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# NX Nastran Characteristic Element length

Experimenter

Hello,

I am currently performing a Explicit analysis (SOL701). For calculation the critical time step a characteristic length is used. In the NX Nastran Advanced Nonlinear Theory and Modeling Guide the characteristic element length is discribed as a function of the volume and area of the sides of the element. However the exact function is not given. Does anybody know how this characteristic element length is calculated?

3 REPLIES

# Re: NX Nastran Characteristic Element length

Community Manager

Mark3 wrote:

Hello,

I am currently performing a Explicit analysis (SOL701). For calculation the critical time step a characteristic length is used. In the NX Nastran Advanced Nonlinear Theory and Modeling Guide the characteristic element length is discribed as a function of the volume and area of the sides of the element. However the exact function is not given. Does anybody know how this characteristic element length is calculated?

Hi Mark3,

Here is what a colleague of mine says in response to your question:

The characteristic element length in an explicit analysis is – to my knowledge - not a scientifically derived quantity (not to be mistaken with the characteristic element length in damage mechanics!). Based on strict mathematics, it would be the length the pressure travels within the critical time increment. However, this would mean that one small element would kill the performance of the whole run. Therefore, an average of the element dimensions is calculated that allows minor violations of the stable time increment but where experience has shown that this does not cause inaccuracy.

Regards,
Ganesh

Simcenter 3D Community Manager
Simulation and Test Solutions

# Re: NX Nastran Characteristic Element length

Genius

Hi Mark3,

please reffer to the "critical time step".I am pretty confident that the thing you mean is the smallest element edge witch governs the critical time step value.

Regards,

Tomek.

# Re: NX Nastran Characteristic Element length

Siemens Valued Contributor

Guidance from Adina is as follows:

The computation of the characteritic length is involved. But for estimation purposes the length for an undistorted 8-node brick element is approximately the volume of the element divided by the largest element face area, and for an undistorted 4-node shell element, it would be approximately the area of the element divided by the longest element edge.