Low-frequency Cycles Research Articles

Transepithelial Laser versus Alcohol Assisted Photorefractive Keratectomy Safety and Efficacy: 1-Year Follow-up of a Contralateral Eye Study.

PurposeTo compare single-step transepithelial photorefractive keratectomy (TPRK) to conventional alcohol assisted epithelial removal then photorefractive keratectomy (AAPRK) regarding pain, epithelial healing, visual acuity, corneal haze measured subjectively and objectively, higher order aberrations changes, contrast sensitivity and vector analysis of astigmatic correction with one year follow-up.MethodsA prospective double-blind randomized study of 29 subjects (58 eyes) who underwent myopic aberration-free laser correction by smart pulse technology using Schwind Amaris 1050 Hz with 1-year follow-up. Right eye was randomly treated by AAPRK or TPRK. Postoperative assessment was performed on day 1 and 3, at 1st week, and 1st, 3rd, 6th, and 12th months. Patients were assessed for pain, epithelial healing, visual acuity, corneal haze, astigmatic correction, higher order aberrations and contrast sensitivity.ResultsEpithelial healing was complete by the 3rd day in 62.1% of AAPRK eyes and in 89.7% of TPRK eyes. First day postoperative pain was higher in TPRK group (p = 0.0134). The decimal uncorrected visual acuity at 12 months was 1.47 ± 0.39 and 1.57 ± 0.38 in the AAPRK and TPRK groups respectively (p = 0.3719). Post-photorefractive keratectomy haze reached a final level of 0.04 ± 0.14 and 0.02 ± 0.1 in AAPRK and TPRK groups respectively (p = 0.5607). Contrast sensitivity was comparable in low and high frequency cycles per degree. Vector analysis of astigmatic correction showed correction index at one year of 0.99 and 1.05 for AAPRK and TPRK groups respectively.ConclusionsAlcohol assisted and transepithelial photorefractive keratectomy have comparable results regarding safety and efficacy.

COVID-19, stock market and sectoral contagion in US: a time-frequency analysis

We assess the conditional relationship in the time-frequency domain between the return on S&P 500 and confirmed cases and deaths by COVID-19 in Hubei, China, countries with record deaths and the world, for the period from January 29 to June 30, 2020. Methodologically, we follow Aguiar-Conraria et al. (2018), by using partial coherencies, phase-difference diagrams, and gains. We also perform a parametric test for Granger-causality in quantiles developed by Troster (2018). We find that short-term cycles of deaths in Italy in the first days of March, and soon afterwards, cycles of deaths in the world are able to lead out-of-phase US stock market. We find that low frequency cycles of the US market index in the first half of April are useful to anticipate in an anti-phasic way the cycles of deaths in the US. We also explore sectoral contagion, based on dissimilarities, Granger causality and partial coherencies between S&P sector indices. Our findings, such as the strategic role of the energy sector, which first reacted to the pandemic, or the evidence about predictability of the Telecom cycles, are useful to tell the history of the pass-through of this recent health crises across the sectors of the US economy.

Identifying high-frequency shocks with Bayesian mixed-frequency VARs

We contribute to research on mixed-frequency regressions by introducing an innovative Bayesian approach. We impose a Normal-inverse Wishart prior by adding a set of auxiliary dummies in estimating a Mixed-Frequency VAR. Based on this new “high-frequency” identification scheme, we illustrate our method by identifying uncertainty shock for the U.S. economy. As the main findings, we document a “temporal aggregation bias” when we adopt a common low-frequency model instead of estimating a mixed-frequency framework. The bias is amplified in case of a large mismatching between the high-frequency shock and low-frequency business cycle variables.

On the non-monotonic behaviour of fire spread

A conceptual model based on the dynamic interaction between fire, the fuel bed and the surrounding flow to explain the non-monotonic or intermittent behaviour of fires is proposed. According to the model, even in nominally permanent and uniform boundary conditions, the fire-induced flow modifies the geometry of the flame and its rate of spread. After an initial acceleration, there is a reduction in the rate of spread followed by one or more cycles of growth. Carefully controlled experiments of fires in slopes and canyons show that the evolution of fire properties, namely flame angle and rate of spread, have high-frequency oscillations superimposed on the low-frequency fire growth cycle described above.

Numerical Investigation of Wing Section Undergoing Low-Frequency Aeroelastic Limit Cycle Oscillations

An in-house-developed computational fluid dynamics code that allows for grid motion was implicitly coupled with the differential equation for a harmonic oscillator. Simulations were carried out for a wing section with an airfoil from the X-56A with a chord-based Reynolds number of and 12 and 15 deg angles of attack. Results for 12 deg angle of attack show that for a wing section with low inertia and highly flexible support, the amplitude of the structural motion of the wing section motion is relatively weak. For a wing section with higher inertia and stiffer support, the structural response matches the natural eigenfrequency. For both cases, compared with results for the rigid (nonmoving) wing section, the frequency content of the aerodynamic loads is noticeably reduced near the structural eigenfrequency. For the wing section with stiffer support at 15 deg angle of attack, a laminar separation bubble is repeatedly bursting and reestablishing itself at a relatively low frequency. When the stiffness of the support is reduced so that the low-frequency of the bubble bursting couples with the structural eigenfrequency, the amplitude of the unsteady aerodynamic forces is doubled and the amplitude of the structural motion increases nearly 30 times compared with the stiffer support.

Employing a Laser Headlight Electrical System to Measure and Calculate Electro-Optic Conversion Efficiencies of Blue-Beam Laser Diodes

In this study, a laser headlight electrical system was developed and implemented to measure and compare the electro-optic characteristics of the blue-beam laser diodes (BBLDs). To observe different temperatures (from negative to positive temperatures) that cause optical output power (OOP) and electrical power variations of the BBLDs, BBLDs were placed in an ambient temperature testing chamber, and the electro-optic characteristics of the BBLDs were measured to estimate the electro-optic conversion efficiencies. The different OOPs and electro-optic conversion efficiencies at different temperatures can be obtained when OOP adjustment was performed by average current and low-frequency pulse-duty cycle modulation technologies. Moreover, this study developed a human–machine interface using the LabVIEW software; therefore, the laser headlight electrical system can be controlled and monitored by a computer with the controller area network (CAN) bus communication. Finally, the prototype of the laser headlight electrical system was achieved, and three BBLDs were driven; furthermore, the white light of the headlight can be produced by mixing blue laser beams with a yellow fluorescence ceramic wafer.

The co-movements between geothermal energy usage and CO2 emissions through high and low frequency cycles.

Geothermal energy is considered environmentally friendly than fossil fuel sources, and geothermal power plants are expected to have a low carbon footprint. It is renewable that can last million years. There exist, however, several gases stored under the earth's surface which are released into the atmosphere during digging (TWI 2020). This research paper aims at monitoring the potential positive and negative effects of geothermal on environmental quality (CO2 emissions) in the USA for the period January 1980 to August 2019. The paper employed wavelet and partial wavelet coherence computations to explore the impacts of geothermal energy usage on the environment. The concluding remarks from the estimations can be classified into short-term (1-3-year cycle) results and long-term (3-8-year cycle) results. It is found in the short term that (i) geothermal usage increased CO2 emissions during 1980-1983 and 1993-1997 and (ii) CO2 emissions caused geothermal usage to increase for the period 2000-2009 and after 2015. The paper reveals also that in the long term, (a) geothermal energy consumption increased CO2 emissions during the periods 1985-1990, 1993-1996, and 2013-2016 and (b) geothermal energy consumption decreased CO2 emissions for the period 1996-2008 in the USA. This research work eventually yields some relevant geothermal energy policy suggestions for US policymakers to make geothermal more environmentally friendly.

Exchange Rates and Endogenous Productivity

Real exchange rates (RERs) display sizable uctuations not only over the business cycle, but also at lower frequencies, resulting in large and persistent swings over decades|facts that many business cycle models struggle to match. We propose an international macroeconomics model with endogenous productivity to rationalize these facts. In the model, endogenous growth amplifies stationary uctuations generating persistent productivity differences between countries that trigger low-frequency cycles in the RER. The estimated model effortlessly replicates the empirical spectrum, autocorrelation, and half-life of the RER. In addition, we document that low frequency movements in aggregate trade ows are crucial to discipline the RER cycles. Finally, we validate the model-implied co-movement between relative prices and technology differentials using a panel of cross industry-country data on patent and industry prices.

In Vivo Optogenetic Modulation with Simultaneous Neural Detection Using Microelectrode Array Integrated with Optical Fiber.

The detection of neuroelectrophysiology while performing optogenetic modulation can provide more reliable and useful information for neural research. In this study, an optical fiber and a microelectrode array were integrated through hot-melt adhesive bonding, which combined optogenetics and electrophysiological detection technology to achieve neuromodulation and neuronal activity recording. We carried out the experiments on the activation and electrophysiological detection of infected neurons at the depth range of 900–1250 μm in the brain which covers hippocampal CA1 and a part of the upper cortical area, analyzed a possible local inhibition circuit by combining opotogenetic modulation and electrophysiological characteristics and explored the effects of different optical patterns and light powers on the neuromodulation. It was found that optogenetics, combined with neural recording technology, could provide more information and ideas for neural circuit recognition. In this study, the optical stimulation with low frequency and large duty cycle induces more intense neuronal activity and larger light power induced more action potentials of neurons within a certain power range (1.032 mW–1.584 mW). The present study provided an efficient method for the detection and modulation of neurons in vivo and an effective tool to study neural circuit in the brain.

Моделирование напряженно-деформированного состояния поршня дизеля с учетом неупругих деформаций

The failure of aluminum pistons of diesel engines is often associated with formation of cracks originating at the bowl rim. The appearance of cracks is a consequence of thermal fatigue of the material due to low-frequency cycles of heating and cooling of the piston during the engine start-up, operation at various speed and load conditions, and subsequent shutdown. To assess the lifetime of the bowl rim, it is necessary to simulate non-isothermal elastoplastic deformation of the alloy using material plasticity and creep models available in finite element analysis software (e.g. ANSYS). This paper presents the results of uniaxial tensile and creep tests of proportional specimens made from piston blanks of the V-type diesel engines YaMZ-658. The piston material is AlSi12CuNiMg silumin alloy. The article describes methods for determining constants in plasticity and creep models. The results of numerical simulation of the piston’s stress-strain state for the start — nominal power mode — stop cycle using the finite element method are presented. Conclusions about the presence of plastic and creep strains at the piston edge are drawn.

On the comprehensive stability analysis of axially loaded bistable and tristable metastructures

Approaches to systematic analysis of essentially nonlinear structures with multistable responses, controlled buckling and snapping behavior have received much attention recently in the context of mechanical metamaterials design. A snapping bistable element is generally a highly efficient damper, performing well even at very low forcing frequencies. In this paper, we transfer basic tools of the metamaterials analysis to macroscopic systems relevant to civil and mechanical engineering applications. Such systems are comprised of only several bistable elements. Followed by analysis of a single snapping bistable axial (two-force) element, we consider a combination of two elements with antisymmetric properties and demonstrate a robust tristable performance of the resultant structure in low-frequency or quasistatic tension-compression loading cycles. The tristability provides an overall response that is symmetric for tension and compression, which makes it interesting for applications in machinery and large-scale seismic structures.

Defining the exploitation patterns of groundwater heat pump systems

Shallow geothermal systems are the most efficient and clean technology for the air-conditioning of buildings and constitutes an emergent renewable energy resource in the worldwide market. Undisturbed systems are capable of efficiently exchanging heat with the subsurface and transferring it to human infrastructures, providing the basis for the successful decarbonisation of heating and cooling demands of cities. Unmanaged intensive use of groundwater for thermal purposes as a shallow geothermal energy (SGE) resource in urban environments threatens the resources' renewability and the systems' performance, due to the thermal interferences created by a biased energy demand throughout the year. The exploitation regimes of 27 groundwater heat pump systems from an alluvial aquifer were firstly examined using descriptive statistics. Linear relationships between abstraction and injection temperatures of the systems were assessed by calculating Pearson's r correlation coefficient, and used as an evidence of thermal interferences. Then, time series of flow rate, operation temperature and energy transfer were modelled by means of spectral analysis and sinusoidal regression methods, followed by the definition of the relative exploitation patterns. The exploitation regimes examined presented a clear cooling bias and a similar cyclicality. The amplitudes correlated with the different end-user's activities (e.g. medical centres) when high frequency cycles were observed, while climatization strategies (e.g. constant flow rates and modulation of injection temperatures) did so when low frequency cycles were detected. The time series models allowed defining the relative operational pattern of a system and the groups of systems following such patterns. The biases in exploitation regimes of groundwater heat pump systems existing in Mediterranean areas require correction measures to ensure a more balanced exploitation of the SGE resources. The definition of the characteristic exploitation pattern proposed could be applied to guide resource managers by identifying unbalanced systems, understanding existent exploitation strategies and proposing corrective alternative plans.

Discharge regimes and emission characteristics of capacitively coupled radio frequency argon plasma with a square wave input

This work presents a computational and experimental investigation of capacitively coupled radio frequency argon plasma in a roll-to-roll chemical vapor deposition system for carbon nanostructure growth. The system operates at moderate pressures (less than 30 mbar) with an 80 kHz square wave voltage input. The computational model aids the understanding of plasma properties and α–γ transition parameters which strongly influence the nanostructure deposition characteristics in the system. A 1D plasma model is developed to characterize the effects of input voltage, gas pressure, frequency, and waveform on the plasma properties. A hybrid mode which displays the characteristics of both α and γ discharges is found to exist for the low cycle frequency 80 kHz square wave voltage input due to the high frequency harmonics associated with a square waveform. The modeled gas temperatures are 22.5% higher than the experimentally measured values due to the presence of molecular species in the experiments. Collision radiative modeling is performed to predict the argon emission intensity in the discharge gap. The results are found to lie within 16% of the optical emission spectroscopy measurements with better agreement at the center of the discharge, where the measurement uncertainty is low and the emission by ions is not significant.

Atmospheric pressure high-power impulse plasma source for deposition of metallic coatings

Abstract

Extrinsic and intrinsic dynamics in movement intermittency.

This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that all the issues have been addressed (see decision letter).

Numerical investigation of the low-frequency flow oscillation over a NACA-0012 aerofoil at the inception of stall

High-fidelity large eddy simulation is carried out for the flow field around a NACA-0012 aerofoil at Reynolds number of [Formula: see text], Mach number of 0.4, and various angles of attack around the onset of stall. The laminar separation bubble is formed on the suction surface of the aerofoil and is constituted by the reattached shear layer. At these conditions, the laminar separation bubble is unstable and switches between a short bubble and an open bubble. The instability of the laminar separation bubble triggers a low-frequency flow oscillation. The aerodynamic coefficients oscillate accordingly at a low frequency. The lift and the drag coefficients compare very well to recent high-accuracy experimental data, and the lift leads the drag by a phase shift of [Formula: see text]. The mean lift coefficient peaks at the angle of attack of [Formula: see text], in total agreement with the experimental data. The spectra of the lift coefficient does not show a significant low-frequency peak at angles of attack lower than or equal the stall angle of attack ([Formula: see text]). At higher angles of attack, the spectra show two low-frequency peaks and the low-frequency flow oscillation is fully developed at the angle of attack of [Formula: see text]. The behaviour of the flow-field and changes in the turbulent kinetic energy over one low-frequency flow oscillation cycle are described qualitatively.

Links between different classes of storm tracks and the flood trends in Spain

The identification of storm tracks that generate the annual maximum floods and the quantification of their air-moisture content is proposed to understand better the atmospheric generation processes of floods in Spain, as well as their decreasing trends identified previously. In this work, the role of the atmospheric component on hydrological changes on Spain by using storm track data generated by the hybrid single particle Lagrangian integrated trajectory model (HYSPLIT) is examined. Storm tracks associated with annual maximum flood series from 14 streamflow gauges located across Spain are obtained by using NCEP/NCAR Reanalysis data. They are classified into five clusters through use of the K-means algorithm. It was also shown that the use of five clusters was able to reproduce the five major types of storms identified in Spain reported in the literature. The posterior grouping of the five associated storm types into two bigger groups (Oceanic and Continental storms), led to a coherent seasonal and spatial behaviour of hydrological regimes. For some gauges, it was observed that distinct flood statistics, such as mean and variance, differ significantly as a result of atmospherically distinct generation processes, suggesting that local annual maximum flood series may be non-homogeneous as a result of contrasting atmospheric generation processes. By means of logistic regression, it is estimated that the probability of occurrence of Oceanic storms reduced significantly in the Spanish Atlantic region along the studied period. Furthermore, the existence of low-frequency cycles introduces significant variation in the occurrence of storm types in the country, with them being much more frequent during the period 1975–1979, while Oceanic storms were more frequent during the 1983–1987 and 1996–1999 periods. Those storms classified as Continental are observed in 59.3% of the studied cases, while those termed Oceanic in 40.7%. Continental storms also contribute with more moisture in the studied cases, that is to say, 63.1% of total moisture content, while Oceanic storms contributed with 36.9%.

Effects of single and double pulses on microstructure and mechanical properties of weld joints during high-power double-wire GMAW

High-power double-wire gas metal arc welding (GMAW) experiments were conducted with the single and double pulses technology. The effects of single and double pulses on the weld bead formation, microstructure and mechanical properties of the weld joints were investigated. The experimental results demonstrated that through the high-power double-wire single and double pulsed GMAW stable welding could be achieved. Also, the high quality weld beads were acquired with slight spatter, without undercut or hump. Double-wire double pulsed GMAW was more stable, producing more perfect weld shape and higher microhardness compared to double-wire single pulsed GMAW. The stirring effect of alternating strong and weak pulses on the weld pool during the double-wire double pulsed GMAW could produce significant fish-scale ripples and refine the grains, resulting in finer grain microstructure with higher hardness. The results proved that the low-frequency pulse cycle controlled the formation of each weld pool precisely. This meant that a weld pool was formed in a low-frequency pulse cycle.

What causes business cycles to elongate, or recessions to intensify?

In this paper we use both New Keynesian and Classical monetary models to explain why volatility transfers from high to low frequency cycles occur, causing the business cycle to elongate, and how they could reverse. Our results show that an increase in inflation aversion or a reduction in the commitment to output stabilization would create volatility transfers sufficient to give rise to a great moderation, while a reversal of those commitments would take it away again. In short, the lower frequency cycles become more volatile at the expense of traditional business cycle frequencies, even though there are no reversals in the parameters that govern the economy’s underlying economic behaviour, dynamics or volatility. This implies we should expect less frequent but more severe recessions, with smoother business cycle expansion phases in between. We identify moderate expectations for output or earnings growth to be the key element that under-pins these results.

Feasibility and effects of intra-dialytic low-frequency electrical muscle stimulation and cycle training: A pilot randomized controlled trial.

Background and objectivesExercise capacity is reduced in chronic kidney failure (CKF). Intra-dialytic cycling is beneficial, but comorbidity and fatigue can prevent this type of training. Low–frequency electrical muscle stimulation (LF-EMS) of the quadriceps and hamstrings elicits a cardiovascular training stimulus and may be a suitable alternative. The main objectives of this trial were to assess the feasibility and efficacy of intra-dialytic LF-EMS vs. cyclingDesign, setting, participants, and measurementsAssessor blind, parallel group, randomized controlled pilot study with sixty-four stable patients on maintenance hemodialysis. Participants were randomized to 10 weeks of 1) intra-dialytic cycling, 2) intra-dialytic LF-EMS, or 3) non-exercise control. Exercise was performed for up to one hour three times per week. Cycling workload was set at 40–60% oxygen uptake (VO2) reserve, and LF-EMS at maximum tolerable intensity. The control group did not complete any intra-dialytic exercise. Feasibility of intra-dialytic LF-EMS and cycling was the primary outcome, assessed by monitoring recruitment, retention and tolerability. At baseline and 10 weeks, secondary outcomes including cardio-respiratory reserve, muscle strength, and cardio-arterial structure and function were assessed.ResultsFifty-one (of 64 randomized) participants completed the study (LF-EMS = 17 [77%], cycling = 16 [80%], control = 18 [82%]). Intra-dialytic LF-EMS and cycling were feasible and well tolerated (9% and 5% intolerance respectively, P = 0.9). At 10-weeks, cardio-respiratory reserve (VO2 peak) (Difference vs. control: LF-EMS +2.0 [95% CI, 0.3 to 3.7] ml.kg-1.min-1, P = 0.02, and cycling +3.0 [95% CI, 1.2 to 4.7] ml.kg-1.min-1, P = 0.001) and leg strength (Difference vs. control: LF-EMS, +94 [95% CI, 35.6 to 152.3] N, P = 0.002 and cycling, +65.1 [95% CI, 6.4 to 123.8] N, P = 0.002) were improved. Arterial structure and function were unaffected.ConclusionsTen weeks of intra-dialytic LF-EMS or cycling improved cardio-respiratory reserve and muscular strength. For patients who are unable or unwilling to cycle during dialysis, LF-EMS is a feasible alternative.