Main Mode Research Articles

Novel Combustion Instability Diagnosis Method With Upstream Pulsation of Repetitive Laser-Induced Plasmas

Abstract In this experimental study, we are presenting the ability of laser-induced plasmas with successive pulsation to identify combustion instabilities (CI) of a premixed lab-scale combustor. An acoustic disturbance equivalent to a shockwave perturbation is generated in the main air supply line of a swirled injector prior to the fuel addition by focusing nanosecond laser pulses of 1.6 W average power at 10 Hz. The shockwaves are attenuated to be strong pressure waves when reaching the combustor and impact the pressure field for short periods. After plasma breakdowns, the system returns back to its original state after 4 ms once the added acoustic energy has been fully dissipated. Given a set geometry, it is observed that the laser-induced breakdown amplifies the characteristic frequency peaks of the combustor system when actuated in cold flow. Furthermore, when applied to reacting flows, the pulsating acoustic perturbations impact the pressure fluctuation in the combustor, e.g., reducing the amplitude of the primary characteristic frequency peak at certain conditions. The identification of the main instability modes thanks to the plasma shockwave provides proof of the potential use of this novel diagnosis strategy in various and complex combustion systems.

Feasibility Study of Three-Phase Modular Converter for Dual-Purpose Application in DC and AC Microgrids

The modern concept of a universal converter is intended as a power converter suitable for application in both dc or ac grids using the same external connectors. This novel family was recently proposed to allow an easier integration of renewable energy sources and energy storage systems, interfacing with dc/ac grids and/or loads with a minimum redundancy of power switches and passive elements. This kind of solution and applications are expected to be a reality in the nearest decade, as ac and dc low voltage distribution networks will coexist. Nevertheless, the initial solutions proposed as universal converter were focused on ac single-phase power conversion systems. In this sense, this article proposes and describes a new member of the universal converter family suitable for dc to dc and dc to ac power conversion (both in three-phase three-wire and in three-phase four-wire). The proposed power topology is derived as a modular extension from the single buck-boost bidirectional cell. Its main operation modes (buck and boost) are discussed, and a pulse-width-modulation technique is developed to generate the corresponding switching patterns. The proposed solution is successfully validated in open-loop mode both in simulation and experimentally with a laboratory prototype. The measured efficiency of the power converter was above 97% in the dc to ac mode and around 99% in the dc to dc mode.

Microstructure evolution and mechanical properties of new Co-free maraging steel produced by wire arc additive manufacturing

In this work, P-MIG is used as additive process, a new low-cost Co-free maraging steel wire is used as raw material to additive manufacturing block component, and the microstructure and mechanical properties of the component is investigated. The microstructure of the component is composed of martensite and austenite, and the closer to the bottom, the more serious the elemental segregation and the more austenite content. From the top to the bottom the high-angle grain boundaries increase and the grain size decrease, the difference of the heat dissipation condition and the thermal cycle are the reasons for this phenomenon. The texture of austenite is mainly Cube texture, and the texture of martensite is Rotated Cube texture and Goss texture. The tensile strength of XD, YD, ZD three directions is 1172 ± 15 MPa, 1021 ± 7 MPa, 1139 ± 16 MPa, respectively, impact toughness is 64.7 ± 1.6 J/cm2, 51.1 ± 5.6 J/cm2, 51.4 ± 3.1 J/cm2, respectively, TRIP effect occurs in the process of tensile, the main mode of fracture in tensile and impact is toughness fracture. The microhardness decreases gradually from bottom to top, which is due to the short-lived aging strengthening by thermal cycle, the change of grain size and GND density. The results of the work reveal the successful application of the new low-cost Co-free maraging steel wire in WAAM and provide an important reference for the adjustment of additive process, subsequent heat treatment and the adjustment of wire composition.

Study of vibration shock processes of non-linear mechanical systems with distributed parameters

In practice, under the conditions of perfection and constructive development of modern equipment and machines, nonlinear mechanical systems with distributed parameters are often encountered, which, depending on the principles of operation, are affected by vibration shock. Therefore, the study of vibration shock processes of the mentioned systems has great theoretical and practical importance and as a result to determine the optimal parameters of vibration protection devices to ensure their safe operation. In our case, the displacement field of two interacting non-linear mechanical systems with distributed parameters is considered, when their interaction is of vibration shock nature. Obviously, the mentioned events are more pronounced when the self-oscillation frequency of one or both systems momentarily approaches the frequency of forced vibration shock processes. In addition, critical moments are fixed during the phase shifts of forced oscillations of oscillatory systems, in this case, the frequencies of forced oscillations approach mutually opposing phase moments. By choosing the optimal parameters of hysteresis losses, it is possible to almost exclude sub-harmonic modes superimposed on the main resonance modes in vibration shock processes. During hysteresis losses of the parabolic type, the value of µ changes automatically in connection with impulsive loads, which will allow us to transfer the vibration shock processes to automatic modes and, accordingly, the practically safe operation of the mentioned systems.

Novel Combustion Instability Diagnosis Method With Upstream Pulsation of Repetitive Laser-Induced Plasmas

Abstract In this experimental study, we are presenting the ability of laser-induced plasmas with successive pulsation to identify combustion instabilities (CI) of a premixed lab-scale combustor. An acoustic disturbance equivalent to a shockwave perturbation is generated in the main air supply line of a swirled injector prior to the fuel addition by focusing nanosecond laser pulses of 1.6 W average power at 10 Hz. The shockwaves are attenuated to be strong pressure waves when reaching the combustor and impact the pressure field for short periods. After plasma breakdowns, the system returns back to its original state after 4 ms once the added acoustic energy has been fully dissipated. Given a set geometry, it is observed that the laser-induced breakdown amplifies the characteristic frequency peaks of the combustor system when actuated in cold flow. Furthermore, when applied to reacting flows, the pulsating acoustic perturbations impact the pressure fluctuation in the combustor, e.g., reducing the amplitude of the primary characteristic frequency peak at certain conditions. The identification of the main instability modes thanks to the plasma shockwave provides proof of the potential use of this novel diagnosis strategy in various and complex combustion systems.

A novel metagenomic approach uncovers phage genes as markers for increased disinfectant tolerance in mixed Listeria monocytogenes communities

Listeria monocytogenes is an important human pathogen with a high mortality rate. Consumption of contaminated ready-to-eat food is the main mode of transmission to humans. Disinfectant-tolerant L. monocytogenes have emerged, which are believed to have increased persistence potential. Elucidating the mechanisms of L. monocytogenes disinfectant tolerance has been the focus of previous studies using pure cultures. A limitation of such approach is the difficulty to identify strains with reduced susceptibility due to inter-strain variation and the need to screen large numbers of strains and genes. In this study, we applied a novel metagenomic approach to detect genes associated with disinfectant tolerance in mixed L. monocytogenes planktonic communities. Two communities, consisting of 71 and 80 isolates each, were treated with the food industry disinfectants benzalkonium chloride (BC, 1.75 mg/L) or peracetic acid (PAA, 38 mg/L). The communities were subjected to metagenomic sequencing and differences in individual gene abundances between biocide-free control communities and biocide-treated communities were determined. A significant increase in the abundance of Listeria phage-associated genes was observed in both communities after treatment, suggesting that prophage carriage could lead to an increased disinfectant tolerance in mixed L. monocytogenes planktonic communities. In contrast, a significant decrease in the abundance of a high-copy emrC-harbouring plasmid pLmN12–0935 was observed in both communities after treatment. In PAA-treated community, a putative ABC transporter previously found to be necessary for L. monocytogenes resistance to antimicrobial agents and virulence, was among the genes with the highest weight for differentiating treated from control samples. The undertaken metagenomic approach in this study can be applied to identify genes associated with increased tolerance to other antimicrobials in mixed bacterial communities.

Reinforcement of concrete beams using waste carbon-nanoclay-fiberglass laminate pieces

In the last few decades, strengthening of structures in need of repair with fiber reinforced polymer (FRP) composite materials produced with different fiber types has gained great importance. Within the scope of this experimental study, the usability of hybrid glass and carbon composite laminates produced for different purposes and later cut into waste was investigated for concrete reinforcement. Hybrid composite laminates were produced in the form of glass-carbon-glass and carbon-glass-carbon, and the effect was investigated in two different sequences in the study. In addition, there are 3 different rates of nanoclay (0.50%, 0.75% and 1.25%) in the production of composite materials, and the effect of nanoclay ratio was investigated. In the study, two different numbers of composite laminates were adhered to the concrete samples produced in 70x70x280 mm dimensions and subjected to flexural strength test. In the Carbon-Glass-Carbon series using triple waste laminate pieces, the highest flexural strength was reported in the CGC-0.75-3 series, which achieved an increase of approximately 55% and 42% compared to the Control and Control-E series. It was determined that the effectiveness of the reinforcement technique of concrete with laminates in flexure did not change significantly depending on the number of laminate pieces. The main mode of failure in the experimental work was due to concrete fracture.

Bond behaviour of steel rebar in polyethylene engineered cementitious composites under various load conditions

Engineered Cementitious Composites (ECC) are a special type of ultra-ductile fiber-reinforced concrete that uses microfibers. The bond strength of Polyethylene ECC with steel reinforcement was found to be not well investigated in the literature under both monotonic loading and cyclic loading conditions. This study aims to investigate the bond strength properties of Polyethylene ECC with steel reinforcement under monotonic and cyclic loading conditions and to evaluate the effect of embedment length, loading rate, and the cover on bond strength experimentally. The beam-end test was selected for this experiment since it is a relatively simple test, yet it produces a realistic stress field. The results indicated that ECC's main failure mode is splitting-pullout, with high slippages occurring. Furthermore, increasing the cover thickness from 3db to 4db enhanced the bond strength by 91%, while raising the anchorage length from 5db to 7db improved it by 105%. Increasing the load rate from 0.01 mm/s to 32 mm/s resulted in a significant 63% improvement in bond strength. Moreover, ECC performed exceptionally well under cyclic loading, displaying increased peak loads by an average of 76.8% compared to monotonic loading. The study findings are expected to guide the formulation of development length equations and finite element models incorporating ECC.

Creep-fatigue life estimation of circumferential welds in thermal high energy piping systems

ABSTRACT To prevent accidents in high energy piping systems of thermal power plants, it may be important to accurately estimate the creep-fatigue life of circumferential piping welds. The main failure mode in the damage cases was Type IV damage which was creep-dominated damage occured at the fine-grained HAZ(FGHAZ). The damege mode is considered to change from longitudinal failure at the base metal to circumferential Type IV failure when the ratio of axial stress to hoop stress exceeds a certain value. The allowable thermal expansion stress in the design, the stress relaxation property of the material, and the creep rupture strength of welded joints are investigated, as factors affecting creep damage of circumferential piping welds. Fundamental creep-fatigue tests were perfomed using cross-weld specimens and their lives were estimated by a linear damage summation rule. In the life estimation, it seems necessary to consider elastic follow-up phenomena where strain transfers to the HAZ.

Influence of soil - foundation - tank interaction on buckling strength of liquid storage tanks

Large earthquakes in the last 25 years have caused significant damage to buildings and infrastructure, including the partial or total collapse of storage tanks in various industries. Elephant foot buckling, or local buckling at the base, is one of the main failure modes observed in these structures, and this failure mode can lead to their collapse and/or complete loss of contents. Although hydrostatic and hydrodynamic loads typically affect the seismic response of tanks, the effect of soil type on tank buckling behavior has not been widely studied or recognized. This research aims to evaluate the effect of soil type on seismic fragility of tanks by analyzing typical storage tanks used in the wine industry. The work focuses on elephant foot buckling for tanks with both unanchored and anchored bases and compares the influence of three different types of soil and two different tank geometries. The approach uses the capacity spectrum method, as opposed to the more commonly used incremental dynamic analysis, to determine a critical peak ground acceleration to cause buckling at the tank. The tanks were subjected to 21 Chilean seismic records with three different soil types and a no-soil condition. From the results a lognormal fragility curve, and its median and standard deviation, are calculated. The results indicate that unanchored tanks built softer soils exhibit poorer performance, while tanks in competent soils and rock exhibit good performance. Anchored tanks show less sensitivity to soil types than unanchored tanks. The study demonstrates the importance of considering soil-foundation-structure interaction for wine storage tanks, but the results indicate that many comparable storage structures will be similarly affected.

Compressive performance of full-scale GFRP composite sandwich wall panels with wood core

This paper investigated the compressive behaviors of sandwich wall panels consisting of glass fiber-reinforced polymer (GFRP) face sheets and Paulownia wood core experimentally and numerically. Vacuum-assisted resin infusion molding (VARIM) process was used to produce GFRP sandwich wall panel specimens. Four full-scale wall panels with different heights were tested under axial loading to study their failure modes and load-bearing capacities. The results revealed that buckling was the main failure mode of GFRP wall panels under the axial compression and the ultimate load-bearing capacities of specimens were improved the decrease of the height-to-thickness ratio. Experiments showed that decreasing the height-to-thickness ratio from 51.5 to 38.6 can effectively improve the ultimate load-bearing capacities of the wall panels by 124%. The experimental results were compared with finite element analysis (FEA) and analytical results were in good agreement. In addition, an equation was used to predict the load-bearing capacities of the wall panels suitable for the specimens with smaller height-to-thickness ratio.

The relationship between repeated impacts and CAI strength of sandwich structure considering different impactor shape in individual impact

Sandwich structures are susceptible to repeated impact loads, and each individual impact is not nearly identical to the previous one, especially for repeated impacts under different impactor shapes such as hail impact. Therefore, the repeated impact behavior using different impactor shape in individual impact of polymethacrylimide (PMI) foam sandwich structure is studied and discussed through experiments and finite element (FE) simulations in this paper. The representative mechanical characteristics such as impact responses, damage modes, and energy absorption are investigated until penetration occurs, and the mechanism of impact damage accumulation between impact events with different impactor shapes is revealed. Besides, the compression after impact (CAI) strength is utilized to assess the residual strength of sandwich structure under repeated impacts. It indicates when the contact area between the specimen and impactor is larger, it can result in a higher impact force, a shorter contact duration, and potentially reduce the extent of damage. The matrix damage is the main damage mode, and the proportions of matrix tension and compression are nearly constant, with a ratio of approximately 2:1. In addition, the proportion of the impact energy absorbed by foam core increases as the number of impacts increases. When the impact energy is low, the CAI strength is sensitive to the number of impacts, the shape of impactor, and the sequence of impactors, but the similar phenomenon does not occur at higher impact energy. The present study provided a series of experimental and numerical results, which could be utilized as design reference guidance in the designing of composite sandwich structures.

Diurnally Varying Ekman Layer in a Rossby Wave

Abstract Weak but persistent synoptic-scale ascent may play a role in the initiation or maintenance of nocturnal convection over the central United States. An analytical model is used to explore the nocturnal low-level jets (NLLJ) and ascent that develop in an idealized diurnally varying frictional (Ekman) boundary layer in a neutrally stratified barotropic environment when the flow aloft is a zonally propagating Rossby wave. Steady-periodic solutions are obtained of the linearized Reynolds-averaged Boussinesq-approximated equations of motion on a beta plane with an eddy viscosity that is specified to increase abruptly at sunrise and decrease abruptly at sunset. Rayleigh damping terms are used to parameterize momentum loss due to radiation of inertia–gravity waves. The model-predicted vertical velocity is (approximately) proportional to the wavenumber and wave amplitude. There are two main modes of ascent in midlatitudes, an afternoon mode and a nocturnal mode. The latter arises as a gentle but persistent surge induced by the decrease of turbulence at sunset, the same mechanism that triggers inertial oscillations in the Blackadar theory of NLLJs. If the Rayleigh damping terms are omitted, the boundary layer depth becomes infinite at three critical latitudes, and the vertical velocity becomes infinite far above the ground at two of those latitudes. With the damping terms retained, the solution is well behaved. Peak daytime ascent in the model occurs progressively later in the afternoon at more southern locations (in the Northern Hemisphere) until the first (most northern) critical latitude is reached; south of that latitude the nocturnal mode is dominant.

Shear performance of high-strength bolt-epoxy bonding composite connector in steel-concrete composite structure

To clarify the shear mechanism of high-strength bolt-epoxy bonding composite connectors in prefabricated steel-concrete composite structures, twenty-one direct shear specimens and six double shear specimens were carried out in this study. The pretension force, interface type, epoxy curing time and loading pattern were considered. Results revealed that the main failure mode of high-strength bolt-epoxy bonding composite connectors was that the large area of epoxy matrix was separated from the steel plate, and the bolt was sheared off. The load-slip curve of composite connector specimens included the elastic stage, epoxy failure stage, frictional slip stage, bolt force transmission stage and failure stage. Compared to the specimens with a bolt connector under recommended pretension, the average initial slip load of composite connector specimens was 437.7% higher. Additionally, the average initial slip load of specimens with 1-day epoxy curing time was 24.4% lower than 7-day curing time, but the average ultimate load and ultimate slip were 12.4% and 29.4% higher, respectively. When the pretension ratio decreased from 1.0 to 0.6, the average initial slip load of specimens with a bolt connector was significantly decreased by 33.0% and the average ultimate slip of composite connector specimens was increased by 18.1%. The average initial slip load and ultimate load of composite connector specimens were comparable under cyclic loading compared to monotonic loading. Upon the experimental results and existing formulas, some equations were proposed for predicting the initial slip load and bearing capacity of steel-concrete composite structures with composite connectors.

Progressive damage analysis of double-layer variable thickness 3D woven composite scaled engine casing

Due to the excellent mechanical properties and strong design flexibility, the 3D woven composite engine casing shows great potential in high performance fields. This article adopted the method of double-layer weaving and adding yarn to achieve the variable thickness of the 3D woven composite engine casing. The purpose of this article is to study the compression performance and failure mechanism of this variable thickness casing in different thickness zones through experiments and numerical simulations to lay a foundation for future optimization and inclusiveness research on this casing with complex thickness changes. Three types of representative volume cells are established for progressive damage analysis. 3D-Hashin criteria and von-Mises stress criterion are used as damage criteria for yarns and matrix. The progressive damage process and the proportion of damage in the inner and outer layers of three types of 3D woven tubes under axial compression load are analyzed. Results show that the main failure modes of the three types of tubes are yarn-matrix compressive cracking along direction 3 and matrix failure. The damages are mostly concentrated in the warp bending area. The proportions of warp yarn damage in the inner and outer layers of three tubes are different.

Spatial structure of resonance cavities in sunspots

ABSTRACT We present a study of wave processes in sunspots from active regions NOAA 11131 on 2010 December 10 and NOAA 12565 on 2016 July 14 observed by SDO/AIA in the 1600, 304, and 171 Å temperature channels. To study the spatial structure of the resonance cavities previously found by Jess et al., we applied spectral data processing techniques such as pixelized wavelet filtering and mode decomposition. For the first time, we found stable regions as waveguides of the oscillations in the sunspot umbra, occupying specific frequency ranges without spatial overlap. The sizes of these regions depend on the frequency oscillations, and the maximum frequency coincides with the values of the harmonics of the main oscillation mode. Frequency drifts were observed in the band occupied by these regions, with different spectral slopes depending on the location of the sources in the sunspot umbra. We suggest that the observed distribution of wave sources in the umbra is a set of resonant cavities where successive amplification of oscillations at selected multiple harmonics is observed. The distribution of sources at low frequencies indicates the influence of the atmospheric cut-off due to the inclinations of the magnetic field lines.

Current fluctuations in a partially asymmetric simple exclusion process with a defect particle.

We study an exclusion process on a ring comprising a free defect particle in a bath of normal particles. The model is one of the few integrable cases in which the bath particles are partially asymmetric. The presence of the free defect creates localized or shock phases according to parameter values. We use a functional approach to Bethe equationsresulting from a nested Bethe ansatz to calculate exactly the mean currents and diffusion constants. The results agree very well with Monte Carlo simulations and reveal the main modes of fluctuation in the different phases of the steady state.

General multi-attack detection for continuous-variable quantum key distribution with local local oscillator

Abstract Continuous-variable quantum key distribution with a local local oscillator(LLO CVQKD) has been extensively researched due to its simplicity and security. For the practical security of the LLO CVQKD system, there are two main attack modes referred to as reference pulse attack and polarization attack presently. However, there is currently no general defense strategy against such attacks and the security of the system needs further investigation. Here, we employ a deep learning framework called generative adversarial networks (GANs) to detect both attacks. We first analyze the data in different cases, derive a feature vector as input to a GAN model, and then show the training and testing process of the GAN model for attack classification. The proposed model has two parts, a discriminator and a generator, both of which employ a convolutional neural network (CNN) to improve accuracy. Simulation results show that the proposed scheme can detect and classify attacks without reducing the secret key rate and the maximum transmission distance. It only establishes a detection model by monitoring features of the pulse without adding additional devices.

Tensile strength analysis of sandwich composite-to-metal joints based on experimental investigation and progressive failure method

In this study, the tensile strength characteristics of the hybrid sandwich composite-to-metal specimen consisting of two steel I-beams and a T-shaped sandwich composite structure were studied. The tensile strength of the sandwich hybrid joints under design load and ultimate load has been tested. The progressive failure model is established according to the modified Shokrieh-Hashin criterion. The interface cohesive zone model is applied to reveal the damage development of the bonding layer. The stresses, strains, ultimate strength, and main damage modes of the hybrid joint under design load and ultimate load conditions are simulated and the predicted results are compared with those from tests. The experimental and numerical results show that the strain, the trend of load-displacement curves and the damage modes are in good agreement. The strength of matrix is the main factor limiting ultimate strength. Therefore, the influences of the overlapping width and thickness on ultimate bearing capacity were analyzed to obtain the optimized structural strength of the sandwich composite-to-metal bolted joints.

Investigation of Degradation in 20-Year-Field-Exposed PV Modules From India by Cross-Characterization Using Electroluminescence Imaging and Thermography

The reliability of photovoltaic (PV) modules is essential for ensuring a smooth operation over the anticipated timespan while operating outside. Examining the degradation in field-exposed photovoltaic (FEPV) modules will help to identify the possible degradation modes that can affect PV module performance and functioning. In this paper, to identify the major modes of degradation in Indian subtropical climate conditions, 20-year-old FEPV modules have been investigated by cross-characterization using dark lock-in thermography (DLIT) and electroluminescence (EL) imaging. Cross-characterization using EL and DLIT images has been helpful in investigation of various modes of degradations in FEPV modules in the presence of multiple degradations. Encapsulant and busbar ribbon interface degradations have been identified as the two main modes of degradation observed in Indian sub-tropical climate conditions. Minor degradations include finger interruptions and cell cracks. A major effect of degradation has been observed on the fill factor and short circuit current, which have decreased by up to 30% and 40%, respectively. The results presented in this paper can be used to understand degradation occurring in a sub-tropical climate and for the non-destructive analysis of degradations in the FEPV modules.