Consistent Units for Thermo-Mech Analysis
Consistent Units for Thermo-Mech Analysis
(OP)
Hi all!
I have a question regarding the selection of suitable units for the calculation of a thermal-mechanical analysis. Inorder to have the stress in Mpa what should be the unit of:
-Specific Heat
-Conductivity
-Mass Density
If the units of force is N and the distances are in mm.
Thanks in advance,
Eser
I have a question regarding the selection of suitable units for the calculation of a thermal-mechanical analysis. Inorder to have the stress in Mpa what should be the unit of:
-Specific Heat
-Conductivity
-Mass Density
If the units of force is N and the distances are in mm.
Thanks in advance,
Eser





RE: Consistent Units for Thermo-Mech Analysis
Everything else then follows automatically.
Mass. Must be in kg (to be "consistent" with N for force).
Stress. Will be in N/mm^2 (ie MPa).
Acceleration. Will be in mm/s^2 (and this will apply to "g" as well).
Density. Will be in kg/mm^3.
Energy. Newtons times millimetres, ie millijoules (mJ).
Thermal conductivity. mJ/mm/s/degC.
Specific heat. mJ/kg/degC.
Etc.
RE: Consistent Units for Thermo-Mech Analysis
Length: mm
Mass: tonne (T)
Density: T/mm3
Time: second
Energy / Work: Joule
Power: Watt (=J/s)
Force: Newton
Modulus / Stress: N/mm2 (=MPa)
Temperature: Kelvin (for temperature differentials, 1 K = 1 degree C)
Conductivity: W/mm/K (= J/s/mm/K)
Specific Heat: J/T/K
Flux: W/mm2 (=J/s/mm2)
Convection: W/mm2/K (= J/s/mm2/K)
RE: Consistent Units for Thermo-Mech Analysis
(Now I know why my bridges fall UP.)
RE: Consistent Units for Thermo-Mech Analysis
Energy/Work: Joule = N*m
That is NOT consistent with N and mm. I have not studied the details but the proposed is not consistent as far as I can see.
Please correct me if I'm wrong.
Regards
Thomas
RE: Consistent Units for Thermo-Mech Analysis
Length unit: mm
Mass unit: tonne
Time unit: s
1 force unit = tonne x mm / s^2 = 1N
1 work unit = N x mm = 1E-3 J
1 stress unit = N / mm^2 = 1E6 Pa
1 density unit = tonne/mm^3 = 1E-12 kg/m^3
etc.
Now I have to hope I'm corrrect.
Eser, I assume that this is a set of units that you are not familiar with. Do you have to use this set of units? My experience of using a set of units that I am not familiar with is that it is easier to make a mistake and harder to spot it (see above).
Regards
James
RE: Consistent Units for Thermo-Mech Analysis
RE: Consistent Units for Thermo-Mech Analysis
There is, in my opinion, no "best" unit system. Use whatever system you are comfortable with. But so far I have never seen anybody who got it correct on the first try when creating a "new" system. There are a lot of misstakes to be made, unfortunately.
Good Luck
Thomas
RE: Consistent Units for Thermo-Mech Analysis
Oops! You are quite correct - in the N/mm system, the consistent unit of energy / work is the N.mm or the mJ (millijoule). the correct list is as follows - I hope!
Length: mm
Mass: tonne (T)
Density: T/mm3
Time: second
Energy / Work: millijoule (mJ)
Power: milliwatt (= mW = mJ/s)
Force: Newton
Modulus / Stress: N/mm2 (=MPa)
Temperature: Kelvin (for temperature differentials, 1 K = 1 degree C)
Conductivity: mW/mm/K (= mJ/s/mm/K)
Specific Heat: mJ/T/K
Flux: mW/mm2 (= mJ/s/mm2)
Convection: mW/mm2/K (= mJ/s/mm2/K)
It is for precisely this reason that i customarily use SI units whenever possible. Whenever you need a material parameter or similar, you can be sure that the data should be expressed in basic SI units. Getting such material properties in derived units can be problematical, to say the least!
RE: Consistent Units for Thermo-Mech Analysis
are we being careless with mass and force ...
the last time i checked a tonne was a force,
and THE unit for mass was the gram (i'll accept kg)
1N = 1g*1m/sec^2
we, over here on this side of the pond, still have the fun of using slugs as the imperial unit of mass
RE: Consistent Units for Thermo-Mech Analysis
Wrong - tonne is another way to write the Mg (1e6g or 1e3kg). Also, the Newton is defined as 1N=1[(kg*m)/(s^2)].
Also, it doesn't matter what you use as the BASE units:
length
time
mass
energy
temperature
so long as the derived units are CONSISTENT. Isn't that the definition of a consistent set of units. For all I care, one could use km as length, Mg as mass, year as time and GJ for energy, and K for temperature so long as the other derived units are consistent with this set of BASE units.
Perhaps a refresher on units is needed for some... For example, force is [M]*[L]/([t]^2), and pressure is [M]/([L]*[t]^2).
RE: Consistent Units for Thermo-Mech Analysis
That so few people seem to fully understand the meaning and use of units, especially the correct definition of the SI units. TGS4 is right in saying that "it doesn't matter what you use as the BASE units, so long as the derived units are CONSISTENT" and that your work is clearly marked with your choice of units, but straying from SI units to a derived set of metric units appears to create many problems for many people, whilst users of the Imperial system of measurement IMHO appear not to have the same problems. Is this because an "inch" is more natural and easier to work with than a "metre" ? In the mechanical world, how many 3D models and 2D drawings use the metre as the unit of measurement ? Every drawing or model I see uses the "millimetre" in the metric world or the "inch" in the Imperial world. Is it then any wonder that things go wrong in the metric world if the product definitions do not use SI units ?
RE: Consistent Units for Thermo-Mech Analysis
In my experience problems can occur when someone is comfortable with "his own" unit system and then work outside his/her normal scope. Stress engineer working with dynamics for example.
Newton and millimeters gives MPa but is usually the best choice for dynamics.
As for THE unit for mass it is kg, it's a flaw in the SI-system that means that the base unit has a prefix. As for tonne it's mass but tonne force is force.
For more on SI-system I recommend this:
http://physics.nist.gov/cuu/Units/units.html
and if you want to play with units:
http://www.digitaldutch.com/unitconverter/
Regards
Thomas
RE: Consistent Units for Thermo-Mech Analysis
Rant over.