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    SimScale DocumentationSimulation SetupContactsContacts in Conjugate Heat Transfer

    Contacts in Conjugate Heat Transfer

    In a Conjugate Heat Transfer (CHT) analysis, an interface defines the physical behavior between the common boundaries of two regions that are in contact, e.g. solid-solid, or solid-fluid.

    Important

    In conjugate heat transfer simulations, all the fields at the interfaces are fully constrained by the Interface type. Therefore, defining boundary conditions to faces assigned to interfaces is not allowed. This would result in an overconstrained model.
    If an interface is assigned to a boundary condition, the following error message is displayed when the user tries to start a simulation:

    multiple boundary conditions or interface definitions error
    Figure 1: Error message when defining boundary conditions to an interface in CHT simulations

    To solve this error, the user needs to unassign the interfaces from the boundary conditions.
    Additionally, interfaces between two flow regions are not possible and will result in an error when running simulations.

    Automatic Interface Detection

    When creating a new CHT simulation,  all possible interfaces will automatically be detected and populated in the simulation tree. Interfaces will be grouped together and defined as Coupled thermal interface.

    How To Modify Specific Interfaces?

    Individual interfaces or a group of interfaces can be filtered via entity selection. Select the entities (faces or volumes) for which you want to select all interfaces that exist between them.

    workflow to filter contacts in CHT analyses by selection
    Figure 2: Specific interfaces can be selected individually or in bulk by selecting one or multiple entities in the viewer. The interface settings can be changed by filtering the interfaces by selection.

    Once you filter the interfaces of interest, a window opens with additional options for the Interface type for the selected contacts.

    filtered interfaces being exposed individually in the contacts tree
    Figure 3: All interfaces that are returned by the filter will be selected in bulk and exposed individually in the contacts tree. By customizing their settings, individual interfaces will stay exposed in the tree.

    Interfaces which differ in settings from the standard bulk interfaces group will stay exposed individually in the simulation setup tree.

    Contact Detection Errors

    As all possible interfaces are detected automatically, it is not possible to manually add an interface or to change the entity assignment for a specific interface. In case no interfaces can be detected automatically, SimScale will show a warning message before running the simulation.

    warning message in case of failed automatic contact detection contacts in cht
    Figure 4: This warning indicates that the bodies in the simulation domain are not fully connected through interfaces, suggesting either that the model is not contiguous or that some interfaces are missing

    In this case, it is possible to create a mesh and start a simulation run for this simulation. However, it is recommended to investigate the CAD model for potential errors that prevent successful contact detection. Some common causes are:

    Small gaps between the faces: If the face pair is separated by a gap higher than the CAD tolerance, the interface will not be detected. Sometimes, the gap is so small that it is difficult to find it by visual inspection. Please move the parts together or extrude the faces to create the proper contact.

    Duplicate parts: Sometimes the CAD modeling history makes it so that parts can be duplicated upon export. Please check that such a condition is not present in your model.

    Interfering parts: If the face pair is intersecting into each other by a margin higher than the CAD tolerance, the contact will not be detected. Please move the parts away so that the faces are in perfect contact, or you can also perform a Boolean operation to create coincident faces. However, in the case the overlap is done on purpose because of the use of an advance concept, please ignore the warning and press OK.

    warning message in case interfering parts were detected during the geometry swap in cht
    Figure 5: This warning indicates that the assigned geometry contains interfering parts. Such interferences, particularly between parts with material assignments, will prevent meshing.

    Some of the fix operations described above can be performed in the CAD edit. If you are unable to complete the modifications using the operations available in the platform, proceed with your dedicated CAD software.

    Interface Type

    The Interface type options define the heat exchange conditions at the interface. The types available for the interfaces in CHT are reported below:

    Coupled

    The coupled thermal interface models a perfect heat transfer across the interface. This is the default setting, in case an interface is not defined by the user.

    Thin Layer Resistance

    The Thin layer resistance allows modeling a layer with thickness \(t\) and thermal conductivity \(\kappa\), and \(\rho\) electric resistivity between the two interface regions.

    thin layer resistance contact in cht
    Figure 6: Settings panel for the thin layer resistance model

    For example, it is possible to model the thermal paste between a chip and a heat sink without needing to resolve it in the geometry. Adding a thin layer to the geometry and meshing it is a problem, considering that the thickness of these layers is two or three orders of magnitude smaller than other components in the assembly.

    When Joule heating is activated under global settings, it is important to take into account the electric properties at the contact surface. Electric resistivity (\(\rho\)) is a material property that quantifies how strongly a material opposes the flow of electric current. It depends on the material itself and is measured in Ohm-meters \([Ω·m]\).

    Last updated: September 1st, 2025

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