VWT Results Information
So you have run a simulation, now what do all of those results mean!
The web results page shows the Drag in Newtons at 13m/s (46.8 Km/H) in the X, Y and Z directions. The X direction is the one we are most interested in for Greenpower cars. A lower number means better aerodynamics and increased speed in the race. The Drag forces based on direct preasure and viscous drag are given. The Total forces are the ones we are interested in.
The results also display the calculated Cd value and the frontal area together with a combine Cd * area figure.
Viewing the VWT output data using Paraview
You will need the "paraview" application installed on your computer to do this http://www.paraview.org.
First download the result data from the VWT's list of results (Results vtk.zip). This is in the form of a "zip" archive of VTK files. Unpack this archive onto your local hard disk.
We have provided a basic Paraview state file, vwt.pvsm, that can be loaded using the "Load State" button of Paraview. This loads the main data files and sets up some basic views. You will be presented with a dialog to chnage the path to the run_200.vtk and car/car_200.vtk files sets the path to these to match where you unpacked them from the ZIP file. The run_200.vtk file contains all the "wind" data from the VWT and the car_200.vt file contains the actual cars surface data.
Alternatively you can manually open the "VTK/vwt_200.vtk" and the "VTK/car/car_200.vtk" files using the Paraview program. Click on the "Apply" buttons to load the data sets.
Then to view streamers:
- Select the vwt_200.vtk data set.
- Click on the Stream Tracer Icon. In the "Seeds" settings set the point to be (0,0,0.3), set the Number of Points to be 200 and set the Radius to be 0.7. The click on "Apply". This gives useful starting values.
- Click on the Stream Tracer's "Display" tab and set the "Color by" option to "p" (pressure). Yo may want to manually set the data range colour map so that the streamers are displayed in the full colour range. A range of -50 to 50 works quite well.
To view pressure areas in color on the car:
- 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. A range of -50 to 50 works quite well.
The paraview application provides a lot of facilities for viewing your data including making animation videos.
What are we looking for
Generally we are looking for where the airflow separates from the surface of the car rather than follows it and especially turbalance. This will happen behind features of the car. You can zoom into these areas with Paraview and perhaps increase the streamer count to see more detail.
Preasure areas as can be seen with colours, are a useful tool. Red high preasure areas will appear on the front of things and blue low preasure areas on the rear. High preasure in front push on the car but are difficult to avoid. Low preasure areas on the rear of featues "suck" the car back. These can generally be reduced by better aerodynamic profiles.
Details of Simulation
The VWT software creates a VWT of size 8x4x3 meters. It creates a 80x60x40 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 cells just around the surface of the car and a little way beyond. The VWT is set to use just under 1 million cells. The cells that cross the cars surface are marked.
The software then sets up some initial conditions with a 13m/s airflow from the left and the road moving at 13m/s (46.8 Km/H). It then calculates the air pressure, air velocity and other values for each cell 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 about 120 itterations the complete set of data for all VWT cells 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.
Picture of simple car in the VWT with streamers
Picture of Cube Showing turbulence
Some Rotary Racer Pictures
Old Bits need update ...
A Greenpower car, with 17 Amps flowing into the motor uses 400 Watts of power (24V * 17A = 408W). Assuming a motor and gearing efficiency of about 65% this results in 256 Watts available to push the car forward.
The speed of the car, assuming the gearing is set correctly, is given by:
dragForce = (carRollingFrictionCoeff * weight * 9.8) + (0.5 * airDensity * carAirDragCoeff * carFrontArea * (speed * speed))
Assuming the cars rolling resistance is about 10N and the car moves at 13m/s (46.8Km/H) at 17 Amps, then the air drag is 30 - 10 = 20.0 Newtons.
The air drag is proportional to the square of the velocity. So reducing the cars airdrag by 10% will yield, approximately, a 3.00% increase in speed.
So reducing the drag from 20N to 18N, at the same speed, would increase the cars maximum speed to 13.39m/s (48.2Km/H).