Viewing a fluid world

New moving grid features make it possible to use computational fluid dynamics to simulate internal gear pumps and substantially improve their performance.

The challenge of simulating a gear pump is that the walls containing the fluid flow are continually moving, requiring that the shape of the CFD mesh change as well.

In the past, applying CFD techniques to gear pumps was somewhat problematic in that traditional CFD codes are designed for problems in which the solid walls that confine the fluid flow are fixed in position. This restriction rules out useful modeling of internal gear pumps because it can’t account for the motion of the gear teeth.

Recently, however, CFD codes have added moving grid features that allow for the motion of walls in the computational model.

This feature expands the capabilities of CFD to a wide range of new problems such as internal combustion engines, positive displacement pumps, etc.

In one case, CFX-TASCflow, a general 3-D Navier-Stokes fluid flow solver developed by AEA Technology, was used to model a ge-rotor (generated rotor) gear pump with nine lobes on the outer gear and eight lobes on the inner gear.

CFX-HEXA was first used to create the simple suction and discharge port grids. The generalised grid interface was used to connect the suction and discharge port grids to the rotor grid. This feature provides attachment or unstructured periodic connection between any type of hexahedral grid regardless of the size or shape of the geometry and mesh topology.

Since the nodes at the mesh connections need not match, the grid generation process is simplified and greatly accelerated. The grid interface is updated by CFX-TASCflow at the start of each new time step after the rotor grid has been moved to its new position.

Starting with the known gear teeth profile at one position, the analyst formulated a simple transformation that specified the gear teeth coordinates for all other positions.

This transformation was coded into a Fortran user subroutine. A local executable was created that compiled and linked the analyst’s source code. At each time step, new positions for the gears were determined and the grid between the gears was recomputed.

The analyst enabled the required control parameters for the moving grid feature as well as the transient output of the grid coordinates and ran the simulation using the local executable that included the custom user subroutine. They then post-processed the results, taking care to account for the fact that the grid coordinates change at each time-step.

The simulation results showed the flow velocity and pressure in the inside of the gear pump over a time sequence as it revolved. They illustrated losses produced by gear meshing and the influence of the suction and discharge ports that reduce the efficiency of the pump.

The information that is generated by the simulation is far greater than what can be achieved with physical testing. In addition, CFD models can be created and modified in much less time and at a lower cost than is required to build a hardware prototype.

The bottom line is that CFD now makes it possible to design better gear pumps in less time and at a lower cost.

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