The "Pneumatic Sponge Ball Accelerator" is installed at the Tschumi Pavilion in Groningen / The Netherlands. The apparatus contains 1000 black sponge balls, which are sucked through 150m of transparent pneumatic tubes with the power of a normal household vacuum cleaner. Visitors can operate the machine with a touch sensor: They can change the direction of the airflow and watch the balls speed up, slow down and turn around. The balls reach a top speed of about 4m/s.
You can read more about the project on my website: niklasroy.com/project/174/PSBA