Modeling axisymmetry (SOL 414)

You can create and solve axisymmetric models in all SOL 414 types. Axisymmetric modeling can reduce the time needed to create and solve a full 360 degree model.

When you create the axisymmetric model, you can mesh it with CQUADX4, CQUADX8, CTRAX3 and CTRAX6 elements. The nodes on these elements must all lie in either the XZ plane, or all in the XY plane of the absolute coordinate system.

The CQUADX4, CQUADX8, CTRAX3 and CTRAX6 elements can behave as Fourier axisymmetric elements. Fourier axisymmetric elements allow for non-axisymmetric loading while improving the accuracy of the analysis results.

The displacement degrees-of-freedom for Fourier elements are represented as a series, where each term in the series represents additional harmonic behavior. The R, θ, and Z displacements are represented as follows:

{u_r} = {u_{0r}}\left( {r,z} \right) + \sum {\left( {\cos i\theta } \right)} {u_{ir}}\left( {r,z} \right) + \sum {\left( {\sin i\theta } \right)} {u_{ - ir}}\left( {r,z} \right)

{u_\theta } = {u_{0\theta }}\left( {r,z} \right) + \sum {\left( {\sin i\theta } \right)} {u_{i\theta }}\left( {r,z} \right) + \sum {\left( {\cos i\theta } \right)} {u_{ - i\theta }}\left( {r,z} \right)

{u_z} = {u_{0z}}\left( {r,z} \right) + \sum {\left( {\cos i\theta } \right)} {u_{iz}}\left( {r,z} \right) + \sum {\left( {\sin i\theta } \right)} {u_{ - iz}}\left( {r,z} \right)

The software solves for the displacement results of each harmonic number i independently. The zeroth harmonic represents the behavior of a pure axisymmetric element. Each harmonic above the zeroth harmonic adds additional behavior.

Concentrated mass, forces, boundary conditions, and bearings are usually defined on the axis of a rotor. To connect them to the rotor, use the FOU3 coupling element. The FOU3 coupling element connects a node on the axis of a rotor to a node in the axisymmetric portion of the model. Another option is coupling a 2D axisymmetric model (such as a shaft) to a 3D cyclic symmetry model (such as a rotor) using the Fourier Multi Harmonic and 3D Coupling simulation object. Both the FOU3 element and the Fourier Multi Harmonic and 3D Coupling simulation object are explained in Coupling 2D and 3D portions of a model (SOL 414).

Fourier harmonic definition

To specify the harmonic numbers for which you want the software to calculate results, use the Harmonics Set and Harmonic to Fix modeling objects. The Harmonics Set specifies the harmonic numbers globally. That is, they are applied to each subcase. However, you can use the Harmonic to Fix modeling object inside a subcase to remove harmonic numbers.

Because every subcase of a given type must request the same harmonic numbers, if you include a Harmonic to Fix modeling object in a subcase, you must also include it in all of the other subcases of that type.

Fourier element output

The Fourier axisymmetric element results depend on a summation of the harmonics at specific values of the tangential coordinate, θ. You can use the Simcenter Nastran FHPOST and FHPNST parameters to recombine the results into 3D.

To specify these parameters, edit the solution. In the Solution dialog box, on the Parameters page, use the options in the Recombination group.

Generating superelements of axisymmetric models

You can condense a 2D axisymmetric model to a superelement. For more information, see Rotor dynamic analysis with superelements (SOL 414,103).

Where do I find it?

Creating a set of harmonic numbers

Application	Pre/Post
Prerequisites	A Simulation file as the work and displayed partSimcenter Nastran as the specified solverRotor Dynamics as the specified analysis type
Command Finder	Modeling Objects
Location in Dialog Box	Type list→Harmonics Set→Create

Specifying a set of harmonic numbers to remove from a subcase

Application	Pre/Post
Prerequisites	A Simulation file as the work and displayed partSimcenter Nastran as the specified solverRotor Dynamics as the specified analysis type
Command Finder	Modeling Objects
Location in Dialog Box	Type list→Harmonics to Fix→Create

How do I

Create connection elements between coincident nodes with CBEAR2 elements

Define rotor bearing or bushing properties

Create a rotor region

Define a rotor

Define the rotor dynamics solution parameters

Create an unbalance mass

Define forcing frequencies

Define nonlinear transient parameters

Set the duration of the simulation

Create a modeling object

Assign a modeling object to a solution or solution subcase

Learn more

Rotor dynamic analysis (SOL 414)

Modeling cyclic symmetry (SOL 414)

Creating connection elements on a rotor model (SOL 414)

Damping (SOL 414)

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Modeling axisymmetry (SOL 414), Simcenter 3D 2021.1 Series

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