Mode Transition Research Articles

Segmented variable-frequency ultrasound synergistic hot-air drying of Rhubarb: Effect on drying characteristics and quality and thermal analysis

This study employed segmented variable-frequency ultrasound synergistic hot-air drying (SVFU-HAD) for Rhubarb slices, selected two sets of time nodes for frequency conversion (60 min, 120 min, and 90 min, 150 min), and two sequences of frequency conversion (high-frequency to low-frequency, and low-frequency to high-frequency). It aimed to investigate the effects of SVFU-HAD on the drying characteristics, quality, and heat transfer of Rhubarb slices. The findings indicated that segmented variable-frequency ultrasound has advantages in increasing drying rate and improving uniformity of cavitation effects compared to constant-frequency ultrasound. Analysis of physical properties revealed that the rehydration performance of dried products subjected to ultrasonic variable-frequency treatment (90 min, 150 min) according to the drying rate was better (RR > 3.3). The transition mode from high-frequency to low-frequency in variable-frequency ultrasonic treatment contributes to maintaining the overall color of Rhubarb. Analysis of chemical properties unveiled that Rhubarb treated with 40 kHz (0 min)–28 kHz (60 min)–25 kHz (120 min) segmented variable-frequency ultrasound contained overall higher levels of tannins, dianthrones and free anthraquinones content, which exceeded the average values by 3.24%, 26.65%, and 14.42%, respectively. In addition, thermal analysis results based on ANSYS Workbench software demonstrated that the drying uniformity of SVFU-HAD is superior to that of hot-air drying and constant-frequency ultrasound synergistic hot-air drying (CFU-HAD). Overall, the SVFU-HAD method employed in this study presents an innovative approach to ultrasound synergistic hot-air drying research with promising potential for enhancing the efficiency and quality characteristics of Rhubarb slices.

Influence of curing conditions on the interface bonding properties of CFRP plate–steel

To understand the influence of different curing conditions on the interface bonding performance of CFRP-reinforced steel plates, tensile tests were conducted on epoxy resin dog bone specimens and CFRP-reinforced steel plate double lap specimens. The study explored the tensile performance of dog bone specimens under different curing temperature/time conditions and the mechanical properties of double lap specimens under different curing pressures and curing temperature/time conditions. The results indicate that increasing the curing temperature can shorten the curing time of dog bone specimens and also improve their elastic modulus and tensile strength. Elevating the curing pressure, curing temperature, and extending the curing time cause a transition in the failure mode of the double lap specimens from surface fiber stripping of the CFRP plate to a mixed failure dominated by surface fiber stripping and adhesive–steel interface debonding, and the bearing capacity, interface ultimate slip, and peak shear stress of the specimens significantly increase, with the shear stress concentration zone gradually expanding from the loading end towards the free end, simultaneously, the ductility of the specimen increased, and the failure process was delayed. The ultimate load, interface peak shear stress, and ultimate slip of the specimens cured at 90 °C are significantly higher compared to the control group cured at 20 °C and 0.01 MPa. However, with increasing curing time, the rate of growth shows a decreasing trend.

Simple model of turbine-based combined cycle propulsion system and smooth mode transition

Abstract Turbine-based combined cycle (TBCC) propulsion system is becoming one of the most promising propulsion systems for two-stage-to-orbit reusable launch vehicle. Mathematic model of this combined cycle engine is helpful for the basic understanding of performance analysis of this propulsion system. We developed mathematic model of TBCC propulsion system based on C++ platform in this paper. Firstly, turbojet engine was built on component level and ramjet engine was calculated through stream thrust function. Then, performance of turbine-based combined cycle propulsion system along a specific flight trajectory was investigated. According to the thrust of this combined cycle engine, mode transition point was suggested at Ma 2.5, which may achieve smooth mode transition from turbine mode to ramjet mode. Finally, mode transition based on smooth mass flow and smooth thrust criteria were studied. The thrust gap arises during smooth mass flow mode transition, particularly when the turbojet engine afterburner is powered off at the start of the mode transition, and it meets 34 % of the total thrust. Smooth thrust mode transition may be achieved by delaying the turbojet engine afterburner power off and injecting additional fuel into ramjet burner.

Atmospheric oxidation drove climate change on Noachian Mars

Modern Mars is bipolar, cold, and oxidizing, while early Mars was characterized by icy highlands, episodic warmth and reducing atmosphere. The timing and association of the climate and redox transitions remain inadequately understood. Here we examine the spatiotemporal distribution of the low surface iron abundance in the ancient Martian terrains, revealing that iron abundance decreases with elevation in the older Noachian terrains but with latitude in the younger Noachian terrains. These observations suggest: (a) low-temperature conditions contribute to surface iron depletion, likely facilitated by anoxic leaching through freeze-thaw cycles under a reducing atmosphere, and (b) temperature distribution mode shifted from elevation-dominant to latitude-dominant during the Noachian period. Additionally, we find iron leaching intensity decreases from the Early to Late Noachian epoch, suggesting a gradual atmospheric oxidation coupled with temperature mode transition during the Noachian period. We think atmospheric oxidation led to Mars becoming cold and bipolar in its early history.

Experimental study on the mechanical characteristics of weakly cemented mudstone under different loading rates

With the gradual shift of coal mining to the western coal mining region of China, floor heave in weakly cemented mudstone roadways has become an issue affecting the safety and efficiency of coal mine production. Additionally, different mining rates can lead to fluctuating support stresses on the roof and floor of weakly cemented mudstone roadways. Therefore, obtaining a comprehensive understanding of the mechanical properties of weakly cemented mudstone at different loading rates is conducive to improving the issue of floor heave in such roadways and provides a theoretical basis for further study. In this context, a series of uniaxial mechanical tests with concurrent acoustic emission monitoring were conducted on specimens of weakly cemented mudstone under various loading rates (0.005, 0.01, 0.05, and 0.1 mm/s). The stress‒strain and acoustic emission response curves were obtained to effectively characterize the strength, deformation, damage, macroscale instability, and crack propagation characteristics of the mudstone under the influence of loading rate effects. The research results support the following findings: (1) With increasing loading rate, the peak strength and elastic modulus of weakly cemented mudstone significantly increase, while the peak axial strain and peak radial deformation significantly decrease. (2) With increasing loading rate, the stress required to trigger the expansion of weakly cemented mudstone gradually increases, and a significant power-law relationship arises between the strain of the mudstone at the start of expansion and the loading rate. (3) With increasing loading rate, the acoustic emission ringing count of weakly cemented mudstone increases: The failure of weakly cemented mudstone changes from small-range progressive failure to sudden failure, and the failure mode transitions from shear failure to tensile‒shear composite failure. (4) The studied mudstone damage variables increase with increasing loading rate, following an approximate exponential function. The conclusions obtained in this work can provide a theoretical basis for the evolution mechanism and control of floor heave in deep roadway mining.

An experimental and computational investigation of discharge mode transitions in a partially magnetized radio frequency capacitively coupled oxygen discharge

AbstractA magnetized capacitively coupled oxygen plasma was studied synergistically using phase resolved optical emission spectroscopy and particle‐based kinetic simulation. Discharge mode transitions from ambipolar mode into drift mode and finally into α mode induced by increasing the magnetic field were observed at different driving frequencies and different electrode gaps. The simulation results demonstrate that the discharge operating in the same mode exhibits a similar degree of electronegativity. By increasing driving frequency or electrode gap, the same discharge mode transition tends to occur at a lower magnetic field, and, meanwhile, the high electric field and electron power absorption shift from the bulk region to the sheath edge.

Unsteady flow characteristics in an over-under TBCC inlet during mode transition under unthrottled and throttled conditions

The study presents an experimental exploration into the mode transition of an over-under TBCC (Turbine-Based Combined Cycle) inlet, with a specific emphasis on the flow characteristics at off-design transition Mach number. A systematic investigation was undertaken into the mode transition characteristics in both unthrottled and throttled conditions within a high-speed duct, employing high speed Schlieren and dynamic pressure acquisition systems. The results show that the high-speed duct faced flow oscillations primarily dictated by the separation bubble near the duct entrance during the downward rotation of splitter, leading to the duct’s unstart under the unthrottled condition. During the splitter’s reverse rotation, a notable hysteresis of unstart/restart of the high-speed duct was observed. Conversely, hysteresis vanishes when the initial flowfield nears the critical state owing to downstream throttling. Moreover, the oscillatory diversity, a distinctive characteristic of the high-speed duct, was firstly observed during the mode transition induced by throttling. The flow evolution was divided into four stages: an initial instability stage characterized by low-frequency oscillations below 255 Hz induced by shock train self-excitation oscillation and high-frequency oscillations around 1367 Hz caused by the movement of separation bubble. This stage is succeeded by the “big buzz” phase, comprised of pressure accumulation/release within the overflow-free duct and shock motion outside the duct to retain dynamic flow balance. The dominant frequency escalated with the increase of the internal contraction ratio in the range of 280 Hz to 400 Hz. This was followed by a high-frequency oscillation stage around 453 Hz dominated by a large internal contraction ratio with low pulsating energy, accompanied by a continuous supersonic overflow. Lastly, as the splitter gradually intersected the boundary layer of the first-stage compression surface, the capture area and the turbulence intensity of the incoming flow underwent a sudden shift, leading to a more diverse flow oscillation within the duct, manifested as various forms of mixed buzz.

Reinforcement Learning-Based Control Sequence Optimization for Advanced Reactors

The last decade has seen the development and application of data-driven methods taking off in nuclear engineering research, aiming to improve the safety and reliability of nuclear power. This work focuses on developing a reinforcement learning-based control sequence optimization framework for advanced nuclear systems, which not only aims to enhance flexible operations, promoting the economics of advanced nuclear technology, but also prioritizing safety during normal operation. At its core, the framework allows the sequence of operational actions to be learned and optimized by an agent to facilitate smooth transitions between the modes of operations (i.e., load-following), while ensuring that all safety significant system parameters remain within their respective limits. To generate dynamic system responses, facilitate control strategy development, and demonstrate the effectiveness of the framework, a simulation environment of a pebble-bed high-temperature gas-cooled reactor was utilized. The soft actor-critic algorithm was adopted to train a reinforcement learning agent, which can generate control sequences to maneuver plant power output in the range between 100% and 50% of the nameplate power through sufficient training. It was shown in the performance validation that the agent successfully generated control actions that maintained electrical output within a tight tolerance of 0.5% from the demand while satisfying all safety constraints. During the mode transition, the agent can maintain the reactor outlet temperature within ±1.5 °C and steam pressure within 0.1 MPa of their setpoints, respectively, by dynamically adjusting control rod positions, control valve openings, and pump speeds. The results demonstrate the effectiveness of the optimization framework and the feasibility of reinforcement learning in designing control strategies for advanced reactor systems.

Wake interference effects on flow-induced vibration of flexible membrane wings

This work investigates the effect of wake interference on the nonlinear coupled dynamics and aerodynamic performance of flexible membrane wings at a moderate Reynolds number. A high-fidelity computational aeroelastic framework is employed to simulate the flow-induced vibration of flexible membrane wings in response to unsteady vortex wake flows produced by an upstream stationary circular cylinder. The coupled dynamics of the downstream membrane are investigated at different gap ratios, aeroelastic numbers, and offset distances. The variations in flow features, membrane responses, and frequency characteristics are analyzed to understand the wake interference effect on membrane aeroelasticity. The results indicate that the aerodynamic performance and flight stability of the downstream membrane are degraded under the wake interference effect. Four distinct flow regimes are classified for the cylinder–membrane configuration, namely (i) single body flow, (ii) co-shedding I, (iii) co-shedding II, and (iv) detached vortex-dominated vibration, respectively. The mode transition is found to build new frequency synchronization between the flexible membrane and its own surrounding flows, or the wake flows of the cylinder, to adjust the aerodynamic performance and membrane vibration. This study sheds new light on membrane aeroelasticity in response to wake flows and enhances understanding of the fluid–membrane coupling mechanism. These findings can facilitate the development of next-generation bio-inspired drones that have high flight efficiency and robust flight stability in gusty flows.

Avoiding heat source calibration for finite element modeling of the laser powder bed fusion process

The melt pool characteristics govern process stability and microstructure development during Laser Powder Bed Fusion (LBPF). The melt pool is strongly affected by the laser parameters and based on the melt pool depth-to-width ratio, three melting modes can be distinguished: conduction, transition, and keyhole mode. Finite element (FE) simulations have been extensively used to investigate the cyclic thermal history of the LPBF process and the effects of different process parameters on the part quality. However, the heat sources applied in such simulations can only accurately predict the melt pool behavior, and thus the temperature profile, when processing in conduction mode. As for the transition and keyhole regime modeling, heat source parameters or material properties are usually adjusted to make the simulated melt pool dimensions match the real values. This study proposes a novel approach to bypass the conventional heat source calibration strategy to simulate the LBPF process across different melting regimes. The predicted melt pool dimensions are validated for a wide range of process parameters applied to three different alloys: Ti-6Al-4 V, 316 L austenitic stainless steel (316 L), and 2507 super duplex stainless steel (SDSS). The average errors obtained on the melt pool width, depth, length, and aspect ratio are 9.7 %, 12.4 %, 14.5 %, and 14.3 % respectively. These results demonstrate how the proposed approach can capture the melt pool dimensions throughout different materials and melting modes eliminating the time-consuming heat source calibration steps and facilitating the LPBF modeling workflow.

A highly sensitive temperature sensor based on long period fiber grating coated with polymer-bilayer-membrane

This study introduces a highly sensitive temperature sensor based on long-period fiber grating (LPFG). The sensor is composed of a long period fiber grating covered by a higher RI (RI) layer of polyvinyl alcohol (PVA) and a lower RI layer of polydimethylsiloxane (PDMS). The PVA layer enables the sensor to operate in the mode transition region with high sensitivity, and the PDMS layer further enhances the temperature sensitivity owing to its high thermos-optic coefficient. In addition, the sensor can mitigate the influence of humidity due to the hydrophobic properties of PDMS. Experimental results show that the temperature sensitivity can achieve to 1232.3 pm/℃ within the range of 20℃-100℃ for an LPFG with a period of 363 μm coated with a 265 nm PVA layer and a 20 μm PDMS layer.

The relationship between threshold power of wave mode transition and tube radius in argon helicon discharge

We report in this Letter a geometric similarity of threshold radio frequency (RF) power absorbed by the helicon wave coupled mode in argon helicon discharge. Helicon plasma was excited by a half-helical antenna in tubes of different radii at a pressure of 0.3 Pa. Plasma density was measured by using a Langmuir probe, and wave structure was measured by a B-dot probe. The results show that the threshold powers for triggering helicon wave mode are proportional to the tube cross sections, and the plasma densities at the transition point are the same, indicating a geometric scaling law between the RF power and the tube size. This similarity of threshold power might provide a comparison of results in different systems.

Dynamic flow behaviors of an inlet isolator in embedded rocket-driven mode transitions

Dynamic flow behaviors of an inlet isolator in embedded rocket-driven mode transitions

Kinetic simulations of capacitively coupled plasmas driven by tailored voltage waveforms with multi-frequency matching

Impedance matching is crucial for optimizing plasma generation and reducing power reflection in capacitively coupled plasmas (CCP). Designing these matchings is challenging due to the varying and typically unknown impedance of the plasma, especially in the presence of multiple driving frequencies. Here, a computational design method for impedance matching networks (IMNs) for CCPs is proposed and applied to discharges driven by tailored voltage waveforms (TVW). This method is based on a self-consistent combination of particle in cell/Monte Carlo collision simulations of the plasma with Kirchhoff’s equations to describe the external electrical circuit. Two Foster second-form networks with the same structure are used to constitute an L-type matching network, and the matching capability is optimized by iteratively updating the values of variable capacitors inside the IMN. The results show that the plasma density and the power absorbed by the plasma continuously increase in the frame of this iterative process of adjusting the matching parameters until an excellent impedance matching capability is finally achieved. Impedance matching is found to affect the DC self-bias voltage, whose absolute value is maximized when the best matching is achieved. Additionally, a change in the quality of the impedance matching is found to cause an electron heating mode transition. Poor impedance matching results in a heating mode where electron power absorption in the plasma bulk by drift electric fields plays an important role, while good matching results in the classical α-mode operation, where electron power absorption by ambipolar electric fields at the sheath edges dominates. The method proposed in this work is expected to be of great significance in promoting TVW plasma sources from theory to industrial application, since it allows designing the required complex multi-frequency IMNs.

Influence of inhomogeneous distribution of inflow equivalence ratio on oblique detonation morphology and characteristic

In this paper, numerical simulations using Euler equations coupled detailed chemical reaction model are performed to reveal the influence of inhomogeneous distribution of inflow equivalence ratio (ER) on the morphology and characteristic of oblique detonation in hydrogen/oxygen/argon mixtures. The purpose of this study is to better understand the key parameters’ variation law of oblique detonation flow field under practical flight conditions so as to guide the design of oblique detonation chamber. Within the scope of our simulations, the results show that the oblique detonation wave (ODW) can still be standing under a large ER gradient. The thermodynamic state and characteristic sizes of the flow field reach the maximum value around ER = 0.8. First, the ODW angle and the post-wave temperature/pressure increase with the homogeneous inflow ER. Then, the inhomogeneity of inflow ER is introduced by assuming a lateral linear distribution covering the whole inflow boundary. When the ER increases along the inflow boundary with ER = 0 at the wedge tip, the overall morphology of the ODW presents a concave structure. Inversely, the ODW is convex with ER = 0 at the top outlet. The morphology and characteristic sizes of ODW are determined by the mixture composition in front of the corresponding wave surface. The transition mode of ODW is mainly determined by the ER of the incoming flow in front of the induction region.

Experimental study on the discharge characteristics of an air rotating gliding arc

In this study, the discharge characteristics of an air rotating gliding arc (RGA) are investigated by synchronous measurements of a digital oscilloscope and a high-speed camera, and the emission spectrum measurement. The discharge evolution in one complete motion cycle exhibits a ‘breakdown-elongation-extinction’ process accompanied by a jump phenomenon of the arc root and a back-breakdown phenomenon. The discharge evolves from the unstable breakdown mode (U-B), to the transition mode and finally to the stable gliding mode (S-G) by increasing the input voltage or decreasing the tangential and axial gas flow rates. The U-B mode at an input voltage of 120 V is featured by the large reduced electric field and high electron temperature of 1.90 eV, but the arc length and existence time are very short. The S-G mode at an input voltage of 270 V has a relatively low breakdown frequency of 0.33 kHz and an average breakdown current of 1.31 A, implying that the arc steadily glides and rotates along the spiral electrode. The average electron temperature is 0.64 eV in the S-G mode, while the arc length and existence time are longer. The rotational and vibrational temperatures of the N2(C3∏u) state are respectively measured at 2200 K and 4400 K in the U-B mode, and in the S-G mode are 2600 K and 4820 K. From the evolution of emission intensities of measured excited species, it is found that the NOγ band emission intensity generally rises from the U-B mode to the S-G mode since the gas temperature and arc existence time rise, indicating that the S-G mode may be beneficial for the vibrationally promoted Zeldovich reactions. This study could deepen the understanding of arc characteristics in air RGA for selecting a suitable mode to achieve better plasma performance in practical applications.

Peridynamic fracture analysis of film–substrate systems

When subjected to mechanical, thermal, or other loads, film–substrate systems may undergo complex cracking behaviors, which encompass film and substrate cracking, interfacial debonding, and their combinations, exhibiting rich fracture patterns, such as three-dimensional helical cracks. Identifying the mechanisms underlying these fracture phenomena may lead to more advanced strategies for technologically significant applications. In this paper, we develop an interfacial cohesive peridynamic method for fracture analysis of multiple-phase materials. Particularly, we focus on the modeling of coupled film cracking and interfacial debonding in film–substrate systems. By introducing cohesive interfacial bonds to describe the mechanical properties of the interfaces and adopting a displacement-based cohesive failure criterion, the model is able to predict the critical condition and path of interfacial crack propagation. The robustness of the interfacial cohesive peridynamic method is validated through a series of representative examples. We also demonstrate its efficacy in simulating three-dimensional cracks and identify the essential role of the interfacial energy release rate in controlling the cracking mode transition from a restricted pattern to a helical pattern. The numerical predictions of cracking paths and stress distributions agree well with the previous experimental results. This study provides a valuable tool for analyzing different cracking patterns in film–substrate systems and composite materials.

High temperature tensile properties and deformation behavior of CoCrFeNiW0.2 high entropy alloy

High entropy alloys (HEAs) have been a new-type of structural materials, which show good performance in various extreme conditions. In the present study, we systematically investigated the high temperature deformation behavior of as-cast CoCrFeNiW0.2 HEA under different temperatures from 298 to 1173 K and various strain rate of 1 × 10−4 s−1 to 5 × 10−3 s−1. An obvious temperature dependence of yield strength can be found with a remarkable decrease from 362 MPa at 298 K to 119 MPa at 1173 K. The normal Portevin–Le Chatelier (PLC) effect was presented in the intermediate temperature range of 673 K to 1073 K, the serrated flow transformed in the sequence of A → A + B → A + C → B + C → C with the increasing temperature, which is caused by dynamic strain aging (DSA). W atoms played an important role in the interaction between solute atoms and dislocations. Fractography analysis suggests the failure mode transition from transgranular fracture to intergranular fracture. Electron backscatter diffraction (EBSD) demonstrates that the high temperature deformation mechanism of the HEA is dominated by dynamic recovery (DRV). Additionally, transmission electron microscopy (TEM) was performed to observe the dislocation configurations and the interaction between dislocations and μ phase precipitates at various temperatures, and these results further identify the high temperature deformation mechanism. To summarize, this study indicates the proposed HEA has excellent high temperature mechanical properties and a wide range of potential applications in many fields.

Behavioral differences between young adults and elderly travelers concerning the crowding effect on public transit after the COVID-19 pandemic

After the COVID-19 pandemic, passengers' concerns about crowding on public transit vehicles has increased due to their fear of being infected. Since WHO has announced that the elderly are the most vulnerable to COVID-19, it is important to understand the impact of crowding on the elderly and provide policy implications on public transit vehicles. The aim of this study is to analyze the behavioral differences between young adults and the elderly concerning the crowded conditions on public transit with a stated preference survey in Seoul metropolitan area. Crowding multipliers are estimated to quantify the impedance on crowding based on the mixed logit model. The results indicate that the elderly are more sensitive to crowding on public transit vehicles than younger people, and subway passengers are affected more by crowding than passengers who are using other modes of transit. In addition, it is confirmed that the preference of the elderly for public transit varies according to gender, income, and the purpose of travel. These analysis results can provide implications for setting new policy directions to improve the public hygiene and comfort of elderly people when they use public transit in the post-COVID-19 world.

Experimental study on the flexural behaviors of ECC-reinforced masonry beams with GFRP mesh

This study aims to investigate the flexural behaviors of ECC-reinforced masonry beams with GFRP mesh under various interface treatment methods. Eleven sets of ECC-reinforced masonry beams with GFRP mesh were fabricated and subjected to a four-point bending test. The effects of the fibre volume fraction and thickness of ECC, interface treatment methods, GFRP mesh strength and layers on the flexural behavior of masonry beams were investigated and analyzed, including the failure mode, crack distribution, flexural strength, and load-deflection curves. The test results revealed that a transition in the failure mode of masonry beams from brittle failure to interfacial stripping failure or ductile failure after reinforcement. Increasing the number of GFRP mesh layers, the fibre volume fraction and thickness of ECC improved the flexural performance of the masonry beams, while increasing the GFRP mesh strength had the opposite effect. Among the variables, the interface treatment method exerted the greatest influence on the flexural performance of the masonry beams. The masonry beams treated with a bolt interface exhibited the best flexural performance, with flexural toughness and ultimate deflection increasing by 1.69 times and 0.32 times, respectively. Furthermore, the flexural performance of the masonry beams reinforced with a 15 mm thick ECC and single-layer GFRP mesh demonstrated similar reinforcement effects to the masonry beams reinforced with a 20 mm thick ECC, while reducing costs by 23 %. Based on the existing codes governing the calculation method of flexural strength and considering the influence of various factors, a calculation model for the flexural strength of ECC-reinforced masonry beams with GFRP mesh was established.