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-   -   UDF at pressure inlet[pleas help me!!] (https://www.cfd-online.com/Forums/fluent/84562-udf-pressure-inlet-pleas-help-me.html)

kokoory February 2, 2011 12:55

UDF at pressure inlet[pleas help me!!]
 
Hello All,

I would like to increase pressure and temperature at pressure inlet boundary.

As like this,

1~1000 iteration : pressure - 101325 pa, temperature - 250K
1001~2000 iteration : pressure - 101325* 2 , temperature - 500K

How do i do complete my goal?

I could compile and hook, but it is all!!
I try to code UDF to find a solution by myself. but it does not work!!

ComputerGuy February 2, 2011 22:09

Kokoory,

A few things:
1) Are you really trying to change boundary conditions on a per-iteration basis? Is this a steady state or a transient problem? If you're actually trying to do a steady state simulation and you want to change inlet conditions after a specified number of iterations, don't. Simply put your boundary conditions to what you want them to be at t=infinity and let the simulation solve.
2) If you're trying to solve a transient case, and by iterations you mean "time steps" or "simulation time, in seconds" have a look at this Time dependent flows in Fluent with UDFs
3) Finally, more of a numerics issue...The changes you're imposing are fairly dramatic over what, presumably, will be a short period of time. This often causes convergence difficulties, and rightfully so -- the physics you're trying to impose are almost non-physical. In one timestep (very short) the pressure and temperature across an entire face double. Typically, one would impose a slightly more gentle ramp than a step change, and take fractional timesteps in the "in between" part to allow the solver to get to a more stable answer.

I hope this helps!

ComputerGuy

kokoory February 3, 2011 09:40

Thanks, ComputerGuy.

I refer your opinion for the model.

So, I use the below code for pressure and temperature.
Code:

#include "udf.h"
DEFINE_PROFILE(temperature_magnitude, t, i)
{
real temperature_mag;
real current_time;
face_t f;
current_time = CURRENT_TIME;
temperature_mag=300+220*current_time;
begin_f_loop(f,t)
{
F_PROFILE(f,t,i) = temperature_mag;
}
end_f_loop(f,t)
}

Code:

#include "udf.h"
DEFINE_PROFILE(pressure_magnitude, t, i)
{
real pressure_mag;
real the_current_time;
face_t f;
the_current_time = CURRENT_TIME;
pressure_mag=101325*the_current_time;
begin_f_loop(f,t)
{
F_PROFILE(f,t,i) = pressure_mag;
}
end_f_loop(f,t)
}

after that, I interpreted_ and compiled and hooked to pressure_inlet B.C.
For pressure code, I hook to Gauge total pressure and for temperaturee code, to temperature.

Next, I set the time step size = 0.001 and number of time steps = 10000
and Max iterations per step = 1. Because I would like to increase the pressure at 1013250 and the temperature at 2500K after all calculating.

Is it OK what I thought to be?


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