A Study on Universal Factor Determining the Evolution of Surface Roughness During Electrochemical Polishing

Electrochemical polishing (ECP) is widely used for scratch- and damage-free finishing of metal components. Though the polishing effect of ECP has been confirmed in many researches, the influence of polishing parameters on evolution of surface roughness is still ambiguous owing to the use of different ECP systems. In this paper, the universal factor determining the evolution of surface roughness during ECP is studied by theoretical analysis as well as experiments. Theoretical analysis based on viscous layer mechanism demonstrates that the material removal thickness is the key parameter governing the roughness evolution of the polished surface regardless of other parameters including the voltage, current and electrolyte concentration and so forth. A series of experiments were designed and carried out to verify the proposed hypothesis. Both the experimental results and already published researches proved the validity and universality of the newly developed hypothesis on surface roughness evolution. This work is of great significance for further understanding the finishing mechanism of ECP and process control for its practical applications.

Impact of fibre incorporation and compaction method on properties of pervious concrete

This paper deals with the possibility of the improvement of pervious concrete properties by incorporation of different types of fibres and studies the effect of short duration vibration of pervious concrete properties in comparison with compaction with wooden lath and hammer. Ten mixtures of pervious concrete were prepared, five of which were compacted with wooden lath and hammer and five by short duration vibration. Density, porosity, permeability and mechanical properties were tested for in hardened pervious concrete specimens. It was concluded that mixtures compacted by short duration vibration had better mechanical properties due to the formation of a viscous layer at the contact surface between the aggregate grain and the cement matrix during the compaction, as well as pore-related properties. The addition of fibres negatively affected porosity and permeability but generally improved mechanical properties of concrete. The positive effect of fibre addition was more emphasised in cases of vibrated mixtures.

Are complex rift patterns the result of interacting crustal and mantle weaknesses, or of multiphase rifting? An analogue modelling study

<p>During extension of the continental lithosphere, deformation often localizes along pre-existing weaknesses originating from previous tectonic phases. When simulating such structures with analogue or numerical methods, modellers often focus on either crustal or mantle heterogeneities. By contrast, here we present results from 3D analogue models to test the combined effect and relative impact of (differently oriented) mantle and crustal weaknesses on rift systems.</p><p>Our model set-up involves a rigid base plate fixed to a mobile sidewall. When this sidewall moves outward, the edge of the base plate induces a “velocity discontinuity” (VD) that acts as an upper mantle fault/shear zone in a strong upper mantle. The VD is either parallel to the model axis, or 30˚ oblique. On top of this base plate, we apply a viscous layer representing the ductile lower crust, followed by a sand cover that simulates the brittle upper crust. Crustal weaknesses were either imposed by implementing “seeds” (i.e. ridges of viscous material at the base of the sand layer), or by pre-cutting the sand. Similar to the basal plate edge, we apply different crustal weakness orientations as well.</p><p>Without weaknesses in the model crust, an axis-parallel VD forms an axis-parallel rift basin above along the VD. When adding oblique seeds, they strongly localize deformation, creating a series of obliquely oriented graben. Yet the VD still induces faulting along the model axis, leading to the development of offset axial graben as well. Pre-cut faults also localize deformation but are less dominant than the seeds. As a result, the VD has more control and the axial rift structures are much more pronounced. In the oblique VD case, the reference model develops a series of en echelon graben along the VD. Axis-parallel seeds strongly localize faulting, to such a degree that the effect of the VD is very much overruled. Pre-cut faults allow more influence from the VD, but still dominate the system. Doubling the extension rate increases the strength of the viscous layer, enhancing coupling between the VD and sand cover, so that a series of en echelon graben crosscutting the seed-induced structures develop.</p><p>We find that the orientation and relative weakness of inherited weaknesses in the mantle and crust, as well as extension rates control subsequent rift structures. These structures and their relative evolution can be complex due to the interplay of the above factors, and importantly, all develop under the same pure shear extensional boundary condition. Our results show that very differently oriented rift structures can form during one phase of extension without the need to invoke multiple rift phases. Furthermore, coupling can change over time due to changes in extension velocity or gradual thinning of the lower crust, thus affecting rift evolution. These findings provide a strong incentive to reassess the tectonic history of various natural examples.</p>

Zero viscosity and thermal diffusivity limit of the linearized compressible Navier–Stokes–Fourier equations in the half plane

We study the zero viscosity and thermal diffusivity limit of an initial boundary problem for the linearized Navier–Stokes–Fourier equations of a compressible viscous and heat conducting fluid in the half plane. We consider the case that the viscosity and thermal diffusivity converge to zero at the same order. The approximate solution of the linearized Navier–Stokes–Fourier equations with inner and boundary expansion terms is analyzed formally first by multiscale analysis. Then the pointwise estimates of the error terms of the approximate solution are obtained by energy methods. Thus establish the uniform stability for the linearized Navier–Stokes–Fourier equations in the zero viscosity and heat conductivity limit. This work is based on (Comm. Pure Appl. Math. 52 (1999), 479–541) and generalize their results from isentropic case to the general compressible fluid with thermal diffusive effect. Besides the viscous layer as in (Comm. Pure Appl. Math. 52 (1999), 479–541), the thermal layer appears and couples with the viscous layer linearly.

Rayleigh–Taylor and Kelvin–Helmholtz instability studied in the frame of a dimension-reduced model

Introducing an extension of a recently derived dimension-reduced model for an infinitely deep inviscid and irrotational layer, a two-layer system is examined in the present paper. A second thin viscous layer is added on top of the original one-layer system. The set-up is a combination of a long-wave approximation (upper layer) and a deep-water approximation (lower layer). Linear stability analysis shows the emergency of Rayleigh–Taylor and Kelvin–Helmholtz instabilities. Finally, numerical solutions of the model reveal spatial and temporal pattern formation in the weakly nonlinear regime of both instabilities. This article is part of the theme issue ‘Stokes at 200 (Part 1)’.

The thermal boundary layer due to viscous dissipation in impulsively started Poiseuille flow

Viscous dissipation occurs in the boundary layers on the walls of a channel in which a flow is accelerated from rest by the sudden imposition of a pressure gradient. We analyse the thermal boundary layer due to this dissipative heating, obtaining numerical solutions and also asymptotic solutions for the cases of both large and small Prandtl number, with both isothermal and adiabatic wall conditions. With large $\mathrm{Pr}$ the temperature rise is controlled by the viscous layer, so is independent of $\mathrm{Pr}$ and of the wall condition. With small $\mathrm{Pr}$ heat is conducted away from the viscous layer more rapidly, so the temperature rise is reduced as $\mathrm{Pr}$ decreases.