The use of FLUENT software enabled students at Queen’s University, Ontario, Canada, to improve the aerodynamics of their solar car. By eliminating 500 Newtons of negative lift, they helped the car to finish second in the 10-day, 1425-mile Sunrayce 99. The Queens University car relies entirely on solar power to drive an electrical motor that generates only 2 horsepower at cruising speed, so aerodynamics are crucial to the vehicle’s performance. The original design of the Queen’s University vehicle featured a symmetrical airfoil that generated a great deal of negative lift, increasing the rolling resistance of the vehicle. There wasn’t time or money to build models and test them in a wind tunnel. Instead, the students used CFD results to guide them.

Queens University’s Solar Car on its way to a second place finish in Sunrayce 99.

The team performed a simulation with FLUENT of the original design concept. They were astonished at the amount of negative lift generated. It was like adding 50 kg to the weight of the car. The graphical output allowed the students to see why this was happening. The symmetrical airfoil basically split the air in half, and since the amount that went under the car had to pass through a much smaller space than the air that went over the top, it had to travel much faster. This was indicated on the analysis plot by velocity vectors. With the air moving faster under the airfoil, the car was functioning in a manner opposite to an airplane wing, with air pressure forcing the vehicle down toward the ground.

Z-axis velocity from a slice around the edge of the solar car. The fairings in the mesh can be seen. Near the rear edges of the fairings, the z-velocity is downward. Near the front of the fairings, the velocity is upward.

The students made changes to the airfoil design to reduce negative lift. Their approach was to lower the nose to force more of the air to flow over the top of the airfoil. It took about one hour to generate a mesh for each new shape. The students put each design change through a CFD analysis until they found one that eliminated negative lift. This turned out to be a design that positioned nose of the airfoil so that three-quarters of the air volume went over the airfoil and one-quarter of the air went under. This ratio equalized the pressure above and below the car, achieving the goal of zero lift.

Pressure contours of the bottom of the solar car. Air acceleration between the fairings and higher pressure areas in front and behind the fairings are clearly shown.

A dedicated team and high technology tools were the keys to success.

FluentNEWS