CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > General Forums > Main CFD Forum

Time step for implicit solver

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

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   March 24, 2013, 10:27
Default Time step for implicit solver
  #1
Far
Super Moderator
 
Sijal
Join Date: Mar 2009
Location: Islamabad
Posts: 4,553
Blog Entries: 6
Rep Power: 54
Far has a spectacular aura aboutFar has a spectacular aura about
Send a message via Skype™ to Far
I am simulating low pressure turbine unsteady flow and I have couple of questions:

1. How to decide time step for the implicit solver?

2. Do we need to satisfy CFL < 1 for implicit solver?

3. Why implicit solver is preferred in commercial codes and as I understand for unsteady flows explicit solver would be better option.

4. Or explicit solver is only efficient in LES where turbulent scales to be resolved are the same order as the time step dictated by CFL < 0.2 condition?

5. For the same geometry and flow, time step is inversely proportional to Reynolds number?
Far is online now   Reply With Quote

Old   March 24, 2013, 11:10
Default
  #2
Super Moderator
 
flotus1's Avatar
 
Alex
Join Date: Jun 2012
Location: Germany
Posts: 3,395
Rep Power: 46
flotus1 has a spectacular aura aboutflotus1 has a spectacular aura about
A CFL number smaller than 1 is a stability constraint for some explicit solvers which does not apply to the implicit ones.

This partly answers question 3, because with the implicit solvers, the timestep size can be chosen according to the flow considered and does not have to fulfill a CFL criterium.

Since a CFL number below 1 is a good guess for the timestep in LES, the advantage of implicit solvers allowing for larger timesteps becomes irrelevant, since the timestep has to be small anyway.

According to Kolmogorovs turbulence theory, the timescale of the smallest eddies is proportional to Re^{-1/2}. Yet this is only relevant for DNS.
In a LES, there are different approaches to estimate an appropriate time step size, like CFL<1 for example.
flotus1 is offline   Reply With Quote

Old   March 24, 2013, 16:53
Default
  #3
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,756
Rep Power: 71
FMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura about
the choice is driven by the numerical stability as well as accuracy requirements.... often, implicit scheme are not "pure", in the sense they are mixed in an implicit and an explicit part. For example, a classical integration used in DNS/LES is the Crank-Nicolson for the diffusion and the Adams-Basforth for the convection. This result in a conditionally stable scheme that is not stable only under the satisfation of the cfl<1 condition. It exists a stability region in the cfl-Reh plane.
FMDenaro 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
Time step size and max iterations per time step pUl| FLUENT 33 October 23, 2020 23:50
Transient simulation not converging skabilan OpenFOAM Running, Solving & CFD 14 December 17, 2019 00:12
InterFoam negative alpha karasa03 OpenFOAM 7 December 12, 2013 04:41
Upgraded from Karmic Koala 9.10 to Lucid Lynx10.04.3 bookie56 OpenFOAM Installation 8 August 13, 2011 05:03
Problems with simulating TurbFOAM barath.ezhilan OpenFOAM 13 July 16, 2009 06:55


All times are GMT -4. The time now is 09:59.