Non-expert summaryMany cleaning and decontamination operations employ a flow to detach or move a particle or foreign element (e.g. a piece of dust, baterium or a spore) from a surface. The presence of the element disturbs the flow near the wall. Knowledge of the flow field and the forces imposed on the element is useful for predicting or quantifying removal. This paper considers a simple shear flow of a Newtonian fluid over an axisymmetric protuberance (with uniform shape) on a plane wall. The full 3-D problem is formulated in terms of three scalar Fredholm integral equations of the first kind and is solved using a boundary-element method. The hydrodynamic force and torque exerted on the protuberance, and distribution of shear stresses, are calculated. There is good agreement with previous analytical computations for hemi-spherical and spherical shapes.
Non-expert summaryThe use of a gas to purge a viscous liquid from a straight cylindrical pipe of finite length is investigated for the case where the liquid is viscoelastic. The rheology of the liquid is described using the Giesekus and the Phan-Thien-Tanner (PTT) models. The focus of the work is on the numerical aspects of the simulations. A parametric analysis is made in order to determine the effects of elastic and inertial forces, and the Newtonian viscosity, on the process.
Non-expert summaryThe use of pressurised air to displace a viscoplastic liquid from a complex duct geometry is simulated using detailed numerical modelling. The transient displacement of Newtonian and viscoplastic liquids by air in cylindrical tubes of finite length with a concentric expansion followed by a contraction in their cross section is considered. The change in diameter is not sudden. Various expansion and contraction ratios are studied. Papanastasiou's formula is employed to regularize the discontinuous Bingham model. Results are presented for a range of fluid and geometrical parameters, and some cases are compared to analytical results.
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.
Non-expert summaryTheoretical study of the decontamination of a 2D porous material. The decontaminant is immiscible with the contaminant and neutralize it through interfacial reaction. Two scenarios are considered with either the contaminant filling partially the pore space, or fully. This reactive-diffusive problem is solved using a homogenisation technique that separates the local pore-scale dynamics from the macro-scale temporal evolution. The effect of the porous material is accounted for by the model.
Non-expert summaryExperimental exploration of the exchange of two immiscible fluids in a wedge. Due to antagonist principal curvature, the more wetting fluid exchange with the less wetting fluid in the wedge, through a capillary instability. The instability develops in the form of fingers pinching into droplets. The less wetting fluid is thus displaced out of the wedge. Measurements are performed for varying geometry and viscosity. Discussion but no conclusion of the physical mechanisms are proposed in light of the more well known viscous fingering instability.
Non-expert summaryThis is the first paper to present a coherent treatment of the formation of a hydraulic jump by a smooth, coherent liquid film impinging normally on to a smooth horizontal plate. The liquid spreads out radially in a thin layer bounded by a circular hydraulic jump, outside which the depth is much greater. The motion in the layer is analyzed in terms of boundary-layer theory, both for laminar and for turbulent flow, and relations are obtained for the radius of the hydraulic jump. These relations are compared with experimental results obtained for water jets in air.. The analogous problems of two-dimensional flow are also considered.
Non-expert summaryThe Lattice-Boltzman numerical method is used to solve the Navier-Stokes equations describing the displacement of a viscoplastic fluid from a cylindrical pipe by the injection of a Newtonian liquid. The equations are written in dimensionless form so no particular time or length scale is specified. Gravity and surface tension effects are included. The average velocity of the Newtonian liquid is such that its flow is laminar. The numerical code is able to resolve several features of the process: the growth of a 'finger' of Newtonian liquid as it pushes the viscoplastic fluid out and the development of waves at the liquid-fluid interface. The impact of key dimensionless groups, particularly the Bingham number (ratio of critical or yield stress to stress induced by the flow) and Reynolds number.