Cases

Following a successful benchmark case study, Westland Helicopters Limited (WHL) has decided to use 3-matic software for generating surface meshes for their CFD1 projects. WHL wished to expand the range aerodynamic data available for this helicopter beyond that accessible from wind tunnel testing. This required the generation of a new CFD model of the helicopter. During the project, Westland’s Fluid Dynamics and Environmental team faced several challenges, which they discovered could be tackled easily using 3-matic.

3-matic-Remesh reduces time needed to generate watertight surface meshes

Lacking a 3D computer model of the helicopter, a subcontractor scanned the surface of a wind tunnel model. Reverse engineering the scanned data resulted in a file that contained too many surfaces (534MB STEP file) to read into their surface-meshing tool, Gambit2, on the first try. The data could be processed after the number of surfaces was reduced manually. Unfortunately, two complex but significant details, the FLIR and winch, were omitted completely, because these objects were poorly scanned and the subcontractor couldn’t repair the data. In addition, the supplied surfaces didn’t join to form a consistent shell. At least one of the surfaces supplied was physically impossible to create a real solid. It wasn’t feasible to proceed with this project without a significant effort to repair the bad geometry. However, the missing components would still be missing. At this stage Materialise flew to Westland’s aid, providing them in a few days with a high quality surface mesh that comprised all components.

Fast preparation and repair of the defective scanned data

The omitted elements were essential in this project. That’s why the first thing the 3-matic team did was to repair the defective data. This process involved segmentation of the FLIR and bracket and repair work to the surface mesh, like the removal of all overlapping and intersecting triangles. Next, our 3-matic engineer gave the winch body model the same treatment. The scanning process didn’t capture all features completely. The winch brackets – connecting the winch to the helicopter body – for example could only be scanned partly (see Fig. 1a). However, these brackets were easily reconstructed in 3-matic by sketching and creating a cylinder on the scanned data (see Fig. 1b).

Rapid import and repair of the reverse engineered data

Materialise staff also generated the surface meshes for the bad geometry in the reverse engineered surfaces supplied by the subcontractor. See figure 2.a & b for an example.

All this repair work took about 17 hours. Had the original scanned data – instead of the often imperfectly reverse engineered surfaces – been available, the 3-matic team estimates that they would have been able to create a quality surface mesh in less than a third of that time.

3-matic-Remesh reduces time needed to generate watertight surface meshes

As things stood, the surface mesh still contained many skewed triangles. Such triangulation effectively prevents the creation of a good volume mesh. Using 3-matic’s range of highly automated tools, we were able to remesh the surface mesh quickly and easily (see Fig. 3a&b). The degree of skew in the resulting high quality triangle surface mesh conformed to the conditions imposed by CFD software, in this case Gambit or Tgrid from Fluent.
- More about mesh optimization via 3-matic's Remesh module

The optimized mesh was then grouped into faces for export to the Fluent file format (see Fig. 4). With a single mouse click we created a box around the model, which is a highly labour-intensive task in CFD pre-processing software. The box enabled the creation of volume meshes representing the air around the helicopter, necessary to analyze the air flows. It took us only ten hours to transfer the 3D model into a mesh ready for flow analysis.

Good CFD results of optimised model

Further, Westland could turn these faces into volume meshes using TGRID. Later, they solved and post-processed the volume meshes using FLUENT software (see Fig. 5a & b). Mr. Baverstock adds: “The CFD results of the model supplied by Materialise were very good; for example, the drag of the aircraft at the design point was overestimated by less than 4% compared with the wind tunnel tests.”

Some stunning numbers:

File provided by subcontractor

• 534MB STEP file

File generated by 3-matic team:

     Final surface mesh:

• 224 184 triangles
• 0,5 max skew

     Final volume mesh:

• 1 244 584 volume cells
• 0,86 max skew

Repair and re-meshing of the scanned and reverse engineered data took only 27 hours.