Convex meshes for FEM acoustics analyses

Use the Convex Mesh command to create a convex mesh of 2D elements around an existing 2D mesh. In Simcenter Nastran FEM acoustic and vibro-acoustic analyses, you must define the envelope that bounds the acoustic fluid mesh. For example, in an exterior acoustic analysis, you must define the envelope for the exterior fluid domain. The envelope also defines the region of interest for the analysis. You can use the Convex Mesh command to create a 2D mesh that:

• Envelops the structural mesh.

• Is offset from the structural mesh.

The amount that you offset the convex mesh from the structural mesh affects the size of the fluid mesh. The convex mesh should be large enough to allow you to create a fluid mesh that is at least one or two elements deep in all locations. However, you should ensure that the convex mesh is not larger than necessary, because a larger fluid mesh can result in longer solve times.

Convex mesh that was created with a 20mm offset around a structural mesh	Translucent view of the convex mesh that shows the enclosed structural mesh

After you create a convex mesh, you can:

• Create the 3D fluid mesh that fills the area between the inside surface of the convex mesh and the 2D mesh that defines the inner boundary of the fluid mesh. For example, you can use the Solid from Shell command to fill the gap between the original 2D mesh and the convex mesh with tetrahedral elements.

• Create an Automatically Matched Layer (AML) on the surface of the convex mesh in the Simulation file. An AML is a special type of boundary condition that you can use to model a non-reflecting layer that absorbs outgoing waves.

Note:

A convex mesh is an intermediate mesh that is necessary for creating the fluid mesh and the AML. When you solve your model, you should not export the convex mesh to the solver input file.

Offsetting the convex mesh from the structural mesh

A convex mesh must be offset from the structural 2D mesh to create the space for the fluid domain. The amount that you offset the convex mesh from the structural mesh affects the size of the fluid mesh. The convex mesh should be large enough to allow you to create a fluid mesh that is at least one or two elements deep in all locations. However, you should ensure that the convex mesh is not larger than necessary, because a larger fluid mesh can result in longer solve times.

In the Convex Mesh dialog box, you can use the Offset Method option to define how the software should offset the convex mesh from the 2D mesh.

• With the Scale Factor option, the software offsets each node in the convex mesh by scaling vectors according to the specified value beginning with the node located at the center of gravity of the convex mesh.

• With the Distance Along Normals option, the software offsets each node in the convex mesh the specified distance in the direction of the average element face normal at that node.Note: In general, the offset distance that you specify should be related to the lowest frequency of the frequency range. This distance should represent a significant portion of the wavelength, such as ƛ/20 or ƛ/30 for the automatically matched layer approach to work efficiently at low frequencies.

• With the None option, the software creates the convex mesh directly on top of the elements in the underlying 2D mesh.

In the Offset Distance box, you can either specify the distance to offset the convex mesh from the 2D mesh, or you can use the SizeForAcoustics function to define the distance to offset the convex mesh as a function of frequency:

offset distance = speed of sound/(maximum frequency value * the number of elements per wavelength)

After you define the required maximum frequency value, you can use the following optional arguments to specify values for:

• Speed of sound (soundVel)

• The maximum number of elements per wavelength (elemPerWavelength)

If you do not specify values for the speed of sound or for the number of elements per wavelength, the software uses default values (soundVel=340000.0 mm/s and elemPerWavelength=6). You can use the Acoustic Parameters options in the customer defaults to set these default values.

Specifying infinite planes

You can create up to three infinite planes to define symmetry conditions in the convex mesh. Use the options in the Infinite Plane list to specify whether you want to create the plane parallel to the XY, XZ, or YZ plane in the model's absolute coordinate system.

If you define an infinite plane, all elements that are adjacent to the elements on the infinite plane are oriented at 90° to the elements on the infinite plane.

Structural mesh as the underlying mesh for the convex mesh with an infinite plane to define symmetry in the mesh.	Cross-section view of the structural mesh and convex mesh.	Full view of the convex mesh that is created with an infinite plane in the XZ direction.

If you specify multiple infinite planes, the software always projects the 2D mesh onto the X, Y, and Z planes in an ordered sequence, regardless of the order in which you specified the planes. The software first projects the 2D mesh onto the x plane. Then, the software projects both the original 2D mesh and its projection to the first plane onto the Y plane. The software then repeats this process for the Z plane, if necessary.

Trimming a convex mesh at the infinite plane

If the infinite plane that you specify intersects the convex mesh, you can use the Trim Convex Mesh option to trim the convex mesh where that intersection occurs.

• The source 2D mesh must include free element edges.

• Nodes on the free element edges must lie within the specified Projection Tolerance either above or below the infinite plane.

The software projects nodes from the source mesh that lie within the Projection Tolerance value onto the infinite plane. The software then imprints the loop of element edges from the source mesh onto the infinite plane and trims the convex mesh back to that loop. The trimming process helps ensure that the convex mesh is completely closed and fully encompasses the intersection of the 2D input mesh and the infinite plane.

The 2D source mesh with the plane to use as the infinite plane for the convex mesh.	A view using the Clip Section command that shows the convex mesh around the 2D source mesh. Notice how the software trims the convex mesh where it intersects the infinite plane.

Working with the convex mesh

A convex mesh is FE-based and is not associated with the underlying geometry. The software does not update FE-based meshes if the underlying part geometry is modified. When you create a convex mesh, the software stores the options and settings you used to create the mesh. If you edit the mesh and change a setting, such as the element type or offset distance, the software recreates the mesh with the new settings.

The software stores meshes created with the Convex Mesh command in a 2d_convex_mesh node in the Simulation Navigator.

Where do I find it?

Application	Pre/Post
Prerequisite	A FEM file as the work part and displayed partAn existing mesh of 2D elements
Command Finder	Convex Mesh

Acoustics Parameters defaults

Menu	File→Utilities→Customer Defaults
Location in dialog box	Simulation→Pre/Post→Meshing→General tab

How do I

Reset node and element IDs

Learn more

Manually creating elements

Extruding elements

Projecting elements

Reflecting elements

Revolving elements

Rotating elements

Translating elements

Splitting shell elements

Swapping the diagonals between triangular elements

Splitting 1D elements

Detaching elements from a mesh

Attaching elements into a mesh

Creating a mesh from a cloud of points

Rib detection and removal within a mesh

Thickening a 2D mesh

Acoustic chamber meshes for panel transmission loss analyses

Open duct end meshing for acoustics

Combining triangular elements

Modifying element order

Modifying the type of elements

Moving nodes in 2D elements

Modifying element labels

Modifying element connectivity

Deleting elements

Extracting elements from a mesh

Locking and unlocking a mesh

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Convex meshes for FEM acoustics analyses, Simcenter 3D 2021.1 Series

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