I was thinking about how a person could create and maintain a vacuum at sea level. I was thinking of any supersonic objects (bullets, etc.), and that a vacuum (or partial) may be directly behind the object as it travels, as the air wouldn't have time to move in to replace the void faster than the object. I'm sure this creates tremendous drag also. Anyway, let's say we had a centrifugal fan. Ignoring any leaks, let's say we have a large one that was 1.0 meter in diameter. I say a large one, as we're trying to get the blade tips to the speed of sound. Also, let's say we prevented air from flowing in the normal sense by blocking the inlet, so no new air can flow in to replace the air that's being moved unless it comes back. Let's say we spin this fan at 5000 RPM somehow (blade speed close to the speed of sound). So, if no new air could rush and replace the vacated area, would there be a vacuum in the space between the blades? I would think that as you approach Mach 1.0, that no new air could really enter the space... Is this true?