The model is a small cube translating within a larger cube, which can be visualized as a smaller rectangle moving inside a larger rectangle. In Gambit, there's nothing particularly special about meshing or defining boundaries; it's the same process for both tasks. Next, set up in Fluent: first select the unsteady model so that the dynamic mesh option becomes available. Then, under "dynamic mesh," choose "parameters." There are many options here, mainly for setting dynamic mesh parameters, depending on the method of dynamic mesh you choose—there are three methods: smoothing, layering, and remeshing. Smoothing is suitable for small deformations, layering is for regular grids, and remeshing is for large deformations. There are also "in-cylinder" and "sixdof" options, but I haven't used them. After setting these, go to "dynamic mesh zones" to define which boundary needs to move. Here, you can choose either movement or deformation—for example, in the case I mentioned, it's movement. If a rod bends under pressure, that would be deformation. The most important part here is calling your own UDF (User-Defined Function). Specifically, call several macros for dynamic meshing. For more details, you can refer to the help documentation. Let me give an example with the simplest case.
```c
DEFINE_CG_MOTION(piston, dt, vel, omega, time, dtime)
{
Thread *t;
face_t f;
vel[0] = 10;
}
```
This is the simplest and most commonly used example. It means the object moves at a speed of 10 m/s in the x-direction. `vel[]` represents velocity, and the brackets can define three directions: 0, 1, and 2, corresponding to x, y, and z respectively. What does `Thread *t` mean? It tells Fluent which boundary is moving. Each boundary has its own ID value, which is used for feedback. That's pretty much it for the setup. Next, before calculating, check if the mesh moves correctly. First, initialize, then go to solver->mesh motion and click apply. If you see the mesh move, congratulations! Your settings are successful.
Let me explain this simple UDF in more detail. This is found under Define->User Defined. Some macros require compilation, while others need interpretation. Dynamic meshing only works with compiled macros. You can write UDFs using Notepad, but for compiled macros, you must use a C compiler. For interpreted macros, you don't need to install a C compiler—you can use text-based interpretation. However, for compiled macros, you need to use C.