Inexactness of the Hydro-Thermal Coordination Semidefinite Relaxation

Hydro-thermal coordination is the problem of determining the optimal economic dispatch of hydro and thermal power plants over time. The physics of hydroelectricity generation is commonly simplified in the literature to account for its fundamentally nonlinear nature. Advances in convex relaxation theory have allowed the advent of Shor's semidefinite programming (SDP) relaxations of quadratic models of the problem. This paper shows how a recently published SDP relaxation is only exact if a very strict condition regarding turbine efficiency is observed, failing otherwise. It further proposes the use of a set of convex envelopes as a strategy to successfully obtain a stricter lower bound of the optimal solution. This strategy is combined with a standard iterative convex-concave procedure to recover a stationary point of the original non-convex problem.Comment: Submitted to IEEE PES General Meeting 201

A holistic framework for the strategic management of first tier managers.

Purpose – The intention of this research paper is to shed light on the key factors influencing the human resource management (HRM) performance of first tier managers (FTMs), and the vital importance that a holistic strategic framework1 can have in this regard. Design/methodology/approach - The research paper uses an up to date historical approach, and additionally, draws on detailed case study evidence from an aerospace manufacturing/engineering company via manually and electronically analysed in-depth semi-structured interview and company documentation. Findings - The case study findings reveal that the FTM’s HRM performance is greatly influenced by a group of four key factors, and their interrelationships: Research limitations – The literature survey was mainly industry specific (manufacturing), and the sample was restricted to one sector (areospace), as well as one case study organisation. Furthermore, the views of FTMs’ staff are not directly represented. Originality – The FTM’s HRM role shows no signs of abating; in addition, questions regarding their performance of this role have increasingly emerged. This has become more evident as organisations have continued to expand the FTM’s role by devolving greater HRM responsibilities to them, and their performance of the role demonstrates that many FTMs are ill equipped to perform their HRM duties to the required standard. However, the vital importance that a holistic strategic framework (which encompasses the four key factors influencing their performance) can have on their performance of this role is not acknowledged. The present research paper, gives an empirical example of a case study, and therefore provides a springboard to address the weakness in the literature

HR leaders hold the key to effective diversity management: … as more and more important decisions are taken at local level

The article observes that diversity management (DM) is central to the HRM function in work organizations, and the past decade has seen employers seeking to establish more devolved management structures that place more emphasis on local-level managerial decision-making. An important component of this shift in many organizations has been the reconfiguration of the role of fist-line managerial leader (FLML) to encompass a wider range of HRM responsibilities. Hence, diversity management and leadership in this regard are a key HRM role for leaders and managers

Probability distribution of the order parameter

The probability distribution of the order parameter is exploited in order to obtain the criticality of magnetic systems. Monte Carlo simulations have been employed by using single spin flip Metropolis algorithm aided by finite-size scaling and histogram reweighting techniques. A method is proposed to obtain this probability distribution even when the transition temperature of the model is unknown. A test is performed on the two-dimensional spin-1/2 and spin-1 Ising model and the results show that the present procedure can be quite efficient and accurate to describe the criticality of the system.Comment: 5 pages, 7 figures, to appear in Braz. J. Phys. 34, June 200

Controlled Shock Shells and Intracluster Fusion Reactions in the Explosion of Large Clusters

The ion phase-space dynamics in the Coulomb explosion of very large ($\sim 10^6 - 10^7$ atoms) deuterium clusters can be tailored using two consecutive laser pulses with different intensities and an appropriate time delay. For suitable sets of laser parameters (intensities and delay), large-scale shock shells form during the explosion, thus highly increasing the probability of fusion reactions within the single exploding clusters. In order to analyze the ion dynamics and evaluate the intracluster reaction rate, a one-dimensional theory is used, which approximately accounts for the electron expulsion from the clusters. It is found that, for very large clusters (initial radius $\sim$ 100 nm), and optimal laser parameters, the intracluster fusion yield becomes comparable to the intercluster fusion yield. The validity of the results is confirmed with three-dimensional particle-in-cell simulations.Comment: 25 pages, 11 figures, to appear in Physical Review

Evidence for quasi-chemically homogeneous evolution of massive stars up to solar metallicity

Long soft gamma ray bursts (LGRBs) are usually associated with the death of the most massive stars. A large amount of core angular momentum in the phases preceding the explosion is required to form LGRBs. A very high initial rotational velocity can provide this angular momentum. Such a velocity strongly influences the way the star evolves: it is chemically homogeneously mixed and evolves directly towards the blue part of the HR diagram from the main sequence. We have shown that chemically homogeneous evolution (CHE) takes place in the SMC, at low metallicity. We want to see if there is a metallicity threshold above which such an evolution does not exist. We perform a spectroscopic analysis of H-rich early-type WN stars in the LMC and the Galaxy. We use the code CMFGEN to determine the fundamental properties and the surface composition of the target stars. We then place the stars in the HR diagram and determine their evolution. We show that both the LMC and Galactic WNh stars we selected cannot be explained by standard stellar evolution. They are located on the left of the main sequence but show surface abundances typical of CN equilibrium. In addition, they still contain a large amount of hydrogen. They are thus core-H burning objects. Their properties are consistent with CHE. We determine the metallicity of the Galactic stars from their position and Galactic metallicity gradients, and conclude that they have 0.6<Z<1.0. A moderate coupling between the core and the envelope is required to explain that stellar winds do not extract to much angular momentum to prevent a blueward evolution. In view of the findings that some long gamma ray bursts appear in solar environments, CHE may be a viable way to form them over a wide range of metallicities.Comment: 10 pages, 10 figures. Accepted in Astronomy and Astrophysic