1,097 research outputs found

    A computational efficient modelling of laminar separation bubbles

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    In predicting the aerodynamic characteristics of airfoils operating at low Reynolds numbers, it is often important to account for the effects of laminar (transitional) separation bubbles. Previous approaches to the modelling of this viscous phenomenon range from fast but sometimes unreliable empirical correlations for the length of the bubble and the associated increase in momentum thickness, to more accurate but significantly slower displacement-thickness iteration methods employing inverse boundary-layer formulations in the separated regions. Since the penalty in computational time associated with the more general methods is unacceptable for airfoil design applications, use of an accurate yet computationally efficient model is highly desirable. To this end, a semi-empirical bubble model was developed and incorporated into the Eppler and Somers airfoil design and analysis program. The generality and the efficiency was achieved by successfully approximating the local viscous/inviscid interaction, the transition location, and the turbulent reattachment process within the framework of an integral boundary-layer method. Comparisons of the predicted aerodynamic characteristics with experimental measurements for several airfoils show excellent and consistent agreement for Reynolds numbers from 2,000,000 down to 100,000

    A computationally efficient modelling of laminar separation bubbles

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    In order to predict the aerodynamic characteristics of airfoils operating at low Reynolds numbers, it is necessary to accurately account for the effects of laminar (transitional) separation bubbles. Generally, the greatest difficulty comes about when attempting to determine the increase in profile drag that results from the presence of separation bubbles. While a number of empirically based separation bubble models have been introduced in the past, the majority assume that the bubble development is fully predictable from upstream conditions. One way of accounting for laminar separation bubbles in airfoil design is the bubble analog used in the design and analysis program of Eppler and Somers. A locally interactive separation bubble model was developed and incorporated into the Eppler and Somers program. Although unable to account for strong interactions such as the large reduction in suction peak sometimes caused by leading edge bubbles, it is able to predict the increase in drag and the local alteration of the airfoil pressure distribution that is caused by bubbles occurring in the operational range which is of most interest

    SolarStat: Modeling Photovoltaic Sources through Stochastic Markov Processes

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    In this paper, we present a methodology and a tool to derive simple but yet accurate stochastic Markov processes for the description of the energy scavenged by outdoor solar sources. In particular, we target photovoltaic panels with small form factors, as those exploited by embedded communication devices such as wireless sensor nodes or, concerning modern cellular system technology, by small-cells. Our models are especially useful for the theoretical investigation and the simulation of energetically self-sufficient communication systems including these devices. The Markov models that we derive in this paper are obtained from extensive solar radiation databases, that are widely available online. Basically, from hourly radiance patterns, we derive the corresponding amount of energy (current and voltage) that is accumulated over time, and we finally use it to represent the scavenged energy in terms of its relevant statistics. Toward this end, two clustering approaches for the raw radiance data are described and the resulting Markov models are compared against the empirical distributions. Our results indicate that Markov models with just two states provide a rough characterization of the real data traces. While these could be sufficiently accurate for certain applications, slightly increasing the number of states to, e.g., eight, allows the representation of the real energy inflow process with an excellent level of accuracy in terms of first and second order statistics. Our tool has been developed using Matlab(TM) and is available under the GPL license at[1].Comment: Submitted to IEEE EnergyCon 201

    From complementary currency to institution: a micro-macro study of the Sardex mutual credit system

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    The remarkable growth of Sardex as a local currency throughout the island of Sardinia over the past 5 years motivated an in-depth look at its starting assumptions, design and operational principles, and local context. The paper looks at Sardex as a social innovation start-up, a complementary currency, a mutual credit system, and a socio-economic «circuit». The analysis relies on interviews of circuit members and its founders. The main findings are that trust was and continues to be fundamentally important for the creation and operation of the mutual credit system, and that Sardex encompasses both economic and social value(s) in a process of re-embedding of the economy. Sardex configured itself as a crucial mediator of economic exchanges and became a valuable actor acting as an institution at the regional level. These properties make it an ideal space for experimentation in socio-economic innovation that can be characterized as a «laboratory for multi-level governance»

    Community currencies as laboratories of institutional learning: emergence of governance through the mediation of social value

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    This paper is motivated by a long-standing curiosity about the role of scale in explanatory theories of socio-economic action. Introducing scale as an analytical variable implies the coexistence of individuals alongside institutions. We make the case that economic activity becomes more sustainable when it is ‘colonized’ by ‘social value’ whereby market activity is complemented with community and democratic values, by which we mean the opposite of the commodification of e.g. social networks analytics. We take the Sardex mutual credit system as an empirical context from which to begin exploring the extent to which such community-based economic practices offer a democratic and social alternative to, a questionable substitute for, or a functional supplement to the capitalist market, the welfare state, and public enterprises administered by state bureaucracies. We broach critically the emergence of institutional collective structures from the perspective of social constructivism, post- anarchist theory, economic anthropology, and post-capitalist studies of economic action. In particular, we focus on how Graeber’s ideas on the history of debt apply to these points. We propose a recursive constructive framework for socio-economic action whereby money as a social construction is itself a medium of economic construction and, as such, becomes an important lever subject to “design” inputs by socio-economic stakeholders engaged in the development of an inclusive and participatory governance process of institution construction

    European intellectuals follow Charlie Brown!

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    Mathematical Foundations for Balancing the Payment System in the Trade Credit Market

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    The increasingly complex economic and financial environment in which we live makes the management of liquidity in payment systems and the economy in general a persistent challenge. New technologies make it possible to address this challenge through alternative solutions that complement and strengthen existing payment systems. For example, interbank balancing and clearing methods (such as real-time gross settlement) can also be applied to private payments, complementary currencies, and trade credit clearing to provide better liquidity and risk management. The paper defines the concept of a balanced payment system mathematically and demonstrates the effects of balancing on a few small examples. It then derives the construction of a balanced payment subsystem that can be settled in full and therefore that can be removed in toto to achieve debt reduction and payment gridlock resolution. Using well-known results from graph theory, the main output of the paper is the proof—for the general formulation of a payment system with an arbitrary number of liquidity sources—that the amount of liquidity saved is maximum, along with a detailed discussion of the practical steps that a lending institution can take to provide different levels of service subject to the constraints of available liquidity and its own cap on total overdraft exposure. From an applied mathematics point of view, the original contribution of the paper is two-fold: (1) the introduction of a liquidity node with a store of value function in obligation-clearing; and (2) the demonstration that the case with one or more liquidity sources can be solved with the same mathematical machinery that is used for obligation-clearing without liquidity. The clearing and balancing methods presented are based on the experience of a specific application (Tetris Core Technologies), whose wider adoption in the trade credit market could contribute to the financial stability of the whole economy and a better management of liquidity and risk overall.Peer reviewe