Understanding Rotating Regions in SOLIDWORKS Flow Simulation

By GSC
SOLIDWORKS Flow Simulation offers some great tools for analyzing rotating region problems. There are a lot of capabilities within the tool but, often times, there isn’t a lot of information about these capabilities. This article is meant to be a comprehensive guide to everything related to rotating region problems within SOLIDWORKS Flow Simulation. This will include explaining the different types of rotating region problems as well as the best practices associated with each one of them. There are two main types of rotating regions in Flow Simulation; global and local rotating regions. Both of these types are available for all fluid types; Newtonian and Non-Newtonians fluids.

Global Rotating Region

The global rotating region assumes you have a model that is completely symmetric about the rotating axis and all the components within the computational domain are rotating at the speed of the rotating reference frame.
Due to the fact this type of rotating region has such narrow requirements, this type is often not suitable for most applications because most industrial applications have components that are not rotating or are not symmetric about the rotational axis. If the case comes up where all components are symmetric about the rotational axis but there is a stator (components that are not rotating), stator faces can be specified through the use of a moving wall boundary condition.

Local Rotating Region

The local rotating region is very similar to the global except here you can specify a subset of components within the assembly that can be part of the rotating region. This greatly increases the amount of applications where rotating reference frames within SOLIDWORKS Flow Simulation can be used. The local rotating region requires that you specify a separate component that includes all the rotating components. The software will calculate the flow fields outside of the rotating reference frame and then transfer them to the rotating region at the boundary. There are two additional formulations that can be used for a local rotating region, the Averaging method and the Sliding Mesh method. The characteristics of the rotating components and the type of flow analyzed will determine which of these methods is appropriate.

Averaging

The Averaging method, also sometimes called the Mixing Plane method, is an approach developed in order to approximate the flow of rotating blades such as fans, propellers, etc. It does a good job of producing results that correlate well with experimental data for these types of applications. It’s important to be aware that using this method for anything other than geometry that reassembles a hub with axisymmetric blades may result in data that does not match reality; this includes things like helixes or geometries with holes. It is important to be aware of the following requirements when using the Averaging method:

• The inlet and outlet flow fields should be axisymmetric with respect to the rotational axis; this simply means the fluid needs to enter and exit the blades along their center axis.
• If gravity needs to be considered in the analysis, it needs to be along the axis of rotation.

Important to mention are a couple of limitations also associated with the averaging approach:

• This approach cannot be used with high Mach number flow fields
• The Averaging method assumes a steady state solution within the rotating region even for transient analyses

Sliding Mesh

The Sliding Mesh method is for more complex applications that do not fall under the specific requirements for the Averaging method. This is useful in situations where the flow field is non-uniform along the circumferential direction. This can be very useful in both traditional rotating components that resemble fans as well as non-traditional components such as Archimedes screw or helixes. The following are a few guidelines when it comes to using the Sliding Mesh method:

• Use only for rotating geometry, translational geometry such as pistons or hydraulic cylinders cannot be analyzed
• There is no limitation on the direction of gravity if it is considered in the analysis
• Due to the unsteady nature of the flow fields, Sliding Mesh only works in time-dependent analyses
• Only useful in cases where the rotating geometry drives the fluid
• It is assumed the boundary between the rotating geometry and enclosure (rotors/stator) does not change
  • Such an example cannot be simulated in the closing of a valve
• The rotating region cannot intersect with the non-rotating (stator) walls

General Best Practices

There are several best practices that go along with working with rotating region problems. These best practices help rotating region problems converge and give more accurate results.

• It is recommended when working with rotating reference frames that an internal analysis is used rather than an external analysis. This tends to be a bit counter-intuitive but the reasoning lies within how external analysis is formulated behind the scenes. External analyses use something called far-field pressure conditions at the boundaries of the computational domain. This can cause issues with convergence in rotating region problems. In cases like this, internal analyses are preferred. It is suggested that the rotating component is encapsulated by a hollow cylinder and environmental pressure conditions be applied to the inside faces of the cylinder. It’s important to make sure the cylinder is large enough to not interfere with any flow field gradients.
• In cases where two or more local rotating regions are required in the analyses, it is important to make sure the rotating regions do not intersect with each other.
• If the rotating region is centered between an inlet and outlet, it is recommended the inlet condition is a flow rate or a pressure boundary condition and the outlet condition is a pressure boundary condition. Flow Simulation will automatically calculate the inlet and outlet values at the rotating region. It is not necessary to define boundary conditions at these locations.

Rotating Region Shape

In order to ensure accurate results, it is important to pay attention to the shape defined as your rotating region component. The requirements for the shape of your rotating region component are the same no matter what type of rotating reference frame you are using; Averaging or Moving Mesh.

• The rotating region must fully enclose all of the rotating components unless it is combined with a Moving Wall boundary condition
• It’s recommended that a Moving Wall is applied to rotating faces that are axisymmetric to the rotational axis. This means whenever possible try and have the rotating region only encompass “blade-like” components.
• The rotating region should extend passed the tips of the blades to allow for adequate mesh refinements
• The rotating region must be axisymmetric and centered on the axis of rotation
• The rotating region can be in the shape of a torus but cannot be a sphere

In the case of the Averaging method, if the rotating components are enclosed by a non-rotating axisymmetric component it is recommended the rotating region extends into the non-rotating component and the intersecting faces defined as a Stator wall.
The following is an example of how to apply a rotating region to this wheel component. The rotating region should fully encapsulate the blades and extend a bit beyond the blades. It should be centered half way through the thickness of the flanges. All faces are not fully covered by the rotating region should have a Moving Wall boundary condition applied to them so they have zero movement relative to the rotating region.

Time Step

The automatic time step chosen by Flow Simulation will always give good results but often times this time step is very conservative and smaller than is necessary. For rotating region problems, the time step used can be approximated by the following equation:

This equation becomes invalid with a small number of channels between the blades. In this case or when in doubt, 1/200th of the period of rotation should be used.
It’s important to note the manual time step is different than how often results are saved.

• Don’t save results more than one full rotation
• Make sure that the results are saved often enough to capture both the rotation as well as the development of the flow fields

Mesh

• A trial and error process may be required to get an adequate mesh for rotating region problem. The goal is the get a uniform mesh throughout the rotating region with 3-5 cells between the blades.
• It’s also important to have 3-5 cells past the end of the blades in the downstream direction in order to accurately capture the flow field.
• At a bare minimum, it is suggested there are at least 2 cells between the end of the blades and the boundary of the rotating region. This rule of thumb should also be applied to any gaps between the rotating region and non-rotating component outside of this .

GSC

GSC fuels customer success with 3D engineering solutions for design, simulation, data management, technical documentation, and 3D printing, as well as the most comprehensive consulting, technical support, and training in the industry. As a leading provider of SOLIDWORKS solutions and Stratasys 3D printing technologies, GSC’s world-class team of dedicated professionals have helped numerous companies innovate and increase productivity by leveraging advanced technologies to drive 3D business success. Founded in 1989, GSC is headquartered in Germantown, WI. For more information about GSC, please visit www.gsc-3d.com