CFD Online Logo CFD Online URL
Home > Forums > Main CFD Forum

Real transient behaviour

Register Blogs Members List Search Today's Posts Mark Forums Read

LinkBack Thread Tools Display Modes
Old   August 17, 2006, 02:49
Default Real transient behaviour
Posts: n/a

I have a transient simulation where I expect the solution never to reach a steady state (perhaps a periodic steady state though). However this is not the case (it reaches steady state quite rapidly). How come? I have a few suggestions, can you please tell me what you think?

1. Using an implicit method I might have too high time step at some points in the simulation, thus violating the CFL condition, and hence not describing the transient behaviour adequately.

2. Using an implicit method of 1st order the resolution in time might not be enough (coupled to nr 1 above).

3. The simulated system is symmetrical, but the physical system might be - not fully symmetrical.

Best Regards

  Reply With Quote

Old   August 17, 2006, 04:42
Default Re: Real transient behaviour
Posts: n/a
Dear Sara,

My suggestions are as follows

1. Implicit methods cannot be used in the framework of transient solvers as you do it for a steady state framework. You can definitely use it in a dual time framework, violate CFL condition and update your variables, though.

2. First order implicit scheme are not generally preferred fot time -accurate simulations. Temporal accuracy of 2 (BDF/Crank-Nicholson) is usually favoured.

3. The response of the system you attempt to simulate is closely tied to the physics. If you have a symetrically oscillating airfoil, you attempt to numerically simulate the response of this oscillation, which expectedly must mimick the symmetry of oscillation.

If you are trying to use an implicit scheme the way you use it on a steady state solver and simulate a transient problem, you would possibly end up in steady state and quite quickly. If you are using a steady state solver and want to simulate an unsteady phenomenon, with least change to your code, I suggest that you use a first order explicit time stepping with CFL criterion to see the "periodic behaviour", though it could take quite a lot of iterations before you actually achieve the transient response.


  Reply With Quote

Old   August 17, 2006, 09:47
Default Re: Real transient behaviour
Posts: n/a
Inadequate spatial resolution or a low-order approximation for the spatial gradients can sometimes lead to numerical damping. Also use of upwinding for advective terms must be carefully considered.
  Reply With Quote


Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

Similar Threads
Thread Thread Starter Forum Replies Last Post
error: uninitialized local variable 't' used MASOUD Fluent UDF and Scheme Programming 3 November 26, 2012 06:24
how to hook this udf shanu FLUENT 1 August 26, 2011 10:48
defining a term for a domain using DEFINE_ADJUST MASOUD Fluent UDF and Scheme Programming 1 September 24, 2010 05:08
enum MASOUD Fluent UDF and Scheme Programming 0 June 5, 2010 00:49
udf error Rashmi FLUENT 0 December 27, 2005 06:35

All times are GMT -4. The time now is 14:07.