Non-expert summaryThe displacement of one liquid by another in a channel of constant height occurs in flushing and cleaning operations. The authors study the effect of buoyancy (arising from different fluid densities) on a pressure-driven flow of two miscible fluids in inclined channels using direct numerical simulations DNS). The flow dynamics are governed by the continuity and Navier–Stokes equations, without the Boussinesq approximation for buoyancy, coupled to a convective-diffusion equation for mass transfer between the two liquids. The effect of concentration on viscosity and density is modelled. The effect of varying the density ratio, Froude number, and channel inclination on the flow dynamics is examined, for moderate Reynolds numbers. These detailed simulations give insights into mixing and cleaning behaviour.
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.
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.
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 summaryThis is a short review presenting and comparing five apparatuses that are used in laboratory scale investigations of cleaning: the parallel plate flow cell, the impinging jet, the radial flow cell, the rotating disk and fluid dynamic gauging. These systems are of particular relevance to the study of surface fouling, surface cleaning or adhesion on solid surfaces in laboratory environment. The key features of their hydrodynamics, as well as their practical advantages and drawbacks, are discussed. Examples of applications fields are also listed. A useful introduction to these devices.
Non-expert summaryAn important part of hygienic design is knowledge of the forces imposed on a surface by the flow of a process liquid, and whether those forces are large enough to remove any contaminants. This is a chiefly experimental study investigating the numbers of adhering Bacillus cereus spores before and after cleaning in place of part of a dairy processing line. Non-intrusive flow characterization, of local wall shear stress and velocity profiles, were measured using electrochemical and ultrasonic Doppler velocimetry techniques. The geometries studied inlcuded gradual asymmetric pipe contractions and pipe expansions and bends. The recirculation zone resulting from the flow detachment after the expansion resulted in a large number of adherent spores downstream of this geometrical change. Welding and gasket regions also retained spores, proving difficult to clean despite high-shear stress forces involved. The flow measurements are compared with some theory so the level of modelling is not high but the data sets present important cases for new investigations, e.g. using CFD methods.