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Non-expert summaryThe flushing of a viscous fluid from a pipe or cavity by a less viscous one is widely used in cleaning operations. This paper reports the numerical modelling of the transient displacement of a viscoplastic material from straight or suddenly constricted (square entry) concentric cylindrical tubes of finite length, simulating a gas at higher pressure displacing the geometry initially filled with liquid. A mixed finite element method is coupled with a quasi-elliptic mesh generation scheme in order to follow the very large deformations involved. The gas bubble grows in length and leaves a thin liquid film on the duct wall (so it does not remove all the liquid). The shape of the bubble is computed for various Reynolds and Bingham numbers (ratio of yield stress to viscous stress). The 'tip splitting' instability that can arise in flow of a gas along a tube filled with a viscous Newtonian fluids is suppressed with viscoplastic fluids at higher Bingham numbers. The shape of the bubble as it passes through the constriction is also studied.

Non-expert summaryModelling study of the decontamination of a porous material. An immiscible cleanser react with a contaminant, producing reactants which can be partially soluble in either phases. The diffusive-reactive transport problem is solved taking into account the moving boundary through asymptotic analysis and compared with numerical simulations. It is found that the partition coefficient of the product is more important in determining the removal rate than the reaction rate, as products of the reaction can prevent further reaction if they accumulate at the interface.