64 research outputs found
Turbulent shear layers in confining channels
We present a simple model for the development of shear layers between
parallel flows in confining channels. Such flows are important across a wide
range of topics from diffusers, nozzles and ducts to urban air flow and
geophysical fluid dynamics. The model approximates the flow in the shear layer
as a linear profile separating uniform-velocity streams. Both the channel
geometry and wall drag affect the development of the flow. The model shows good
agreement with both particle-image-velocimetry experiments and computational
turbulence modelling. The low computational cost of the model allows it to be
used for design purposes, which we demonstrate by investigating optimal
pressure recovery in diffusers with non-uniform inflow
The interplay of crack hopping, delamination and interface failure in drying nanoparticle films
Films formed through the drying of nanoparticle suspensions release the build-up of strain through a variety of different mechanisms including shear banding, crack formation and delamination. Here we show that important connections exist between these different phenomena: delamination depends on the dynamics of crack hopping, which in turn is influenced by the presence of shear bands. We also show that delamination does not occur uniformly across the film. As cracks hop they locally initiate the delamination of the film which warps with a timescale much longer than that associated with the hopping of cracks. The motion of a small region of the delamination front, where the shear component of interfacial crack propagation is believed to be enhanced, results in the deposition of a complex zig-zag pattern on the supporting substrate
How ice grows from premelting films and water droplets
Close to the triple point, the surface of ice is covered by a thin liquid
layer (so-called quasi-liquid layer) which crucially impacts growth and melting
rates. Experimental probes cannot observe the growth processes below this
layer, and classical models of growth by vapor deposition do not account for
the formation of premelting films. Here, we develop a mesoscopic model of
liquid-film mediated ice growth, and identify the various resulting growth
regimes. At low saturation, freezing proceeds by terrace spreading, but the
motion of the buried solid is conveyed through the liquid to the outer
liquid-vapor interface. At higher saturations water droplets condense, a large
crater forms below, and freezing proceeds undetectably beneath the droplet. Our
approach is a general framework that naturally models freezing close to three
phase coexistence and provides a first principle theory of ice growth and
melting which may prove useful in the geosciences.Comment: 32 pages, 10 figure
Crust formation in drying colloidal suspensions
During the drying of colloidal suspensions, the desiccation process causes the suspension near the air interface to consolidate into a connected porous matrix or crust. Fluid transport in the porous medium is governed by Darcy's law and the equations of poroelasticity, while the equations of colloid physics govern processes in the suspension. We derive new equations describing this process, including unique boundary conditions coupling the two regions, yielding a moving-boundary model of the concentration and stress profiles during drying. A solution is found for the steady-state growth of a nedimensional crust during constant evaporation rate from the surface. The solution is used to demonstrate the importance of the system boundary conditions on stress profiles and diffusivity in a drying crust. © 2011 The Royal Society
Mud peeling and horizontal crack formation in drying clays
Mud peeling is a common phenomenon whereby horizontal cracks propagate parallel to the surface of a drying clay. Differential stresses then cause the layer of clay above the crack to curl up to form a mud peel. By treating the clay as a poroelastic solid, we analyze the peeling phenomenon and show that it is caused by the gradient in tensile stress at the surface of the clay, analogously to the spalling of thermoelastic materials. For a constant water evaporation rate at the clay surface we derive equations for the depth of peeling and the time of peeling as functions of the evaporation rate. Our model predicts a simple relationship between the radius of curvature of a mud peel and the depth of peeling. The model predictions are in agreement with the available experimental data. Copyright 2011 by the American Geophysical Union
Ice-lens formation and geometrical supercooling in soils and other colloidal materials.
We present a physically intuitive model of ice-lens formation and growth during the freezing of soils and other dense, particulate suspensions. Motivated by experimental evidence, we consider the growth of an ice-filled crack in a freezing soil. At low temperatures, ice in the crack exerts large pressures on the crack walls that will eventually cause the crack to split open. We show that the crack will then propagate across the soil to form a new lens. The process is controlled by two factors: the cohesion of the soil and the geometrical supercooling of the water in the soil, a new concept introduced to measure the energy available to form a new ice lens. When the supercooling exceeds a critical amount (proportional to the cohesive strength of the soil) a new ice lens forms. This condition for ice-lens formation and growth does not appeal to any ad hoc, empirical assumptions, and explains how periodic ice lenses can form with or without the presence of a frozen fringe. The proposed mechanism is in good agreement with experiments, in particular explaining ice-lens pattern formation and surges in heave rate associated with the growth of new lenses. Importantly for systems with no frozen fringe, ice-lens formation and frost heave can be predicted given only the unfrozen properties of the soil. We use our theory to estimate ice-lens growth temperatures obtaining quantitative agreement with the limited experimental data that are currently available. Finally we suggest experiments that might be performed in order to verify this theory in more detail. The theory is generalizable to complex natural-soil scenarios and should therefore be useful in the prediction of macroscopic frost-heave rates
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