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3 | 3 | To run an FSI simulation we need a mesh for both the structural domain and the fluid domain. These two meshes must have their interface nodes coincide exactly in order to satisfy the interfacial conditions that result from conservation of mass and momentum. The coincident nodes of the fluid mesh are mapped onto the corresponding nodes on the structural mesh and the solution of velocity, displacement, pressure, etc. are treated as equal in the structural and fluid domains. |
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5 | | -The fluid domain geometry for patient-specific anatomies are generated using the usual [SimVascular modeling pipeline](http://simvascular.github.io/docsModelGuide.html). To create the geometry for the structural domain, we will make use of the [boundary layer meshing feature](https://simvascular.github.io/docsMeshing.html#tetgenboundarylayer) in the `Meshing` module. The usual case for boundary layer meshing involves extruding this thin layer of elements *inwards*, starting from the walls and going into the direction of the vessel centers. To make a wall mesh, we will instead use the boundary layer meshing feature to extrude elements *outwards* to effectively make a new mesh with a specified thickness that surrounds our fluid domain. This new mesh will form the geometry of our structural domain. |
| 5 | +The fluid domain geometry for patient-specific anatomies are generated using the usual [SimVascular modeling pipeline](http://simvascular.github.io/modeling.html). To create the geometry for the structural domain, we will make use of the [boundary layer meshing feature](https://simvascular.github.io/meshing.html#tetgenboundarylayer) in the `Meshing` module. The usual case for boundary layer meshing involves extruding this thin layer of elements _inwards_, starting from the walls and going into the direction of the vessel centers. To make a wall mesh, we will instead use the boundary layer meshing feature to extrude elements _outwards_ to effectively make a new mesh with a specified thickness that surrounds our fluid domain. This new mesh will form the geometry of our structural domain. |
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7 | 7 | Before we use the boundary layer meshing to extrude outwards, it is extremely important that we thoroughly smooth our model. The outward extrusion of elements has the potential to cause some elements to extrude into each other and overlap, which will cause the extrusion to fail. This is especially true at bifurcations, where the extruded elements from the two daughter branches could easily run into each other near the junction if not properly smoothed. We will therefore make thorough use of the local smoothing tools in the Models module before meshing. Below is an example of a bifurcation that would normally occur after lofting segmentations together without smoothing. If we try to extrude a boundary layer mesh from this, the elements at the sharp corner will intersect with each other and cause it to fail. |
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@@ -48,9 +48,9 @@ Now that we have a smoothed model, we can open the "Meshes" module. Right-click |
48 | 48 | <figcaption class="svCaption" >SV Meshing.</figcaption> |
49 | 49 | </figure> |
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51 | | -We must first select an appropriate "Global Max Edge Size" for our model. This edge size will determine the thickness of our wall mesh. We will remesh the volume of the fluid and solid domains later, so select the edge size to be *double* what your desired thickness is. We select this to be double to cut down the meshing time. If you are not sure how to select the thickness of your model, choosing a thickness that is 10% of the mean radius in your model is a reasonable assumption used throughout the literature. Below the edge size selection, you should see a box to select boundary layer meshing. First, click the checkbox next to "Boundary Layer Meshing" to turn it on. Below this, you should notice three spaces to select parameters of the boundary layer meshing. Since we are producing this boundary layer for the wall mesh, we can use the same settings for these. "Portion Edge Size" determine the overall thickness of our boundary layer mesh, as a fraction of the "Global Max Edge Size" selected above. Since we selected a "Global Max Edge Size" to be double our desired thickness, we want this parameter to be 0.5. Next, you will have to choose the "Number of Layers" in your boundary layer mesh. Increasing this number will increase the accuracy of your structural domain calculations but also increase the number of elements and thus increase your cost. A reasonable number for this parameter is 2. Last, we must select the "Layer Decreasing Ratio", which is a parameter that allows subsequent layers to be a smaller size than the one before it. Since this parameter does not matter too much for creating a wall mesh, we can select this to be 1.0 to make it so all our layers are the same size. |
| 51 | +We must first select an appropriate "Global Max Edge Size" for our model. This edge size will determine the thickness of our wall mesh. We will remesh the volume of the fluid and solid domains later, so select the edge size to be _double_ what your desired thickness is. We select this to be double to cut down the meshing time. If you are not sure how to select the thickness of your model, choosing a thickness that is 10% of the mean radius in your model is a reasonable assumption used throughout the literature. Below the edge size selection, you should see a box to select boundary layer meshing. First, click the checkbox next to "Boundary Layer Meshing" to turn it on. Below this, you should notice three spaces to select parameters of the boundary layer meshing. Since we are producing this boundary layer for the wall mesh, we can use the same settings for these. "Portion Edge Size" determine the overall thickness of our boundary layer mesh, as a fraction of the "Global Max Edge Size" selected above. Since we selected a "Global Max Edge Size" to be double our desired thickness, we want this parameter to be 0.5. Next, you will have to choose the "Number of Layers" in your boundary layer mesh. Increasing this number will increase the accuracy of your structural domain calculations but also increase the number of elements and thus increase your cost. A reasonable number for this parameter is 2. Last, we must select the "Layer Decreasing Ratio", which is a parameter that allows subsequent layers to be a smaller size than the one before it. Since this parameter does not matter too much for creating a wall mesh, we can select this to be 1.0 to make it so all our layers are the same size. |
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53 | | -Below these parameter are three checkboxes. The "Extrude Boundary Layer Inward from Wall" checkbox will extrude the boundary layer mesh inwards if selected. Since we want to extrude the boundary layer mesh *outwards*, we will uncheck this box. The next box is a setting for "Use Constant Boundary Layer Thickness", which will attempt to make the entire boundary layer the same thickness if selected. We recommend leaving this box unchecked so the boundary layer mesh can adaptively change thickness in areas of tricky geometry. The last checkbox, "Convert Boundary Layer to New Region/Domain", is very important. We want to check this so that we will have a way to separate the fluid and structural domains later. |
| 53 | +Below these parameter are three checkboxes. The "Extrude Boundary Layer Inward from Wall" checkbox will extrude the boundary layer mesh inwards if selected. Since we want to extrude the boundary layer mesh _outwards_, we will uncheck this box. The next box is a setting for "Use Constant Boundary Layer Thickness", which will attempt to make the entire boundary layer the same thickness if selected. We recommend leaving this box unchecked so the boundary layer mesh can adaptively change thickness in areas of tricky geometry. The last checkbox, "Convert Boundary Layer to New Region/Domain", is very important. We want to check this so that we will have a way to separate the fluid and structural domains later. |
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55 | 55 | Now, you are ready to run the mesher. Click "Run Mesher" near the top right of the "SV Meshing" window to run the mesher. If the meshing was successful, you should see a window pop up to inform you of the statistics of your mesh. If the meshing was unsuccessful, it is likely that your model may need to be smoothed more to avoid intersecting elements. Once you are successful in producing this boundary layer mesh, right-click it from the SV Data Manager, and click "Export Mesh-Complete" and choose a location to send the mesh. |
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