Airpower VWT

Content

View
View Content

Account

Username
Password

Airpower VWT

T.Barnaby - Chipping Sodbury School

Introduction

This page gives information on the Airpower Car Virtual Wind Tunnel system developed to simulate the aerodynamics performance of the schools air powered cars. It has been developed using the Open Source OpenFOAM CFD (Computational Fluid Dynamics) software. This uses heavy weight mathematics to simulate the airflow over the cars shape in a virtual wind tunnel.
To use, the cars design has to be drawn, to scale, in a 3D CAD program. We have used the Open Source Blender program to achieve this although other programs can be used such as Desktop/Pro. The input to the VWT is a STL file. The cars design can then be simulated in the wind tunnel and the overall aerodynamic drag calculated. The VWT software can calculate the cars drag force in Newtons as well provide data for a 3D data viewing package, such as paraview, to allow streamers, arrows and coloured "hot spots" to be shown.
We have made this VWT software available from the web so that the pupils can use it to simulate the cars performance. This does not have the full features of the native software but is more accessible and easier to use. The software is not fully tested ...
The VWT simulator is at: http://portal.beam.ltd.uk/greenvwt

How To Use the Web VWT

  1. Create the design of the car using a 3D drawing program. The car should be drawn to scale in mm (ie about 295mm long). Its centre should be at X=0, Y=0, Z=0. The left hand side (-x) should be the front of the car.
  2. Export the drawing as a STL file (eg car1.stl) with a scale of 1.
  3. Use the Greenpower VWT simulation page and enter the mode of "airpower", the title, your email address and the filepath of the STL file to be simulated and click on the simulate button. The link to the VWT simulation page is here. The system will assign a case number to your simulation run. Note only one simulation can run at a time.
  4. Wait some time. The simulation will take around 3 minutes.
  5. Keep reloading the VWT simulation page using the browser back button. Eventually you will see the results from the simulation or an error message for your case number.
  6. The main aerodynamic drag in Newtons at 12m/s (43.2Km/H) is displayed for the x, y and z directions. You can also see the full results by clicking on the "output" link.
  7. Only one simulation can be run at a time.
Note that the 3D cad design must fit within the VWT sizes (X(-0.5m - +0.5m) Y(-0.2m - +0.2m) Z(.20m - 0.2m)). 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 Quad core XEON server with 3GBytes of RAM.

Information on Results

The web results page shows the Drag in Newtons at 12m/s (43.2 Km/H) in the X, Y and Z directions. The X direction is the one we are most interested in for Airpower cars. A lower number means better aerodynamics and increased speed in the race.

Details of Simulation

The VWT software creates a VWT of size 1x0.4x0.4 meters. It creates a 10x10x10 set of cubes within this space. The cars design is then placed in this cubic grid and the grid is "refined" around the edges of the cars surface. This subdivides the cubes until there are a lot of cubes just around the surface of the car and a little way beyond. The cubes that cross the cars surface are marked.
The software then sets up some initial conditions with a 12m/s airflow from the left and the road moving at 12m/s (43.2 Km/H). It then calculates the air pressure, velocity and other values for each cube in the VWT taking into account all the neighboring cubes. This set of calculations is repeated to see how the airflow changes with time. After 4 seconds the complete set of data for all VWT cubes is written to a set of files and the pressure on each cars surface is calculated to determine the overall force in Newtons in each of the major axis. The results are stored for retrieval later.

Using Paraview

Paraview can be used to view the resulting data with coloured hot spots and streamers. Paraview needs to be installed on the PC. See the links below to do this.
First download the result data from the "Result Data vtk.tar.gz" or "Result Data vtk.zip" link on the VWT web site. The choice of these depends on whether your PC can handle TAR or ZIP file archives. The results data is in the form of a "tar" archive of VTK files. Unpack this archive onto your local hard disk. You should then be able to open the VTK/vwt_40.vtk and VTK/car/car_40.vtk files using the Paraview program. Click on the "Apply" buttons to load the data sets.

To view streamers:
  1. Select the vwt_40.vtk data set.
  2. Click on the Stream Tracer Icon. In the "Seeds" settings set the point to be (0,0,0), set the Number of Points to be 200 and set the Radius to be 0.1. The click on "Apply". This gives useful starting values.
  3. Click on the Stream Tracer's "Display" tab and set the "Color by" option to "p" (pressure). You may want to manually set the data range colour map.
To view pressure areas in color on the car:
  1. Click on the Car's "Display" tab and set the "Color by" option to "p" (pressure). Yo may want to manually set the data range colour map.

Installing Paraview program on Microsoft Windows

A CD is available with the necessary Microsoft Windows applications for editing car 3D CAD drawings and viewing the resulting data from the VWT. Alternatively they can be downloaded over the Internet. The actual VWT simulation is performed on a Linux system accessed via the Internet.

For the CD Installation, perform the following actions:

  1. Insert the CD into the system
  2. Open up a Windows Explorer window onto the CD
  3. Install Paraview: Double click on the "paraview-3.2.1-win32-x86.exe" file.
The Paraview application is used to view the VWT's output data in graphical form.

The Programs can be downloaded and installed from the following Internet web sites:

Item Url
Paraview http://www.paraview.org/New/download.html

Web Links