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-   -   transient profile-time step (http://www.cfd-online.com/Forums/fluent/102679-transient-profile-time-step.html)

 Lilly May 31, 2012 08:15

transient profile-time step

Dear all,

I have got a question concerning the time of my transient boundary profile. I want the velocity of my simulation to vary in time and wrote a profile that looks like this:
((profile transient 3 0)
(time
0
1
2
)
(velocity
2
6
3)
)

When I import this profile to FLUENT I can pick the velocity as b.c. at the inlet. But what is about the time? Can I pick it somewhere else too? Or is the selected time step size used for the velocity variation? I couldn't find this at the FLUENT manual. Does somebody know, where I can find it?
Thank you for all your help!
Lilly

 ghost82 May 31, 2012 09:06

1 Attachment(s)
The profile you wrote is represented in the attached picture.
If you use a profile you are performing a transient simulation, so choose an appropriate time step in the "Run" window, considering your profile.

In other words "Or is the selected time step size used for the velocity variation?" this is true.

If you want to change the profile other than linear you have to interpret/compile an udf.

Daniele

 Lilly May 31, 2012 09:51

Thank you, ghost82,

the values of my velocity profile were just picked as an example :).
But that means (in case of that example), if I would use a time step size of 0.1 s,the new time axis of my velocity profile would be 0,0.1,0.2,...Have I got that right?
Lilly

 ghost82 May 31, 2012 11:46

Lilly, take this as an example:

you want a profile as velocity=time
This means at t=0 s, v=0 m/s; t=1 s, v=1 m/s and so on..

You want to study the system from t=0 till t=10 s.

Your profile will be of 2 points: 0;0 and 10;10 (time;velocity)
Fluent interprets this profile as a linear profile.
Your time step will be for example of 1 s.
So at t=0 fluent calculates v=0, at t=1 it calculates v=1 and so on (because profile are always linear between 2 points).

This 2 points profile is EQUAL for example to this 4 points profile: 0;0 2;2 6;6 10;10 (time;velocity): this is the same as the 2 points profile.

Your profile file has to include points when the slope of your profile changes:
for example if your velocity is 0 m/s at t=0s, increases linearly till 10 s, reaching a velocity of 3 m/s, it maintains at 3 m/s till 15 s and then it decreases to 0 m/s at t=20 s your profile will be:
0;0 10;3 15;3 20;0 (time;velocity)

You can see I include only points where slope of profile changes.

Then you can choose the time step you want; fluent will calculate velocity value assuming a linear profile between 2 points.

Hope now it's more clear.

Daniele

 Lilly June 4, 2012 06:33

Hi Daniele,

thanks a million for your detailed explanation!
I got it now!

Lilly

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