Two Approaches for Rotating Frames in Flow Simulation

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There are two approaches that SOLIDWORKS Flow Simulation utilizes to handle rotating reference frames. The rotating reference frames can either be applied globally or locally. When they are applied globally, the model assumes that all of the walls rotate at the same speed as the reference frame and that the corresponding Coriolis and centrifugal forces are taken into account. Alternatively, when they are applied locally, the rotating region is only applied to that area, such as a fan blade. This local region must be defined as a component in the model defined as rotating.

For locally applied rotating frames, there are two approaches:

In the averaging or Mixing Plane method, the fluid flow within the rotating region is calculated within the reference frame. Flow field parameters are transferred from adjacent flow regions to the rotating region’s boundary as a boundary condition. The flow field must be axially symmetric at the rotating region’s boundary and the rotating regions must not intersect each other.

In the second method, called the Sliding Mesh method, it is assumed that the flow field is unsteady, and it is available for transient analysis only. This method allows us to obtain a more numerical solution, so it will take longer to solve than the Mixing Plane method.

To show the difference between the Mixing Plane and Sliding Mesh methods, we will use the example of a fan.

Mixing Planes Method

Below we have used the rotating local region option in the Flow Simulation Wizard. For this analysis, there is a cylinder model surrounding the fan (so it is an internal analysis) including the rotating region.

mixing plane region option

To show the difference between the Mixing Plane and Sliding Mesh methods, we will use the example of a fan.

mixing plane z direction

To define the rotating regions, click on rotating regions from the Flow Simulation Study Tree and choose to ‘Insert Rotating Region’. There is a fan blade in the assembly as well as an additional part called ‘Part1’ that is a solid cylinder that encloses the fan blade (rotating region). When defining the Rotating Region, we will use the ‘Part1’ that was added and not the actual fan blade itself. We will also need to enter the rotational speed for this region.

mixing plane rotating regions

After defining the rotating region, right-click on ‘Input Data’ at the top of the Study Tree and go to Component Control. Under Component Control, we will want to disable the ‘Part1’ used to define the region that is rotating.

mixing plane input data

We then applied a global mesh, local mesh, environmental pressure, and then added a surface goal for Force and Torque. Before running, we also added a calculation control for the number of iterations and then ran. After running, here are some results of the Mixing Plane method:

mixing plane mesh result
mixing plane result 2

We will next show an example of the second approach called Sliding Mesh. The Sliding Mesh approach is considered a more robust approach and assumes an unsteady flow field. It is available for transient studies and since it is more computationally demanding, generally takes longer than the previous method to solve.

For this example, we will use a blower fan. There is a strong interaction between the rotor and stator, which the Sliding Mesh approach is a very good fit for. The inlet of this example is simulated using a dome-shaped lid for inlet distribution. As before in the wizard, we will select rotating region, local regions, and this time, use Sliding Mesh rather than averaging.
solidworks flow sim blower fan

After meshing the model, we need to define the rotating region just like in the averaging approach.

blower fan rotating region
rotating region 2

Remember, the part we are using to define the rotating region is disabled in the Component Control.

We will also define the Real Wall condition in this study. When we have an entire wall moving in the tangential direction relative to the fluid, it is recommended to use a Real Wall Boundary condition. The Real Wall condition recommendation applies to both faces inside and outside the rotating region. We have captured the Real Wall condition for this example by selecting the three faces on top as well as the three faces on the bottom of the rotor and applied an angular velocity. We are using an absolute value for velocity since the rotating region lies both inside and outside the rotating region.

flow sim real wall

We will need to add inlet and outlet boundary conditions as well as refined the mesh when the flow is unsteady by select the rotating region. We also set a few goals for the project. On the Calculation Control Options panel, we set it to finish by a specific Physical Time.

flow sim specific physical time

The time we entered should be two full revolutions. As stated before, the Sliding Mesh method does take longer to run, but the results you’ll receive are very valuable.

sliding mesh
sliding mesh results

To summarize, the Mixing Plane, or averaging method, is a simpler approach that ignores rotational characteristics and must have an asymmetrical flow. You should use this method when the flow field enters and exits axially. The Sliding Mesh method is more robust in that it accommodates unsteady flow fields as well as accounts for the flow field not entering and exiting axially.

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