Stability Analysis of Gravity Currents of a Power-Law Fluid in a Porous Medium

We analyse the linear stability of self-similar shallow, two-dimensional and axisymmetric gravity currents of a viscous power-law non-Newtonian fluid in a porous medium. The flow domain is initially saturated by a fluid lighter than the intruding fluid, whose volume varies with time astα. The transition between decelerated and accelerated currents occurs atα= 2 for two-dimensional and atα= 3 for axisymmetric geometry. Stability is investigated analytically for special values ofαand numerically in the remaining cases; axisymmetric currents are analysed only for radially varying perturbations. The two-dimensional currents are linearly stable forα 2 (two-dimensional accelerated currents) andα> 3 (axisymmetric accelerated currents) the linear stability analysis is of limited value since the hypotheses of the model will be violated

Prospecting of industrial minerals: a geophysical approach

This work deals with the evaluation of the electrical resistivity method for the identification and characterization of industrial minerals deposits. The ERT (Electrical Resistivity Tomography) technique is demonstratively applied to 4 different areas of the central and northern Sardinia. The research focused on clays, which generally have conductivity values greater than those of the host lithotypes. In this regard, a bentonite deposit close Ozieri (N-Sardinia) was investigated by 2D and 3D ERT surveys, and a preliminary study was conducted in the northern part of Nurra (NW-Sardinia) in order to discriminate Messinian clay deposits from the underlying bedrock. The ERT method was also tested on cineritic levels, which for their absorption properties are increasingly required as litter for pets. Potentially exploitable cineritic levels have been identified in two different sites of central Sardinia (Macomer and Zerfaliu), within the Oligo-Miocene calcalkaline volcanic sequence. In all the 4 case studies it is attempted: 1) to define the geometry of the deposits; 2) to estimate the reserves and locate the main faults useful to decipher the ore-forming processes; 3) to evaluate the reliability of the ERT method by cross-checking the results with boreholes data. The results indicate that ERT method may represent a relatively cheap and powerful tool to obtain high-resolution geological and stratigraphic information on clayey and cineritic bodies.</br

Shear-Thinning Fluid Flow in Variable-Aperture Channels

Non-Newtonian fluid flow in a single fracture is a 3-D nonlinear phenomenon that is often averaged across the fracture aperture and described as 2-D. To capture the key interactions between fluid rheology and spatial heterogeneity, we adopt a simplified geometric model to describe the aperture variability, consisting of adjacent one-dimensional channels with constant aperture, each drawn from an assigned aperture distribution. The flow rate is then derived under the lubrication approximation for the two limiting cases of an external pressure gradient that is parallel/perpendicular to the channels; these two arrangements provide upper and lower bounds to the fracture conductance. The fluid rheology is described by the Prandtl–Eyring shear-thinning model. Novel closed-form results for the flow rate and hydraulic aperture are derived and discussed; different combinations of the parameters that describe the fluid rheology and the variability of the aperture field are considered. The flow rate values are very sensitive to the applied pressure gradient and to the shape of the distribution; in particular, more skewed distribution entails larger values of a dimensionless flow rate. Results for practical applications are compared with those valid for a power-law fluid and show the effects on the fracture flow rate of a shear stress plateau

PROPAGATION OF GRAVITY CURRENTS OF NON-NEWTONIAN POWER-LAW FLUIDS IN POROUS MEDIA

A comprehensive analytical and experimental framework is presented to describe gravity-driven motions of rheologically complex fluids through porous media. These phenomena are relevant in geophysical, environmental, industrial and biological applications. The fluid is characterized by an Ostwald-DeWaele constitutive equation with behaviour index n. The flow is driven by the release of fluid at the origin of an infinite porous domain. In order to represent several possible spreading scenarios, we consider: i) different domain geometries: plane, radial, and channelized, with the channel shape parameterized by ; ii) instantaneous or continuous injection, depending on the time exponent of the volume of fluid in the current, ; iii) horizontal or inclined impermeable boundaries. Systematic heterogeneity along the streamwise and/or transverse direction is added to the conceptualization upon considering a power-law permeability variation governed by two additional parameters  and . Scalings for current length and thickness are derived in self similar form coupling the modified Darcy’s law accounting for the fluid rheology with the mass balance equation. The speed, thickness, and aspect ratio of the current are studied as a function of model parameters; several different critical values of  emerge and govern the type of dependency, as well as the tendency of the current to accelerate or decelerate and become thicker or thinner at a given point. The asymptotic validity of the solutions is limited to certain ranges of model parameters. Experimental validation is performed under constant volume, constant and variable flux regimes in tanks/channels filled with transparent glass beads of uniform or variable diameter, using shear-thinning suspensions and Newtonian mixtures. The experimental results for the length and profile of the current agree well with the self-similar solutions at intermediate and late times

Vestibular schwannoma and ipsilateral endolymphatic hydrops: an unusual association

Vestibular schwannoma and endolymphatic hydrops are two conditions that may present with similar audio-vestibular symptoms. The association of the two in the same patient represents an unusual nding that may lead clinicians to errors and delays in diagnosis and clinical management of affected subjects. We discuss the case of a patient with an intrameatal vestibular schwannoma reporting symptoms suggestive for ipsilateral endolymphatic hydrops. The patient presented with uctuating hearing loss, tinnitus, and acute rotatory vertigo episodes, and underwent a full audiological evaluation and imaging of the brain with contrast-enhanced Magnetic Resonance Imaging. Clinical audio-vestibular and radiological examination con rmed the presence of coexisting vestibular schwannoma and endolymphatic hydrops. Hydrops was treated pharmacologically; vestibular schwannoma was monitored over time with a wait and scan protocol through conventional MRI. The association of vestibular schwannoma and endolymphatic hydrops is rare, but represents a possible nding in clinical practice. It is therefore recommended investigating the presence of inner ear disorders in patients with vestibular schwannoma and, similarly, to exclude the presence of this condition in patients with symptoms typical of inner ear disorders

Audio-vestibular symptoms in systemic autoimmune diseases

Immune-mediated inner ear disease can be primary, when the autoimmune response is against the inner ear, or secondary. The latter is characterized by the involvement of the ear in the presence of systemic autoimmune conditions. Sensorineural hearing loss is the most common audiovestibular symptom associated with systemic autoimmune diseases, although conductive hearing impairment may also be present. Hearing loss may present in a sudden, slowly, rapidly progressive or fluctuating form, and is mostly bilateral and asymmetric. Hearing loss shows a good response to corticosteroid therapy that may lead to near-complete hearing restoration. Vestibular symptoms, tinnitus, and aural fullness can be found in patients with systemic autoimmune diseases; they often mimic primary inner ear disorders such as Menière’s disease and mainly affect both ears simultaneously. Awareness of inner ear involvement in systemic autoimmune diseases is essential for the good response shown to appropriate treatment. However, it is often misdiagnosed due to variable clinical presentation, limited knowledge, sparse evidence, and lack of specific diagnostic tests. The aim of this review is to analyse available evidence, often only reported in the form of case reports due to the rarity of some of these conditions, of the different clinical presentations of audiological and vestibular symptoms in systemic autoimmune diseases

Porous gravity currents: A survey to determine the joint influence of fluid rheology and variations of medium properties

We develop a model to grasp the combined effect of rheology and spatial stratifications on two- dimensional non-Newtonian gravity-driven flow in porous media. We consider a power-law constitutive equation for the fluid, and a monomial variation of permeability and porosity along the vertical direction (transverse to the flow) or horizontal direction (parallel to the flow). Under these assumptions, similar- ity solutions are derived in semi-analytical form for thin gravity currents injected into a two-dimensional porous medium and having constant or time-varying volume. The extent and shape of the porous domain affected by the injection is significantly influenced by the interplay of model parameters. These describe the fluid (flow behaviour index n ), the spatial heterogeneity (coefficients β, γ, δ, ω for variations of per- meability and porosity in the horizontal or vertical direction), and the type of release (volume exponent α). Theoretical results are validated against two sets of experiments with α= 1 (constant inflow) con- ducted with a stratified porous medium (simulated by superimposing layers of glass beads of different diameter) and a Hele-Shaw analogue for power-law fluid flow, respectively. In the latter case, a recently established Hele-Shaw analogy is extended to the variation of properties parallel to the flow direction. Comparison with experimental results shows that the proposed model is able to capture the propagation of the current front and the current profile

Human Ergonomic Simulation to Support the Design of an Exoskeleton for Lashing/De-Lashing Operations of Containers Cargo

Lashing and de-lashing operations of containers cargo on board containerships are considered as quite strenuous activities in which operators are required to work continuously over a 6 or 8 hours shift with very limited break. This is mostly because containerships need to leave the port as soon as possible and containers loading and unloading operations must be executed with very high productivity (stay moored in a port is a totally unproductive time for a ship and a loss-making business for a shipping company). Operators performing lashing and de-lashing operations are subjected to intense ergonomic stress and uncomfortable working postures. To this end, the authors of this article are participating to a research project for the design of an exoskeleton that will help operators to reduce ergonomics and working posture problems while increasing, at the same time, the productivity. This paper presents the results of a human ergonomic simulation devoted t

Functional outcomes in supracricoid laryngectomy

Supracricoid laryngectomies (SCLs) are conservative surgical techniques for the treatment of selected laryngeal carcinomas and are considered an organ-sparing alternative to total laryngectomy and chemo-radiotherapy. The main characteristics of SCLs are the preservation of the main laryngeal functions as respiration, phonation and swallowing, without a permanent tracheostomy. Supracricoid laryngectomies have been questioned for many years as regarding functional and oncological outcomes and are currently accepted, although patient selection criteria and functional results are still debated. The mainstream of this surgery is the maintenance of one functioning cricoarytenoid unit to allow restoring of swallowing and phonation. Thus, post-operative rehabilitation protocol is required to archive functional outcomes and avoid functional failure of this surgery; an early rehabilitation protocol improves functional results, in particular regarding swallowing. Swallowing and voice functional outcomes differ among several centres and are often related to the post-operative management, although SCLs provide commonly good swallowing and respiratory outcomes. To date, SCLs are proven surgical procedures for the treatment of laryngeal cancer and should be a valuable option to total laryngectomy and chemo-radiotherapy for selected advanced laryngeal squamous cell carcinoma. In this clinical review, we discuss the clinical outcomes in patients treated with SCLs with particular attention to rehabilitation protocol and functional outcomes for swallowing and voice rehabilitation