Problem with units, Normal Modes. 1

[bocchiardoje]

Hello, this is my first thread.
I want to calculate the normal modes of a structure.
Im working the model in millimeters [mm], the mass in kilogram [kg] for Mass elements, and the Stiffnes Modulus (E) in [daN/mm2] for material and the density in [kg/mm3]. So, the forces result in daN -> (10 N are 1daN).
I have a couple of springs in my model too and the Stiffnes in [daN/mm].
If the natural frecuencies are: Fn=(1/2*PI)*SRQT(K/M) in Hertz [Hz] or [1/seg]where K (Newton/meters) is the stiffnes of springs and M the mass in Kilogram.
Using my unit sistem (daN/mm, mm, and Kg)... My question is this: I have to multiplicate the result (Hz) in 100??? For consistence units
Only have accurate results in frecuencies when use Newton, meters and Pascal.
Sorry for my poor english! Thanks.

 

[ThomasH]

Hi

You can find examples of consistent units at this web page:

http://femci.gsfc.nasa.gov/units/index.html

I have not studied your example in detail but based on the web page I doubt they are consistent. Why don't you try something simple? Be aware that Newton is kg*m/s^2 and so on.

Good Luck

Thomas

 

[rb1957]

replace 1dN with 10N ... use consistent units

 

[rb1957]

oh, and 1mm = 0.001m ... again, consistent units.

else if K is dN/mm, and M is kg then the consistent K = 10N/0.001m = 10^4 N/m

 

[bocchiardoje]

Thanks ThomasH and Rb1957. Rb1957, yes I do that, so; sqrt(10^4)=100, all my results multiplied by 100.

 

[BlasMolero]

Hello!,
I you adjust your model using the following units then the normal modes results (Hz) will be OK:

• Model length in millimeters [mm].
• Mass in metric Tons [Ton] to enter mass for any lumped Mass element like "CONM2"
• Young Modulus (E) in [MPa, ie, N/mm2]
• Density in [Ton/mm3].
• Loads in Newtons, and then Stiffnes in N/mm

Using the above units scheme is very convenient to work with models in milimeters.

Best regards,
Blas.

~~~~~~~~~~~~~~~~~~~~~~
Blas Molero Hidalgo
Ingeniero Industrial
Director

IBERISA
48011 BILBAO (SPAIN)
WEB:

http://www.iberisa.com

 

[rb1957]

aren't you also mixing units, using tonne for mass, not kg ?
also kN for force would be consisent

but why not use metric the way it was meant to be ... N, kg, m, sec ... who cares what the length unit is ... 1m or 1000mm looks the same on the screen

 

[BlasMolero]

Yes, you can use the system of units you like, but "mm" and MPa is very convenient (the same as "mm" and "mN/mm2"), nothing to see with Pa (ie, N/m2) and "meters".

Yes, in fact, the geometry looks the same in the screen, but a pressure or stress = 100 MPa looks in the screen as 0.1e9 N/m2 .... this is the problem.

You can run the way you like, the important is to be coherent system of units.
Bst regards,
Blas.

~~~~~~~~~~~~~~~~~~~~~~
Blas Molero Hidalgo
Ingeniero Industrial
Director

IBERISA
48011 BILBAO (SPAIN)
WEB:

http://www.iberisa.com