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  • Query regarding Lesson 13 ‘Unsteady/Transient’ Behaviour

    Posted by Aditya on September 19, 2023 at 6:48 pm

    Hello everyone,

    I was trying to perform a transient analysis for the flow over a cylinder which was the assignment for this module. However, I am facing some issues in getting a convergent solution. My residuals keep on fluctuating even after trying both manual and automatic CFL numbers. Any help to solve my this issue would be greatly appreciated.

    The geometry is a 3d cylinder of 1 m diameter and 500mm length (it was given a symmetry constraint to make it 1 m long)

    The flow velocity was taken as 20 m/s with air as the medium. I tool density of air as 1.2 kg/m3 and dynamic viscosity as 1.81e-5 kg/(ms) hence it’s in the turbulent region.

    I chose a k-epsilon model to simulate the flow and the following are the graphs I obtained.

    I tried to do a steady state simulation for this and was getting non fluctuating residuals which was confusing because i thought this models was not supposed to give stable resduals when on steady state

    Sandeep replied 7 months ago 2 Members · 1 Reply
  • 1 Reply
  • Sandeep

    September 19, 2023 at 9:56 pm

    Hi Aditya,
    Your results look perfect. That is how you capture the fluctuations of drag, CD etc. as a function of time.
    Regarding the residuals is also correct.
    In transient simulations, you are marching step by step through the time steps (delta t). And for each delta t you will have multiple inner loop iterations ( by default in fluent is 20) sometimes you may have to use more. So with these 20 iterations, you should get converged residuals for that particular timestep. Then for the next time step, the residual will gain shoot up and will converge within those 20 inner loop iterations. This will repeat and that is what you are getting. Which is perfectly alright.

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