Defining acoustic properties of fluids with CFD results

You can use temperature, pressure, mass density, and speed of sound results from a computational fluid dynamics (CFD) general notation system (CGNS) file to define the acoustic properties of the fluid in a Simcenter Nastran SOL 108 or 111 acoustic analysis. With this capability, you can efficiently account for spatial variations in the acoustic properties of the fluid throughout your model. Thus, you can more accurately model acoustic propagation and radiation in applications where significant gradients in the acoustic properties exist. Examples of such applications include HVAC systems, exhaust systems, and gas turbines. The mesh for these acoustic simulations can be a standard FE or adaptive-order FE (FEMAO).

To use this capability, you create a Model and Load Pre-processing solution process in the context of a frequency response acoustic simulation, to map CFD results from the CFD mesh to the mesh of the acoustic simulation. Simcenter 3D stores the mapped results in an SCH5 file. You then create a simulation object that references the SCH5 file. During the solve, rather than using the acoustic properties for the material that you assigned to the FE mesh, the software uses the acoustic properties that are associated with the simulation object.

Compatibility of CFD and FE meshes

Because of the algorithm that Simcenter 3D uses to map the CFD results to the acoustics model, the CFD model and the acoustics model can have different spatial ranges, node locations, and mesh densities.

Importing results from a CGNS file

A CGNS file can contain multiple types of results, and the results can be for multiple time steps or iterations. You specify which result types to import and the time step or iteration from which to import the results. The result type combinations that you can import are as follows:

• Temperature only

• Speed of sound only

• Temperature and mass density

• Temperature and pressure

• Speed of sound and mass density

• Speed of sound and pressure

When you select temperature only or speed of sound only, you must specify a value for the pressure that the software uses throughout the model.

Because CGNS files are unitless, you must also specify the units that Simcenter 3D attaches to the data that you are importing.

Mapping results from the CFD mesh to the FE mesh

Because the meshes for the CFD and acoustics models can differ, Simcenter 3D uses a maximum distance algorithm to map the CFD results from nodes (or element centroids) in the CFD mesh to nodes in the acoustic FE mesh. The mapping occurs for only the nodes in the acoustic FE mesh that you specify.

Two maximum distance algorithms are available. The more computationally efficient algorithm works as follows:

1. For each node in the acoustic FE mesh, the maximum distance algorithm identifies the nodes in the CFD mesh that lie within a spherical search region centered about the acoustic FE node. You can specify the size of the spherical search region.

2. For the mapping calculation, the maximum distance algorithm retains the nodes in the CFD mesh that lie within the spherical search region and are closest to the acoustic FE node. You can specify the maximum number of nodes in the CFD mesh that are within the spherical search region that the algorithm retains.

3. For each acoustic FE node, the maximum distance algorithm computes a weighted average of the CFD result at these nodes in the CFD mesh. The weighting is inversely proportional to the distance from the node in the CFD mesh to the acoustic FE node.

4. The algorithm loops over each node in the acoustic FE mesh that you specify.

The other maximum distance algorithm may produce more accurate mappings because it conserves the CFD result at each node (or element centroid) in the CFD mesh. However, it requires more computational effort. This algorithm is referred to as the conservative maximum distance algorithm.

The conservative maximum distance algorithm works as follows:

1. For each node in the CFD mesh, the conservative maximum distance algorithm identifies the nodes in the acoustic FE mesh that lie within a spherical search region centered about the CFD node. You can specify the size of the spherical search region.

2. For the mapping calculation, the conservative maximum distance algorithm retains the nodes in the acoustic FE mesh that lie within the spherical search region and are closest to the CFD node. You can specify the maximum number of nodes in the acoustic FE mesh that are within the spherical search region that the algorithm retains.

3. The conservative maximum distance algorithm distributes the result at the CFD node to these acoustic FE nodes. The distribution conserves the overall CFD result so that the integral of the result over the source CFD mesh is equal to the integral of the result over the target acoustic FE mesh.

4. The algorithm loops over each node in the CFD mesh.

5. At each acoustic FE node that you specified, the algorithm sums the weighted contributions from each node in the CFD mesh to obtain the mapped result at the acoustic FE node.

You can create multiple mappings of the CFD results. Each mapping can have different mesh mapping options specified.

When you create multiple mappings, you must adhere to the following rules:

• For the CFD mesh, you can include the nodes (or element centroids) in multiple mappings.

• For the acoustic FE mesh, you can include each node in a single mapping only. In addition, you must include every node in a mapping so that the acoustic properties of the fluid are defined throughout the entire acoustic FE mesh. Otherwise, an error occurs when you solve the acoustic analysis.

Thus, you can create regions in the model where the software uses the results at a greater or lesser number of nodes (or element centroids) in the CFD mesh to calculate the mapped result at a given acoustic FE node.

For additional information on the mapping algorithms, see Mesh Mapping Data dialog box.

SCH5 file

When Simcenter 3D maps the CFD results to the acoustic FE mesh, it writes the mapped results to an SCH5 file. The SCH5 file is a binary file. You select the SCH5 file when you create the acoustic temperature mapping simulation object. When used in the acoustic simulation, the acoustic temperature mapping simulation object causes Simcenter 3D to use the acoustic property data in the SCH5 file rather than the acoustic properties for the material that you assigned to the mesh.

Converting properties to speed of sound and mass density

Simcenter Nastran requires speed of sound and mass density values when it calculates the element matrices in an acoustic analysis. Thus, if you import temperature or pressure data from the CGNS file, the data must be converted to speed of sound and mass density data.

Simcenter 3D uses the following equation to convert temperature data to speed of sound data:

c = \sqrt {\gamma RT}

where c is the speed of sound, γ is the isentropic expansion factor, R is the gas constant, and T is the (absolute) temperature.

Simcenter 3D uses the following equation to convert pressure data to mass density data:

\rho = \frac{{\gamma p}}{{{c^2}}}

where ρ is the mass density, γ is the isentropic expansion factor, p is the (absolute) pressure, and c is the speed of sound.

During the acoustic analysis solve, Simcenter Nastran uses the shape functions for the elements in the acoustic FE mesh to interpolate the speed of sound and mass density data from the nodes to the Gauss points of the elements.

Where do I find it?

Creating the solution process

Application	Pre/Post
Prerequisites	A Simulation file as the work part and displayed partSimcenter Nastran as the specified solverAcoustic or vibro-acoustic as the specified analysis typeSOL 108 or 111 as the specified solution type
Simulation Navigator	Right-click the simulation node→New Solution Process→Model and Load Pre-processing

Selecting the CGNS file

Application	Pre/Post
Prerequisites	A Simulation file as the work part and displayed partSimcenter Nastran as the specified solverAcoustic or vibro-acoustic as the specified analysis typeSOL 108 or 111 as the specified solution typeAn existing model and load pre-processing solution process
Simulation Navigator	Right-click the model and load pre-processing node→Add Load→Input File

Mapping CFD results to the acoustic FE mesh

Application	Pre/Post
Prerequisites	A Simulation file as the work part and displayed partSimcenter Nastran as the specified solverAcoustic or vibro-acoustic as the specified analysis typeSOL 108 or 111 as the specified solution typeAn existing model and load pre-processing solution process
Simulation Navigator	Right-click the model and load pre-processing node→Add Operation→Mesh MappingRight-click the mesh mapping node→Edit

Creating the acoustic temperature mapping simulation object

Application	Pre/Post
Prerequisites	A Simulation file as the work part and displayed partSimcenter Nastran as the specified solverAcoustic or vibro-acoustic as the specified analysis typeSOL 108 or 111 as the specified solution typeAn existing model and load pre-processing solution processThe CFD results mapped to the FE or FEMAO mesh
Command Finder	Acoustic Temperature Mapping

How do I

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Defining acoustic properties of fluids with CFD results, Simcenter 3D 2021.1 Series

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