ON STEADY THIRD GRADE FLUIDS EQUATIONS ADRIANA VALENTINA BUSUIOC, DRAGOS¸ IFTIMIE AND MARIUS PAICU Abstract. Let ? be a simply connected, bounded, smooth domain of R2 or R3. We consider the equation of steady motion of a third grade fluid in ? with homogeneous Dirichlet boundary conditions. We prove that the monotonicity technique used by Paicu [22] in the full space for unsteady motion allows to obtain the existence of a W 1,40 solution provided that the forcing belongs to W ?1, 43 . The size of the forcing is arbitrary. Key words: Non-Newtonian fluids, third grade fluids. 1. Introduction Fluids of grade three are a subclass of the family of fluids of complexity three for which the constitutive law is given by the formula (1) T = ?pI + ?A+ ?1A2 + ?2A 2 + ?1A3 + ?2(AA2 + A2A) + ?|A| 2A where A1 ? A, A2 and A3 are the first three Rivlin-Ericksen tensors (or rate-of-strain tensors) defined recursively by A = A1 = ?u+ (?u) t, An = A˙n?1 + (?u) tAn?1 + An?1?u, where the dot denotes the material derivative and u is the velocity field. Relation (1) arises when the fluid is assumed incompressible and the constitutive law is polynomial of degree less than 3 in the first three Rivlin-Ericksen tensors.
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