Past exam of the mathematics course of the University of Cambridge 2007 iii Paper 76 5 Solution Created 2026-10-03 Updated 2026-10-14
Write for the Kirchhoff stress tensor and for the second Piola-Kirchhoff stress tensor. Since and for the velocity gradient,Therefore the upper-convected rate as a stress push-forward is
For the simple shear flow, put and . The velocity gradient and rate-of-strain tensor areTheir upper-convected derivative givesDenote the symmetric extra stress by , and the relaxation time by to distinguish it from . The Oldroyd-B model in the question becomesFor the steady constitutive response, , and . HenceA constant imposed shear rate can still have startup transients; these values describe the steady response.
For circular Couette flow , the velocity gradient in the orthonormal polar basis isFor completeness, in fixed Cartesian coordinates let and . Then and . Along a particle, the polar basis rotates at angular speed . Its contribution to the material derivative cancels the rigid-rotation part of in the upper-convected derivative. The rotating-frame reduction of circular viscoelastic shear therefore leavesThe flow direction is now and the gradient direction is . With , the same simple-shear constitutive calculation gives
Angular force balance implies is constant. Integrating gives . With and ,The radial force balance is . Thus the inertialess Oldroyd-B circular Couette pressure isThe constant is fixed by a pressure or normal-traction boundary condition.