Gerotor chamber Meshing

This section explains the settings for the gerotor chamber mesh generated using the Rotor Template Mesher. This involves selection of the surfaces, directional characteristics of the mesh and other advanced mesh settings.

The parameters related to Gerotor meshing can be accessed by setting the Setup Options to Advanced Mode, as shown in Figure 6.71

Figure 6.71 - Gerotor template mesher

Inner Rotor

This is used to assign the inner rotor of the gerotor as follows:

Select the inner rotor surface under CAD Surfaces in the Geometric Entities Panel.
Click Add Surfaces icon to the right of Inner Rotor, or click Select Inner Rotor in the Properties Panel.

Outer Rotor

This is used to assign the outer rotor of the gerotor as follows:

Select the outer rotor surface under CAD Surfaces in Geometric Entities Panel.
Click Add Surfaces icon to the right of Outer Rotor, or click Select Outer Rotor in the Properties Panel.

Note: The boundary conditions are automatically set by the template for the assigned surfaces.

Rotor Mesh Size

This allows to control the resolution of the mesh created in the gerotor pumping chamber as:

Coarse
Normal
Fine
User Input

Figure 6.72 - Rotor mesh size- Gerotor

Figure 6.73 - Gerotor mesh sizes

The control parameters associated with User Input (see, Figure 6.74) are:

Points in Rotational Direction: Corresponds to the number of cell nodes in the rotational direction along the circumference of the pumping chamber. The number of control cells in this direction is (Points in Rotational Direction – 1).
Points in Radial Direction: Corresponds to the number of cell nodes in the radial direction from the center to the edge of the pumping chamber. The number of control cells in this direction is (Points in Radial Direction – 1).
Points in Axial Direction: Corresponds to the number of cell nodes in the axial direction from the bottom to the top of the pumping chamber. The number of control cells in this direction is (Points in Axial Direction – 1).

Figure 6.74 - User input mesh size- Gerotor

Shaft Type

The template supports two different shaft types:

Fixed: This shaft type is used for modelling gerotor pumps and motors with an inner gear shaft that rotates around a fixed center. The outer gear set is driven by the inner gears and rotates as well.
Orbiting: This shaft type is used for modelling gerotor pumps and motors, in which the inner gear shaft orbits and the outer gear set stays fixed. This parameter sets the outer rotor to remain stationary and the center of rotation of the shaft to orbit.

Note: Orbiting shaft type also activates Rotating Port as an additional boundary condition.

Figure 6.75 - Shaft type- Gerotor

Number of Inner Teeth: Specify the number of teeth in the inner rotor.

Number of Outer Teeth: Specify the number of teeth in the outer rotor.

Inner Rotor Center: Specify the center point of the inner rotor in the pump. In practice, it can be specified anywhere along the axis of symmetry in coordinates.

Outer Rotor Center: Specify the center point of the outer rotor in the pump. In practice, it can be specified anywhere along the axis of symmetry in coordinates.

Figure 6.76 - Rotor teeth and centers- Gerotor

Note: Although the Number of Inner Teeth or Number of Outer Teeth do not affect the mesh generation, they are typically specified during the meshing operation and then automatically copied over to the Pump Configuration menu.

Rotational Axis Vector

The direction of the axis of rotation of the inner rotor in the laboratory reference frame, specified in coordinates.

Figure 6.77 - Rotational axis vector-Gerotor

Meshing Method

This provides the following two methods:

Radial: The mesh lines radiate from the rotor center (intersecting circumferential mesh lines) and terminate at points on the outer rotor outline. This is the default method of meshing.
Linear: This is designed to mesh rotors with complex outlines, where the mesh nodes are distributed on the inner/outer rotor outline directly. It is an advanced method, currently only recommended if meshing difficulties arise using the Radial method. Once Linear meshing is selected, Smooth Rotor Mesh is also activated since it usually requires a certain amount of smoothing to remove any negative interior cell volumes which might be created. The Linear method is better suited for gerotor outlines shaped less smoothly in the circumferential direction (i.e. containing protrusions or indentations).

Figure 6.78 - Meshing method-Gerotor

Smooth Rotor Mesh

This allows to improve the gerotor mesh using a mesh smoothing algorithm, which maximizes cell face orthogonality and optimizes the cell density distribution.

Figure 6.79 - Smooth Rotor Mesh

Max Iterations: Specifies the number of iterations the smoothing algorithm has to run for. The default value is 15, usually optimal for most cases. However, user can adjust this to capture any desirable mesh characteristics which could prove useful for a specific application. To illustrate the effect of this value, as shown in Figure 6.80, the choice of Max Iterations = 100 proved to be excessive, since the mesh quality was reduced compared to the mesh created using the value Max Iterations = 15.

Relaxation (0 < r < 2): Specifies what percentage (i.e. 70% = 0.7 for relaxation value) of the calculated node (cell volume corner point) position displacement increment calculated during the smoothing process is used for the final mesh point locations at a given time-step. Low values of relaxation show that the resulting mesh is almost indistinguishable from a mesh made using Meshing Method: Radial. A relaxation value of 0.5 creates a mesh that appears to be an average of a mesh created using the default Radial meshing method and a mesh created using the Linear meshing method. The default relaxation value of 1.0 (see, Figure 6.81).

Figure 6.80 - Smooth Rotor Mesh-Max Iteration

Figure 6.81 - Smooth Rotor Mesh-Relaxation (0<r<2)

Figure 6.82 - Visual Representation of Relaxation

Helix Angle Rotor

This allows to specify the angle between the rotor axial inclination and the rotor axis. It can be specified for either the Inner Rotor or Outer Rotor. During the mesh generation, the Helix Angle is used to specify the correct slant of the rotor whereas, the CAD Surfaces are used as bounds for the mesh to obtain a water-tight volume.The helix angle value is prescribed under Angle.

Figure 6.83 - Helix Angle