1,721,284 research outputs found
Celle circolari e reti di clasti in letti fluviali a sedimenti grossolani. Alcuni risultati sperimentali
No full textCircle cells and clast networks in gravel-bed rivers. Some experimental results. - Extensive field work, begun in 1995, in highly diverse morphoclimatic environments (Northern England, Iceland, Italy, Southern Africa, Southern Australia and Antarctica) has shown the presence, in gravel-bed rivers, of widespread clast cells, covering sometimes 50% or more of river bar areas. In order to reproduce the same processes in controlled conditions, a large number of flume experiments was carried out from 2003 onwards. We have been able to pinpoint two processes: an alluvial process with a trident-shaped flow which bifurcates in correspondence to the keystone, especially at the thalweg, and a random colluvial process whereby cells formed by bank landslides are exhumed by removal of fines at sub-critical flow rates. A dependence of flow patterns from keystone size has been demonstrated. Clast networks exercise a considerable sheltering effect against bed mobilization, as the network elements tend to dampen the energy of the flow. Similar networks are found in many other natural phenomena, suggesting the existence of an underlying order in nature
Development of a Variable Activation Energy Model for Biomass Devolatilization
No full textA thermogravimetric balance is used in this work to characterize different classes of biomass fuels: residues (rice husks, olive cake, cacao shells), woods (poplar, beech, pellets), and grasses (mischantus). The effect of the heating rate is evaluated in the range 10-80 K/min providing significant parameters for the fingerprinting of the fuels. Kinetic parameters are obtained by applying traditional isoconversional methods. The activation energy as a function of the conversion reveals the multistep nature of the biomass devolatilization. Although average values allow the reactivity of different fuels to be compared, a first-order reaction model can hardly predict the biomass devolatilization in the whole range of conversions. A VEB (variable activation energy model for biomass devolatilization) model is developed, based on the results of the kinetic analysis, maintaining a simple kinetic scheme. A good agreement is obtained for the biomass residues in all HR runs in the entire range of temperatures. The multistep mechanism can be studied without assuming any chemical components or pseudocomponents, thus limiting the number of model parameters. Similarities in the optimized VEB curves for the fuels studied in this work give useful generalization parameters for biomass devolatilization modeling
Verso una definizione di complessità delle conoscenze sulla mente; l'esplicitazione delle attribuzioni nei racconti di bambini di età prescolare
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Guidelines for the diagnosis and treatment of non-ST-segment elevation acute coronary syndromes: the task force for the diagnosis and treatment of non-ST-segment elevation acute coronary syndromes of the European Society of Cardiology
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