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]{style=”font-weight: bold;”.  CEL_CHAIN will attempt to bisect edges that are longer than [bisection_length]{style=”font-weight: bold;”.  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]{style=”font-weight: bold;”. RECON will attempt to create ‘nice’ elements by using face swapping.  (The LaGriT command [MODE]{style=”font-weight: bold;”/[RECON]{style=”font-weight: bold;” can alter the meaning of ‘nice’.  The default is to reconnect to restore the delaunay criterion.  Calling [MODE]{style=”font-weight: bold;”/[RECON/GEOM ]{style=”font-weight: bold;”prior to the [MASSAGE ]{style=”font-weight: bold;”call will create ‘plumper’ elements).  SGD will attempt to improve element aspect ratios by moving nodes.

The actions of [MASSAGE]{style=”font-weight: bold;” are controlled by values of these four parameters:

[toldamage]{style=”font-weight: bold;” 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]{style=”font-weight: bold;”. So if [toldamage]{style=”font-weight: bold;” is set to an extremely small number, one can expect hardly any node movements, annihilations, or face swaps will be allowed.  Conversely, if [toldamage]{style=”font-weight: bold;” is set too large, physical interfaces may be significantly deformed by the action of MASSAGE.  Experience has shown that setting [toldamage]{style=”font-weight: bold;” equal to approximately .01 times the diameter of the mesh frequently gives acceptable results.  

The guidelines for selecting [bisection_length]{style=”font-weight: bold;”, [merge_length]{style=”font-weight: bold;”, [toldamage]{style=”font-weight: bold;” , and [tolroughness]{style=”font-weight: bold;” are as follows.  [bisection_length]{style=”font-weight: bold;” should not be smaller than [merge_length]{style=”font-weight: bold;”, 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]{style=”font-weight: bold;” together can easily create edges of length roughly 3 *[merge_length]{style=”font-weight: bold;” in the mesh.  Hence it is recommended that [bisection_length ]{style=”font-weight: bold;”be at least three times as large as merge length.

Merges of edges of length <= [merge_length]{style=”font-weight: bold;” 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]{style=”font-weight: bold;”) is meant to be considerably less than [merge_length]{style=”font-weight: bold;”.

On the other hand, the maximum roughness tolerated in the graph ([tolroughness]{style=”font-weight: bold;”) should be considerably more than [toldamage]{style=”font-weight: bold;”, or roughness refinement will be triggered by actions such as flipping or merging.

Hence, our guidelines for selecting the parameters are:

[bisection_length]{style=”font-weight: bold;” >= 3 *[merge_length]{style=”font-weight: bold;”>> [toldamage]{style=”font-weight: bold;”

[tolroughness]{style=”font-weight: bold;” >= 10 *[toldamage]{style=”font-weight: bold;”  (for 2-D surface grids).

For example, for a grid with diameter of order three, we have used:

[bisection_length]{style=”font-weight: bold;”, [merge_length]{style=”font-weight: bold;”, [toldamage]{style=”font-weight: bold;”, [tolroughness]{style=”font-weight: bold;” =.3, .1, .01, .1

If one of {[bisection_length]{style=”font-weight: bold;”, [merge_length]{style=”font-weight: bold;” is omitted, the omitted one will be set so that [bisection_length]{style=”font-weight: bold;”=3 *[merge_length]{style=”font-weight: bold;”.

If they are both omitted, they will both be taken to be infinity.

If [toldamage]{style=”font-weight: bold;” is not specified, no node annihilation will take place.

If [tolroughness]{style=”font-weight: bold;” 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 [nosmoot]{style=”font-weight: bold;”h, norecon, [lite, ignoremats, strictmergelength, checkaxy, semiexclusive]{style=”font-weight: bold;”, and [exclusive]{style=”font-weight: bold;”. 

- Specifying [nosmooth]{style="font-weight: bold;" will turn off the 'smooth' step by skipping the call to SGD. - Specifying [norecon]{style="font-weight: bold;" will turn off all 'recon' steps. - If [lite]{style="font-weight: bold;" 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]{style="font-weight: bold;" causes **MASSAGE** to process the multimaterial mesh in a single material mode; it ignores the material interfaces.  - The optional argument [strictmergelength]{style="font-weight: bold;" forces strict interpretation of [merge\_length]{style="font-weight: bold;" so that there is no merging along the edges of flat elements.  This is important if [ignoremats]{style="font-weight: bold;" is specified to avoid losing the interfaces. -  If [checkaxy]{style="font-weight: bold;" 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]{style="font-weight: bold;" 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]{style="font-weight: bold;" 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]{style="font-weight: bold;" 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]{style="font-family: monospace;"]/[ifirst,ilast,istride]/     [**nosmooth**]/[**norecon**][**strictmergelength**]/[**ignoremats**]/[**lite]**/[**checkaxy**]/[**semiexclusive**]**/[exclusive**] **EXAMPLES:** **massage**/[0.3/0.1/0.01]{style="font-family: courier new,courier,monospace;"/  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**/[[0.3/0.1/0.01/0.1]{style="font-family: courier new,courier,monospace;"/]{style="font-family: courier;" Same as above but for 2-D surface meshes, roughness greater than 0.1 will trigger refinement. **massage**/0.3/0.1/0.01/[]{style="font-family: courier;"**pset,get,**[pset1]{style="font-family: courier new,courier,monospace;" 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[,]{style="font-family: courier new,courier,monospace;"**[pset1]{style="font-family: courier new,courier,monospace;"**[/]{style="font-family: courier new,courier,monospace;"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]{style="font-weight: bold;",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]{style="font-weight: bold;" is allowed **massage**[/1.e+20/1.e-9/1.e-9/1,0,0]{style="font-family: courier new,courier,monospace;"**/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).