# How to set deltaT in controlDict?

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 June 12, 2019, 10:31 How to set deltaT in controlDict? #1 Senior Member   Raza Javed Join Date: Apr 2019 Location: Germany Posts: 183 Rep Power: 7 Hello Everyone, I have a question related to deltaT in controlDict. What things do I need to consider while choosing the deltaT? I am running my simulation, there is a fluid flow in my case, and velocity of fluid is in range of 1e-3 m/s to 1.5 m/s. but my residuals for velocity are not getting down. I exactly don't know but could it be also because of my deltaT because solver seems to be working, but the velocity residuals are very high (between 0.003 to 0.01). Do I need to set deltaT by keeping in mind my velocity of fluid? I shall be very thankful if someone can help me out in this. Thank you

 October 9, 2023, 19:30 #2 New Member   Johannes Join Date: Feb 2023 Posts: 3 Rep Power: 3 Have a look at the Courant Number and the Courant–Friedrichs–Lewy condition.

October 10, 2023, 14:08
#3
Senior Member

Saeed Jamshidi
Join Date: Aug 2019
Posts: 214
Rep Power: 7
On the basis of my information, there are two ways for setting delta t:
First and easiest way is working with courant number directly; in controlDict set adjustableTimeStep to true.

Second way is to manually calculate time step. You should take the smallest cell volume (3D) and adapt your time step on the basis of it. See attached file.
However, in my opinion, it's better to use forst way, because the second way automatically is done by openfoam.
Good luck.
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 Tags controldict, openfoam

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