The wind tunnel
Dr Mark Jermy describes how the wind tunnel at the University of Canterbury works and how it can be used to measure forces on cyclists. He describes how the drag force is measured using strain gauges and how voltages produced by the strain gauges are calibrated to give an accurate result for the aerodynamic drag on a cyclist.
Transcript
DR MARK JERMY We’ve got two wind tunnels here at the University of Canterbury. We call it a closed section wind tunnel because it’s got walls all the way around the area where we make the measurements. And that’s our faster wind tunnel. It goes up to a speed of 200 kilometres per hour.
We’ve got another tunnel, which we call the open jet wind tunnel because there are no walls around the area where we put the thing we want to measure, and we use that for the cyclists because it’s much larger, and we can fit a cyclist and a rider into the tunnel.
A wind tunnel – it’s some tens of kilowatts of electrical power runs the fan, and that‘s basically a tube. There’s an inlet tube to bring the air into that fan, and there is an outlet tube to bring it to whatever we are measuring – in our case, a cyclist.
But the wind tunnel is all about control, it’s about controlling the wind speed, and it’s about controlling the distribution of wind so it’s the same velocity everywhere. So we need some control system on those fans, and we need something to measure the speed where the cyclist is.
Wind tunnels are used mostly for measuring forces on objects. With cyclists, we’re most interested in drag, so we use a device which has basically the same technology as a pair of bathroom scales that you might weigh yourself with, but it measures how hard the cyclist is pushed back by the wind. We are measuring the drag force through how much that beam is bent by.
Those strain gauges give us an electrical signal, which is just a few millivolts, so that has to be amplified and then logged onto a computer. But it is still just a voltage, and we have to relate that to a force, so what we do is we calibrate it. ‘Calibrate it’ means we put a known force on it. We turn off the wind tunnel so there is no wind, and then we use a rope and a pulley system with a known weight to pull the cyclist back with a known force. We usually use a weight of about 2 kilograms because that is about the same level as the force which a cyclist feels when travelling.
So before we do the test, we apply our 2 kilogram weight and we see what voltage we get, and we take it off and we see what voltage we get, so that gives us a 2 kilogram mark and a zero force mark. Then we turn the wind on and we see what voltage we get with the cyclist, and it comes somewhere in between those two, and where it comes in between those two calibration levels tells us how much force the cyclist is experiencing.
Acknowledgements: Amaury Abdennebi, ENISE
