Knowledge Base

CategoriesNumerical simulations, Theoretical modelling

Non-expert summaryThis modelling study considers what happens after a droplet is set into motion by the action of an impose shear flow. Inertial effects and contact-angle hysteresis are both considered. A number of flow regimes are investigated, including steadily moving drops, partial and entire droplet entrainment. The critical conditions (capillary number) for the onset of entrainment are determined for pinned as well as for moving drops. The approach to breakup is then investigated in detail, including the growth of a ligament on a drop, and the reduction of the radius of a pinching neck. A model based on an energy argument is proposed for the rate of elongation of ligaments. The paper concludes with an investigation of detachment of a hydrophobic droplet from a solid wall.

Article

Self-cleaning of a hydrophobic surface by a rolling water droplet

Hassan, G., Yilbas, B.S., Al-Sharafi, A., Al-Qahtani, H.

2019Scientific Reports95744

Keywordscleaning, drop, flow displacement, wetting

DisciplinesMechanical Engineering, Surface Chemistry

CategoriesLaboratory experiments, Theoretical modelling

Scale1e-6 to 1e-3 m, 1e-3 m, 1e-3 to 1 m / 1e-3 to 1 s, 1 to 60 s

Non-expert summaryThis is an experimental study of removal of dust particles from a hydrophobic surface by a rolling/sliding water droplet. The effect of surface inclination angle on droplet dynamics and dust removal is analyses and compared with a model. Droplet rolling dominates over sliding. Removal is mainly due to the droplet liquid coating the particles as it passes over the particle, and the removal efficiency is determined by the inclination of the surface.

Article

Convective mass transport from rectangular cavities in viscous flow

Occhialini, J.M., Higdon, J.J.L.

1992J. Electrochem. Soc.139(10)2845-2855

Keywordscavity flow, cleaning, decontamination, mass transfer, shear flow

CategoryNumerical simulations

Non-expert summaryThe authors conduct a comprehensive numerical study of convective mass transport from 2-D rectangular cavities in low‐Reynolds‐number flows, i.e. the flow set up in a long rectangular trench by the motion of a steady shear flow across the top. They calculate the velocity field in the trench and the associated mass transport (enhancement of diffusion). The flow field is calculated by a high‐order implementation of the boundary‐integral method, while the convective diffusion equation is solved using the spectral‐element method. Results are presented in the form of concentraton contours and local mass fluxes, for cavity aspect ratios from 1:1 to 4:1 and for Péclet numbers from 0 to 100,000. They investigate the effects of inlet flow profile and system boundaries on the system.

Article

Transport enhancement mechanisms in open cavities

Horner, M., Metcalf, G., Wiggins, S., Ottino, J.M.

2002J. Fluid Mechanics452199-229

Keywordscavity flow, cleaning, hydrodynamics, mass transfer, shear flow

DisciplinesFluid Mechanics

CategoriesLaboratory experiments, Numerical simulations

Scale1e-3 to 1 m / 1 to 60 s, 1 to 60 min, 1 to 24 hours

Non-expert summaryWhen a steady shear flow passes over a long (2-D) cavity it sets up a circulation cell in the cavity, where the flow in the cavity is contained within the cavity (with a boundary called the separatrix) and mass transport into and out of the cavity is predominantly by diffusion. This is particularly true at small scales. The authors investigate numerically and experimentally two methods for enhancing mass transport from these 'cells' - by geometrically modifying the boundary driving the flow, and making the driving flow time-dependent. Both modifications destabilize one of the wall attachment points of the separatrix, allowing fluid exchange between the cavity and channel. The range of Reynolds and Reynolds-Strouhal numbers studied is 7.7 <= Re <= 46.5 and 0.52 <= ReSr <= 12,55 in the spatially dependent mode and 12 <= Re <= 93 and 0.26 <= ReSr <= 5.02 in the time-dependent mode. The transport is described theoretically via lobe dynamics, which characterizes the instability of the separatrix. They find that the resulting mass transfer between the cavity and the outer shear flow, through the distabilized separatrix is enhanced by several orders of magnitude compared to a diffusive mass transfer.

Article

Transient displacement of a viscoplastic material by air in straight and suddenly constricted tubes

Dimakopoulos, Y., Tsamopoulos, J.

2003J. Non-Newtonian Fluid Mech.11243-75

Keywordscleaning, flow displacement, moving boundary problem, viscoplastic fluid

CategoryNumerical simulations

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.

Article

On the gas-penetration in straight tubes completely filled with a viscoelastic fluid

Dimakopoulos, Y., Tsamopoulos, J.

2004J. Non-Newtonian Fluid Mechanics117(2-3)117-139

Keywordscleaning, flow displacement

CategoryNumerical simulations

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.

Article

Transient displacement of Newtonian and viscoplastic liquids by air in complex tubes

Dimakopoulos, Y., Tsamopoulos, J.

2007J. Non-Newtonian Fluid Mechanics142(1-3)162-182

Keywordscleaning, flow displacement, hydrodynamics, multiphase flow, viscoplastic fluid

CategoriesNumerical simulations, Theoretical modelling

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.

Review article

The fluid mechanics of cleaning and decontamination of surfaces

Landel, J. R., Wilson, D.I.

2021Annual Review of Fluid Mechanics53147-171

Keywordscleaning, decontamination

CategoryReview

Scale1e-9 to 1e-6 m, 1e-6 m, 1e-6 to 1e-3 m, 1e-3 m, 1e-3 to 1 m, 1 to 10 m, 10 to 1000 m, greater than 1 km / less than 1e-3 s, 1e-3 to 1 s, 1 to 60 s, 1 to 60 min, 1 to 24 hours, 1 to 30 days, 1 to 12 months, greater than 1 year

Non-expert summaryBroad review of the fluid mechanics involved in the cleaning and decontamination of surfaces using liquid flows. Three main phases are reviewed: the contacting phase, which requires the liquid cleanser to reach the soil; the action phase, which studies the various physico-chemical effects of the cleanser on the soil; and the removal phase, where the cleanser transports the soil away from the surface through various phenomena.

Article

Mathematical modelling of chemical agent removal by reaction with an immiscible cleanser

Dalwadi, M.P., O'Kiely, D., Thomson, S.J., Khaleque, T. S., Hall, C. L.

2017SIAM Journal on Applied Mathematics77(6)1937-1961

Keywordsasymptotic analysis, decontamination, moving boundary problem, Stefan problem, surface reaction

CategoryTheoretical modelling

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.

Article

Homogenisation problems in reactive decontamination

Luckins, E., Breward, C.J.W., Griffiths, I.M., Wilmott, Z.

2020European Journal of Applied Mathematics31782-805

Keywordsdecontamination, homogenisation, porous material, reaction

CategoryTheoretical modelling

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.

Article

Movement of chemical warfare agent simulants through porous media

Jenkins, R.A., Buchanan, M.V., Merriweather, R., Ilgner, R.H., Gayle, T.M., Watson, A.P.

1994Journal of Hazardous Materials37(2)303-325

Keywordschemical warfare agent simulant, decontamination, permeation, porous material, risk

DisciplinesChemical Engineering, Chemistry, Fluid Mechanics

CategoryLaboratory experiments

Scale1e-3 to 1 m / 1 to 60 min, 1 to 24 hours

Non-expert summaryExperimental study of the permeation of CWA simulants through different layers of porous building materials: brick, cinder block, wall board, wood. The breakthrough time is measured in a specially designed cell. Anisotropy of the permeation is observed for anisotropic material such as wood. Surface decontamination is tested and shown effective.

Article

Washing wedges: capillary instability in a gradient of confinement

Keiser, L., Herbaut, R., Bico, J., Reyssat, E.

2016Journal of Fluid Mechanics790619-633

Keywordscapillary instability, linear stability analysis, multiphase flow

CategoryLaboratory experiments

Scale1e-3 m, 1e-3 to 1 m / 1 to 60 min

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.