Setting Methods for Solid Motion in Fluent (Part 1)

1 Overview

Solid motion is a crucial factor in certain CFD (Computational Fluid Dynamics) problems, such as the rotation of a fan. Relevant issues can be classified as follows:

In Fluent, there are several methods to represent solid motion, including:

1 Multiple Reference Frame (MRF)

2 Sliding Mesh

3 Dynamic Mesh

4 Overset Mesh

This article, as the first part, covers only the setup of MRF and sliding mesh. The sections on dynamic mesh and overset mesh will be covered separately.

2. Applications

MRF and sliding mesh methods are typically used for problems with simple motion patterns such as axis rotation and translation. Axis rotation is more commonly applied in engineering.

MRF method supports steady-state and transient simulations, while sliding mesh method supports only transient simulations.

3. Meshing

The computational domain usually includes two parts: stationary domain and moving domain. The moving domain refers to the region near the moving solid, which is separately partitioned. The stationary domain constitutes the remaining part of the computational domain.

For scenarios involving global movement like centrifuges, the entire computational domain can be treated as the moving domain. Modelling stationary domains is not needed.

For example, the mixer model shown in the image, the yellow area near the blades is seperated as the rotating domain, while the grey area represents the stationary domain.

4. Computational Setup

4.1 MRF Setup

In Fluent, you need to select the "frame motion" option in the moving cell zone conditions panel and define the motion pattern.

Defining rotation requires specifying axis of rotation and angular velocity.

The rotation axis is a straight line in three-dimensional space, defined by direction vectors and points it passes through. The direction vector is set in the "rotation-axis direction" section, and it can be any vector of arbitrary length. For example, (0, 0, 1) or (0, 0, 2) are equivalent for defining the direction of the rotation axis. The point it passes through is set in the "rotation-axis origin" section and can be any point on the rotation axis.

The direction of rotation follows the right-hand rule, and the sign of the direction vector of the rotation axis affects the direction of rotation. For example, (0, 0, 1) and (0, 0, -1) represent opposite rotation directions.

Since the MRF method does not involve grid deformation, there is no need to set additional interfaces between the rotating and stationary domain regions; a shared node grid can be used directly.

4.2 Sliding Mesh Setup

In Fluent, you should select the "mesh motion" option in the motion domain panel and define the motion pattern.

The setup for motion patterns on this panel is similar to MRF.

Since the mesh in the moving domain changes over time, mesh interfacesneed to be created for data interpolation, and a conformal mesh cannot be used.

For existing MRF setups, the Fluent command "mesh/modify-zones/mrf-to-sliding-mesh" can automatically convert them to sliding mesh setups.

4.3 Boundary Condition Setup

In Fluent, the default boundary condition for wall surfaces is a no-slip boundary condition, and is stationary relative to the adjacent cell zone, not absolute coordinates. As a result, the wall boundary velocity is determined by the motion setting of the domain.

Due to geometric constraints and other reasons, it is difficult to ensure that the motion patterns of all walls and their respective domains are the same. For some walls, the motion velocity needs to be set separately to match physical reality.

In the boundary condtions panel, it is recommended to define the motion of walls based on the absolute coordinate system to avoid errors.

5. Post-processing

In post-processing, Fluent velocity can be divided into absolute velocity and relative velocity. The reference frame for velocity is the absolute coordinate system for absolute velocity and the moving coordinate system for relative velocity. Comparison between the two is shown in the figure.

6. Difference Between MRF and Sliding Mesh

At low rotational speeds, the results of MRF and sliding mesh are similar, but the deviation increases with higher speeds.

To obtain precise results and periodic fluctuation characteristics of flow, it is advisable to initially run steady-state simulations with MRF to obtain preliminary results. Then, use sliding mesh for transient simulations, with the steady-state results from MRF serving as initial conditions for transient simulations.