APPROXIMATE ELONGATION FLOW PROPERTIES UTILIZING THE OPPOSED ORIFICE TECHNIQUE - CORRECTION FOR SHEAR AND INERTIA

Mackay et al. (1995) have presented an approximate technique to determ ine the elongation viscosity from pressure drop measurements in a simp le stagnation flow device. In the present paper we describe experiment s using a high viscosity Newtonian oil, aimed at probing some of the a ssumptions made by Mackay et al. We find that Trouton ratios calculate d using the original analysis are well above the value of three expect ed for Newtonian fluids. Finite element simulations of the flow field show this is due to the net pressure drop having a substantial shear c ontribution, which should be corrected for before the Trouton ratios a re evaluated. Interestingly, most of the shear correction is due to sh ear on the inside of the orifice near the exit from the central flow r egion. The shear contribution to the pressure drop occurs for all flow rates, however, at large flow rates there is also an inertial correct ion to the pressure drop. In this paper we describe an approximate met hod that corrects for both shear and inertial effects. With these effe cts recognised and corrected for, the measured Trouton ratios are redu ced to around three.