Hello everyone,
I am trying to run my explicit Ls Dyna model by defining a time step size that would be constant throughout the simulation. So far I have only managed to control the time stepping using the DT2MS parameter in the *CONTROL_TIMESTEP card but it comes with mass scaling. Is there any way to assign an arbitrary fixed time step size throughout the termination time without invoking mass scaling in Ls Dyna? I would highly appreciate any suggestions and insight in this regard.
Thanks,
Shahriar.
In LS-DYNA, the time step is calculated for each individual element in your mesh. The process works like this:
- For each element, LS-DYNA calculates its characteristic length (typically the smallest distance across the element) and uses this along with the material properties (like wave speed) to determine that element's critical time step
- The program then looks at ALL elements in your model and finds the smallest critical time step among them. This becomes your global time step, because the simulation needs to use a time step that's stable for every single element
This is why mesh quality is so important - if you have just one very small or distorted element, it can force the entire simulation to use a tiny time step, significantly increasing computation time.
For example: A nice cubic element might allow a time step of 1e-6 seconds. But if you have one highly distorted or very small element, it might require 1e-8 seconds. The whole simulation would then have to run at 1e-8 seconds to maintain stability.
So to combat this, you can set a minimum timestep in LS-Dyna (DT2MS) which will add mass to elements with timestep below that, which increases their timestep to match the set value.
What is your goal in trying to control every element to have the same constant timestep? If this is a class question, you can accomplish this if every element in your model is the same size / same minimum edge length.