30 Jun Aerodyn Wind Tunnel Test
Hello all! As you may have gathered, the team recently took a cross country road trip to test our 2011 car, Xenith, in the Aerodyn wind tunnel in North Carolina.
Wind tunnel testing was one of our team’s weak points in the past, but going into future build cycles, it will be a large part of our design process. Our purpose in taking our previously raced car to Aerodyn was twofold – to become experienced in how a wind tunnel operates and how to be most efficent with our time, and to do benchmarking on Xenith’s true aerodynamic performance.
Aerodyn is a fairly unique wind tunnel, and is extremely well suited to our purposes. The defining factor that makes Aerodyn such a good choice for our extremely low drag testing requirements is their system of ground-level boundary layer management. When a car is traveling down the road, it is the car that is moving relative to the still air and road, meaning that the airspeed relative to the car is uniform all the way down to the road. In a wind tunnel, however, the car is fixed and the air is moving, which introduces friction drag along the sides of the tunnel – most importantly, at ground level. Due to the viscosity of the moving fluid, this produces a flow speed gradient from the free stream in the middle of the tunnel all the way down to zero at the floor and walls, where (theoretically) the speed is zero. (This is a simple explanation that ignores things like turbulence and assumes what is called ‘laminar’ flow, but it illustrates the concept.) Due to the very small total amount of drag that our solar car develops, this effect could wildly throw off any measurements. Aerodyn allows us to accurately model the airflow around the car because they eliminate this gradient at the floor – through a complex series of ground-level slots that alternately blow and suck air to match the free stream speed, the airflow is made to behave as if the ground were moving underneath the car, just as it is on the road.
Fortunately, the team learned quite a bit from our time at Aerodyn. We found that the aerodynamic performance of our car was more or less in line with the excellent properties that software modeling had predicted. What we were able to see, however, was the sensitivities inherent in such an extreme design, in terms of angle of attack, yaw angle, oscillation, and other characteristics. While the car performed well in simulation and in the wind tunnel, it was these sensitivities that may have had a large detrimental effect in the messy and harsh environment of the outback. It was an extremely informative experience overall, and one that we will certainly be repeating again soon in our current design cycle.
A big thank you to all of the extremely friendly, knowledgeable, and generally helpful people at Aerodyn!