I'm no air expert, but couldn't you use some sort of fan turning at the required speed with a nozzle directed at the blades? That would interrupt the air at the frequency that the blades pass in front of it.
I'm no air expert, but couldn't you use some sort of fan turning at the required speed with a nozzle directed at the blades? That would interrupt the air at the frequency that the blades pass in front of it.