1,721,263 research outputs found
Modeling of segregation in dense granular flows
Full text linkIn this thesis, a continuum framework, composed of equations of mass, momentum and fluctuating kinetic energy, coupled to one capable of describing the evolution of the interface between air and granular material, was used to study the
dense motion of granular materials within two industrially relevant equipments: mixers and silos. The model uses the constitutive equations to close the system of equations and a phenomenological approach to describe the behavior of granular materials. A second model, capable of describing the evolution of the segregation of a bidispersed mixtures through a transport equation, was used to model the segregation. The segregation model is flexible; in fact it is possible to describe the segregation speed using different relationships depending on the segregation mechanisms.
As a first approach, it was decided to adopt a relation deriving from Literature approaches. Despite the use of this relation has led to good results, a further study based on experiments and DEM simulations has been done to improve the
description of the segregation velocity to be used in segregative model
Particle Size Segregation During the Discharge of Binary Mixtures and the Role of Void Saturation
No full textParticles differing in size, under external mechanical activation, typically segregate rather than mix. This phenomenon, known as particle size segregation, is spontaneous and unavoidable in practice. In this work, we present an experimental investigation aiming at better understanding the segregation mechanisms occurring in industrial devices related to the storage of particulate materials. Even if storage in closed vessels is often considered a static operation, loading and unloading steps are always present and they must be considered as inherent part of the whole storage operation. These steps are clearly dynamic in nature. This work concentrates on the discharge step in which the particulate material is strongly sheared, especially close to the outlet of the storage bin giving high chances of segregation to the material. Segregation in dense sheared flows under gravity occurs mainly through the percolation of smaller particles in the voids existing between larger ones. The level of saturation of voids by small particles can strongly impact the segregation rate. It appears therefore that the relative amount of fines with respect to large particles, the level of bed dilatancy induced by shear, the interparticle friction are all variables that can affect fine particles mobility and therefore the extent of segregation. For these reasons, we carried out experiments on small-scale vessels discharging in a funnel flow regime. Binary mixtures at different fine compositions and particle size ratios have been considered. Furthermore, we carried out experiments to a better knowledge of the mixtures voidage. The correlations found in the literature fail to predict the critical fine concentration at which fine particle segregation stops. An Alternative correlation has been therefore proposed based on a micromechanical analysis of particles filling
Characterization of Powder Flowability for Additive Manufacturing
No full textPowders have always been used in industry, spanning from food to metallurgic ones, and in recent decades they have become the protagonists of other innovative process: additive manufacturing (AM). Knowing the flowability of a powder in a specific piece of equipment is fundamental to achieve better manufacturing efficiency and product quality in powder processing industries. A powder used in AM machines is subjected to different flow and stress regimes. There are commercial devices able to measure the flowability of the powder under different levels of consolidation stresses, but they are not able to work at very low stresses. Among the available, alternative, non-commercial tests dynamic BIM is the most promising. In dynamic BIM a spherical indenter impinges on the powder with the velocity of a falling body; it is possible to determine the flowability (through the hardness) of the powder, but the evaluation is complex and must be improved. In this work we investigate the dynamic impact of an indenter in a packed powder, measuring the geometrical characteristic of the crater forming on the powder surface. Furthermore, we try to improve the definition of dynamic hardness and shear rate by rewriting them in terms of granular velocity rather than indenter impact velocity as the most common reports in the literature
Investigation on particle size and packing tortuosity by coupling image analysis and permeability tests
Full text linkThis experimental study aims to enhance the understanding of the correlation among equivalent particle diameters measured using two analytical techniques: optical analysis (assisted by computer aided image analysis) and permeability tests. The presence or absence of a specific analytical method or instrument can lead to the use of an incorrect equivalent diameter. Therefore, it can be beneficial to establish conversion rules between different equivalent particle diameters obtained through various methods and instruments. The optical analysis returns an equivalent diameter value inherently independent of particle arrangement since it deals with isolated particles. In contrast, the permeability test offers an equivalent mean diameter dependent not only on the size of the particles but also on their packed arrangement. A suitable correlation between the two diameters has been proposed, shown to be a decreasing function of porosity following a power law. An unexpected outcome of the comparison between the optical method and permeametry is the possibility to isolate and characterize the effect that the packing arrangement has on pressure losses and to characterize it in terms of the tortuosity of the path that the fluid must travel through the packed bed. Our findings confirm a strong alignment between our tortuosity model, which contains the ratio between the two equivalent diameters considered here, and an empirical correlation from literature often utilized for predicting packed bed tortuosity
Particle Size Characterization through Bed Permeability Tests
No full textThis is an experimental study aiming at understanding the relationship between particle diameters measured with different analytical techniques: optical analysis (microscopy aided by computer image analysis), laser light scattering and permeability tests. Permeability test can provide an equivalent mean diameter that can be used in several cases where particle specific surface is the relevant property to focus on (for example problems related to particle reactivity or involving interactions with fluids as in pneumatic transport or in fluidization). This diameter can be determined by measuring the pressure loss in a granular bed of known porosity at various gas flow rate and by using a proper mathematical model (e.g., Darcy or Karman-Kozeny models) to correlate them. Four different materials (glass beads, granulated microcrystalline cellulose, MCC, sodium chloride crystals, tetraacetylethylenediamine powders, TAED) in the size range 700-1000 μm (by sieve analysis) were analysed. A strong discrepancy was observed by calculating the Sauter diameters from image analysis (and laser light scattering) and those from permeability measurements. The introduction of a function considering the pores morphology and connectivity was able to reconcile the size measured with the three different methods
Shear-induced particle segregation in binary mixtures: Verification of a percolation theory
No full textGranular materials composed of different-sized grains may experience undesired segregation. Segregation is detrimental for a lot of industries because it leads to an increase in production costs and wastes. For these reasons, the segregation phenomena have been intensively studied in the last decades, and a lot of models have been provided by many researchers. However, these models are mainly based on empirical relations rather than physical considerations. This paper aims to confirm the main assumptions made by Volpato, Tirapelle, and Santomaso (2020) in their percolation theory by means of DEM simulations. The simulated geometry is a tilting shear box filled with few tracer particles in a bed of coarser sized grains, and simulations are performed for a range of tilting frequencies and size ratios. The results provide meaningful insight on the mathematical model parameters and allow us to say that the percolation theory relies on physically consistent assumptions
Chronic pain and disability in diabetes
No full textThere are several potential sources of acute and chronic pain in older diabetic patients, many of whom have two or more painful conditions. Major contributors to chronic lower extremity pain in older diabetic patients include peripheral neuropathy and peripheral arterial disease. Although chronic pain is always present in neuropathies, painful peripheral neuropathies are especially disabling in older patients because of detrimental effects on balance, sensorimotor function, gait, and functional autonomy. Older diabetic patients have worse PAD below the knee than their non-diabetic peers and higher risk of amputation. Research has demonstrated that diabetics have more severe PAD but complain more of atypical leg pain with exertion and rest, rather than the classic intermittent claudication.
Musculoskeletal pain is very common in older persons and it is often difficult to diagnose the underlying pathology. In the absence of a definitive diagnosis, symptom management to prevent functional decline becomes paramount in the care of older patients. Among patients who score positive on a simple pain screen, a more detailed follow-up assessment should include pain history and a detailed description of pain frequency, duration, location, and impact on mood and functioning.
Correct classification of the pain syndrome according to pathophysiologic mechanisms will establish whether pain is neuropathic (e.g. diabetic neuropathy) or nociceptive (e.g. arthritis pain or other somatic or visceral pain), or of mixed pathogenesis.
Regardless of the underlying conditions that cause pain, the presence of persistent pain, particularly back pain and lower-extremity pain, has a marked disabling effect, adding to the risk for functional limitation or worsening disability and further reducing the quality of life of older patients with diabetes. From this point of view accurate assessment and appropriate management of pain will help older diabetic patients to maintain their activities and independence.
Individually tailored therapeutic trials are the hallmark of effective pharmacotherapy for persistent pain in older diabetic patients. Providers face particular challenges in assessment and treatment of persons who have cognitive impairments. These patients are particularly vulnerable to medication side effects and also to undertreatment. The AGS guidelines for pain management emphasize use of an interdisciplinary approach. Referrals to pain specialists, pain clinics, and rehabilitation specialists are important and often underutilized components of chronic pain management for older patients. Efforts within primary care practices to promote better patient self-management of chronic conditions may be beneficial for older adults with diabetes
Modeling and experimental investigation of shear-induced particle percolation in diluted binary mixtures
No full textA Frail Health Care System for an Old Population: Lesson form the COVID-19 Outbreak in Italy
No full textAbstract not availabl
Clinical Features of SARS-CoV-2 Infection in Older Adults
No full textThe COVID-19 clinical presentation is extremely heterogenous and, in older people, it is influenced not simply by chronologic age but also by common geriatric syndromes, such as multimorbidity, motor disability, and frailty. Consequently, although typical respiratory symptoms remain the most frequent clinical presentation of COVID-19 in all age classes, in older patients, atypical symptoms (including but not limited to delirium and hyporexia) are more common than in middle-aged adults and have been associated with adverse outcomes. Moreover, some studies described the tendency of COVID-19 presenting symptoms to aggregate in clusters, and this approach seems to better capture the complexity of COVID-19 disease. The prognostic value of COVID-19 symptom clusters, however, is currently poorly investigated, especially in the older population
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