[CFD-user]

Hi,

I am writing an UDF, in which I would like to use the previous time step data. Although, in FLUENT we can access previous time step data for many variables with '_M1', I would need user defined variables previous timestep rather than the standard variables.

For e.g. m(t+dt) = m(t) + ...............

so to solve the above equation for the new time step I will be needing m(t) i.e., previous time step data.

I have been keep on trying this from last few days, but could not crack.

It will be appreciated, if you can throw some light on it.

Regards...

[Amir]

Hi,

Why didn't use other UDM for this purpose?

[CFD-user]

Originally Posted by Amir

Hi,

Why didn't use other UDM for this purpose?

Hi,

Thank you for your reply.

I tried earlier with UDMIs also and did not work. But, I think even UDMIs also store the current time step data only. I will try once again & see.

Regards

[Amir]

Originally Posted by CFD-user

Hi,

Thank you for your reply.

I tried earlier with UDMIs also and did not work. But, I think even UDMIs also store the current time step data only. I will try once again & see.

Regards

It depends how you use and update such memories in order to refer to desired time step.

Bests,

[sbaffini]

What Amir meant to say is that you can save your current time variable in some UDMI and the older one in another UDMI and basically keep updating them as the time passes. In this case you will never need the old time values as you will always reference current time variables

[CFD-user]

Originally Posted by Amir

It depends how you use and update such memories in order to refer to desired time step.

Bests,

Can you please give some example in terms of UDMIs?

[CFD-user]

Originally Posted by sbaffini

What Amir meant to say is that you can save your current time variable in some UDMI and the older one in another UDMI and basically keep updating them as the time passes. In this case you will never need the old time values as you will always reference current time variables

Can you please give some example in terms of UDMIs? Now I will trying with fprintf and fscan options from the file.

[Amir]

Originally Posted by CFD-user

Can you please give some example in terms of UDMIs?

Consider for example 2 UDMs at first: UDM0, UDM1 and initialize them.
Then in your desired define-macro which does a process in each time step you have e.g.:
DEFINE_X(.....)
{
C_UDMI(...,1)=f(....)+C_UDMI(...,0); //here UDM0 refers to previous time step
.
.
.
// At last you have:
C_UDMI(...,0)=C_UDMI(...,1); // prepare UDM0 for the next time step

}

Just note to provide these commands each time steps not each iteration; i.e., if your macro executes each iteration, you need to use a if-clause to restrict the commands for each time step.

Bests,

[CFD-user]

Originally Posted by Amir

Consider for example 2 UDMs at first: UDM0, UDM1 and initialize them.
Then in your desired define-macro which does a process in each time step you have e.g.:
DEFINE_X(.....)
{
C_UDMI(...,1)=f(....)+C_UDMI(...,0); //here UDM0 refers to previous time step
.
.
.
// At last you have:
C_UDMI(...,0)=C_UDMI(...,1); // prepare UDM0 for the next time step

}

Just note to provide these commands each time steps not each iteration; i.e., if your macro executes each iteration, you need to use a if-clause to restrict the commands for each time step.

Bests,

Thanks for your reply.
I have following doubts:
1. For the first time step what would be the value of C_UDMI(...,0)
2. Suppose even if you include it in 'if-else loop' i.e., say for the first time step I will initialise C_UDMI(...,0) in 'if' loop. And subsequently I would use this in else loop. I think I have tried this without inlcuding the if loop for each time step and did not work. I would try for each time-step instead of each iteration and come to you.

Thank you again.

[Amir]

Originally Posted by CFD-user

1. For the first time step what would be the value of C_UDMI(...,0)

The UDM0 value would be the initial value for desire variable. (you can set it appropriately via initialize->patch or via other pre-used UDF)

Originally Posted by CFD-user

2. Suppose even if you include it in 'if-else loop' i.e., say for the first time step I will initialise C_UDMI(...,0) in 'if' loop. And subsequently I would use this in else loop. I think I have tried this without inlcuding the if loop for each time step and did not work. I would try for each time-step instead of each iteration and come to you.

It should work if executed at each time step not each iteration.

Bests,