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UDF Fluent running in Parallel mode

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Old   August 31, 2016, 05:54
Default UDF Fluent running in Parallel mode
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Abhiroop Bhadra
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Hi, I have a udf in Ansys Fluent and it is suppose to run only once in a timestep.

/************************************************** **********/
static int last_ts = -1;
DEFINE_DPM_LAW(trial1,p,ci)

{
int curr_ts;
curr_ts = N_TIME;
if (last_ts != curr_ts)
{
last_ts = curr_ts;
Message("Called");
}
}

/************************************************** **********/
On running fluent in serial mode it is being called once.But on running in parallel mode, it is called thrice in a timestep. I am new to udfs. Can anyone tell me why is this happening ?
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Old   August 31, 2016, 06:04
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Davoud Malekian
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Hello,
How many processors you have allocated to your calculation? probably 3?
actually parallel mode divide your domain into several subdomains (for parallel calculation as far as i know!! check it urself for making sure), in your case your domain has been divided into 3 subdomains by parallel mode and thats why you are getting 3 results in each time step.
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Old   September 1, 2016, 17:19
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When you start a parallel computation, your mesh is divided among the computer cores you allocated. Any UDF that you use is sent to every core by default. If you want to change it, you need to use directives, such as "if RP_NODE", "if RP_HOST" etc. First, read a bit about UDF parallelization in the UDF/Customization Manual. Once you understand how the commands are passed around the cores, you will find out that to write a paralllelized UDF is not hard.

The code you posted is just simple printing of a message in the console. It gets printed once for each core. If you want to have it printed only once in total, use Message0 instead of Message. This commands performs the print only from node 0.

Some applications can become a bit more complicated. For example, if you want to integrate a quantity over the whole domain, you need to do it at each core, then send the partial results from all nodes to node 0 and then to sum it up. If you want to write the result in a file, you need to pass it to the host process. Anyway, that's just a very brief overview, I would recommend the manual, it is well understandable.
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