“…Such a method is based on the concept of variational inequalities and its application to Bingham fluids was pioneered by Duvaut and Lions [23]. Since that time, a series of researches have been performed on pipe flows of Bingham fluids [24][25][26][27][28] and general viscoplastic fluids [20,29], flows past a single sphere [23] and two collinearly translating spheres [30], flow past a cylinder [31], multi-layer lubrication flow and counter-current exchange flow [22], cavity driven flow [32] and other problems [33][34][35]. Now, in pipe flows, the existence and uniqueness of a solution to the variational inequality when the problem is posed in a Hilbert space is well known [23], and is applicable to Bingham fluids.…”