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-   -   Effect of the time step size on the calculatino result (http://www.cfd-online.com/Forums/ansys/93737-effect-time-step-size-calculatino-result.html)

norman_bates October 25, 2011 06:42

Effect of the time step size on the calculatino result
 
Hello everyone,

im quite a rookie at fluent and after spending a lot of hours trying to find a solution to my problem i thought i could share iht with you. The problem im trying to solve is just a simple transient heat transfer problem. I define one part of a sphere to be warmer than the rest and watch the temperature to disperse.
When i now solve this for the same time period but using different time steps (i.e. i divide the same time span into a different amount of steps) the result varies pretty strong. I have 2 question:

1. The reason for the difference should be the dependency between the time step size and the element size right? This would mean, that if my time steps are smaller than the time it takes for the temperature to travel through one finite element the result should always be the same?

2. Is there an automated way within fluent to select the appropriate time step?

Thanks a lot in advance

mohammadkm October 29, 2011 03:53

Dear norman
1)You should decrease the time step until the difference between the restults become independent of time step.
2)in implicit model there is no criteria for time step

karthickeyan December 15, 2011 06:03

simple logic
 
time step is element size/velocity.which should not be equal to 1




Quote:

Originally Posted by norman_bates (Post 329335)
Hello everyone,

im quite a rookie at fluent and after spending a lot of hours trying to find a solution to my problem i thought i could share iht with you. The problem im trying to solve is just a simple transient heat transfer problem. I define one part of a sphere to be warmer than the rest and watch the temperature to disperse.
When i now solve this for the same time period but using different time steps (i.e. i divide the same time span into a different amount of steps) the result varies pretty strong. I have 2 question:

1. The reason for the difference should be the dependency between the time step size and the element size right? This would mean, that if my time steps are smaller than the time it takes for the temperature to travel through one finite element the result should always be the same?

2. Is there an automated way within fluent to select the appropriate time step?

Thanks a lot in advance



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