In 2001, DUTís car, the Nuna 1, won the race at a speed of 92 km/hr. DUTís second car, the Nuna 2 won the race in 2003 at a speed of 97 km/hr. After the 2003 race, DUT decided to use CFD and wind tunnel experiments to improve the design of their next car, the Nuna 3, for the 2005 race.
DUT choose to use Gridgen as the preprocessor for their CFD analysis. They used FLUENT as the flow solver and FIELDVIEW for postprocessing.
Gridgen was used to create a hybrid grid with prism blocks on the wall and unstructured blocks on the rest of the control volume. A higher density grid was applied under the belly pan and in the wake. The position of transition from laminar to turbulent boundary layer on the body and the wheel boxes is taken from oil flow patterns and stethoscope measurements on the wind tunnel model. In the calculations, the turbulent viscosity is switched off in the laminar zone. It turned out that CFD calculations with the k-ε realizable model produced lift and drag results comparable to the wind tunnel experiments.
Originally, the wheel fairings were causing an area of adverse pressure gradient on the belly pan. This triggered premature transition from laminar to turbulent flow, increasing skin friction drag.
On the design of the Nuna 3, the bump in the pressure distribution was eliminated by modifying the belly pan, allowing laminar flow up to the rear wheel fairing.
The students were also able to shape the front wheel fairings such that a suction effect is created under crosswind conditions that reduces pressure drag.
Using CFD, DUT once again was successful in designing the winning car in the 2005 World Solar Challenge. The Nuna 3 won the race with an average speed of 103 km/hr.
The Delft Nuna3 solar powered car.
The hybrid grid around the car's wheelbox.
Pressure distribution on the solar powered car.