CFD Wind Tunnel System
T.Barnaby - Rotary Racer - Chipping Sodbury School
This page gives information on how to use the Greenpower Car Virtual Wind Tunnel system that was developed to simulate the Rotary Racer's aerodynamics performance. Under the hood it uses the Open Source OpenFOAM CFD (Computational Fluid Dynamics) software together with some pre-processing, post-processing and web interface code. This uses heavy weight mathematics to simulate the airflow over the cars shape in a virtual wind tunnel.
To uses the system the following tasks have to be undertaken:
- Create the cards 3D design: This is the hard part! You need to create a suitable 3D drawing of your car to scale. This only needs to define the outside shape of the car so details such as axles, motors etc are not needed. Various 3D CAD packages can be used for this see the information later.
- Export the design in STL Format: The VWT needs the cars design in an STL format file. Most 3D CAD packages can export to this format.
- Start off a VWT Simulation: You now need to login to the VWT website and add a new simulation. The STL file is uploaded to the website at this stage and a simulation processing run is started. The simulations are added to a queue and the results will be abailable after some time depending on the resolution of the simulation used and how many others are using the system. The minimum time is about 5 minutes.
- Look at the Results: The VWT website will list the results of your simulation including models frontal area, Cd factor and the forces etc. It will also provide the wind tunnels 3D data for viewing streamers etc. This is in a VTK format file than can be dowenloaded from the VWT website and viewed using the paraview program.
Although the VWT produces an absolute drag force in Newtons and a Cd value, these values will not be that accurate. The VWT is best used to determine if design changes increase or decrease the drag value with respect to another similar design.
Creating a 3D car design
It takes a lot of knowledge and experience to effectively use a 3D CAD package. This section just provides a basic overview, please follow your 3D packages tutorials to learn how to effectively use it.
For the VWT, the cars design has to be drawn, to scale, in a 3D CAD program.This only needs to define the outside shape of the car so details such as axles, motors etc are not needed. Any 3D CAD program can be used that can export the design in STL (stereolithography) format. These include: Autodesk Inventor, Siemens SolidEdge, PTC Creo, FreeCAD, Google Sketchup or Blender. We will provide some general information on using each of these if/when possible. Some points on creating the design:
- The design should be created/exported in mm or metres scale. If given in metres the VWT will automatically scale to mm.
- Ideally the designs orientation should be such that the cars side is in the XZ plane with the cars front to the left. So the cars length is along the X axis, the width is in the Y direction and the height is in the Z direction. If not it is possible to rotate the design during import to the web based VWT.
- The design should be a full enclosed single surface model. The quality of the surface can cause issues if there are gaps in the surface etc.
- The 3D cad design must fit within the VWT size (X(-4m - +4m) Y(-2m - +2m) Z(0m - 3m)). The VWT will position the car automatically.
Most 3D CAD packages work in a similar way. Generally you draw 2D sketches and extrude these into 3D to form the model or to cut away parts from the model. It is generally worth creating at least two parts, a body part and a wheel part and assembling these into a car assembly. Some notes:
- To draw a basic car body you can create a 2D sketch of the cars side in the XZ plane and then extrude to get the width in the Y direction. This gets a basic body shape.
- Looking down on top of the car the shape from the top can be created by cutting away from the body to give the basic curves.
- The edges of this body can now be rounded to get a reasonable shape.
- The cockpit can then be cut out from this and roll-bar, fairings and drivers helmet added as parts.
- Wheels can be quickly created by drawing a 2D profile from the axle out to the tyre looking from the top of the wheel and then creating a 3D module by rotating this 2D sketch about the axle.
The following videos, by Gareth of the Rotary Racer team shows how to create a basic car. These were done using PTC Creo, but the overall method is applicable no matter what package you use.
Usage of various 3D CAD tools:
Start off a VWT Simulation
In order to run simulations you need an account on the Greenpower VWT website. Please contact email@example.com to obtain one of these. There are only a few teams allowed access at the moment for resource reasons.
- Login to the VWT website: http://vwt.beamweb.co.uk
- Go to the "VWT" page using the top menu: http://vwt.beamweb.co.uk/vwt
- Click on the Add new Simulation link in the left hand menu.
- Fill in the title and add the STL file containing the simulation, the other fields can be left alone. Click on the "Upload file" button.
- A picture of the design should appear. Check the design is in the correct orientation: cars side show with the front at the left. If this is of the wrong rotation enter in values (normally 90 or -90) in the RotateX/RotateY and/or RotateZ fields and click on the "Change parameters" . Note that the system applies the x then y then z rotations and so thought as to which values to enter needs to be done!
- Click on the "Process in VWT" button to queue the job for simulation.
- Go back to the list of jobs and wait until the status for your job is finished (reload web page occasionally).
- You will now see the results and can download the VTK file to view the streamers pressure etc.
Generally the default parameters are fine for a car simulation run. They can be changed if wanted. The parameters are:
|Title||A suitable title for this simulation. It is worth adding shorthand information on which design this is based on and the changes made to it. So something like: "RR9E-longertail1" would be good.|
|Public||Make this simulation viewable by all|
|Resolution||The approximate number of cells for the VWT processing run. The more the better the VWT accuracy but the longer the processing run. 1000000 normally gives good results.|
|Height||The height of the car above the ground. This does need to be at least 20mm otherwise the simulation run is un-predictable.|
|Speed||The airflow speed in km/H|
|Rotate[XYZ]||The angles to rotate the design prior to simulation. Note that these are applied in order X, Y, Z.|
|Description||A user description of this design/simulation run|
VWT View Results
The website lists the core overal results on the VWT page. Howvere it is worth downloading the full 3D results in the VTK file and using the paraview program to view the results using streamers and preasure colours etc. See the following page for more details.
The VWT mathmatics is quite complicated and can fail for certain designs. If you get an error the "error" staus will be flagged. You can look at the processing run's log to see what may have happened.
Generally check your cars design to make sure the STL file is a single enclosed design with no gaps etc. There are some STL "clean up" tools that can help.
VWT Website Operation
Each VWT user has an individual account of the VWT website. They can also be associated with a Greenpower team (group). Each group has its own web page that a groups member can edit if we provide them with access permissions to do so.
By default VWT simulations are only visible to the user and anyone in their team. However, individual simulations can be marked as public ones if wanted so that anyone can view them.
The simulations are queued for processing. Our main servers may be in use by the company or powered off to save power when you submit the simulation. If powered off they will be automatically powered up to run the simulation and powered off automatically later. So it may take some time before the results are available. A typical simulation run at the 1000000 cels resolution will take about 5 minutes depending on the number of servers available.
The system is designed so that minimal special IT is needed in the schools. The only things needed are a suitable 3D CAD package and the paraview 3D viewer to be installed on schools computers. All of the VWT processing is done on our parallel servers.
If you can provide any textual help for pupils please let us know and we will ad this to this website.
the VWT size (X(-4m - +4m) Y(-2m - +2m) Z(0m - 3m)). Also the design should have just one complete surface with no breaks. Errors can easily occur. A "cryptic" error message will be produced in this case.
The web VWT is simple to use, but limited in features. It runs on a Linux based computer cluster with 16 upwrads CPU's.