A Library for Self-Adjusting Computation

We present a Standard ML library for writing programs that automatically adjust to changes to their data. The library combines modifiable references and memoization to achieve efficient updates. We describe an implementation of the library and apply it to the problem of maintaining the convex hull of a dynamically changing set of points. Our experiments show that the overhead of the library is small, and that self-adjusting programs can adjust to small changes three-orders of magnitude faster than recomputing from scratch. The implementation relies on invariants that could be enforced by a modal type system. We show, using an existing language, abstract interfaces for modifiable references and for memoization that ensure the same safety properties without the use of modal types. The interface for memoization, however, does not scale well, suggesting a language-based approach to be preferable after all.     

Ultrasonic characterization of lactose crystallization in gelatin gels

Ultrasonic velocity and attenuation measurements (2.25 MHz center frequency) were used to follow bulk crystallization of lactose (43% and 46%) from gelatin (1.5% and 3%) gels at 25 °C, and compared to turbidity (500 nm) and isothermal calorimetric measurements. Ultrasonic velocity decreased slightly (approximately 0.5%) during crystallization while ultrasonic attenuation was low in the absence of lactose crystals and increased progressively during crystallization. The lag time before the onset of crystallization decreased and the maximum rate of increase in attenuation during crystallization increased with increasing lactose supersaturation but was not affected by gelatin concentration (P < 0.05). Similar results were seen in turbidity and isothermal calorimetric measurements. Ultrasonic attenuation measurements have the potential to measure crystallization kinetics in complex food matrices and to be applied on-line. Practical Application: Many foods contain crystals that affect their taste and texture (for example, lactose crystals can give a grainy defect in ice cream). The formation of crystals is often hard to predict so methods to measure the development of crystals inside real foods are useful. In this study, we show that as lactose crystallizes in a gelatin gel the ultrasonic attenuation--capacity to absorb sound--increases and can be related to the amount of crystals present. Ultrasound is easier to apply in real food processing than the existing methodologies.     

Socio-Economic Determinants of Student Mobility and Inequality of Access to Higher Education in Italy

Networks and Spatial Economics - This paper introduces a modified version of the Hansen-gravity model as a framework to estimate the accessibility of higher education (HE) institutions in Italy...     

Equilibrium beach profiles: erosion and accretion balanced approach

Estimating the magnitude of profile scale factor is currently based on the assumption that beach profiles are in equilibrium condition. However, this assumption cannot be applied to all beach profiles. In this article, a boundary based profile scale factor is proposed that through a normalized coordinate system leads to a unique global profile scale factor. The global profile scale factor helps to determine an initial linear beach profile. The amount of erosion and accretion that causes the initial linear profile to transform to the natural equilibrium beach profile (EBP) can be calculated. Accordingly, the balance between the amount of erosion and accretion will identify a turning point distinguishing the erosion and accretion areas on the profile. This turning point helps to evaluate whether the profile is in equilibrium condition or not. The proposed model was validated through various beach profiles resulting in high degrees of accuracy and reliability.     

Semiempirical thermodynamic modeling of a direct methanol fuel cell system

In this study, a thermodynamic model of an active direct methanol fuel cell (DMFC) system, which couples in‐house experimental data for the DMFC with the mass and energy balances for the system components (condenser, mixing vessel, blower, and pumps), is formed. The modeling equations are solved using the Engineering Equation Solver (EES) program. This model gives the mass fluxes and thermodynamic properties of fluids for each state, heat and work transfer between the components and their surroundings, and electrical efficiency of the system. The effect of the methanol concentration (between 0.5 and 1.25 M) and air flow rate (between 20 and 30 mL cm^?2 min^?1) on the net power output and electrical efficiency of the system and the condenser outlet temperature is investigated. The results essentially showed that the highest value for the electrical efficiency of the system is 23.6% when the current density, methanol concentration, and air flow rate are taken as 0.2 A cm^?2, 0.75 M, and 20 mL cm^?2 min^?1, respectively. In addition, the air flow rate was found to be the most significant parameter affecting the condenser outlet temperature.     

Relay Multi-hop Communications for Next Generation Mobile Networks: Investigation of Path-Loss Models

In next generation communication systems, a multi-hop scheme, in which stations between a mobile terminal and a base station relay signal transmissions, seems a promising ap-proach for wide-area coverage and system capacity enhancement. Another prospective benefit for multi-hop scheme is the reduction of transmission power for a link. This reduction is expected by splitting the transmission into a series of hops using the other mobile stations as repeaters which provide a gain from receiver (Rx) to transmitter (Tx). In this way, the transmission power is expected to become less due to the non-linear nature of the path loss. In this work, different path loss models are examined for multi-hop communication systems. Direct communication and the multi-hop communication cases are compared according to different path loss models. The question of how path loss changes by splitting the transmission into a series of sub-paths is also investigated. The investigations show that it is not possible to reduce the path losses except for some cases thus the transmit power. On the other hand using the multi-hop scheme makes possible to increase the coverage area of the network and also reduce the dead spots in the cell.     

On the feasibility of re-stimulation of shale wells

As a result of advances in horizontal completions and multi-stage hydraulic fracturing, the U.S. has been able to economically develop several decades of worth of natural gas. However, a considerable concern has risen on the economic viability of shale gas development for reasons associated with the fast production declines as well as recent down-turns of natural gas prices besides rises in the costs of new technologies. Therefore, an economic analysis is required to investigate the profitability of the refracturing treatment of unconventional gas resources. Net present value of cash flows and internal rate of return are calculated for a range of gas prices considering 20 years of natural gas production from a typical unconventional shale gas reservoir. A systematic comparison is then accomplished for three scenarios: (1) re-fracturing versus no refracturing, (2) combination of re-fracturing and drilling new wells, and (3) time-dependent re-fracturing treatment. Further, this paper incorporates the cost of re-fracturing treatment, the cost of drilling a new horizontal well, the water treatment cost, as well as the current and future price of natural gas in the model. The findings of this work would help the future re-stimulation development plans of the emerging unconventional shale gas plays.