature. I have written a udf for free stream temperature (ambient) as funciton of time but it has unexpected effect in the convection boundary condition at the wall. when I am changing the temperature value in the udf it has no effect on solution. but for expression and as constant free stream temperature it works fine.

[Rob]

I assume you've got DEFINE_PROFILE returning a temperature in Kelvin and hooked into the highlighted box below?

[iitdhn.a]

Tahnks you for the reply!

I used the following udf code to define varing ambient temperature and the unit I used in degree C

DEFINE_PROFILE(ambient_temperature_profile, t, i)

{

    real time = CURRENT_TIME;

    real hour = time / 3600.0;

    real temp;

    face_t f;

 

    while (hour >= 24.0)

        hour -= 24.0;

 

    if (hour >= 8.0 && hour <= 13.0)

    {

        temp = -20.0 + ((0.0 + 20.0) / 5.0) * (hour - 8.0);  // -20°C → 0°C

    }

    else if (hour > 13.0 && hour <= 18.0)

    {

        temp = 0.0 - (15.0 / 5.0) * (hour - 13.0);  // 0°C → -15°C

    }

    else if (hour > 18.0 && hour < 24.0)

    {

        temp = -15.0 - (5.0 / 6.0) * (hour - 18.0);  // -15°C → -20°C

    }

    else

    {

        temp = -20.0;  // Constant at night (midnight → 8 AM)

    }

 

    begin_f_loop(f, t)

    {

        F_PROFILE(f, t, i) = temp;

    }

    end_f_loop(f, t)

}

 

[Rob]

 

Alter to Kelvin, UDFs use SI units regardless of other settings. Also avoid the arrow symbols in comments, they should be OK but there’s a risk Fluent will see it as something else because of the UNIX legacy. 

To add, given flow timescales of under a second, and your code being hours you may want to review just how long you intend this model to run for.