Generate Mesh and Split Its Elements into Two Groups

Create a PDE model.

modelSingleDomain = createpde;

Import the geometry.

importGeometry(modelSingleDomain,'Block.stl');

Generate and plot a mesh.

msh = generateMesh(modelSingleDomain);

figure
pdemesh(modelSingleDomain)

Obtain the nodes and elements of the mesh.

nodes = msh.Nodes;
elements = msh.Elements;

Find the x-coordinates of the geometric centers of all elements of the mesh. First, create an array of the same size as elements that contains the x-coordinates of the nodes forming the mesh elements. Each column of this vector contains the x-coordinates of 10 nodes that form an element.

elemXCoords = reshape(nodes(1,elements),10,[]);

Compute the mean of each column of this array to get a vector of the x-coordinates of the element geometric centers.

elemXCoordsGeometricCenter = mean(elemXCoords);

Assume that all elements have the same region ID and create a matrix ElementIdToRegionId.

ElementIdToRegionId = ones(1,size(elements,2));

Find IDs of all elements for which the x-coordinate of the geometric center exceeds 30.

idx = mean(elemXCoords) > 30;

For the elements with centers located beyond x = 30, change the region IDs to 2.

ElementIdToRegionId(idx) = 2;

Create Geometry with Two Cells

Create a new PDE model.

modelTwoDomain = createpde;

Using geometryFromMesh, import the mesh. Assign the elements to two cells based on their IDs.

geometryFromMesh(modelTwoDomain,nodes,elements,ElementIdToRegionId)

ans = 
  DiscreteGeometry with properties:

       NumCells: 2
       NumFaces: 108
       NumEdges: 205
    NumVertices: 100

Plot the geometry, displaying the cell labels.

pdegplot(modelTwoDomain,'CellLabels','on','FaceAlpha',0.5)

Highlight the elements from cell 1 in red and the elements from cell 2 in green.

elementIDsCell1 = findElements(modelTwoDomain.Mesh,'region','Cell',1);
elementIDsCell2 = findElements(modelTwoDomain.Mesh,'region','Cell',2);

figure
pdemesh(modelTwoDomain.Mesh.Nodes, ...
        modelTwoDomain.Mesh.Elements(:,elementIDsCell1), ...
        'FaceColor','red')
hold on
pdemesh(modelTwoDomain.Mesh.Nodes, ...
        modelTwoDomain.Mesh.Elements(:,elementIDsCell2), ...
        'FaceColor','green')