Attempt at Visualization and Measurement of Shear Stress Distribution in Unsteady Flow Using Flow-Induced Birefringence

We attempted non-invasive quantitative visualization of shear stress filed in a fluid based on flow-induced birefringence. The method provides instantaneous shear-stress field without the need of calculating the gradient of the velocity distribution. Two-dimensional distribution of the shear stress in a flow is obtained by measuring the birefringence from the phase difference of polarized incident light transmitted through the working fluid and applying the stress-optic law. For measuring the shear stress values, we need photoelastic coefficients to relate the measured birefringence to the shear stress. We performed calibration experiments to determine the photoelastic coefficients using a circular Couette flow. Based on the calibration, shear stress distribution was obtained in a laminar flow in a rectangle duct. The shear stress distribution exhibited qualitatively consistent results with the ones predicted in a numerical simulation. The minimum values showed good agreement with the prediction, while the maximums differed by a factor of three in the quantitative comparison. Further improvement is expected to provide quantitatively reliable distribution of the shear stress.