Reduction of beam refraction in optical pipe flow experiments by use of sheet-fabricated pipe walls

Abstract

A measurement section is presented that allows for the study of liquid flows in straight pipes with noninvasive optical techniques. The amount of refraction of light rays is minimized by the use of short pipe sections manufactured of transparent film. For the situation of water, it is shown that walls as thin as 85 μm can be used in turbulent flows at high velocities. In our specific case, a flow of water at Reynolds numbers upto ReD=300 000 in a pipe of O 70 mm diameter (average velocity 4 m/s) was realized without unacceptable vibrations of the pipe wall. In this situation laser Doppler measurements can be carried out without the need to correct for the position of the measurement volume or changes in the velocity response of the system.