Spatio-temporal measurements of wall friction using three-dimensional photoelasticity*

Accurate measurements of wall shear stress in turbulent, low-speed, incompressible flows remain a challenge as it requires a non-intrusive tangential force sensor that can respond swiftly, is ultra-sensitive and noise free. We are proposing a novel method, employing a three-dimensional photo-elastic material, to spatio-temporally measure the wall shear stress under a high Reynolds number turbulent boundary layer. We fabricate a low modulus of elasticity, stress-free photo elastic sheet, using epoxy resin (SQ-2001) and hardener (SQ-3154) that can discriminate the tangential stresses from normal forces via an optical fringe pattern. The experimental setup consists of white light, an optical arrangement of a polarizer and analyzer, and a Single Lens Reflex camera. The shear stress information is extracted using a neural network-based approach to fringe patterns. The estimated Young’s modulus of elasticity of the sheet with dimensions 20 cm x 20 cm x 0.1 cm is 0.05Mpa. The temporal response the photoelastic sheet, measured using an acoustic transducer, had a 20 microsecond delay with a 5 milli-Newton sensitivity.