Shell Objects
In addition to solid objects, Mechanical – Thermal models can contain shell objects, which you can draw in the mechanical design or import from a different design or CAD model. Shell objects are sheet objects for which you have selected the Shell Element attribute. When you select this attribute, in the object Properties dialog box or the docked Properties window, the following two additional attributes become available:
- Material: Choose a material from the library, as you do for solid objects, or define your own material properties.
- Effective Thickness: The shell is always represented as a zero-thickness object within the model geometry, but the thickness is defined numerically, and the specified value is passed to the solver.
The default thickness for shell objects in Mechanical – Thermal solutions is 1E‑7 mm. For many situations, this thickness can be considered to be infinitesimally small, preventing the shell object from providing a path for significant heat flow in parallel with the object on which it is overlaid.
Thermal results are calculated at the midplane of the shell (that is, at the middle of the shell's thickness). The face of the sheet, as represented in the model geometry, corresponds to the midplane of the shell object.
Shell objects provide the following capabilities in Mechanical – Thermal solutions:
- Excitations and boundaries in other design types are frequently applied to sheet objects. An example is a sheet object in an HFSS model with a Wave Port excitation applied to it. You can use a corresponding shell object in the Mechanical design as the target of an imported surface EM Loss excitation.
- Shell objects can represent thin conductors on the face of, or passing through, a solid object (such as the copper traces of a printed circuit board). In this case, you will need to specify the actual thickness of the trace instead of using the default shell thickness. A shell can also represent a free-standing thin conductor spanning between adjacent objects (not overlaid on or embedded in a solid object). The ribbon of conductor spanning between the end caps of a fuse is an example of a free-standing shell. Fields overlays can be displayed on shell objects in the same way as they can on solid objects.
- You can overlay a shell object on the face of a solid object as a means of superimposing two or more boundary conditions or excitations that cannot be combined on a single face.
- You can use a shell object to encompass multiple contact areas between a large object (such as a printed circuit board) and several adjacent objects (such as surface-mounted electronic components). Then, assign a singe contact definition to the shell to take care of all object-to-object contact areas within its perimeter. Keep the default infinitesimal thickness to prevent the introduction of a significant heat conduction path parallel to the contact plane. See Contact Guidelines and Examples.
- You can overlay a shell object on the face of a solid object as a means of limiting a boundary or excitation to only a portion of a larger face. The shell object effectively becomes a subface of the solid object's larger face.
- Alternatively, you can create a sheet object or a non-model solid object to use temporarily as a tool part for imprinting onto the face of a solid part, creating a subface. You can assign thermal boundaries and excitations directly to the resulting subface of the solid part. The advantage of this alternative method is that there are no shell elements to potentially act as a parallel path for heat flow.
- You must use the Tau mesher when you have performed imprinting and have not kept (that is, Cloned) the sheet objects used as the Tool Parts. The Classic mesher will simplify coplanar and fully tangential subfaces by merging them back into the original, larger face (unless there is an adjacent sheet or solid body at the imprinted subface). The model geometry will not be altered, but the Classic mesher will ignore the subfaces when generating the solid mesh.
Shell objects in Mechanical–Thermal designs do not support a change in temperature across the shell's thickness. There is only one temperature value at any particular point along the shell, and it is at the mid-plane. Temperature variations can occur only in the in-plane directions. Therefore, do not attempt to use a shell object's material properties and specified thickness to represent thermal contact resistance. Rather, assign Contact to the shell, or to one or more of the adjacent object faces, and specify the desired thermal resistance in the contact definition.
See Thermal Contact and How to Assign Thermal Contact for more information.
An example of such a model is a heat sink with a shell object representing the contact area of a semiconductor. You could apply a heat flux excitation to the shell object rather than including the semiconductor body in the model and assigning heat generation to it. As stated in the previous note, the extremely thin default shell element thickness will not conduct a significant amount of heat in most cases.
The method of imprinting an object on a flat or curved face, for the purpose of limiting boundaries or excitations to a portion of a larger face, is the same for Mechanical – Modal and Mechanical – Thermal solutions. See the Related Topics below for more information.