Autodesk CFD has the capability to analyze rotating devices such as fans or impellers using a locally rotating frame of reference. The region is indicated as a material that fully envelops the rotating object. All other volumes in the analysis will remain static, while the Rotating Region uses its own relative frame of reference. This approach is more accurate than a true angular motion simulation and has a significantly shorter runtime. As a note, this type of study requires the Motion solver that is only part of the CFD Ultimate platform.
The first and most important step of analyzing a rotating region is creating the correct CAD geometry. There are some preparations and simplifications that need to be made for the CFD solver to work properly. Here are some of the key elements:
- The rotating part must be completely immersed in the rotating region. The parts can be modeled as voids or solid bodies. Immersed solids will rotate at the same speed as the region.
- The diameter of the rotating region cylinder should extend roughly halfway between the outer edge of the blades and the closest point of the non-rotating wall.
- The volume surround the rotating region (periphery zone) should be completely filled with a fluid such as air or water.
- Inlet and outlet volumes should typically extend at least 3 times the fan diameter. This will help with achieving a converged result.
As shown in the figure below - a simple cylindrical volume is created that envelops the fan geometry. This will represent the rotating region. The remaining volume is filled by air.
Looking at the cross section of the Fusion 360 CAD model, you can see that there is a small amount of fluid between the edge of he rotating region and the side wall of the assembly. This is an important part of the modeling process.
CFD Materials and Boundary Conditions
Once the model is loaded into CFD, the boundary conditions are added. Rotating regions are added as a material assignment. They can be applied to analyze 3 different scenarios based on certain known values:
- Know rotational speed (RPM)
- Known Driving Torque
- Free Spinning (flow driven)
In this example, the rotational speed of the fan is known to be 2500 rpm. This analysis can be used to determine the Volume Flow Rate based on the geometry of the fan blades. The rotating volume is defined as a rotating region with a RPM of 2500 about the Y-axis.
The CFD solver should automatically reduce the mesh size of the rotating region. If it doesn't, make sure the mesh is sufficiently fine to prevent convergence issues. Apply the desired boundary conditions for pressure, temperature, etc. For an open condition - apply a Pressure = 0 at the inlet and outlet.
For the solver settings, it's recommended to start with a time step of 3 deg / step. The time step calculator uses the RPM to calculate an appropriate time step for you. Save intervals are only necessary if you're planning to create an animation. The additional result sets will use up a lot of memory, so try to save no more than 20-30 steps for the full analysis. Cloud solving is also a great option if your computer can't handle the motion solver efficiently.
Cut planes and traces are useful for observing velocity and cavitation. To determine the Volumetric Flowrate, use the Bulk Results calculator on a plane near the inlet or outlet.