Single stage axial fans can reach only a low aerodynamic efficiency of around 50%. This can be drastically improved with a second contra rotating stage. However, the wakefield of the first stage will interact with the second stage, leading to undesirable flow separation and associated noise.

Careful design of the rotor blades can help to prevent these effects, thereby limiting noise generation at the interaction frequencies. The optimized design reaches an aerodynamic efficiency of 76%. It was developed for ebm-papst and presented at the ASME Turbo Expo 2014.