Cubit 16.18 User Documentation
The metrics used for hexahedral elements in CUBIT are summarized in the following table:
Function Name
|
Dimension
|
Full Range
|
Acceptable Range
|
Reference
|
Aspect Ratio
|
L^0
|
1 to inf
|
1 to 4
|
1
|
Skew
|
L^0
|
0 to 1
|
0 to 0.5
|
1
|
Taper
|
L^0
|
0 to +inf
|
0 to 0.4
|
1
|
Stretch
|
L^0
|
0 to 1
|
0.25 to 1
|
2
|
Diagonal Ratio
|
L^0
|
0 to 1
|
0.65 to 1
|
3
|
Dimension
|
L^1
|
0 to inf
|
None
|
1
|
Condition No.
|
L^0
|
1 to inf
|
1 to 8
|
5
|
| Mass Increase Ratio |
L^0
|
1 to inf
|
None
|
|
Node Distance
|
L^1
|
-inf to inf
|
None
|
|
Scaled Jacobian
|
L^0
|
-1 to +1
|
0.5 to 1
|
5
|
Shear
|
L^0
|
0 to 1
|
0.3 to 1
|
5
|
Shape
|
L^0
|
0 to 1
|
0.3 to 1
|
5
|
Relative Size
|
L^0
|
0 to 1
|
0.5 to 1
|
5
|
Shear & Size
|
L^0
|
0 to 1
|
0.2 to 1
|
5
|
Shape & Size
|
L^0
|
0 to 1
|
0.2 to 1
|
5
|
Timestep
|
Seconds
|
0 to inf
|
None
|
6
|
| High Order Metrics | ||||
Distortion
|
L^0
|
0 to 1
|
0.6 to 1
|
7
|
Element Volume
|
L^3
|
-inf to inf
|
None
|
1
|
Jacobian
|
L^3
|
-inf to inf
|
None
|
5
|
With a few exceptions, as noted below, Cubit supports quality metric calculations for linear hexahedral elements only. When calculating quality metrics, that only support linear elements, for a higher order hexahedral element, Cubit will only use the corner nodes of the element.
Aspect Ratio: Maximum edge length ratios at hex center.
Skew: Maximum |cos A| where A is the angle between edges at hex center.
Taper: Maximum ratio of lengths derived from opposite edges.
Stretch: Sqrt(3) * minimum edge length / maximum diagonal length.
Diagonal Ratio: Minimum diagonal length / maximum diagonal length.
Dimension: Pronto-specific characteristic length for stable timestep calculation. Char_length = Volume / 2 grad Volume.
Condition No. Maximum condition number of the Jacobian matrix at 8 corners.
Mass Increase Ratio: This metric stems from the global target time step and the element time step. The density required to fulfill the target time step (via mass scaling) divided by the block density is termed the mass increase ratio. Because the density within each element is constant, a ratio in the element density is equivalent to a ratio in the element mass. This metric calculates the requisite density for each element to attain the prescribed target time step. If that density is greater than the defined density, the metric yields a value greater than one. This desired global time step is set by the user with the command:
[Set] Target Timestep <value>
As stated, this metric computes the element based timestep metric and consequently element blocks must be defined with material properties of Young’s modulus, Poisson’s ratio, and a target timestep must be set.
If this metric is computed in the context of a block ('quality block 1 mass increase ratio') an accompanying printout of the mass increase per block is given.
Node Distance: Minimum distance between any two adjacent corner nodes.
Scaled Jacobian: For linear elements the minimum Jacobian divided by the lengths of the 3 edge vectors.
Shear: 3/Mean Ratio of Jacobian Skew Matrix
Shape: 3/Mean Ratio of weighted Jacobian Matrix
Relative Size: Min(J, 1/J), where J is the determinant of weighted Jacobian matrix
Shear & Size: Product of Shear and Size Metrics
Shape & Size: Product of Shape and Size Metrics
Timestep: The approximate maximum timestep that can be used with this element in explicit transient dynamics analysis. This critical timestep is a function of both element geometry and material properties. To compute this metric on hexes, the hexes must be contained in an element block that has a material associated to it, where the materials poisson's ratio, elastic modulus, and density are defined.
The preceding metrics will measure quality based only on the 8 corner nodes of the hexahedron. The following metrics also take into account the mid nodes.
Distortion: {min(|J|)/actual volume}*parent volume, parent volume = 8 for hex. Cubit also supports Distortion calculations for hex20 elements.
Element Volume: For linear hexes, the jacobian at hex center. For higher-order hexes, the hex is subdivided into sub-tets, the volumes of which are summed.
Jacobian: Minimum pointwise volume of local map at 8 corners at center of hex. Cubit also supports Jacobian calculations for hex27 elements.