I asked this question about what it is (if anything) that provides and upper limit to the speed of a turbine in a fluid stream: Maximum rotation rate of a turbine in a fluid with a given speed
It was closed as "engineering". The linked meta post says:
Are any engineering questions allowed?
Questions inspired by engineering considerations can be on-topic on this site. For instance, it would be valid to ask if there is a fundamental physical limit to
- How well one could resolve an object with a telescope,
- How energy-efficient a computer can be, or
- How small a current can flow through a wire.
Similarly, questions like What really allows airplanes to fly? that ask how a man-made system works are also on topic, as long as they aren't too broad. Questions about the physical reasoning and analysis that lead to design decisions are on topic; questions seeking help designing or building something is off topic.
It seems to me that this question, about a physical limit, is explicitly allowed under that guidance. It's not asking how you could build such a turbine (in fact, it's explicitly not concerned with the exact geometry of the turbine and allows for a zero-friction mechanism, so clearly not related to any real device), just asking what physical rules any turbine operates under with respect to its speed relative to the fluid it's driven by.