CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > ANSYS > CFX

Physical Timescale

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

Like Tree2Likes
  • 1 Post By ghorrocks
  • 1 Post By ghorrocks

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   August 3, 2014, 19:02
Default Physical Timescale
  #1
Senior Member
 
Join Date: Aug 2014
Posts: 160
Rep Power: 9
MissCFD is on a distinguished road
Hello,

I don't understand what is exactly the physical timescale in a steady flow and how it works for the Navier Stoke equations ? Is someone can explain me?

Thank you for your answer
MissCFD is offline   Reply With Quote

Old   August 4, 2014, 06:50
Default
  #2
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 16,840
Rep Power: 132
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
A short quote from the CFX theory manual "For steady state problems, the time-step behaves like an ‘acceleration parameter’, to guide the approximate solutions in a physically based manner to a steady-state solution. This reduces the number of iterations required for convergence to a steady state"

In other words, the time scale allows the user to tune how aggressively the solver marches towards convergence. If too slow the simulation will take longer than it could, if too fast the solver will diverge. It is up to the user to find a time step size which converges reasonably quickly, but remains numerically stable.
aero_head likes this.
ghorrocks is offline   Reply With Quote

Old   August 4, 2014, 11:35
Default
  #3
Senior Member
 
Join Date: Aug 2014
Posts: 160
Rep Power: 9
MissCFD is on a distinguished road
Thank you for your answer.

So, this mean that the instationnary term in Navier Stoke equation is present. But I don't see very well the difference with instationnary flow. Is that at each iteration, physical time step increases ? Or how that works exactly ?

How can I estimate this physical time step, Is it with experience ? Is it better to start with a auto timescale ?
MissCFD is offline   Reply With Quote

Old   August 4, 2014, 18:20
Default
  #4
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 16,840
Rep Power: 132
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
For steady state simulations you are not looking to get a true time accurate history of the flow, just use psuedo-time as a method to converge to the final steady solution. Because of this some of the time-accurate parts of the time step calculation are ignored as they go to zero as you head towards a converged solution. This is why it is called psuedo-time, as it is similar to real time but is simplified.

The documentation describes how to estimate the required time step. Auto-time step is also often a good starting point, but you can use "edit run in progress" to make it bigger or smaller depending on how the convergence is going.
aero_head likes this.
ghorrocks is offline   Reply With Quote

Old   August 25, 2021, 19:14
Default
  #5
Senior Member
 
Brett
Join Date: May 2013
Posts: 191
Rep Power: 10
Bdew8556 is on a distinguished road
so does the time step just act as a relaxation factor?
Bdew8556 is offline   Reply With Quote

Old   August 25, 2021, 19:21
Default
  #6
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 16,840
Rep Power: 132
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
The time step is used as the method to advance the solution. Under relaxation is where you only advance a proportion of the calculated change per iteration to maintain stability. While they are both related to an accurate and stable solution, they are different things. You should read a basic textbook on CFD to have these concepts explained in more detail.
__________________
Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.
ghorrocks is offline   Reply With Quote

Old   August 25, 2021, 19:22
Default
  #7
Senior Member
 
Brett
Join Date: May 2013
Posts: 191
Rep Power: 10
Bdew8556 is on a distinguished road
any links you could recommend?
Bdew8556 is offline   Reply With Quote

Old   August 25, 2021, 19:49
Default
  #8
Senior Member
 
Brett
Join Date: May 2013
Posts: 191
Rep Power: 10
Bdew8556 is on a distinguished road
Could I maybe clarify what I think is correct and where my misunderstanding comes from.
I think I was thinking that if the problem is steady state then you automatically delete/cross out the transient terms. THat could be fine if you're looking at solving it analytically but not numerically.

so taking this example:
https://en.wikipedia.org/wiki/Finite_difference_method

the time step is used even though it's a steady state problem. The relaxation is when we weight the new calculated value for the next step.
Bdew8556 is offline   Reply With Quote

Old   August 25, 2021, 22:02
Default
  #9
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 16,840
Rep Power: 132
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
Do not confuse the equations you are solving with the numerical method you are using to solve them.

The steady state NS equations have no transient term, obviously. This is the equation you wish to solve.

The numerical approach CFX uses to solve the steady NS equations is by using a term which is very similar tot he transient term, so the numerical solution method approaches converging to the steady state NS equation by marching through time. This is only one numerical method, there are others. But the whole idea of the iterative numerical method is that from an initially poor solution of the Steady NS equations the method gives you a new solution which is a better solution. Then you keep iterating until the solution is accurate enough. So in CFX, with the psuedo-transient approach, this iterative new solution is from an approach similar to a transient solution of the NS equations.
__________________
Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.
ghorrocks is offline   Reply With Quote

Old   August 27, 2021, 13:00
Default
  #10
Senior Member
 
Brett
Join Date: May 2013
Posts: 191
Rep Power: 10
Bdew8556 is on a distinguished road
Thanks Glenn,

What is the name of the method you've described there? What others are available? thats quite interesting and you've explained it well (in my opinion).

Sorry to hear about what's happening in Sydney. I'm from Australia too although I live in Florida now.

B
Bdew8556 is offline   Reply With Quote

Old   August 27, 2021, 18:41
Default
  #11
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 16,840
Rep Power: 132
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
Alternatives are things like SIMPLE (https://en.wikipedia.org/wiki/SIMPLE_algorithm), and all its variations like SIMPLER, SIMPLEC, PISO.

Also there are fundamentally different approaches, like streamfunctions based methods, Godunov etc.

And as a segway, do not forget that there are CFD approaches which model fluids but are not based on the Navier Stokes equations at all. Lattice Boltzmann and spherical particle hydrodynamics are examples of this.
__________________
Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.
ghorrocks is offline   Reply With Quote

Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search
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 Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Superlinear speedup in OpenFOAM 13 msrinath80 OpenFOAM Running, Solving & CFD 18 March 3, 2015 05:36
When to use local timescale or physical timescale xyq102296 CFX 23 June 25, 2014 07:55
Determination of physical timescale Chander CFX 2 October 19, 2011 19:47
Advice on Physical Timescale Oli CFX 0 January 16, 2007 10:41
Controling Physical Timescale petr CFX 1 January 5, 2007 18:57


All times are GMT -4. The time now is 04:16.