When the outer cup of a co-axial rheometer oscillates at a relatively high frequency , the moment measured by the wire connected to the inner bob shows a phase lag, even if the fluid under measurement is Newtonian(“the paradox of the elastic component of the water”). It is believed that such a phase lag is attributed to the inertia due to the fluid as well as due to the inner bob. Without special modifica-tions , significant error may be incurred in the measurements of the viscoelasticity. The present... When the outer cup of a co-axial rheometer oscillates at a relatively high frequency , the moment measured by the wire connected to the inner bob shows a phase lag, even if the fluid under measurement is Newtonian(“the paradox of the elastic component of the water”). It is believed that such a phase lag is attributed to the inertia due to the fluid as well as due to the inner bob. Without special modifica-tions , significant error may be incurred in the measurements of the viscoelasticity. The present paper is a new step of the authors, attempts to account for these iner-tial effects. Comprehensive theoretical analysis is made about the inertial effects both due to the fluid and due to the bob,by adopting the new scheme of reducing to a two-point boundary value problem of ordinary differential equations , proposed earlier by the present authors; It is indicated that the harmful resonance phenomena can be eliminated in the working range of oscillation frequency by decreasing therotational inertia moment of the bob and increasing the rigidity of the wire ,or by increasing the damping of the bob by the air. Under such circumstances the inertial effect due to the bob may be neglected (with the bob being considered stationary)and only the inertial effect due to the fluid needs considering,thus greatly simplify-ing the problem. |