Guy Martin and the 112 mph Push Bike
Since I am fascinated by this type of crazy stunt and was involved a few years ago in a similarly crazy project to propel a lawnmower at 100 mph (watch this space!), I couldn't resist checking out the fluid flows for myself using SOLIDWORKS Flow Simulation.
The set up is actually very simple. You just model a truck and a land surface in SOLIDWORKS 3D CAD and run the Flow wizard within SOLIDWORKS Flow Simulation. All you need to do is add the global velocity (in this case 112 mph) and adjust the size of the domain (the volume of space that includes the truck, the ground and enough air to see the upstream and downstream airflow). An additional refinement was to model in the effect of the moving road - or in this case the sand on which the record attempt was made in South Wales. I did this with a 'Moving Wall' boundary condition.
I chose to do this initially as a steady state simulation to get the basic flow patterns and to see if I achieved the same results as on TV. I then reran it as a transient study to capture the chaotic oscillations that occur in this highly turbulent environment.
Below is my basic truck model. It is just a SOLIDWORKS multi-body part. I have used a couple of appearances and applied the new sunlight option to make it look like daylight.
The plots below show the reason why this seemingly impossible speed can be achieved. The air dam at the back of the truck creates large vortices that rotate directly behind it - just where the cyclist (Guy Martin) is positioned. This not only dramatically reduces the aerodynamic drag forces but actually creates a positive airflow that assists Guy as he pedals furiously.
Here is the airflow plotted as trajectories. I have only plotted half of the results so you can see what is happening at the centre. Note the blue low velocity arrows at the back.
Below is an even clearer plot showing the airflow hitting the rear of the air dam. It shows that the air is flowing forwards and therefore is helping Guy.
In 2D, the airflow is shown below with vectors and streamlines.
This is the view from above. You can see side vortices as well as the ones behind the air dam.
Another type of plot is a 'Cut Plot' with a '3D offset'. Below I have plotted the dynamic pressure results on a 2D plane exactly through Guy's cycling position (the black rectangle). I have then offset the values in the 3rd direction to produce this 'mountain map'. The plot shows very clearly that Guy would be in a pool of relatively slowly moving air (the blue region) and that this is quite wide and deep. In reality it oscillates as I was able to prove in a transient study but there is a sufficiently large stable region for Guy to be protected - provided he stays within the pool. Moving outside the pool would be very dangerous - it would mean hitting a wall of air rushing at over 100 mph and could have been fatal.
The final image shows a view from the rear which explains why the TV programme had trouble filming the truck. The truck creates large vortices behind the air dam and these pick up the sand and create a maelstrom of fine sand particles.
I think this is a great example of British courage, technical ingenuity and application of simulation tools.
by Andy Fulcher
Solid Solutions Management Ltd