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Computational fluid dynamics (CFD) is often used by design engineers today as a tool for the analysis and optimisation of the fluid mechanics and thermal properties of machine and system components.

An important prerequisite for the development of efficient calculation models capable of making reliable predictions is the creation of numerically high-quality computational grids that mimic the computational domain as precisely as possible, but without producing a total of too many elements, as increasing model size (number of grid elements) can significantly lengthen the computation times of computer models.

The creation of such computational grids poses two particularly significant challenges for calculation engineers when working with rotary pumps, gear pumps and screw compressors, which make numerical simulation (CFD) of only very limited use for these machines. The complexity of the geometry to be recorded by rotational displacement machines results from the flow volumes in the working chambers of the machine which vary over time. The second challenge are the extremely small gaps between the rotors and between the rotors and the housing, because to calculate the gap flows properly, these areas must be resolved with a very large number of very small grid elements. There are unfortunately not yet many practical approaches for addressing the particular meshing challenges of the rotational displacement machines.

CFX Berlin Software GmbH rose to the challenge of tackling the problem with a brand new approach. With the help of new meshing software TwinMesh developed by CFX Berlin, for the first time, the time-varying flow volumes in the working space of rotational displacement machines can be automatically meshed with high-quality structured hexahedral meshes.

The workflow of TwinMesh thus needs only a few steps. First, the flow volumes of the machine in the stationary and rotating spaces are broken down in the CAD system or in the ANSYS DesignModeler. In the field of stationary components (inflow, outflow), automated high-quality grids can be generated as usual with e.g. ANSYS Meshing or ANSYS ICEM CFD. The rotating parts of the machine are read in as curves of the rotors and the housing contours in TwinMesh and then meshed in an automated fashion for a number of angular positions specified by the user.

The resulting computational grid consists exclusively of hexahedral elements (cuboid-shaped). The axial and radial grid resolution are defined by the user and special areas (boundary layer and column) can be refined separately on request. By using a smoothing algorithm contained in TwinMesh, the node distributions always remain homogeneous, and the interior element angle remains nearly orthogonal.  The generated computational grids for the rotating and the stationary parts of the machine are now assembled into a simulation model in the software in the final step. And here, too, CFX Berlin has come up with something special: for all typical displacement machines, TwinMesh provides special macros which also take care of the actual simulation setup in ANSYS CFD at the press of a button.

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