SL(2,R)-geometric phase space and (2+2)-dimensions

We propose an alternative geometric mathematical structure for arbitrary phase space. The main guide in our approach is the hidden SL(2,R)-symmetry which acts on the phase space changing coordinates by momenta and vice versa. We show that the SL(2,R)-symmetry is implicit in any symplectic structure. We also prove that in any sensible physical theory based on the SL(2,R)-symmetry the signature of the flat target "spacetime" must be associated with either one-time and one-space or at least two-time and two-space coordinates. We discuss the consequences as well as possible applications of our approach on different physical scenarios.Comment: 17 pages, no figure

Hip Arthroscopic Surgery for Femoroacetabular Impingement: A Prospective Analysis of the Relationship Between Surgeon Experience and Patient Outcomes.

Background:Hip arthroscopic surgery is a rapidly growing procedure, but it may be associated with a steep learning curve. Few studies have used patient-reported outcome (PRO) surveys to investigate the relationship between surgeon experience and patient outcomes after the arthroscopic treatment of femoroacetabular impingement (FAI). Hypothesis:Patients undergoing hip arthroscopic surgery for the treatment of FAI in the early stages of a surgeon's career will have significantly worse outcomes and longer procedure times compared with patients treated after the surgeon has gained experience. Study Design:Cohort study; Level of evidence, 2. Methods:Patients undergoing hip arthroscopic surgery for FAI and labral injuries were prospectively enrolled during a sports medicine fellowship-trained surgeon's first 15 months of practice. Patients were stratified into an early group, consisting of the first 30 consecutive cases performed by the surgeon, and a late group, consisting of the second 30 consecutive cases. Radiographic and physical examinations were performed preoperatively and postoperatively. PRO surveys, including the 12-item Short Form Health Survey (SF-12), the modified Harris Hip Score (mHHS), and the Hip disability and Osteoarthritis Outcome Score (HOOS), were administered preoperatively and at a minimum of 1 year postoperatively. Results:There was no difference between the early and late groups for patient age (37.2 ± 11.5 vs 35.3 ± 10.8 years, respectively; P = .489), body mass index (25.6 ± 4.0 vs 25.1 ± 4.5 kg/m2, respectively; P = .615), or sex (P = .465). There was a significantly increased procedure time (119.3 ± 21.0 vs 99.0 ± 28.6 minutes, respectively; P = .002) and traction time (72.7 ± 21.4 vs 59.0 ± 16.7 minutes, respectively; P = .007) in the early group compared with the late group. Mean postoperative PRO scores significantly improved in both groups compared with preoperative values for all surveys except for the SF-12 mental component summary. No differences were found in PRO score improvements or complication rates between the early and late groups. Conclusion:The total procedure time and traction time decrease after a surgeon's first 30 hip arthroscopic surgery cases for FAI and labral tears, but patient outcomes can similarly improve regardless of surgeon experience in the early part of his or her career

Signatures of rocky planet engulfment in HAT-P-4. Implications for chemical tagging studies

Aims. To explore the possible chemical signature of planet formation in the binary system HAT-P-4, by studying abundance vs condensation temperature Tc trends. The star HAT-P-4 hosts a planet detected by transits while its stellar companion does not have any detected planet. We also study the Lithium content, which could shed light on the problem of Li depletion in exoplanet host stars. Conclusions. The exoplanet host star HAT-P-4 is found to be ~0.1 dex more metal rich than its companion, which is one of the highest differences in metallicity observed in similar systems. This could have important implications for chemical tagging studies, disentangling groups of stars with a common origin. We rule out a possible peculiar composition for each star as lambda Boo, delta Scuti or a Blue Straggler. The star HAT-P-4 is enhanced in refractory elements relative to volatile when compared to its stellar companion. Notably, the Lithium abundance in HAT-P-4 is greater than in its companion by ~0.3 dex, which is contrary to the model that explains the Lithium depletion by the presence of planets. We propose a scenario where, at the time of planet formation, the star HAT-P-4 locked the inner refractory material in planetesimals and rocky planets, and formed the outer gas giant planet at a greater distance. The refractories were then accreted onto the star, possibly due to the migration of the giant planet. This explains the higher metallicity, the higher Lithium content, and the negative Tc trend detected. A similar scenario was recently proposed for the solar twin star HIP 68468, which is in some aspects similar to HAT-P-4. We estimate a mass of at least Mrock ~ 10 Mearth locked in refractory material in order to reproduce the observed Tc trends and metallicity.Comment: 5 pages, 6 figures, A&A Letters accepte

Cattle Diet Composition Under a Short- Duration Grazing With and Without Deferment in Puna Rangelands

The treatments of rest and deferment periods have shown to be effective tools to improve range condition, but little is known about their effect on the diet of animals. The present research aimed to evaluate the effect of the deferment period when applied to a short-term grazing system. The experimental design was a completely random factorial where the treatments resulted from combination of two grazing management systems: short-duration with deferment period (WD) and no deferral period (ND) both evaluated for three years. Forage yield (4.9 vs. 4.5 Tm DM ha-1 year-1) like range condition (55.5 vs. 50.7%) improved significantly (P\u3c0.05) under a deferment period regime, unlike diet quality that decreased, evidenced by a low in vitro dry matter digestibility level (40.6 vs. 45.7 %), a higher neutral detergent fiber content (79.0 vs. 74.6 %) and a lower crude protein content (10.0 vs. 11.4%). We conclude deferment period treatments improved rangeland condition and forage availability but not the quality of the diet

Rangeland Health Status and Condition Two Different Yet Complementary Concepts: National Reserve Pampa Galeras Barbara D´Achille Case

Pampa Galeras Barbara D´Achille National Reserve has an extension of 21,030 hectares constituted mainly by puna rangeland and has played a key role in the recovery of the vicuña (Vicugna vicugna) population from extinction. This reserve is divided into two zones, known as the Rigid zone with a total area of 6,500 ha, and the Buffer zone of 14,530 ha. The present study aimed to implement an Ecological Response Unit system (ERU) to create an evaluation and monitoring system of rangeland health and condition, that could serve as a model for the rest of the Protected National Areas destinated for the conservation of vicuña. A total of 29 ERUs were delimited combining fieldwork and spatial information analysis. The results revealed that rangeland health is still at risk due to domestic herbivorous pressure. This suggests the need to implement a Reference Area system (RA) to evaluate and monitor rangeland health and condition indicators and thus improve management and livestock grazing control systems. In this manner, the bases for the execution of an adaptative management plan will settle with the participation of Servicio Nacional de Áreas Naturales Protegidas (SERNANP) and the local rural communities, landowners, and usufructuaries of the economic value of the vicuña fiber

PARACHUTES: A computer program for calculating ram-air parachutes. Theory document

PARACHUTES is a computer program developed at the International Center for Numerical Methods in Engineering (CIMNE) for the simulation of ram-air (gliding) parachute systems. The solution methodology in PARACHUTES is based on two unsteady calculation modules which work in a coupled manner. The aerodynamics of the gliding parachute is solved by a low-order panel method and a finite-element technique is used for the structure. The latter allows modelling the suspension lines, textile fabric and suspended payloads of a typical parachute system by means of cable, membrane and solid linear elements. For specified flight conditions, the simulation program solves the fluid-structural problem governing the behavior of arbitrary parachute-payload configurations and also allows analyzing user-defined maneuvers. This document describes the theoretical formulation and the numerical procedures adopted in PARACHUTES

On the solution of Lambert’s problem by regularization

Lambert’s problem is the two-point boundary-value problem resulting from a two-body orbital transfer between two position vectors in a given time. It lies at the very heart of several fundamental astrodynamics and space engineering problems and, as such, it has attracted the interest of scientists over centuries. In this work, we revisit the solution of Lambert’s problem based on Levi-Civita regularization developed by Carles Simó in 1973. We offer an exhaustive derivation of the theory, including proofs of all the results and the formulae employed, and we extend the algorithm to deal with multi-revolution transfers. Then, after investigating a range of initial guess search techniques and testing different numerical methods to approximate the solution, we propose a procedure in which the initial guess is assigned very efficiently by querying a pre-defined interpolating table. Then, in order to achieve both speed and robustness, we combine Newton-Raphson with safety checks to avoid out of boundary deviations to approximate the solution. We validate the method through several tests and applications, and we assess its convergence and performance. The algorithm presents no singularities, converges in all realistic scenarios and its computational cost is comparable with state of the art algorithms

Methodologies for tracking of load extremes and error estimation using probabilistic techniques

This work, conducted at CIMNE under ALEF project task 1.2.3, presents an investigation about the potential capabilities of neural networks to assist simulation campaigns. The discrete gust response of an aircraft has been chosen as a typical problem in which the determination of the critical loads requires exploring a large parameter space. A very simple model has been used to compute the aerodynamic loads. This allows creating a large database while at the same time retaining some of the fundamental properties of the problem. Using this comprehensive dataset the effects of network structure, training method and sampling strategy on the level of approximation over the complete domain have been investigated. The capabilities of the neural network to predict the peak load as well as the critical values of the design parameters have also been assessed. The applicability of neural networks to the combination of multi-fidelity results is also explored