Acoustic Cavity Meshing

Acoustic Cavity meshing generates a fluid volume mesh used to calculate the acoustic modes (or standing waves) inside the air spaces of a vehicle or similarly enclosed structural model.

Acoustic cavity meshing is primarily used by Noise, Vibration, and Handling (NVH) engineers to design quieter interiors.

Acoustic cavity meshing can be a CPU-intensive process, especially with fine and/or complex meshes, but this can be offset by additional CPU cores. The Acoustic Cavity Mesher is multithreaded to take advantage of multi-core environments.

How Cavities are Identified

Hole and gap patching is a critical part of defining cavities enclosed in the structure model. The process refers to patching over inconsequential gaps and holes that inevitably exist in the structure model, such as speaker holes.

Gaps are defined as elongated openings based on their longest dimensions. Holes are openings defined by the radius of a sphere that can pass through them. By specifying the size of the hole and gap patches, you can control how the cavities are defined through auto search.

Typically, a cavity model is intended to be meshed right up to the outer body panel. Plastic and fiber trim panels are often included in a trimmed body model, but not meant to be used for cavity meshing. However, if the trim panels are selected during the AC meshing process they can be confused as outer body panels, leading to incorrect cavity definition. Therefore, it is important to ensure that only the exterior body panels are selected as the structure components to be included in the auto cavity search.

Types of cavities you may wish to model:

Door: Include the cavity between the inner and outer door panels as a part of the interior by specifying a hole patch size smaller than that of the largest opening in the inner panel, so that the opening is not patched. This allows interior mesh to flow into the door cavity.
Instrument Panel (IP): Model the cavity behind the Instrument Panel as a part of the interior cavity by excluding the IP parts from the structure component selection during the auto cavity search. This forces the auto cavity search to ignore the existence of the sealed IP cluster when determining the interior volume of the passenger compartment. If the IP panel needs to be treated as a radiating source, a fluid boundary needs to be created at the location of the IP, similar to the how a package tray can be included as a structure part.
Pillar: Include large pillar cavities, such as a D pillar cavity, as a part of the interior cavity by ensuring that the gap and hole patch size specified are smaller than that of the largest opening. This prevents the opening from being patched and allows interior mesh to flow into the pillar cavity.
Under Seat: Ensure the under-seat spaces are meshed by specifying an element size smaller than the smallest dimension of the space, thus allowing the interior mesh to fill the cavity.
Trim Component: Special functionalities are required to mesh these cavities.

Factors that Influence Cavity Meshing

The ability to model the acoustic cavity and predict acoustic response inside it is a critical part of NVH analysis, as noise level and quality become key product quality differentiators in the marketplace. A number of factors need to be considered when meshing an acoustic cavity model.

Adequate mesh size.

A rule of thumb is that at least six elements are needed per acoustic wavelength. Based on this rule, the minimum acoustic element sizes at various frequencies are:

500 Hz: 114 mm
1000 Hz: 57 mm

Smaller elements mean a more complex cavity model, which takes longer to run and generates a larger output file, particularly when fluid grid participation output is requested. Recall, however, that the mesher is multithreaded to take advantage of multiple CPU cores.

Mesh size can also affect whether smaller cavity areas, such as the cavities underneath seats, get filled. Mesh size should be selected by considering the size of the smallest cavity that needs to be filled.

Mesh quality as defined by Jacobian value, Tetra Collapse, and similar measures.

Poor mesh quality may cause problems when submitted to the solver, or lead to less-accurate results.

How closely the cavity shape matches the actual structure.

This impacts how accurately the cavity model captures acoustic modes, and how difficult it is to obtain good coupling between the fluid and structure. It is important to define the structure panels intended to be coupled to the cavity.

Aesthetics of the cavity mesh.

The model may appear too jagged if the cavity mesh matches the structure closely. Some users prefer a smooth looking mesh for presentation purposes, but care must be taken so that this does not adversely affect the modes calculated or the quality of coupling generated when default ACMODL search parameters are used.

Interior response definition.

Interior response points need to be defined so that they become a part of the mesh definition when cavity mesh is generated.

Seat Foam Cavity definition and coupling.

Seat foams are typically modeled as denser air cavities. Their geometry definition can come in either as CAD data or existing FE mesh. Some seat models contain detailed foam curvature definition, while others may just be blocky boxes. The acoustic mesher can generate a new seat foam mesh and use congruent grids to connect to the interior cavity elements, or generate fluid MPCs to connect grids on a existing foam cavity mesh to the interior cavity mesh.

Trunk Cavity separated from the Interior Cavity.

For passenger sedans, the trunk cavity is typically modeled as a separate cavity from the interior, separated by the rear seat back foam cavity.

Package Tray properly coupled to both interior and trunk cavities.

For passenger sedans, the "package tray" or "parcel shelf" is situated behind the rear seat backs, between the interior cavity on the top and the trunk cavity below. Its vibration should be coupled into both cavities. This means a boundary (or gap) needs to exist in the cavity model where the package tray is located. This is typically accomplished by the two cavities not sharing grids at the boundary.

Once generated, fluid cavities must be coupled to the structure. OptiStruct creates this coupling automatically during solver analysis, storing the information in the ACMODL card. In addition, Radioss generates an .interface file which can be loaded into HyperMesh to verify fluid surface and structure wetted surfaces.

Create an Acoustic Cavity Mesh

Creating an acoustic cavity mesh is two-staged. First, create a voxel-based preview mesh. Second, select individual volumes, set element quality requirements, and create a smoother, more refined computational mesh for the selected volumes.

Acoustic Cavity meshing begins with the Acoustic Cavity Mesh controller. This controller accepts the necessary base input data to generate a voxelated preview mesh for one or more acoustic cavities. Once the preview mesh exists, however, the Acoustic Cavity Browser displays in the tab area; you can use this tool to modify element quality checks, select specific volumes that have a preview mesh, and create the computational mesh for each selected cavity.

From the NVH Tools ribbon, click the Cavity Mesh tool.

Figure 3.
Define acoustic cavity meshing parameters.
1. In the Acoustic Cavity Browser, click .
2. In the Options dialog, define acoustic cavity meshing parameters accordingly.
Using the Acoustic Cavity Mesh controller, generate preview mesh.
1. Use the Structure selector to select the components that surround the desired cavity.
  Select all of the components that enclose the air spaces. You can create meshes for multiple air spaces simultaneously, depending on which components you initially select.
2. Use the Seats selector to select the solid bodies representing the seats.
  Acoustic cavity meshing takes seating into account, and generates a separate volume mesh for each seat (bench seats are correctly treated as a single assembly, not broken up into individual seats based on the number of humans who could sit on them). This distinguishes the seats from the mesh that represents the air space.
3. Select a method for seat coupling.
  - Choose Node to node remesh to obtain new node-connected cavities for the seats, matching seat nodes, and cavity nodes on a 1:1 basis.
  - Choose MPC no remesh to connect the input seat components to the found cavities using MPCs, which allows node mapping at a ratio other than 1:1. Only use this option if the seat components are composed of solid elements. For proper MPC creation, the seat mesh size must be relatively similar to the size used to create the acoustic cavity mesh; in general, the size difference should be no more than 30%.
4. Optional: Create permanent elements from the temporary elements used to patch over the holes, and store them in their own collector called ^patched_holes by selecting the Create hole elements checkbox.
5. Define a target element size.
  Max frequency is calculated based on this target element size and number of elements per wavelength specified in Options dialog, accessed from the Acoustic Cavity browser.
  The actual mesh may vary from this depending on quality requirements.
6. Define the maximum size of internal elements.
  The max frequency will be populated automatically depending on the specified max element size and No. elements per wavelength specified in Options dialog. It is also possible to specify the max frequency value, and then max element size will be calculated accordingly.
  Note: A small, fine mesh may take a considerable amount of time to generate.
7. Define the minimum element size.
  The acoustic cavity mesh created should not have any elements below the specified minimum element size.
8. In the gap patch size field, enter how large of a gap in the geometry the mesher will ignore.
  Gaps in geometry equal to or less than this size (in model units) will be patched or plastered over while the mesh is generated. Gaps larger than this will be taken into account and flooded with mesh to fill the cavities on the other side.
9. In the hole patch size field, enter how large of a hole in the geometry the mesher will ignore.
  This value should be greater than your mesh size.
  Holes in the geometry equal to or less than this size (in model units) will be patched or plastered over while the mesh is generated. This is useful for excluding things like bolt holes. Gaps larger than this will be taken into account and flooded with mesh, which may result in bleed-through that connects different cavities of your model and treats them as a single mesh. For instance, in a passenger car you might end up with a single complex-shaped mesh that encompasses both the passenger cabin and the trunk space, with a narrow connection where a hole existed between the two otherwise separate cavities.
10. Define a feature angle.
11. Click Preview.
A simple preview mesh is generated.
Figure 4. Simple Preview Mesh
In the Acoustic Cavity Browser, select individual volumes, set element quality requirements, and create a smoother, more refined computational mesh for the selected volumes.

Figure 5. Final Acoustic Cavity Mesh