| Both sides previous revision
Previous revision
Next revision
|
Previous revision
|
faq [2022/10/26 15:23]
marijn.nijenhuis [3. Changing units] |
faq [2023/11/09 22:30] (current)
marijn.nijenhuis [3. Changing units] |
| |
| ===== 3. Changing units ===== | ===== 3. Changing units ===== |
| Whenever the system parameters are expressed using very small numbers, you may receive the warning "" or related error "". Changing the units of the parameters involving the length dimension may help. | When the system parameters are expressed with very small numbers, you may receive the warning "Mass coefficients turn out to be very small" or the related error "Singular mass matrix". Changing the units of the parameters involving the length dimension may help. |
| |
| The example file that comes with SPACAR Light assumes meters as standard unit for length. To change to other units, your script should be adjusted in several places: | The example file that comes with SPACAR Light assumes meters as the default unit for length. To change to other units, your script should be adjusted in several places, depending on how each physical quantity scales with the length L: |
| * coordinates of the nodes (''nodes''): default unit is m so the coordinates scale as L. | * coordinates of the nodes (''nodes''): default unit is m so the coordinates scale as L. |
| * input displacements (''nprops(i).displ_*'', ''nprops(i).displ_initial_*''): default unit is m so the displacements scale as L. | * input displacements (''nprops(i).displ_*'', ''nprops(i).displ_initial_*''): default unit is m so the displacements scale as L. |
| * (rotations in radians are independent of length) | * (rotations in radians are independent of length) |
| * moment of inertia (''nprops(i).mominertia''): default unit is kg*m^2, so moment of inertia scales as L^2. | * moment of inertia (''nprops(i).mominertia''): default unit is kg*m^2, so moment of inertia scales as L^2. |
| * forces (''nprops(i).force'', ''nprops(i).force_initial''): default unit is N = 1 kg*m/s^2 so force scales as L. | * forces (''nprops(i).force'', ''nprops(i).force_initial''): default unit is N = kg*m/s^2 so force scales as L. |
| * moments (''nprops(i).moment'', ''nprops(i).moment_initial''): default unit is Nm = 1 kg*m^2/s^2 so moment scales as L^2. | * moments (''nprops(i).moment'', ''nprops(i).moment_initial''): default unit is N*m = kg*m^2/s^2 so moment scales as L^2. |
| * material stiffness properties (''eprops(i).emod'', ''eprops(i).smod''): default unit is Pa = N/m^2 = kg/(m*s^2) so these scale as 1/L. | * material stiffness properties (''eprops(i).emod'', ''eprops(i).smod''): default unit is Pa = N/m^2 = kg/(m*s^2) so these scale as 1/L. |
| * material density (''eprops(i).dens''): default unit is kg/m^3 so density scales as 1/L^3. | * material density (''eprops(i).dens''): default unit is kg/m^3 so density scales as 1/L^3. |
| * cross-sectional dimensions (''eprops(i).dim''): default unit is m so these dimensions scale as L. | * cross-sectional dimensions (''eprops(i).dim''): default unit is m so these dimensions scale as L. |
| * gravitational acceleration (''opt.gravity''): default unit is m/s^2 so acceleration scales as 1/L^2. | * gravitational acceleration (''opt.gravity''): default unit is m/s^2 so acceleration scales as L. |
| | |
| | In other words, if your script assumes meters as the unit for length, and you want to go to millimeters, the ''nodes'' matrix should be multiplied by 10^3. Likewise, the forces should be multiplied by 10^3, the moments by 10^6, the elasticity and shear modulus by 10^(-3), and all other quantities according to the list above. |
| |
| Note that output values, so numbers you see in the ''out'' structure and in Spavisual, are affected similarly. | Note that output values, so numbers you see in the ''out'' structure and in Spavisual, are affected similarly. |
