Mesh stripping and shape optimization procedures may result in some of the finite elements becoming extremely tiny/thin, seriously distorted, or kinematically unstable (wobbly). To some extent such negative effects can be repaired by mesh cleaning, which aims to remove the most problematic elements. In this way the FE mesh can be kept in good condition for further shape optimization.
Tiny or thin finite elements are characterized by extremely short edges, while the distorted elements are characterized by extremely low geometrical quality. Meanwhile, kinematically unstable elements are characterized by two or more free and unsupported nodes, which makes them at least to some extent freely movable (wobbly). ProTOp contains three cleaning modes aimed at removing such problematic elements.
The mesh cleaning process is an extremely non-linear process, typically requiring many cycles. Due to its highly non-linear nature, the stopping criteria are not easily definable, especially with regard to tradeoff between achievable removals and computation time. In practice this means that it might be worth running the process more than once. In such a case, a few recommendations can be given to get the maximal mesh reparation, as follows.
General engagement guidelines
Procedure guidelines
NOTE. Adjust carefully the operation parameters and run the cleaning process several times until no further removals of the elements can be obtained.
Relative edge length threshold
Range: [0.05, 0.50]
This parameter sets the relative edge length threshold for an element to qualify for a potential removal. More precisely, an element may be removed if its shortest edge is shorter than the average element edge, multiplied by this threshold.
Maximal relative degradation
Range: [0.0, 0.3]
This parameter sets the maximal allowed quality degradation of an element which is being affected by removal of problematic elements. More precisely, a problematic element may be removed if its affected neighbors will degrade less than specified by this parameter. For example, a value of 0.2 means that the neighbor elements are allowed to degrade 20% at most.
Element quality threshold
Range: [0.05, 0.50]
This parameter sets the quality threshold for an element to qualify for a potential removal. More precisely, an element may be removed if its SI quality measure is lower than this threshold.
Surface edge threshold angle
Range: [0.1, 5.0]
This parameter is the threshold angle in degrees for a line between two element faces to be classified as a geometrical edge on the surface of the model. If the angle between the normals of the two faces is below this threshold, the faces are considered to belong to a smooth surface.
Max number of cycles
Range: [1, 64]
This parameter sets the maximum number mesh cleaning cycles. It can be set to a low value, for example 2 or 3, if for a large mesh we just need quick information whether any significant progress is possible.
Mesh cleaning operation effectively removes FE nodes and consequently also elements. Such operations may influence slightly the model geometry and boundary conditions, especially the applied loading. To prevent this, the user may declare various node sets such as, for example, the loaded surface nodes, as non-removable.