The use of rheometers for the evaluation of rheological properties
and the establishment of behaviour laws for fluids requires the
knowledge of the shear rate at any moment and everywhere in the
region between the cone and the plate, referred to as “the gap”
throughout this paper. However, the accurate determination of
the shear rate supposes that the constitutive equation of the
fluid is known beforehand. In order to avoid this paradox, rheometers
are generally built such that the shear rate is supposed to be
approximately constant throughout the gap. This approximation
is realistic for steady flow but may be crude for other types
of fluid motion. The aim of the present work is to determine
the limits of validity of such an approximation when testing
complex fluids in a cone and plate geometry. In this paper, only
purely viscous properties are taken into account. The numerical
solution is based on the control-volume method. When non-linear
and time-dependent effects occur, it is shown that the flow cannot
be represented by simple shearing conical surfaces. This result
is especially important for the characterisation of time-dependent
fluids (as thixotropic fluids), typical of unsteady flow. Finally
the abilities of the proposed model which is named “RHEOUTIL”
are highlighted by comparison between numerical simulations and
experimental results. The analysis of non Newtonian fluids emphasises
the limits of the code “RHEOUTIL”. Indeed, the model has to
evolve in order to take into account the whole complexity of
the fluid, which may also exhibit viscoelastic properties, yield
stress and so on.