A double-refraction method of detecting turbulence in liquids

The doubly-refractive properties of a weak solution of benzopurpurin were used in a study of the onset of turbulence when the liquid flowed through a long horizontal glass pipe. A beam of polarized light was arranged to traverse a diameter of the pipe remote from the ends, the nicols being set to give extinction when the liquid was stationary. The emergent light fell on a photo-cell, which was connected through an amplifier to a cathode-ray tube. With unidirectional flow, photographs of the trace on the screen showed a steady straight line when the velocity was small. Disturbances appeared at a Reynolds number R of about 1970, and complete turbulence was established when R attained a value of 2900. With forced oscillating flow, the trace at low velocities was a sinusoidal curve, on which, at a critical Reynolds number, small superposed ripples were observed. Even when the motion was greatly increased the intricate traces recurred perfectly, but this phase finally broke up into complete turbulence as soon as the amplitude of the surface movement in the bottles at the ends of the pipe reached a certain limit.