Streaming Birefringence as a Hydrodynamic Research Tool—Applied to a Rotating Cylinder Apparatus above the Transition Velocity

Abstract

Streaming birefringence was studied in the annular space between a fixed outer and a rotating inner cylinder at velocities on both sides of transition to Taylor vortex flow. With a well‐collimated light beam, quantitative measurements were possible as close as 0.1 mm from a wall. For an aqueous colloidal solution of bentonite, shear stress could be evaluated from birefringence measurements only when, for a given birefringence, the angle of extinction was the same in the unknown flow as in a known laminar flow. In turbulent shear flow this is not generally the case. For a pure liquid, ethyl cinnamate, the angle of extinction remained at 45° to the streamlines, within experimental error, under all flow conditions obtainable; thus even in turbulent shear flow the birefringence appears to arise primarily from the principal stresses. Assuming birefringence proportional to mean shear stress permits construction of a velocity profile across the gap, yielding the correct value of velocity of the moving wall within limits of experimental error. Thus in this type of two‐dimensional shear flow the mean shear stress can be measured by means of the streaming birefringence of a pure liquid.