Barbara Turnbull

Experiments on the non‐Boussinesq flow of self‐igniting suspension currents on a steep open slope

Fiber reinforced structural components, such as panels with honeycomb cores sandwiched between at least two outer skin layers are dried to remove moisture that may have entered into the interior of the structure. For this purpose the component is cooled in a cooling environment below the freezing point to about -5 DEG to -20 DEG C. and then exposed to a reduced pressure of less than 0.5 mbar. Under these drying conditions moisture is driven out of the component by sublimation so that the moisture does not pass through the liquid phase. The drying time may be reduced by a temperature increase to 80 DEG -100 DEG C. while maintaining the low pressure. The freezing increases the capillary action for the moisture removal. However, for an increased efficiency in the moisture removal small diameter holes, e.g. 0.1 mm bores, may be drilled into the component to facilitate the moisture escape. After drying is complete the holes are closed again e.g. by laminating further cover sheets to the component. Drying time may be reduced by a temperature increase to 80 DEG -100 DEG C. while maintaining the low pressure.

Potential flow models of suspension current air pressure

Abstract We present, analyse and discuss air-pressure data from finite-volume chute flows of dry fine snow in air. These experiments have the correct similarity criteria to model powder-snow avalanches and demonstrate the transition from a dense to a suspended flow. We measured the dynamic air pressure at the base of the flow, which features a marked negative pressure peak immediately behind the front. This feature is also seen in observations of natural powder-snow avalanches measured in Russia, Japan and Switzerland in direct numerical simulations of non-Boussinesq suspension flows and in ping-pong ball avalanches. This is evidence for large internal motions and suggests that there is a coherent vortex in the avalanche front. This can result in impact pressures many times larger than those expected from the mean flow velocity. We analyse the external air pressures using three models and show how the geometry and velocity of the flow can be found from this single air-pressure measurement. We also measured flow heights and speeds using image analysis and show that the speed is roughly independent of the slope angle and scales with the release size raised to the power 1/4, as predicted by similarity analysis for pseudo two-dimensional (2-D) flows.

Super-hydrophobic/icephobic coatings based on silica nanoparticles modified by self-assembled monolayers

A super-hydrophobic surface has been obtained from nanocomposite materials based on silica nanoparticles and self-assembled monolayers of 1H,1H,2H,2H-perfluorooctyltriethoxysilane (POTS) using spin coating and chemical vapor deposition methods. Scanning electron microscope images reveal the porous structure of the silica nanoparticles, which can trap small-scale air pockets. An average water contact angle of 163° and bouncing off of incoming water droplets suggest that a super-hydrophobic surface has been obtained based on the silica nanoparticles and POTS coating. The monitored water droplet icing test results show that icing is significantly delayed by silica-based nano-coatings compared with bare substrates and commercial icephobic products. Ice adhesion test results show that the ice adhesion strength is reduced remarkably by silica-based nano-coatings. The bouncing phenomenon of water droplets, the icing delay performance and the lower ice adhesion strength suggest that the super-hydrophobic coatings based on a combination of silica and POTS also show icephobicity. An erosion test rig based on pressurized pneumatic water impinging impact was used to evaluate the durability of the super-hydrophobic/icephobic coatings. The results show that durable coatings have been obtained, although improvement will be needed in future work aiming for applications in aerospace.

Mass and Momentum Balance Model of a Mixed Flowing/Powder Snow Avalanche

This work uses a one-dimensional, depth averaged model to compute the massbalance of a mixed flowing/powder snow avalanche. This model is comprisedof three basic components: the dense flowing avalanche, the powder cloud anda turbulent wake. The dynamics of a mixed avalanche is strongly dependent onthe interaction between the components and also on the snow cover and ambientair, in particular the exchange or entrainment of snow and air mass. Therefore, animportant first step for modelling mixed avalanches is a basic understanding of thenature of these mass exchanges. In this paper, the governing equations of mass,momentum and turbulent energy are briefly presented. Numerical simulations wererun for three avalanche tracks – Aulta, Galtür and Vallée de la Sionne – for whichdata from real snow avalanche events exist. Based on the results, conclusions weredrawn regarding the parameterisation of the mass exchanges. The mass balances forthese three contrasting avalanches are presented.

Role of fluid density in shaping eruption currents driven by frontal particle blow-out

We study the role of suspension density in eruption currents, a regime of gravity-driven flow that is sustained by massive, localized blow-out of particles acting as a steady source of heavier fluid injected into a uniform flow at high Reynolds number. Inspired by the potential flow solution of Saffman and Yuen [“Finite-amplitude interfacial waves in the presence of a current,” J. Fluid Mech. 123, 459–476 (1982)10.1017/S0022112082003152], we show that the relative density difference between the two fluids swells the size of the currents head without changing its shape, while inducing a velocity jump at the interface. We test this inviscid theory against inviscid and large-eddy-simulations. We also conduct experiments in a water flume, where a line source of fluorescent brines of various densities is injected in a cross-stream and visualized with a narrow sheet of light. Simulations and experiments reveal that, with isotropic velocity distribution on a finite source, eruption currents expand further and d...

Volume growth of a powder snow avalanche

Abstract We contrast the frontal dynamics of dilute powder snow avalanches with the behavior of their tail. While the former can be regarded as a fast-moving eruption current dominated by synergistic material injection into a short head, the latter behaves as a nearly arrested dilute cloud of particles expanding by progressive incorporation of ambient air, or by entrainment of snow-cover material by late avalanches trailing well behind the front.

Looking beyond the powder/dense flow avalanche dichotomy

Kohler et al. (2018) deploy a high spatial and temporal resolution GEODAR radar system to reveal the inside of snow avalanches over the entire slope. They detect a rich variety of longitudinal and slope normal flow structures across a data set of 77 avalanches recorded over 6 years. Distinctive features in the radar signatures permit the definition of seven flow regimes and three distinct stopping signatures, illustrating behaviours much richer than the conventional dichotomy between dense flow avalanches and powder snow avalanches. This presents modellers with the challenge of exploring the physics of these regimes, the transitions between them and their relationship with the surrounding conditions.

Mixed ice accretion on aircraft wings

Ice accretion is a problematic natural phenomenon that affects a wide range of engineering applications including power cables, radio masts, and wind turbines. Accretion on aircraft wings occurs when supercooled water droplets freeze instantaneously on impact to form rime ice or runback as water along the wing to form glaze ice. Most models to date have ignored the accretion of mixed ice, which is a combination of rime and glaze. A parameter we term the “freezing fraction” is defined as the fraction of a supercooled droplet that freezes on impact with the top surface of the accretion ice to explore the concept of mixed ice accretion. Additionally we consider different “packing densities” of rime ice, mimicking the different bulk rime densities observed in nature. Ice accretion is considered in four stages: rime, primary mixed, secondary mixed, and glaze ice. Predictions match with existing models and experimental data in the limiting rime and glaze cases. The mixed ice formulation however provides additio...

Role Models in Gender-Skewed Disciplines

Abstract Higher education often displays significant imbalance in gender ratios within both student and staff bodies. This is clearly the case in engineering where undergraduate women are far outnumbered by their counterparts and women professors are outnumbered to an even greater extent. In this study we consider the value of gender-minority instructors, in a variety of roles, to gender-minority undergraduates within a higher-education environment. We explore whether gender-minority students place a significant value on the gender of their instructors and, if so, in which roles. Using conjoint analysis, a statistical process widely used in market research, we quantify the value gender-minority undergraduates place on having gender-minority lecturers/instructors, project supervisors and personal tutors. Students were asked in a questionnaire to rank a set of synthesized academic profiles based on the academic’s experience and educational background only. The ranking also tested for gender-bias without the participants’ knowledge. The results demonstrated that gender was only considered important to undergraduates either when in the gender-majority or within gender-balanced departments, and then only in the role of personal tutor. Women minority students showed a preference for less experienced tutors with less prestigious academic qualifications. We find gender-minority students attach very little value to the gender of the academics they interact with, unlike their peers. We suggest this is because students who opt into the gender-minority at a tertiary level are either indifferent to gender, or have already made a pro-active choice to disregard it. Support for addressing undergraduate and academic gender imbalance might therefore be better targeted prior to higher education.

Reply to comment by P. A. Bartelt and O. Buser on “Role of pore pressure gradients in sustaining frontal particle entrainment in eruption currents: The case of powder snow avalanches”

We present a model that underscores the role played by the porous snow cover in sustaining large, rapid, dilute powder avalanches over weakly cohesive snow. The model attributes massive localized material injection into the avalanche head to synergistic pressure gradients established within the porous cover by the very static pressure field that this influx induces along the pack surface. Treating massive frontal snow entrainment as a source of fluid, we show that static pressure time-histories recorded at the Vallée de la Sionne (Switzerland) conform to the classical two-dimensional Rankine half-body flow field. We calculate pore pressure within the snow cover and, from the resulting pressure gradient, find stresses on a vertical failure plane. After inferring an upper bound for snow cohesion from pressure records, we derive a sufficient condition for steady failure that sets the depth through which the cover changes from porous solid to fluidized suspension. Fluidization of the top surface imposes another relation among maximum density, internal friction and cohesion of the pack, maximum cloud size and minimum avalanche speed. Altogether, these conditions dictate which snow covers can produce powder snow avalanches. We suggest how similar “eruption currents” sustained by massive frontal entrainment may be relevant to other fluid-particle suspensions.