# time step selection

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

 March 11, 2006, 06:19 time step selection #1 Hila Guest   Posts: n/a Hello, I have a CFD project (using Fluent Software) which deals with blood flow within an aortic model. I use the implicit formulation and a periodic inlet velocity profile with period of 0.86 sec. I will greatly appreciate advices regarding the best technique to select the appropriate time step for the unsteady solution, at least how to select the magnitude of order.. Thanks, Hila.

 March 11, 2006, 07:01 Re: time step selection #2 ganesh Guest   Posts: n/a Dear Hila, The time step for unsteady flow is generally taken as a constant value with a tradeoff between temporal accuracy and speed of computation and the stability, which is unfortunately non-linear. A step of 1e-2 to 1e-3 is the most preferred, but it defintely depends on the problem and grid at hand. Hope this helps Regards, Ganesh

 March 11, 2006, 07:16 Re: time step selection #3 Hila Guest   Posts: n/a Dear Ganesh, Thank you for your help. How did you define this time step? according to your experience or the time period? In fact I tried two time steps - 5e-3 and 1e-3 (172 or 860 steps per cycle) which resulted in similar flow field. To validate it I compared the axial veloity at a specific cross section and the general flow field. Are there any additional methods that I may be missing to check stability and accuracy in Fluent? Best Regards, Hila.

 March 11, 2006, 13:15 Re: time step selection #4 ganesh Guest   Posts: n/a Dear Hila, I am not sure about Fluent, because I have my own solver. The stability is unfortunately non-linear and the time step therfore is a fuction of the grid and the problem. The outer time step(as it is generally called, as also in Fluent which uses Dual time stepping)is therfore a matter of experience, with due respet to issues of stability, speed of computation and accuracy. In fact it is always a nice idea to use two time steps as you did, for a cross check, but will not be a good one in the industrial environment, where quick answers are required, wherein the experience of runnung such cases will prove handy. Hope this helps Regards Ganesh

 March 11, 2006, 14:11 Re: time step selection #5 Hila Guest   Posts: n/a No magic formulation, eh? Thanks a lot, Hila.

 March 12, 2006, 06:23 Re: time step selection #6 taw Guest   Posts: n/a Dear Hilla , I dont know how much relaevant it would be to you, if this might help, I recently read in a matter related with fluctuating turbulent variables, the time mean of a turbulent function, when defined i.e. for u,v, p. , goes like an averaging period taken to be longer than any significant period of the fluctuations themselves, For turbulent gas nad water flows an averaging time period of 5 seconds is quite usually adequate, so your quest may answered in such observations in the real world experiments and length scale settings, regards, taw

 March 12, 2006, 07:21 Re: time step selection #7 Hila Guest   Posts: n/a Dear Taw I appreciate your will to help, but I'm afraid that you did not understand my question... Best Regards, Hila

 March 12, 2006, 13:11 Re: time step selection #8 vladimir Guest   Posts: n/a Hi Hila, if you want to see a magic number, try the CFL condition (Courant-Friedrichs-Lewy); usually shortened as Courant number. Unfortunately, it really matters when explicit time differencing scheme is to be imposed because it is a stability criterion. It simply says how long it'll take for a particle (flow) to pass through a computational element. With the implicit scheme there is no problem with stability therefore you can choose whatever time step you like but you should be aware of the physics you want to model. Additionally, the larger the time step, the worse accuracy; this apply to implicit schemes especially. For more information look it up in any CFD book. Hope it helps. vladimir

 March 12, 2006, 13:55 Re: time step selection #10 Hila Guest   Posts: n/a Dear all, Thank you for the great help. I guess that experience is the best teacher... Bests, Hila.

 March 18, 2006, 08:22 Re: time step selection #11 ali mottalea Guest   Posts: n/a Hello. blood is a non-newtonian fluid. so its equations and time step choosing differ from navier stocks.any way for newtonian fluids 1e-6 to 1e-4 is an appropriate selection for combustion. and i think 1e-3 would be appropriate for noncombustion applications. if you want i can explain you a methode valid for combustion.

 Thread Tools Display Modes Linear Mode

 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 OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post dm2747 FLUENT 0 April 17, 2009 01:29 sunnysun OpenFOAM Running, Solving & CFD 5 March 16, 2009 04:32 msrinath80 OpenFOAM Running, Solving & CFD 2 November 7, 2006 16:15 Jackie Main CFD Forum 5 January 12, 2004 13:26 özgür FLUENT 8 January 6, 2004 09:23

All times are GMT -4. The time now is 08:08.