Cubit
15.3 User
Documentation
Cubit
15.3 User
Documentation
The metrics used for quadrilateral 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 0.7
|
1
|
| Warpage |
L^0
|
0 to 1
|
0.9 to 1.0
|
NEW
|
Element Area
|
L^2
|
-inf to inf
|
None
|
1
|
Stretch
|
L^0
|
0 to 1
|
0.25 to 1
|
2
|
Minimum Angle
|
degrees
|
0 to 90
|
45 to 90
|
3
|
Maximum Angle
|
degrees
|
90 to 360
|
90 to 135
|
3
|
Condition No.
|
L^0
|
1 to inf
|
1 to 4
|
4
|
Jacobian
|
L^2
|
-inf to inf
|
None
|
4
|
Scaled Jacobian
|
L^0
|
-1 to +1
|
0.5 to 1
|
4
|
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
|
0.3 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
|
Distortion
|
L^2
|
-1 to 1
|
0.6 to 1
|
6
|
Aspect Ratio: Maximum edge length ratios at quad center
Skew: Maximum |cos A| where A is the angle between edges at quad center
Taper: Maximum ratio of lengths derived from opposite edges
Warpage: Cosine of Minimum Dihedral Angle formed by Planes Intersecting in Diagonals
Element Area: Jacobian at quad center
Stretch: Sqrt(2) * minimum edge length / maximum diagonal length
Minimum Angle: Smallest included quad angle (degrees).
Maximum Angle: Largest included quad angle (degrees).
Condition No. Maximum condition number of the Jacobian matrix at 4 corners
Jacobian: Minimum pointwise volume of local map at 4 corners & center of quad
Scaled Jacobian: For linear elements the minimum Jacobian divided by the lengths of the 2 edge vectors
Shear: 2/Condition number of Jacobian Skew matrix
Shape: 2/Condition number of weighted Jacobian matrix
Relative Size: Min( J, 1/J ), where J is determinant of weighted Jacobian matrix
Shear and Size: Product of Shear and Relative Size
Shape and Size: Product of Shape and Relative Size
Distortion: {min(|J|)/actual area}*parent area, parent area = 4 for quad
Shape, Relative Size, Shape & Size, and Shear are algebraic quality metrics that apply to quadrilateral elements. Cubit encourages the use of these metrics since they have certain nice properties (see reference 5 below). The metrics are referenced to a square-shaped quadrilateral element, thus deviations from a square are measured in various ways.
Shape measures how far skew and aspect ratio in the element deviates from the reference element.
Relative size measures the size of the element vs. the size of reference element. If the element is twice or one-half the size of the reference element, the relative size is one-half. The reference element for the Relative Size metric is a square whose area is determined by the average area of all the quadrilaterals on the surface mesh under assessment
Shape and size metric measures how both the shape and relative size of the element deviate from that of the reference element.
The SHEAR metric is based on the condition number of the skew matrix. SHEAR is really just an algebraic skew metric but, since the word skew is already used in the list of quad quality metrics, Cubit has chosen to use the word 'shear.'
Shear = 1 if and only if quadrilateral is a rectangle.
The Robinson 'skew' metric equals the ideal (zero) if the quad is a rectangle. It also attains the ideal if the quad is a trapezoid, a kite, or even triangular!
The quadrilateral element quality metrics that are calculated are aspect ratio, skew, taper, element area, and stretch. The calculations are based on metrics described in (Robinson, 87). An illustration of the shape parameters is shown in Figure 1, below. The stretch metric is calculated by dividing the length of the shortest element edge divided by the length of the longest element diagonal.
Figure 1. Illustration of Quadrilateral Shape Parameters (Quality Metrics)