Multiphysics > Meshing for Pre/Post Multiphysics analyses

Chocking elements

Chocking elements are a special type of axisymmetric element that you can use in axisymmetric analyses in the Multiphysics environment. They complement the modeling capabilities of axisymmetric elements and plane stress elements as follows:

You use axisymmetric elements to model regions that carry a hoop stress.
You use plane stress elements to model regions that do not carry a hoop stress.
You use chocking elements to model regions that carry a compressive hoop stress but that cannot carry a tensile hoop stress. Chocking elements behave like axisymmetric elements when a compressive hoop stress is present. In all other situations, they behave like plane stress elements.

For example, you can use chocking elements to model certain regions where hoop stresses are likely to occur within an axisymmetric Fourier harmonics analysis.

Note:

Chocking elements are available in NX Nastran version 11 or later, or any version of Simcenter Nastran.

Modeling gaps

You can also use chocking elements to model the presence of gaps within an axisymmetric model. For example, you can use chocking elements to model regions where the axisymmetric geometry is violated by regularly spaced features like holes or keyways. Typically, you use chocking elements where the potential for a compressive hoop stress exists. You can specify:

The number of gaps that exist on each element.
The size of the initial gap opening (or overclosure) at the location of each gap.

The solver evaluates the gap value at each Gauss point during the solve.

One example of the use of chocking elements is to model the shrouding that surrounds the periphery of turbine blades in an aircraft engine. The graphic shows a generic turbine assembly. The shroud is constructed of discrete segments that are attached to each turbine blade. A small gap exists between each segment. If the combination of mechanical and thermal loads is such that these gaps close during operating conditions, the shroud can sustain a compressive hoop stress. To account for this behavior in an axisymmetric model of the engine, you can mesh the cross section of the shroud with chocking elements.

Chocking element support in thermal solver

In a coupled or thermal analysis, the thermal solver treats chocking elements like plane stress elements throughout the entire analysis. However, for radiation calculations, it treats chocking elements like axisymmetric elements.

You can apply Thermal Stream, Thermal Void, and Thermal Convective Zone loads across the chocking element faces.

Types of chocking elements

You can use the 2D Mesh or 2D Mapped Mesh dialog box in Pre/Post to create the following types of chocking elements:

Chocking linear and parabolic triangles (CCHOCK3 and CCHOCK6)
Chocking linear and parabolic quadrilaterals (CCHOCK4 and CCHOCK8)

Behavior of chocking elements

The behavior of chocking elements during a solution depends on whether large displacements are enabled.

Tip:

In Pre/Post, use the Large Displacements option on the Solution Control page of the Solution dialog box to control whether large displacements are enabled for the solution

If you select the Large Displacements check box, the solution is iterative. For the initial iteration, at any Gauss point location where no initial gap exists, the contribution to the elemental stiffness matrix from that Gauss point is based on the axisymmetric formulation of the chocking element.
If you clear the Large Displacements check box, Simcenter 3D Multiphysics uses the gap status at the beginning of the analysis throughout the analysis. At any Gauss point location at which no initial gap exists, Simcenter 3D Multiphysics bases the contribution to the elemental stiffness matrix from that Gauss point on the axisymmetric formulation of the chocking element

Specifying the properties of gap elements

You can use the Chocking Property dialog box to define:

The material to use for the chocking elements.You can use either isotropic (MAT1) or orthotropic (MAT11) materials with chocking elements.
The number of gaps around the circumference.
The size of the gap openings.You can specify a negative value to indicate an initial overlap or interference value. You can define this thickness as a constant or varying value.To define the gap thickness as a constant value, use the Gap Thickness option in the Chocking Property dialog box.To vary the thickness of the gaps through the cross section, use the Gap Thickness Source option in the Mesh Associated Data dialog box to select a table field that specifies the thickness values.To control the size of the gap opening at the corner nodes of either all elements or selected elements, use the Corner Node Gap options in the Element Associated Data dialog box.

The software evaluates the gap value at each Gauss point during the solve. The software switches to a plane stress formulation option during the solution on a Gauss point by Gauss point basis. In Pre/Post, you can use an expression or spatial field to define a varying gap thickness value.

The options in the Chocking Property dialog box correspond to the fields on the Simcenter Nastran PCHOCK bulk data entry.

Requesting gap results output

You can use options on the Chocking Results tab in the Structural Output Request dialog box to request results for chocking elements during the solve. The options on the Chocking Results tab correspond to the parameters for the Simcenter Nastran CKGAP case control command.

Axisymmetric results

After you solve a model that contains chocking elements, you can use Post-processing to display the results on either a 3D segment of the model or on a full 360° model display. For more information, see Fourier axisymmetric 2D displays.

Where do I find it?

Application	Pre/Post
Prerequisite	A FEM file as the work part and displayed partSimcenter 3D Multiphysics as the specified solver2D Solid Option set to either ZX Plane, Z Axis or XY Plane, X Axis

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