Ansys Time step size in Ansys workbench

[Cylo55]

Hello Gents,

I ususally utilize smaller increment time steps to gradually place a load on a solid for example. My question is......... Why doesn't ANSYS after utilizing the minimum time step you indicated and converging through that step, ............attempt to use a larger time step size to finish the solution faster, being that the simulation stabilized.

I hope that made sense.

Thanks for any advise in advance

 

[CESG]

You've already answered your question. Ansys does it in order to try to solve the problem quicker.

There are many ways to avoid that. I will try to explain some of them:

set the substeps size to a fixed value, by modifying "Analysis Settings" > "Step Controls" "Auto Time Step" to "OFF" and choosing a fixed substep size (or number of substeps) or;

set a range for the substep size by setting the same parameter to ON and choosing a range for the substep size (or number of substeps).

 

[Cylo55]

Thank you for your reply.

I usually do turn off auto time stepping, and indicate myself the initial time step, minimum time step and maximum time step.

But my problem with the program, is that if it reaches the minimum time step during the solution, and converges, why doesn't the program attempt to increase the time step size to a normal value that can converge but doesn't need that minimum time step that it utilized just for that instance.

If i reiterated what i just said in a different way, and your answer still applies, please forgive me.

Thanks

 

[CESG]

Oh, I think I got your point.

Ansys do increase the timestep increment but only after some substeps tend to converge within a small number of iterations.

In Ansys Manual the Chapter 15.7 of Theory Reference (try this path on help: //Theory Reference//15. Analysis Tools//15.7. Automatic Time Stepping) is described how Ansys calculates the time step increment. Basicaly what it described there is that the next time step will be increased or decreased at most of a factor by 2 and will never be greater or smaller than limits that are set by DELTIM command.

From my experience Ansys increases the time step after some successful fast converging substeps and it tends to do it by the ratio of 1.5 the actual time step.

So, in resume, if you think Ansys is not increasing the timestep as much as you want try to reduce the value of DTMAX parameter of DELTIM command.


MSc Carlos Simoes
Mechanical Engineer

 

[Cylo55]

Thanks alot Carlos, thats exactly what i was referring to. Too bad, cause alot of my simulations start off needing a small time step and then don't really require all those little iterations.

Anyway,

Thank you