dubstar-04 wrote:Am i over simplifying this?
I think so. First of all please don't pick SI units. Either you use the whole SI-System or don't mention that some of the units you pick are part of the SI-Sytem. The question is which are base units and which are derived ones. As soon as you make mm the length unit, you leave the SI-System behind. To make your new system consistent you need to change all derived units that have a dimension "length" to get coherent derived unit.
The unit system used "for" the solver should be consistent
dubstar-04 wrote:from experience the solvers generally don't understand units so you could even have the user select what units they want.
The fact that the solvers don't recognize units, does not mean that the programmer or user can do what he/she wants. All the solvers manuals tell to use consistent unit systems.
On letting the user choose: This is second step. The first step is to provide a usable and scientifically correct default.
This means that if someone later decides to use calculix for fluid simulations, he can still use our unit system.
If this works well we can make the units user configurable. But to me this would mean to check if the user selected/provided a consistent unit system and to refuse to work if he/she did otherwise.