 ## General Mechanical

#### scale up geometry in cutting process (dynamics analysis) leads to smaller time to solve?!!!

• mona
Subscriber

hello!!

Mr. peteroznewman i readed this post "scale up geometry 1000 times. The reason for this is that the solver calculates a time step based on the dimension of the smallest element"

you mean that scale up just thick or all dimensions??

i know that time step in dynamics analysis based on the numbers of finite elements so the issue is  ratio and proportion

my question: if i have two models

first model : work piece with dimensions (10 x 5 x1) mm

second model: work piece with dimensions (100 x 50 x10) mm

which one will take bigger time to solve ???

• peteroznewman
Subscriber

Hello Mona,

There is an equation Explicit Dynamics uses to calculate the maximum stable time step. It is a function of length and the speed of sound in the material. The speed of sound is a function of density.  When you want to reduce the waiting time for the Explicit Dynamics solver to show something, it is much easier to increase the density by 100 or 1000 times than change the geometry.  To answer your question, the larger geometry will take less time to solve, but read my last paragraph.

I highlight the word something, because when you do this, you are not solving the original problem anymore, you have changed the physics by making the material so dense.

Sometimes I want a "cartoon" animation to show roughly what the end result might look like. I am happy to have a fast solve time so I can check that I have things properly built in the model.  Sometimes that cartoon animation is sufficient to show someone the motions, even though they might deviate from the true solution, and I don't need to wait for the true solution.

If I need to calculate engineering quantities from the solution, then I have to return the density to its original value and wait the required time.

Note that you can minimize your wait time and have accurate physics by carefully meshing the part to avoid a few small elements. There is a mesh metric called Characteristic Length that will highlight the few smallest elements that are dictating the maximum time step. Edit the geometry or the mesh controls in double the size of the smallest element and you will have cut the wait time in half.

• mona
Subscriber

thank you very much for reply

but i want to understand more,,, if length of the finite element (element size) is same for both the models

which one will take bigger time to solve ???

• peteroznewman
Subscriber

If the length of the smallest element is the same for both models, both models will take the same amount of time to solve.

• mona
Subscriber

thank you very much Mr peteroznewman 