A study on measurement technique of planar elongation viscosity on annular abrupt contraction flow around a sliding down disk bob

Abstract

A mechanical measurement technique for planar elongation viscosity was proposed. A disk shaped bob was designed to reduce the shear stress acting on the surface of the test section and it generates the annular abrupt contraction flow through the entry of the gap between the bob and the cup. The disk bob with the edge shaped rim which was designed to ignore the effect of the shear stress was pushed into a sample liquid held in a cylindrical cup with a constant velocity. Then, the resistance force acting on the disk bob was measured. The trial experiments were carried out for two kinds of standard viscous fluids and the M1 fluid. It was assumed that the resistant force was summation of the force caused by the planar elongation stress and the pressure drop by the shear flow at the gap. Therefore, in order to calculate the planar elongation rate and the pressure drop caused by the shear flow, a representative length was supposed to be the hydraulic mean depth. The Trouton ratios of the evaluated planar elongation viscosity were generally consistent with the theoretical values. In this way, it was clarified that this technique was applicable to the planar elongation viscosity of both the low viscosity fluid and the viscoelastic fluid that has the short relaxation time.