Difference in directional displacement for the same mesh and model settings

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Tayyaba posted this 6 days ago

Hi all,

I hope you are doing well,

I am doing FSI analysis of a thin flap, I repeated my simulations for the same geometry,mesh, boundary conditions and Fluent model but I found a difference in directional deformation of the flap although bending of the flap is the same. I am confused why difference is occurring in the directional deformation although everything is same as the previous simulations.

Can anybody help in this regard?

Tayyaba

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rwoolhou posted this 6 days ago

Please post some images. 

Tayyaba posted this 6 days ago

Here I am attaching the total deformation and directional deformation of the flap. FSI 1 is the initial simulation I have done for 10sec and FSI2 is the repeated

simulation just completed for 6sec. but there is a clear difference in directional deformation. I do not know why is it so , therefore a kind response is needed.

 

Regards

Tayyaba

 

 

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rwoolhou posted this 6 days ago

That looks very similar to what I'd expect for differing levels of convergence or solver settings (ie first to second order). Check the solution has converged properly and run again with a refined mesh & smaller time steps (plus more iterations per step). 

Tayyaba posted this 6 days ago

Thank you very much for your reply, But I have done the both simulations for the second order. Do you mean to change the time step and mesh for the second simulation?But I need results at the same time step and mesh as i have done but getting difference in z direction deformation which shouldn't be happen.

Regards

Tayyaba

Tayyaba posted this 5 days ago

I just realized that the difference is in coupling iteration for the previous and this case,initially it was 10 and now it is5. So I think this is a factor which is changing the results.

Tayyaba

rwoolhou posted this 5 days ago

So you're not updating tools as frequently? Means the mesh/flow will be slightly different, and in a transient model the error will accumulate. 

Tayyaba posted this 4 days ago

Thank you very much for your reply, could you please explain your point a little bit further, I didn't understand that.

 

Regards

Tayyaba

rwoolhou posted this 3 days ago

If we run a transient solution and slightly fail to converge a step we get an error. As time progresses the error slowly builds until it's large. At no point do we necessarily fail the convergence check, because each step is nearly OK. 

Simple equivalent. You carry a sack of grain down the road. Every 10 paces I add an extra grain. You won't notice the difference but after a mile or so the sack will be far heavier than it was at the beginning. 

Tayyaba posted this yesterday

Thank you very much for your explanation. Yes you are right, but in my transient simulation, solution is converging at each coupling iteration. And my understanding is because of difference in coupling iteration, there is a difference between directional deformation of the two simulations performed. Is this a matter?

and sorry but I did not understand the last statement of you "At no point do we necessarily fail the convergence check, because each step is nearly OK"

Regards

Tayyaba

 

rwoolhou posted this yesterday

You've not failed the convergence checks, but have you fully converged the calculation?

Effectively by adding more sub steps or by reducing the time step we force the solver to calculate a more accurate solution: similar to using a finer mesh for a mesh dependency study. 

Tayyaba posted this 2 hours ago

Thank you very much for your explanation,

did you mean the following convergence check for system coupling?

Regards

Tayyaba

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