UNSTEADY SHEAR-FLOW OF A VISCOPLASTIC MATERIAL

A viscoplastic, or yield-stress, liquid occupies the space between two infinite parallel plates. Initially the whole system is at rest. The lower plate is suddenly jerked into motion with given speed or shear s tress, while the upper plate is kept fixed. The flow consists of two r egions; (1) a lower sheared region bounded above by the yield surface, (2) an upper unyielded region bounded below by the yield surface. The yield surface propagates to the upper plate as time proceeds. We firs t consider the equivalent one plate problem of flow over a jerked plat e, and find similarity solutions and small time asymptotic solutions f or prescribed shear and speed cases respectively. These solutions are used as initial solutions for the two plate case. The motion of the yi eld surface and the time taken for the entire material to yield are in vestigated. The problems considered here are two dimensional represent ations of some control devices, for example the light duty clutch, whi ch consists of two corotating, coaxial discs separated by a layer of e lectrorheological material. In this application it is useful to know t he time taken for all the material to yield. (C) 1997 Elsevier Science B.V.