In a conventional CFD simulation, the domain is divided into small control volumes. When an object within that fluid moves (e.g., a piston or valve), the original mesh no longer matches the original boundaries of the domain. However, these situations are very common in processes and products, so CFD simulations with moving elements represent an area of high industrial interest.
To perform these simulations, moving meshes are used, in which the movement of the mesh (translation, rotation, etc.) can be defined to adapt to changes in the geometry of the domain during the simulation.
The main difference is that, unlike a fixed mesh (where the fluid passes through immobile cells), mesh nodes have their own velocity and trajectory.
Simulation methods for moving meshes
The choice of the appropriate technique depends on the nature of the movement (rotation, linear translation, or complex trajectory) and the type of interaction being simulated. The three most common methods for addressing the problem are:
- MRF (Multiple Reference Frame): This does not involve physical movement of the mesh during calculation, but rather changes are applied to the reference system to include inertial forces in the fluid equations. It is widely used in turbomachinery (such as fans or mixers) as it is a fast and useful method for stationary analysis.
- Stretching Mesh: Although no new elements are generated, the nodes move as if they were springs, stretching or compressing the existing cells to adapt to the defined movement. Its main use is in linear movements in limited domains, as it avoids interpolation errors with the mesh of the rest of the contour. For example, internal combustion engines or linear motion valves.
- Overset Mesh: This is based on the superimposition of independent meshes, one for the background and others for the objects, which move freely without deforming. The meshes exchange data in the areas where they overlap using an interpolation process for each time step. It is effective for free movements or complex rotations (such as the flight of a drone). The following image shows an example of an industrial mixer.
Example of application
As a simplified case study, we analyze a train entering a tunnel to evaluate the so-called piston effect. As the vehicle moves forward, it compresses the air in front of it, generating a longitudinal pressure wave that travels at the speed of sound. This phenomenon generates critical structural loads and the characteristic “boom” at the tunnel exit.
The study was conducted using the Stretching Mesh technique in Cradle CFD in a 2D domain, meaning that the simulation of the model’s moving elements is based on the progressive compression of the air volume between the train and the tunnel exit.
The results obtained from the transient analysis of the simplified model are presented below, showing the propagation of the pressure wave, the movement of the train, and the deformation of the mesh.
