The Bundoora East campus of the RMIT University in Melbourne boasts a small automotive wind tunnel. Used for determining the aerodynamic characteristics of scale models and some testing of full-size cars, the facility is under the direction of Simon Watkins, Associate Professor of Automotive Engineering. He took AutoSpeed on an exclusive guided tour of his million-dollar baby.
The wind tunnel is constructed so that the air being pushed by the fan is returned to the fan's suction side. "It's the same air going around and around," said Simon Watkins. "This has advantages in controlling the airflow and also minimising the energy usage." At each of the corners where the air must pass around a 90-degree bend, curved turning vanes are fitted, easing the air onto its new path with as little turbulence as possible.
"The turning vanes can be removed on tracks and double doors opened so that you can drive a vehicle straight in, " explains Watkins. He adds, "The motive power is a 300kW DC thyristor-controlled motor. You can go from zero speed up to about 160 km/h in the test section. The number of rotors and stators of the fan assembly are different, so they don't pass at the same time and make noise."
"Noise is an important consideration in this tunnel, both to make a quiet working environment and also because a lot of our work is in assessing wind noise on full-size vehicles. We have done a number of things to make the tunnel as quiet as possible. We've taken the DC motor out of the tunnel and belt drive the fan from outside, so that the motor noise is away in a separate building. We also use acoustically treated turning vanes. The vanes turn the air but they also absorb a lot of the fan noise. There are two sets of acoustically-treated turning vanes, isolating the noise of the fan from the test section."
"We do model scale tests for the car companies, where we test vehicles that the public won't see for typically five years into the future. We do that for the two major players in Australia - clay models to start off with and then usually quite detailed models, which can cost up to $70,000 each. The next stage is when the car companies have actually got a piece of hardware - it might be a mock-up of the front end, on which we do cooling work."
Pipes buried under the floor of the wind tunnel allow the passing of hot and cold water through the mock-up's radiator, with temperatures measured at both the inlet and outlet. This allows the efficiency of the car's cooling system can be assessed quite early in the car's development.
Brake cooling can also be assessed. The car's wheel is driven by a roller, with inspection of the cooling airflow and the behaviour of the brakes examined visually from beneath the floor of the tunnel. A panel of clear acrylic replaces the plywood floor of the tunnel during this process. "You can actually look within the underbody to the road gap. I call it the last frontier of vehicle aerodynamics - the underside. Production cars dump most of their heat there, so you have to make sure that the heat's going to be moved away in a reasonably efficient manner - but you also want low drag. For low drag you want minimum surface area and streamlined panels. But for good heat dissipation you want maximum surface area, as rough and with as much turbulence as possible!"
Traditional ways in which airflow can be seen include using smoke streams and wool tufts. The team wooltufts the undersides of cars and then views the behaviour of the tufts through the clear floor panel. "It's a thick turbulence mess under there - lots of separated wakes all mixed up," says Watkins. And are Australian car companies looking at making advances in this area? "We have a research project with Ford, looking at what can and can't be done."
The force transducers used to measure the behaviour of models are integrated into a turntable, which can be rotated to simulate the effect of crosswinds. Squashed into the cramped concrete channels beneath the floor of the tunnel, Watkins points out the features. "The turntable has a 6-component force balance, so you can measure drag, lift (downforce if you're lucky!), side force, and also the torques about each of those - pitching moment, rolling moment, yawing movement. You can fully document the aerodynamic forces on the vehicle." The turntable can hold models weighing up to 150kg.
While the other Australian automotive wind tunnel at Monash University has force transducers that can be used directly on full-size vehicles, this smaller tunnel has one major advantage - it costs about $5000 per day, about a third that of using the other tunnel.
Adjacent to the test section is a large control room, with extensive glazing allowing inspection of the test vehicle. The outputs of the force transducers are fed into data-logging PCs via analog-to-digital interfaces. A Digital Audio Tape (DAT) is also used to gather data. In addition to force data, audio information is recorded by means of microphones buried in the 'ears' of a special dummy, which sits bolt upright in the driver's seat of the car being tested. "We also use the DATs to record noise on the road," said Watkins who is known to participate in 4 am freeway testing. A regular traveller to the DaimlerChrysler wind tunnel in Germany, Watkins smiles broadly as he describes the airspeeds achieved during autobahn wind noise testing..... yes, they're just a little faster than in Australia!
http://www.mm.rmit.edu.au/
