Road vehicles in the real world experience aerodynamic conditions that might be unappreciated and omitted in wind-tunnel experiments or in numerical simulations. Precipitation can potentially have an impact on the aerodynamics of road vehicles. An experimental study was devised to measure, in a wind tunnel, the impact of rain on the aerodynamic forces of the DrivAer research model.
In this study, a rain system was commissioned to simulate natural rain in a wind-tunnel environment for full-scale rain rates between about 8 and 250 mm/hr. A 30%-scale DrivAer model was tested with and without precipitation for two primary configurations: the notch-back and estate-back variants. In addition, mirror-removal and covered-wheel-well configurations were investigated.
The results demonstrate a distinct relationship between increasing rain intensities and increased drag of the model, providing evidence that road vehicles experience higher drag when travelling in precipitation conditions. At the lowest rain rates examined, representing moderate rain conditions equivalent to about 8 and 29 mm/hr full scale, drag increases on the order of 2% to 4% were measured. Drag increases in excess of 10% were observed for the highest rain rate tested, which represented an unrealistically-high precipitation condition equivalent to about 250 mm/hr full scale. A non-linear increase in drag with rain rate was observed, suggesting that multiple mechanisms of rain-induced drag were present during the experiments. Lift- and side-force variations did not manifest any trends that were beyond the estimated experimental uncertainty. Therefore precipitation is likely not an influence towards aerodynamic stability. Within the experimental uncertainty, the results did not show any evidence of rain affecting the drag changes associated with modifications to the vehicle shape.