Table of Contents
Geometry Generation for Cart3D Tutorial
This component of the Cart3D tutorial will walk through the process of creating a simple geometry to analyze with Cart3D. For this example, we will build a model of the Onera M6 wing.
Onera M6 Definition
The Onera M6 wing is a standard transonic test case frequently used to demonstrate CFD codes. The geometry, experimental setup, and experimental results are documented in AGARD AR-138. These results are also summarized by the NASA NPARC Alliance Validation Archive.
The Onera M6 wing uses uses an Onera D airfoil normal to the 40.18% chord line. VSP generally works with airfoils specified in the streamwise direction. The NPARC Alliance web site includes a Fortran program to transform the airfoil coordinates; it also includes an airfoil file which results from the transformation. That file was modified to create an airfoil in the VSP airfoil format. The Onera M6 airfoil file is available here.
The planform of the Onera M6 wing is depicted in the following figure pulled from AGARD AR-138. This image has been cropped and rotated slightly so it is ready-to-use as a background image in VSP; you may want to save a copy of the image for that purpose. The simplicity of the Onera M6 makes use of a background image somewhat unnecessary, but this does provide a good example of using a background image.
The Onera M6 wing is described by the following parameter values
- Semispan 1.1963 m
- Root chord 0.8059 m
- Taper ratio 0.562
- Aspect ratio 3.8
- Sweep (LE) 30 deg
Building the Onera M6
As you proceed through the tutorial to build the Onera M6, remember that images have been included as thumbnails, so any place you see a picture, click on it for a larger version.
- Run OpenVSP
- Add wing component
- Add a MS_Wing component to your model. Depending on your defaults file, your new MS_Wing component may look different from this one.
- In 'Geom Browser', choose 'MS WING' from the component type pulldown.
- Delete unneeded wing sections
- The Onera M6 wing has no planform or twist breaks and therefore can be modeled with only one wing section.
- In 'Multi Section Wing Geom', select the 'Sect' tab.
- Repeatedly click the right-facing arrow by 'Section ID:' until the most outboard wing section is active.
- Change model view
- Add background image
- VSP can display a background image in the main graphics window. This is often a good way to start a model when all you have is a three-view drawing. The Onera M6 geometry is well specified, so this step is not strictly needed. However, it still provides a good sanity check.
- Save the Onera M6 planform drawing above to your hard drive.
- If needed, you can scale and offset the background image so that it fits well in the display window. Close the 'Background' window when you are satisfied with the background display.
- In the 'View' window, click 'Top' (or press F5) to change the view orientation to a top-view of the model.
- Change driver group
- There are six canonical interdependent parameters frequently used to describe a trapezoidal wing – aspect ratio (AR), taper ratio (TR), Area, Span, tip chord (TC), and root chord (RC). Usually, if three of these parameters are specified, the other three can be calculated. VSP lets the user choose which parameters to specify when building the model.
- Set span and adjust view
- As the model is updated, the view can be adjusted such that the model and the background image coincide.
- In the 'Section Planform' area, enter '1.1963' into the box next to 'Span'. This is the semi-span of the Onera M6 wing in meters.
- Set wing planform
- The published Onera M6 wing parameters over-specify the wing shape and because of the limited precision included in the report, the published parameters are not perfectly self-consistent. Consequently some amount of compromise is required when modeling this wing. Each independent parameter is set by entering the value into the box to the right of the parameter.
- Set the sweep to 30 degrees.
- Set the taper ratio to 0.562.
- Set the aspect ratio to 1.9. VSP uses the aspect ratio of a single section instead of the whole wing. Consequently, the aspect ratio is 1.9 instead of the published value of 3.8.
- Change driver group
- Notice that the calculated root chord (0.8062) does not perfectly agree with the published value (0.8059). This is a direct result of the limited precision of the specified parameters of the Onera M6 wing.
- Set the root chord to 0.8059.
- Note that the aspect ratio is now 1.9004 (instead of 1.9) and the taper ratio is now 0.5622 (instead of 0.562). Increasing the tip chord will drive aspect ratio closer to the target value - but will also drive taper ratio further from the target value. The parameter values match the published values - within the published significant figures.
- Set airfoil section
- As described above, the Onera M6 uses the Onera D section orthogonal to the 40.18% chord line. This section has been transformed into a streamwise airfoil and put into VSP airfoil format.
- Download the transformed Onera M6 airfoil and save it to your hard drive.
- In the 'Select File' window, navigate to the location where you stored the airfoil, select the file, and click 'Accept'. This will set the root airfoil section.
- Click the right-facing arrow by 'Airfoil' to change control to the tip airfoil section.
- Repeat the 'Read File', navigate, 'Accept' sequence to set the tip airfoil to the Onera M6 section.
- Round the wingtips
- The Onera M6 has rounded wing tips – while the VSP default is a truncated wing tip. Fortunately, VSP also has built-in support for rounded wing tips.
- Change part name
- Although this model only has one component, it is a good practice to give each component in a model a meaningful name.
- In this example, the background image has also been deactivated by clicking 'Window', 'Background…', 'JPEG Image', and then 'Cancel'.
- Save model
- The basic definition of the Onera M6 model is now complete, so this is probably a good time to save your work.
- When you are satisfied, click 'Accept'
Once the geometry definition is complete, the next step is to generate a triangulated surface mesh for analysis in Cart3D. This process is detailed on the Mesh Generation page.
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- Geometry Generation (This Page)