Last week we discussed the different ways to create and mesh infinitely thin baffles using the tools in ANSYS Workbench. In this blog, we’ll discuss how to create and modify baffles using the two flagship ANSYS CFD programs: CFX and Fluent.
In CFX, the method you use to specify your baffles depends on your modeling intent, but the way the baffles were meshed will affect some of our choices. The two methods are as a ‘baffle boundary condition’ or defining a ‘baffle interface’.
Baffle Boundary Condition
Baffle walls can be created by inserting a boundary condition into the surrounding domain and selecting the appropriate faces.
Baffles created this way are for directing flow only! Boundaries in CFX are always treated as leading to the exterior of the solved domain, which mean information cannot be passed from one side to the other (e.g. no heat transfer across the faces).
Alternatively, you can connect one side of a baffle face to the one facing the opposite direction by creating an Interface.
Face selection will depend on whether you made a conformal baffle (Default Method, Option 1, or Option 2 in Part 1 of this blog) or a nonconformal baffle (Option 3). For nonconformal baffles, you can create Named Selections for each side of the baffle and then pick those from a list. For conformal baffles, technically only 1 face exists for both sides in the Mesher, so the Named Selection can’t be used to specify side dependent locations. That being said, conformal baffles will create 2 identical face IDs with different body IDs representing each side, and if you pick them for Side 1, CFX will automatically populate Side 2 with the appropriate pair.
The interface will then need to be set as a wall, and additional interface models can be specified for the different equations. For example, for Heat Transfer you can specify that the baffle provides a thermal resistance of a certain thickness of some defined material.
Heat transfer in this case is purely through-thickness. In other words, heat can flow through the thickness of a metal baffle, but a high temperature would not conduct along the length of the baffle; instead the heat would spread through the adjacent fluid (and then interact with the wall at that the next location).
In Fluent, the method you use to specify your baffles is controlled by how the baffles were meshed, but modelling intent will still affect some choices therein. These methods we will separate between Conformal and Non-Conformal.
Baffles formed by conformal meshes within a Part (Default Method, described last week in this blog) will come into Fluent as a coupled wall, consisting of a boundary zone and its shadow. These zones represent the two opposite-facing sides of the baffle.
For the equations other than momentum, each side of the coupled wall can be treated individually or jointly. For example, the default thermal behavior is to couple each side of the wall together using either a direct mapping of temperature (0 Wall Thickness) or using a specified thermal resistance (nonzero Wall Thickness), but you could instead set one side to have 0 Heat Flux and the other side to be a specific temperature.
Much like CFX, entering a Wall Thickness will provide a thermal resistance in the through-thickness direction. However, Fluent additionally has a model called Shell Conduction that allows for heat transfer along the baffle in addition to through it.
Baffles defined by non-conformally meshed faces (i.e. Option 3 in last week’s blog) require different actions depending on whether you are using them to direct flow only or to additionally pass information.
Non-conformal baffles are exterior faces in the mesher, so if you only need them to direct the flow, they will automatically do so without any user intervention (note that if you also have a non-conformal mesh across regions where there are no baffles, you will need to have an interface across those faces).
If you do need to pass information across the baffle (like heat flow), you must create a non-conformal interface using the coupled-wall option.
This will create 4 addition zones: the Boundary Zones (Side 1 & 2) which are used to control the wall information for areas where the two sides do not line up, and the coupled wall Interface Wall Zones (Side 1 & Side 2) which are used to specify the wall behavior when there is overlap between the two sides. From here, the coupled wall Interface Wall Zones can be set as described in the Conformal Baffles section, other than that Shell Conduction is not supported (through-thickness heat transfer is still allowed).
One important thing to remember here is that Fluent non-conformal interfaces regularly experience trouble with significant overpenetration between the two sides of the interface. This typically occurs when there are different mesh sizes on either side of a curved interface. To combat this, you can enable the Mapped Interface Option, which will use an alternative heat transfer calculation across the coupled wall that performs much better with excessive mesh penetration.