Determination of the shear stress-shear rate relation for blood by Couette viscometry

Abstract

Determination with a Couette viscometer of the shear stress-shear rate relation for a non-Newtonian fluid is not simple because of the variation of shear rate within the viscometer gap. Exact methods for calculation of the shear rate have been presented by Krieger and Elrod, but they require extensive manipulation of the viscometric data. The present study shows that use of average values of shear stress and shear rate allows determination of the viscometric curve to within a few per cent of the exact relation. The error is found to remain finite even for extreme deviation from Newtonian behavior. For a pseudo plastic power-law fluid the maximum per cent error in calculated viscosity at a given shear rate is given by $[\mathrm{(}\mathit{s}-1\mathrm{)}\mathrm{/}2\mathit{s}]\times 100$ , where s is the ratio of the outer to inner cylinder diameters. The method has the advantage that the average shear rates used do not depend on the fluid rheology and thus can be selected in advance.