Drivetrain Oiling Simulation

Geometry Cleanup and Model Organization

If you have a gear-train assembly with protruding shafts or axles, make sure you cut them up such that they are slightly offset inward from the wetted surface of the housing.

The gap shown in Figure 1 between the shaft and the housing will allow SimLab to recognize the volume without confusion. The gap between the wetted surface and the shaft is recommended to be on the order of the particle resolution.

Figure 1. . Cross-section view of the shaft protruding outside of the gearbox housing (left) and the correctly shortened shaft with closed housing (right).

There are several scenarios on how to approach bearings modeling:

Bearings are fairly large and there are only a handful of rolling elements: Best practice: Leave the bearings and the rolling elements as they are, no need for any simplification.
Bearings are medium in size and have a considerable cage that holds the rolling elements: Best practice: Remove the cage. The remaining space will typically allow for some flow through the bearing.
Figure 2. . Full bearing geometry, including the cage on the left. Simplified bearing geometry with removed cage on the right. With moderate resolution, typically several particles can pass between the rolling elements.
Many small rolling elements with or without the cage: Best practice: Remove every other rolling element to create sufficient space in the bearing for the flow to go through. Remove the cage as well, if present.

In terms of kinematic approximations, there are three options:

Option 1: keep the entire bearing static.
Option 2: keep the outer ring static and move the rolling elements and the inner ring at the same RPM as the shaft/axle.
Option 3: keep the outer ring static and move the rolling elements at half the RPM of the shaft/axle and the inner ring at the same RPM as the shaft/axle.

While technically option 3 is the most accurate representation, it is also introducing several additional steps during the pre-processing (more motion definitions, introduction of another nanoFluidX phase). Given that completely accurate flow through bearings is typically beyond the reach of the most common nanoFluidX simulations option 2 provides the best trade off between invested pre-processing time and accuracy of the solution.

Technically, it is possible to define the planetary motion of each individual rolling element, but without sufficiently high resolution (extreme resolution), such pre-processing detail would not bring much in terms of solution quality.

Solution Settings

Particle Spacing: Ideally aim to resolve the tip of the gear (have 3 particles at the tip of the gear tooth). However, in reality this is not always practical as it results in an excessive no. of particles. Under-resolved tips start to behave as a porous media, since they would be partially transparent to the surrounding particles. In the majority of cases, it is tolerable to have 2 particles at the gear tooth tip as there is minimal influence on the flow field.
Simulation Duration: For drivetrain applications, it is recommended to simulate 100-150 rotations of the gears. Typically, this results in a duration between 0.5 and 10 s physical time.

Export

Prior to the export of the solver input files, it is recommended to perform a Data Check to avoid possible inconsistencies and errors.