MASSAGE

MASSAGE
MASSAGE creates, annihilates, and moves nodes and swaps connections in a 2D or 3D mesh in order to improve element aspect ratios and establish user-desired edge lengths.

Specifically, MASSAGE performs up to four iterations of a loop which calls AGD3D (a routine for automated merging of nodes), RECON (a routine for automated reconnection of edges), and SGD (a routine for element aspect ratio improvement using smoothing). MASSAGE then calls CEL_CHAIN which performs Rivara edge refinement and then another call to RECON. In the case of 2-D surface grids, this is then followed by a call to CER_CHAIN which is another edge refinement routine and then a final call to RECON if necessary.

AGD3D will attempt to merge edges that are shorter than merge_length. CEL_CHAIN will attempt to bisect edges that are longer than bisection_length. For 2-D surface grids, CER_CHAIN will attempt to bisect edges that deviate from an averaged surface normal ("have a roughness of") greater than tolroughness. RECON will attempt to create 'nice' elements by using face swapping. (The LaGriT command MODE/RECON can alter the meaning of 'nice'. The default is to reconnect to restore the delaunay criterion. Calling MODE/RECON/GEOM prior to the MASSAGE call will create 'plumper' elements). SGD will attempt to improve element aspect ratios by moving nodes.

The actions of MASSAGE are controlled by values of these four parameters:

bisection_length - edge length that will trigger bisection.

merge_length - edge length that will trigger merging.

toldamage - maximum grid deformation of interfaces and external boundaries allowed in a single merge, smooth or reconnection event.

tolroughness - (for 2D surface grids only) measure of grid roughness (deviation from average surface normal) that triggers refinement.

bisection_length can either be a scalar value or a node field (a node attribute). In the first case, the algorithm directly compares the edge length to the bisection_length value. If the edge length is greater than the bisection_length, the edge will be refined. In the second case, the algorithm compares the edge length to the minimum value of the field at the two nodes incident to this edge. If the edge lenght is greater than this minimum value, the edge will be refined. Thus, one should put a minimum floor value (probably equal to twice the desired minimum edge lenth) for the field. Otherwise the code will refine indefinitely. For an example of an appropriate field, see MASSAGE2 at the end.

toldamage is a parameter which controls how much the grid will be deformed. The 'damage' is a measure of how much interfaces and external boundaries are deformed. Roughly, it measures the depths of 'dents' that are invariably introduced when nodes are moved, annihilated, and faces are swapped. We guarantee that the damage of any single node movement, node annihilation, or face swap is bounded by toldamage. So if toldamage is set to an extremely small number, one can expect hardly any node movements, annihilations, or face swaps will be allowed. Conversely, if toldamage is set too large, physical interfaces may be significantly deformed by the action of MASSAGE. Experience has shown that setting toldamage equal to approximately .01 times the diameter of the mesh frequently gives acceptable results.

The guidelines for selecting bisection_length, merge_length, toldamage , and tolroughness are as follows. bisection_length should not be smaller than merge_length, or the action of merging nodes together will be largely pointless because the newer, longer edges created by merging will simply be bisected again. In fact, merging all edges of length > merge_length together can easily create edges of length roughly 3*merge_length in the mesh. Hence it is recommended that bisection_length be at least three times as large as merge length.

Merges of edges of length <= merge_length are meant to coarsen the mesh, but are not meant to deform surfaces and material interfaces on this scale. The amount of material/surface deformation (toldamage) is meant to be considerably less than merge_length.

On the other hand, the maximum roughness tolerated in the graph (tolroughness) should be considerably more than toldamage, or roughness refinement will be triggered by actions such as flipping or merging.

Hence, our guidelines for selecting the parameters are:
bisection_length >= 3*merge_length>> toldamage
tolroughness >= 10*toldamage (for 2-D surface grids).
For example, for a grid with diameter of order three, we have used:

bisection_length, merge_length, toldamage, tolroughness =.3, .1, .01, .1

If one of {bisection_length, merge_length} is omitted, the omitted one will be set so that bisection_length=3*merge_length.
If they are both omitted, they will both be taken to be infinity.
If toldamage is not specified, no node annihilation will take place.
If tolroughness is not specified, no refinement on roughness will occur and thus the format is compatible with old decks where refinement on roughness did not occur.

The final, optional keywork argument(s) can be one or more of nosmooth, norecon, lite, ignoremats, strictmergelength, checkaxy, semiexclusive, and exclusive.

Specifying nosmooth will turn off the 'smooth' step by skipping the call to SGD.
Specifying norecon will turn off all 'recon' steps.
If lite is specified, only one iteration of the merging/reconnection/smoothing loop is executed, and a reconnection after edge refinement is omitted. This is suitable for applications, such as Gradient Weighted Moving Finite Elements, where MASSAGE is called repeatedly.
The optional argument ignoremats causes MASSAGE to process the multimaterial mesh in a single material mode; it ignores the material interfaces.
The optional argument strictmergelength forces strict interpretation of merge_length so that there is no merging along the edges of flat elements. This is important if ignoremats is specified to avoid losing the interfaces.
If checkaxy is given, then we insure that for 2D meshes, the output mesh will have positive xy-projected triangle areas, provided that the input mesh had them in the first place.
If exclusive is given, then edge refinement operations will only be performed on edges whose endpoints are both in the PSET that MASSAGE is working on. (As usual, new nodes created by refinement are added to the PSET so that MASSAGE can refine edges recursively.) The default behavior is 'inclusive', where only ONE edge endpoint has to belong to the PSET for the edge to be eligible for refinement.
If semiexclusive is given, refinement will only be triggered by edges with both endpoints in the PSET, but some edges with less than two endpoints in the PSET might be refined as part of a 'Rivara chain' triggered by the refinement of an edge with both endpoints in the PSET. This represents an intermediate case between 'inclusive' and exclusive

Note: Since CEL_CHAIN is called only once at the end of MASSAGE, it may be necessary to call MASSAGE twice for optimal results. This is because annihilation of nodes is done with an intent to improve element aspect ratios, but cannot be effective if there are too few nodes initially.

Note: The user may wish to issue a "RMPOINT/COMPRESS" after MASSAGE operations that merge a significant number of nodes.

FORMAT:
massage/bisection_length/merge_length/toldamage/[tolroughness]/[ifirst,ilast,istride]/
[nosmooth]/[norecon][strictmergelength]/[ignoremats]/[lite]/[checkaxy]/[semiexclusive]/[exclusive]

EXAMPLES:
massage/0.3/0.1/0.01/
Mesh edges longer than 0.3 will be bisected; mesh edges shorter than 0.1 might be collapsed if that causes damage (normal surface motion) to material interfaces or external boundaries less than 0.01 ; smoothing of nodes causing damage less than 0.01 is allowed ; face swapping causing damage less than 0.01 is allowed.
massage/H_SCALE/0.1/0.01/
Same as above, except that the bisection_length is a node field called H_SCALE in this case.
massage/0.3/0.1/0.01/0.1/
Same as above but for 2-D surface meshes, roughness greater than 0.1 will trigger refinement.

massage/0.3/0.1/0.01/pset,get,pset1
Mesh edges (containing at least one endpoint in pset1) longer than 0.3 will be bisected; mesh edges shorter than 0.1 might be collapsed if that causes damage (normal surface motion) to material interfaces or external boundaries less than 0.01 and if the annihilated node is in pset1; smoothing of nodes in pset1 causing damage less than 0.01 is allowed; face swapping causing damage less than 0.01 is allowed (unfortunately, LaGriT at this time does not restrict swapping to pset1).

massage/0.3/0.1/0.01/pset,get,pset1/nosmooth
As above, but without smoothing.

massage/1.e+20/0.1/0.1/1,0,0/nosmooth
Because of the virtually infinite value of bisection_length,no edges will be bisected. Since merge_length=toldamage=0.1, merging of edges of length less than 0.1 will be considered, and will not be rejected because of excessive damage. Hence we expect that all edges of length less than 0.1 will be merged away (except in those cases where merging would invert tetrahedra or change material topology). Because nosmooth is specified, no smoothing will take place. Face swapping causing damage less than toldamage is allowed

massage/1.e+20/1.e-9/1.e-9/1,0,0/nosmooth/strictmergelength/ignoremats
This set of arguments will remove degenerate elements from a mesh by merging nodes that have the same coordinate values ( within 1.e-9).