How should the corresponding physical parameters change after the dimension changes from meters to millimeters?

Asked by weijie on 2020-06-03

Dear all,

After the dimension changes from meters to millimeters, how should physical parameters such as density, acceleration of gravity, Kn, Ks, viscousDamping, young and poisson change?

Thank you in advance for your advises and help,

Best Regards,
Jie

Question information

Language:
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Answered
For:
Yade Edit question
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Last query:
2020-06-03
Last reply:
2020-06-03
Jan Stránský (honzik) said : #1

Hello,

TLTR: if you have to ask, do not change it
There is no real need and often a confusion is a by-product (or the main product)..
Also such code is more error prone (one easily forget to transform some input/output).
**I personally** strongly suggest to stay in basic SI unit system.

each quantity has a physical dimension, to transform it to another unit system, just substitute the transformation relation (1 m = 1000 mm). For the quantities mentioned:

density:
kg/m^3 = kg/(1000*mm)^3 = kg/(1000^3 * mm^3) = 1/(10^9) kg/mm^3

acceleration, gravity:
m/s^2 = (1000 mm)/s^2 = 1000 mm/s^2

Kn, Ks (assuming N/m, just "Kn" is too ambiguous):
N/m = (kg*m/s^2)/m = kg/s^2 (no change)

young:
Pa = N/m^2 = (kg*m/s^2)/m^2 = kg/(m*s^2) = kg/(1000*mm*s^2) = 1/1000 * kg/(mm*s^2)

poisson: no change (is dimensionless)

cheers
Jan

I think both short and long versions in the first answer are perfect yet here is another possible answer (or comment):

> after the dimension changes from meters to millimeters

You cannot change from meter to millimeter in yade since yade is not in meter. My conclusion is that this question would fit in a physics forum. It doesn't contain any of the words "yade" or "dem" or "simulation". :)
B

> yade is not in meter
To be more clear yade is not in _any_ unit. The units are in our human brains, yade crunches numbers.
B

Jan Stránský (honzik) said : #4

@Bruno

> this question would fit in a physics forum

true, but it also fits here IMO (see below)

> The units are in our human brains, yade crunches numbers.

true again, but the numbers has some meaning. People (usually) does not run Yade (or any similar program) to just "crunch numbers", but to get some reasonable result. The physical units are part of this reasonability.

That the same numbers have infinite many other meanings (for infinite many physical units chosen and their combinations) is another story :-) usually not that important for specific simulation runs..

cheers
Jan

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