In some specific cases, we might not be able to use a constant-sized mesh model. For instance, when simulating air inside an inflated balloon, the outer mesh size, namely the balloon’s diameter, will increase due to fluid pressure.
In more technical cases, such as the up-and-down movement of a piston in a cylinder or the opening and closing of a valve, we are required to use a changing mesh, known as a dynamic mesh.
The local adaptive mesh is sometimes also required to refine some specific areas of mesh based on the fluid motion. For example, refinement of a zone near the spray nozzle or free surface to save the computational effort.
Figure 3.14. Multiphase Unrefined (top) vs Local refined mesh (bottom) in OpenFOAM
For cases involving movements characterized by uniformity in a specific orientation, such as the rotor of a centrifugal blower, it can be modeled by using more than one regions connected with a interface.
A surface that moves relatively between the outer and inner parts is required to connect data from the inside and outside sections. This is also known as a sliding interface, as illustrated in Figure 2.15.
Figure 3.15. Sliding mesh
Due to the relative sliding motion between the outer and inner meshes, this modeling is also known as a sliding mesh. Sliding meshes can be created for both rotational and translational motions.
The input for dynamic mesh motion can be defined as constraints, for instance, linear movement forwards and backward, as seen in valve cases, or it could be a constant rotational speed input, such as in the case of a centrifugal blower.
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