Previous time step variable to be used in UDF
 

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...

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

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

Bests,

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?

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

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.

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

Bests,

Since Fluent does not handle the coupling of the VOF equation and the UDS equation properly and since the source term in the VOF equation is treated implicitly for volume fraction but explicitly for the UDS variable in question, I need to use "some" variables (suppose I store them in a UDM) from the previous time step. How can I do that? I think I am asking how should I loop w.r.t time?

Amir, how and where would you use the if-clause to restrict the commands to each time step?

same question - how to access a variable value from previous time step - no answer in this thread

Very clear answer in this thread! Store that variable in a UDM, and then access the UDM in the next time step!

How is this ensured? i.e. the if-clause?

Previous Time step data in Custom field functions
 

Hello,
can '_M1' Extension be used while defining custom field functions too to get previous time step data?

Unless some radical change happened in the last couple of Fluent versions, custom field functions are quite different objects with respect to the data available in UDF, especially _M1 and _M2 variables. What you can do is defining your custom field function in an UDF and save it in an UDM, than just do what explained here to have current and past values available.

Hi Amir, If I want to get the time-dependent values of the turbulent kinetic energy, how should I use the UDM?

accessing to previous variable can be done by define_execute_at_and macro. at first desired variable should store in a udmi memory at this macro and then this memory can retrieve anywhere in udf. this macro execute at end of last time step so in iterations of new time step its values dont change.
for example
define_execute_at_end(){
domain
*mixture =Get_Domain(2);
fluid_id =8;
Thread t = Lookup_Thread(t,fluid_id);
cell_t c;
begin_c_loop(c,t)
{
C_UDMI(c,t,0) = c_R(c,t);
}
end_c_loop(c,t)
}

define_source(name,c,t,ds,eqn)
{
rho_previous = udmi(c,t,0);
rho = C_R(c,t);
}