The flowmeter / rotameter is a variable-orifice device used to measure the flow of oxygen, air and nitrous oxide in the anesthesia machine

It usually contains a bobbin

The pressure of gas beneath the bobbin pushes it up the flowmeter until the force beneath it is balanced by the force of gravity pulling it back to its resting position.

The variable orifice of the flowmeter means that the flow of gas past the bobbin at low flows is laminar, but at high flows it is turbulent.

The flutes on the bobbin are to ensure continuous movement of the bobbin as gas passes it, showing that it is not stuck.

The variable orifice of the flowmeter is responsible for the turbulent flow.