# Correct method for calculating time step size

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 February 9, 2016, 04:49 Correct method for calculating time step size #1 New Member     Aarthy Meena Join Date: Oct 2015 Location: Trichy Posts: 29 Rep Power: 10 Hello Everyone, I am a newbie to FLUENT & CFD. I wanted to calculate time step size for unsteady flow around a circular cylinder of dia = 0.01m in the Reynolds number of 45 to 50. The working fluid is water with viscosity = 0.001003 kg/ms and density = 998.2 kg/m3. I came across several posts related to this and many methods to calculate the time step size. 1) From Strouhal number (0.2 for circular cylinder- approx) Sr (0.2) = (frequency x diameter) / velocity From the above equation find the frequency. The total time period, T = 1/frequency. Hence, time step t = T/25 (approx). 2) From CFL condition t = C_m X x / velocity where, C_m = 1 (approx) and x = minimum cell size. 3)From domain length t = domain length / (20 x velocity) 1) Which of the above method is correct ? 2) How to calculate the suitable time step size for my condition? 3) Please explain. Thanks in advance. __________________ Aarthy, PG Student, National Institute of Technology, Trichy, India. Last edited by Aarthy Meena; February 9, 2016 at 05:50.

 February 9, 2016, 05:25 #2 Senior Member   Filippo Maria Denaro Join Date: Jul 2010 Posts: 6,675 Rep Power: 71 the time step is constrained by stability criteria that are based not only on the cfl condition but also on the visicous terms. Furthermore, each discretization has a specific stability region

 February 9, 2016, 05:28 #3 New Member     Aarthy Meena Join Date: Oct 2015 Location: Trichy Posts: 29 Rep Power: 10 Dear Sir, Can you please explain me the method for my flow scenario around a circular cylinder. __________________ Aarthy, PG Student, National Institute of Technology, Trichy, India.

 February 9, 2016, 05:36 #4 Senior Member   Filippo Maria Denaro Join Date: Jul 2010 Posts: 6,675 Rep Power: 71 The numerical stability depends upon the method you use for the integration and the sizes of the mesh (in all directions), does not depend on a specific flow problem. Assuming you are using non-dimensional equations, the velocity magnitude is no more than O(1) so that you can use that as estimation in the (multidimensional) CFL condition

 February 9, 2016, 05:46 #5 New Member     Aarthy Meena Join Date: Oct 2015 Location: Trichy Posts: 29 Rep Power: 10 Thank you sir, Actually am very new to FLUENT and I do not understand your reply. In FLUENT, I am using dimensional form of the governing equations. Kindly please suggest me some material where I can learn about this topic. __________________ Aarthy, PG Student, National Institute of Technology, Trichy, India.

 February 10, 2016, 04:20 #6 New Member   Lucas Join Date: Jun 2015 Posts: 20 Rep Power: 10 A theoretical advice: The CFL-number and the according "condition" are only part of the stability analysis of numerical schemes. A good introduction (for Finite Difference methods) can be found on: https://en.wikipedia.org/wiki/Von_Ne...ility_analysis Basically the stability depends on the equation you discretise (including all the terms in it - as FMDemaro added), as well as the discretisation scheme you use. As a "rule of thumb": lower order discretisation schemes (e.g.: Upwind schemes) and implicit time-dependent solution are very stable, whereas the more accurate methods tend to be unstable. For the Finite Element method, this can be proven analytically for certain cases - An example is also contained in the link. A practical advice: I am not familiar with ANSYS Fluent but I guess, that you will use Finite Volume Method with an implicit solver. The theoretical advice from above might therefore be tough to prove for your problem. I would advise you to have look on the user manual and see, if there is any hint. A cylinder in cross flow is also very well researched and there might be a lot of data and advice for your example already available. Regards, Lucas

 February 10, 2016, 05:46 Help please!! #7 New Member   Ravi Join Date: Feb 2016 Posts: 1 Rep Power: 0 I am trying to do a transient analysis of flow through a bifurcation of penstock pipe (wye) using ansys cfx . Here i need to specify the conditions that the valves of the turbine are closed within a 5 seconds of time. I thought of giving pressure inlet and mass flow rate at the outlet. Now i need to vary the mass flow rate at the outlet from 4650kg/s to 0 within a 5 seconds of time and i dont know how to do that in ansys CFX. Can anybody help me please ??

 May 7, 2019, 08:56 question #8 Member   Join Date: Sep 2018 Posts: 31 Rep Power: 7 Dear Aarthy Meena, did you find a good average to calculate the time step and time step size?