Creating an unstructured mesh with OpenVSP
OpenVSP, also known as Open Vehicle Sketch Pad, is an open source parametric aircraft geometry tool originally developed by NASA. It can be used to create 3D models of aircraft and to support engineering analysis of those models. You will use OpenVSP to create an unstructured mesh of a NACA4412 wing with an aspect ratio
of 6 and reference chord
Visit http://openvsp.org/download.php to download OpenVSP. OpenVSP is free and it runs on Windows, MacOS, and Ubuntu. Once you download the OpenVSP archive, extract it and run the OpenVSP executable. The image below shows the main OpenVSP window:
OpenVSP main window
The geometry browser window on the right allows you to create components. Components include but are not limited to:
Select wing from the component dropdown and click the add button. A wing with default parameters will be created. The image below shows the created wing.
Wing with default parameters
Open the Airfoil tab. Change the thickness to chord ratio "T/C", the camber "Camber", and the camber location "CamberLoc" for both the tip and the root airfoils. Use a thickness to chord ratio of 0.12, camber of 0.04, and camber location of 0.4. These values correspond to the NACA4412 airfoil. You can switch between the tip and root airfoils by clicking the "<" and ">" buttons.
Change the tip and root airfoils to NACA4412
Open the "Sec" tab. Change the span "Span", the length of the tip "Tip C" and root "Root C" chord, and the sweep "Sweep" angle to the following values - 3, 1, 1, and 0.
Change the tip and root chord to 1 and the span to 6
You should now have a NACA4412 wing of aspect ratio 6 displayed in the main window of OpenVSP.
Save the model by clicking "File" and then select the "Save As" option. Save the model as naca4412_wing.vsp3.
Any file name can be used. The file name above (naca4412_wing.vsp3) is for illustrative purposes only.
Save the model as .vsp3 file
Click on "Analysis" and select the "CFD Mesh" option.
Select the "Global" tab and click the "Mesh and Export" button. The model will be meshed using the default mesh settings.
Create mesh using the default parameters
The mesh of the model appears in the main window of OpenVSP. Immediately you can see that the mesh is too coarse near the leading edge. The elements in this region are too big and do not accurately represent the geometry of the wing.
Elements in regions with high curvature (such as the leading edge) should be small.
Resulting mesh using default parameters
To reduce the size of the elements in the region of the leading edge you can lower the "Min Edge Len" value. Set the "Min Edge Len" value to 0.025. Select "All" from the "Use set" dropdown and click the "Mesh and Export" button. The new mesh of the model appears in the main window of OpenVSP. Immediately you can see that the elements in the region of the leading edge are smaller and represent more accurately the wing geometry.
When you click the "Mesh and Export" button OpenVSP will create a STL file in the folder of the naca4412_wing.vsp3 file. The STL file contains the mesh of the model.
Always remember to check the coordinate system! As stated in the preprocessor section the mesh must be in a body-fixed coordinate system i.e. the x-axis must point forward, the y-axis must point starboard and the z-axis must point down.
Check the coordinate system of the model before meshing. The model should be in a body-fixed coordinate system i.e. the x-axis points forward, the y-axis points to the starboard, and the z-axis points down.
The model in the above example is not in a body-fixed coordinate system i.e. the x-axis points backward and the z-axis points up. Select the "XForm" tab and change the "YRot" value to 180. The model is now in a body-fixed coordinate system. Repeat step 9 to re-mesh the model.
Change the coordinate system of the model to a body-fixed one
After re-meshing the model you should have a naca4412_wing.stl file in the folder where you saved the naca4412_wing.vsp3 model file.
Congratulations! You created your first mesh using OpenVSP. Now you can use APM to analyse the flow around the NACA4412 wing.