WIND TUNNEL SAFETY & OPERATING PROCEDURE

8. CFD – WING MIRROR ON FLAT PLATE

8.1 GAMBIT: MODELLING

To create this virtual wind tunnel the most commonly applied approach is to:

1. 
   import the model geometry, which can come in various formats from several software types;
   
2. 
   create a real volume of suitable dimensions around the imported model geometry;
   
3. 
   subtract the model geometry from this brick geometry, thus leaving one volume which will represent the flow domain;
   
4. 
   apply geometry clean-up measures to the geometry, to reduce any complexities in the model that could represent problems when meshing;
   
5. 
   mesh all the faces in the model including the domain walls;
   
6. 
   mesh the volume of the model;
   
7. 
   set the boundary conditions and the continuum type;
   
8. 
   export the final mesh for use in a CFD package.
   

To fix this discrepancy, a ‘real’ face was created over the recess, essentially closing it in. This enclosed region was then transformed into a real volume using the ‘stitch faces’ command, and the resultant volume was then merged into the rest of the wing mirror casing using the ‘merge volumes’ command. These operations resulted in a flat surface over the area in which the recess was found (Figure 11).

With this discrepancy resolved, the model was then deemed an accurate virtual representation of the actual wing mirror.

The next step was to create the volume around the wing mirror geometry from which the wing mirror geometry would then be extracted. This was carried out by using the ‘create real brick’ command, and entering the values of length, height and width as 4000x1500x1500mm respectively (a rough approximation of the wind tunnel test section). This volume was then aligned (using the ‘move/align’ function) with the wing mirror base, with the wing mirror centrally positioned on one of the 4000x1500mm faces of the brick.

With all the volumes in their desired position, the volume subtraction could then be performed. The first volume subtracted was that of the wing mirror mount, Gambit performed this ‘subtract volume’ operation successfully and as a result created a new volume which represented the brick volume with a cavity in the form of the wing mirror mount. However, when attempting to subtract the wing mirror housing from this newly formed volume, an error message of ‘coincident face_face_ints with different body vertices’ was displayed. This problem arose because around the area where the wing mirror casing and mount are hinged, some of the faces on each volume were coincident or overlapping. To rectify this issue, the wing mirror casing volume was moved away from the mount by a few millimetres on each axis to prevent face overlapping or coincidence. The volume subtract operation was then tried again and was successful, thus the desired single volume was created.

With the flow volume defined the geometry could then be ‘cleaned’. This process consisted of eliminating any sharp angles or short edges on the geometry and merging small faces together to reduce the complexity of the model. To eliminate short edges on the model, the ‘connect edges’ tool was used to highlight the shortest edge present in the geometry and this was consequently merged with its neighbouring edge to form a larger edge. This process was repeated until the shortest edge highlighted was no longer judged too short to cause meshing problems. The ‘merge faces’ tool was then used to merge any small or awkward shaped faces into its neighbouring face, thus reducing the geometry complexity through producing larger more easily meshed faces.