Units (mass & force)

Replies to This Discussion

Permalink Reply by Petr Skala on November 1, 2012 at 3:32pm

I am afraid that Kangaroo dont use any exact units system.

Permalink Reply by Daniel Piker on November 1, 2012 at 4:57pm

Not so! - See my answer below. I've just never gotten around to writing them up until now!

Permalink Reply by Daniel Piker on November 1, 2012 at 4:56pm

Let's see, it's just a question of being consistent...

Assuming your Rhino dimensions are in metres (m),

then your mass is in kilograms (kg),

and your force in newtons (N)

Then for your axial spring stiffness use EA/L 

where E is Young's modulus in pascals (Pa)

A is cross-sectional area in square metres (m²⁾

and L is the length of your spring in metres (m)

and timestep is in seconds (s)

a few things to bear in mind:

The speed at which your simulation runs at might not be 1 second per second, depending on the complexity of the simulation, the speed of your computer and the timer delay setting. Multiply the timestep by the iteration count to get the time which has elapsed in your simulation.

Using true values for E corresponding to stiff materials such as steel will often cause problems with simulation stability, unless you reduce the timestep and/or increase the mass of your particles (often necessarily by several orders of magnitude).

Remember that this inertial mass is separate from gravitational mass, and will not affect the final displacement of your system, only how long it takes to get there, so you can use a high fictitious mass to keep the simulation stable without changing things like the final displacement of your structure. This is a common technique in dynamic relaxation.

For loads due to gravity (in m/s²⁾, multiply the true mass of your particle by 9.81 (or thereabouts, depending on where exactly you are!) and apply it as a Unary force to the particle (for a linear element, just apply half its weight to the particle at each end).

 

 

 

Permalink Reply by Rupert Todd on November 1, 2012 at 5:21pm

Thanks!

That's brilliant, thanks Daniel, really appreciate it, time to dig out my old tables of moduli!  As a more general side note, Kangaroo is ace, cheers for that also!

All the best!

Rup

Permalink Reply by M NICK on November 26, 2012 at 1:26pm

When we are using meter for length, E is in Pa or GPa?

For example E for Steel= 200 GPa, if A=0.2 m2 and L=1 m, K= (200 x 10^9 x 0.2) / 1  Does it make sense?!!!

Permalink Reply by Michael Roberts on December 3, 2012 at 1:55pm

For those using non-metric, Alternate units seem to work as ft for length, force, kips, Area (ft^2). Or, use # instead of kips: Depend on E, G, etc. ;)

As long as you are consistant with the units throughout, it should all add up.

Also, increase of the particle mass does help the system converge when stiffness values get larger.

I though that additional mass particle meant more load in the model, but no, only inertial.

Very Cool Work DP!