Units in Ansys

sisquet

Hi!

I'm doing my master thesis about designing a offshore wind turbine tower for subarctic conditions and I have a major problem with units

I'm not much familiarised with Ansys, so I use Ansa as preprocessor and launch the solve in Ansys

The problem I have is in units. I'm using Ansys SI units but tweaked
Long. cm
Mass. Kp
Time. s

and therefore all units combinig those
P Kp/cm^2
dens Kp/cm^3
Force kp/cm·s^-2

the problem is that, my supervisor keeps on saying that for density and MASS21 elements i should use t/cm^3 and t and I don't like the reasons he has given me about it. Furthermore when i run a Block lanzos nodal calculation my first mode is around 0.3E-1 which is far away from expected result of 0.3Hz.
So when i run with density 7.85E-3 i and i tell ansys so extract 4 first modes i got in comand window like, not kidding, +400 solutions and this before cancelling the calculation after 20min when with the other density doesnt take more than 1 min to find results

Im kind of stuck, cause if the nodal is not correct it makes no sense that i waste time trying to optimise an estatic and dynamic model

Please any Ansys guru could help? thanks a million

AlephZero

My advice woud be stop wasting your time with these questions and just use basic SI units.

Or if you really want to do this, make a simple model in basic SI units, then convert it you your own unit system and check the answers are the same. (And if they aren't the same, go back to my first piece of advice).

Mech_Engineer

I agree with AlephZero, you're better off just using standard units and converting back to whatever you want AFTER you've gotten your solution.

sisquet

Hi guys!

thanks a million with your answers!! did a quick change to SI units and works perfect!! I think gonna keep with SI units...the problem will be to convince my supervisor about it...cause he was not happy of having results in Pa or J, but whatever, I'm done in going crazy as to set a consistend set of units

Problem solved, mod can close the thread

Mech_Engineer

You can convert the results to any set of units your professor wants, but I'm having trouble understanding why he would want anything other than standard SI...

sisquet

to get results in MPa is my guess...

nvn

sisquet: You can use either of the following sets of consistent units.

(1) newton (N), meter (m), second (s), pascal (Pa), kilogram (kg), joule (J).
(2) newton (N), millimeter (mm), second (s), megapascal (MPa), tonne (t), millijoule (mJ).

Item 1 is standard SI. Item 2 is my favorite. If in doubt, use item 1. Avoid cm.

If you use item 2, then the units of density and mass are t/mm^3 and t, respectively. Therefore, e.g., your density in post 1 would then become 7850.0e-12 t/mm^3, or 7.85e-9 t/mm^3.

By the way, some of your unit symbols are misspelled in post 1. Always use correct spelling and capitalization of units. E.g., the unit symbol for kilopond is kp, not Kp. Kp means kelvin pico.

However, completely avoid using kilopond; it is inconsistent and incoherent.

Also, always leave a space between a numeric value and its following unit symbol. E.g., 0.3 Hz, not 0.3Hz.