[djing]

hi
I want to simulate unsteady flow in FLUENT why do I use a variable speed input and a variable pressure output
to do that I must use one of two macros (udf):
CURRENT TIME or CURRENT TIMESTEP
I want to know what is the difference between the two macros knowing that I found on their deffinition in FLUENT UDF guide as follows:

Macro Name Returns
CURRENT TIME real current flow time (in seconds)
CURRENT TIMESTEP real current physical time step size (in seconds)

thank you in advance for your reply

[LuckyTran]

Originally Posted by djing

CURRENT TIME real current flow time (in seconds)
CURRENT TIMESTEP real current physical time step size (in seconds)

I had thought that:

current time is physical time with units of seconds
current timestep is a counter (a number) to keep track of how many timesteps have been solved

[djing]

thank you for your reply

if current timestep is a counter (a number) to keep track of how many timesteps have been solved as you say
why it is referred as second in FLUENT

[LuckyTran]

Originally Posted by djing

thank you for your reply

if current timestep is a counter (a number) to keep track of how many timesteps have been solved as you say
why it is referred as second in FLUENT

I was mixed up. Now I am wondering which variable holds the current timestep.

CURRENT TIME is current physical time of current iteration
CURRENT TIMESTEP is the timestep size (change in time between two iterations in time).

But does that help? I still think it's odd that TIMESTEP refers to the timestep size and not the current timestep but whatever.

if you want to know how much time has elapsed since the simulation started, you should reference TIME.
If you need to know the timestep size between two iterations you should reference TIMESTEP. Though, I am not sure if the timestep is considered previous and current iteration or current and next iteration. That would take more reading.

[djing]

thank you for your contribution
I think now the difference is clear between the two (current time and current time step) maintenat I have to use current time for my simulation