Is increasing the time step affect the results accuracy in Fluent?

- March 28, 2020 at 10:01 am
  
  khalidb77
  Subscriber
  
  Dear all
  
  I am running a transient case utilizing the Melting/Solidification model in Fluent. I want to know if I increase the time step to make the solution faster to reach the final time is that will influence the accuracy of the results? knowing that the solution is converged in each time step even with the larger steps?
  
  regards
  
  Khalid
- March 29, 2020 at 7:56 am
  
  vinayak chauhan
  Subscriber
  
  hi
  When you reduce the size of element, you are recommended to reduce the time step correspondingly. Hence you need to scale it down or up based on your mesh details for better results. Also time steps judge the number of iterations for convergence.
  However if you are not changing the mesh size then a larger or smaller time step wont affect your convergence condition , hence you can increase your time step for doing the calculation fast , as it will decrease the number of iterations for calculation.
  
  I hope it helps.
- March 29, 2020 at 6:19 pm
  
  khalidb77
  Subscriber
  
  Many thanks Vinayak for the answer. I used a similar size of grids, in this case, do I need to do time step validation?
  
  Khalid
- March 29, 2020 at 8:42 pm
  
  Karthik Remella
  Administrator
  
  Hello,
  
  Just to add to Vinayak's answer, there is a physical timescale associated with any problem. In order to capture the physics correct, you will need to use a time step lower than this time scale. As long as you are mindful of this, and of course, the CFL condition based on the mesh size, you can continue to increase your timesteps. If you use a time-step larger than what the CFL condition prescribes, then your simulation will diverge.
  
  I hope this helps.
  
  Thank you.
  
  Best,
  
  Karthik