Correlation energy of the spin-polarized electron liquid by quantum Monte Carlo

Variational and diffusion quantum Monte Carlo (VMC and DMC) methods with Slater-Jastrow-backflow trial wave functions are used to study the spin-polarized three-dimensional uniform electron fluid. We report ground state VMC and DMC energies in the density range $0.5 \leq r_\text{s} \leq 20$. Finite-size errors are corrected using canonical-ensemble twist-averaged boundary conditions and extrapolation of the twist-averaged energy per particle calculated at three system sizes (N=113, 259, and 387) to the thermodynamic limit of infinite system size. The DMC energies in the thermodynamic limit are used to parameterize a local spin density approximation correlation function for inhomogeneous electron systems.Comment: arXiv admin note: substantial text overlap with arXiv:2209.1022

The Mirage of Triangular Arbitrage in the Spot Foreign Exchange Market

We investigate triangular arbitrage within the spot foreign exchange market using high-frequency executable prices. We show that triangular arbitrage opportunities do exist, but that most have short durations and small magnitudes. We find intra-day variations in the number and length of arbitrage opportunities, with larger numbers of opportunities with shorter mean durations occurring during more liquid hours. We demonstrate further that the number of arbitrage opportunities has decreased in recent years, implying a corresponding increase in pricing efficiency. Using trading simulations, we show that a trader would need to beat other market participants to an unfeasibly large proportion of arbitrage prices to profit from triangular arbitrage over a prolonged period of time. Our results suggest that the foreign exchange market is internally self-consistent and provide a limited verification of market efficiency

Intravascular migration of contraceptive implants: two more cases

Cases: In addition to previously published case reports, further cases of intravascular migration of contraceptive implants have been identified from an information request to two national adverse reaction spontaneous reporting systems. We report on two new cases of insertion into the venous system with subsequent embolism to a pulmonary artery. Conclusion: Incorporating barium sulfate into the implant has facilitated diagnosis of these very rare adverse events with the initial diagnosis of embolism to the pulmonary arterial tree made by chest X-ray. Removal of an implant from a segmental branch of a pulmonary artery is technically challenging and not without risks. Unsuccessful removal appears to be preceded by a delay in diagnosis leading to endothelialization of the implant in the pulmonary arterial wall. Implications: Subdermal placement of contraceptive implants over the anterior surface of the biceps rather than in the sulcus between them biceps and triceps may negate this rare but reported risk

Splitting Spanner Atoms: A Tool for Acyclic Core Spanners

This paper investigates regex CQs with string equalities (SERCQs), a subclass of core spanners. As shown by Freydenberger, Kimelfeld, and Peterfreund (PODS 2018), these queries are intractable, even if restricted to acyclic queries. This previous result defines acyclicity by treating regex formulas as atoms. In contrast to this, we propose an alternative definition by converting SERCQs into FC-CQs - conjunctive queries in FC, a logic that is based on word equations. We introduce a way to decompose word equations of unbounded arity into a conjunction of binary word equations. If the result of the decomposition is acyclic, then evaluation and enumeration of results become tractable. The main result of this work is an algorithm that decides in polynomial time whether an FC-CQ can be decomposed into an acyclic FC-CQ. We also give an efficient conversion from synchronized SERCQs to FC-CQs with regular constraints. As a consequence, tractability results for acyclic relational CQs directly translate to a large class of SERCQs

Space Harpoon Projectile Analysis for Space Debris Capture

Space debris will become a more prevalent issue in this decade as technological advancements and greater dependencies on communications require more satellites in orbit, with some companies already hosting mega-constellations. Mitigating the debris using a space-tether is the most viable method to construct a space debris capture regime with current technology levels. Harpoon heads for the tethers are key design interests as these will penetrate through the satellites and/or debris. The focus of this paper is the analysis of an aluminium 6082, flat and conical head design, used to perforate aluminium 1050A plates using a gas gun laboratory. The aim is to conduct experiments to achieve a low Minimum Ballistic Velocity (MBV) where this velocity is the minimum velocity needed to perforate a material. Maximum perforation intertwined with minimal fragmentation is the desired balance sought from the designs. A high-speed camera records times taken for the events before and after perforation which deduces the MBV. 10 bar, 12.5 bar, and 15 bar of pressure were used, as well as 3mm, 1.5mm and 1mm Aluminium plate thickness, to provide diverse results for analysis. The MBV was calculated at 49.54m/s for 3mm thickness, with the conical head. The plates were cooled using dry ice to mimic space-like environments where tensile and yield strength increased with the cooler climates, resulting in higher MBVs. After impact, perforation profiles are analysed using a DSLR camera, resulting in ‘punches’ of material with the flat head and ‘petaling’ for the conical head. Conical perforation allows for material to be retained within the plate whilst flat head designs possess the potential for further space debris creation. The results retain reliability through validation checks with an oscilloscope and taking tolerances throughout the experiment. The results feed as a foundation to venture into future work with further ergonomic and bespoke designs

Correlation energy of the paramagnetic electron gas at the thermodynamic limit

The variational and diffusion quantum Monte Carlo methods are used to calculate the correlation energy of the paramagnetic three-dimensional homogeneous electron gas at intermediate to high density. Ground state energies in finite cells are determined using Slater-Jastrow-backflow trial wave functions, and finite-size errors are removed using twist-averaged boundary conditions and extrapolation of the energy per particle to the thermodynamic limit of infinite system size. Our correlation energies in the thermodynamic limit are lower (i.e., more negative, and therefore more accurate according to the variational principle) than previous results, and can be used for the parameterization of density functionals to be applied to high-density systems

Fluid–structure interaction of free convection in a square cavity divided by a flexible membrane and subjected to sinusoidal temperature heating

Purpose: The purpose of the present paper is to model a cavity, which is equally divided vertically by a thin, flexible membrane. The membranes are inevitable components of many engineering devices such as distillation systems and fuel cells. In the present study, a cavity which is equally divided vertically by a thin, flexible membrane is model using the fluid–structure interaction (FSI) associated with a moving grid approach. Design/methodology/approach: The cavity is differentially heated by a sinusoidal time-varying temperature on the left vertical wall, while the right vertical wall is cooled isothermally. There is no thermal diffusion from the upper and lower boundaries. The finite-element Galerkin technique with the aid of an arbitrary Lagrangian–Eulerian procedure is followed in the numerical procedure. The governing equations are transformed into non-dimensional forms to generalize the solution. Findings: The effects of four pertinent parameters are investigated, i.e., Rayleigh number (104 = Ra = 107), elasticity modulus (5 × 1012 = ET = 1016), Prandtl number (0.7 = Pr = 200) and temperature oscillation frequency (2p = f = 240p). The outcomes show that the temperature frequency does not induce a notable effect on the mean values of the Nusselt number and the deformation of the flexible membrane. The convective heat transfer and the stretching of the thin, flexible membrane become higher with a fluid of a higher Prandtl number or with a partition of a lower elasticity modulus. Originality/value: The authors believe that the modeling of natural convection and heat transfer in a cavity with the deformable membrane and oscillating wall heating is a new subject and the results have not been published elsewhere

Chatter, process damping, and chip segmentation in turning: A signal processing approach

An increasing number of aerospace components are manufactured from titanium and nickel alloys that are difficult to machine due to their thermal and mechanical properties. This limits the metal removal rates that can be achieved from the production process. However, under these machining conditions the phenomenon of process damping can be exploited to help avoid self-excited vibrations known as regenerative chatter. This means that greater widths of cut can be taken so as to increase the metal removal rate, and hence offset the cutting speed restrictions that are imposed by the thermo-mechanical properties of the material. However, there is little or no consensus as to the underlying mechanisms that cause process damping. The present study investigates two process damping mechanisms that have previously been proposed in the machining literature: the tool flank/workpiece interference effect, and the short regenerative effect. A signal processing procedure is employed to identify flank/workpiece interference from experimental data. Meanwhile, the short regenerative model is solved using a new frequency domain approach that yields additional insight into its stabilising effect. However, analysis and signal processing of the experimentally obtained data reveals that neither of these models can fully explain the increases in stability that are observed in practice. Meanwhile, chip segmentation effects were observed in a number of measurements, and it is suggested that segmentation could play an important role in the process-damped chatter stability of these materials