Transition Criteria Research Articles

Friction-induced vibration of a stick–slip oscillator with random field friction modelling

In this paper, stochastic planar stick–slip motions are investigated using a slider-on-belt model where the coefficient of friction (COF) of the contact interface is modelled as a random field. New three-variable stick–slip transition criteria are proposed to improve the accuracy and robustness of the algorithm. Stochastic analyses are performed concerning the Peak-to-Valley value of the displacement and friction forces and the time duration of the stick state. It is found that the correlation length of the COF random field is dominantly responsible for the stochastic behaviours of the system. In contrast, the belt velocity and the mean value of the COF have a significant influence on the time duration of the stick state compared with the corresponding deterministic slider-on-belt model.

An improved chaotic recognition method for weak signal frequency and its application to fault diagnosis of planetary gearboxes

Planetary gearboxes are very prone to failure when they are used in low-speed and heavy-load conditions for a long time. Due to strong background noise in the industrial field and the signal decay in the process of fault signal transmission, the weak fault characteristics of planetary gearboxes are submerged by noise and difficult to extract and identify. Aiming at this problem, an improved chaos detection method for weak signal frequency recognition is proposed. In this paper, the maximum weighted kurtosis is selected as the fitness function, and the Aquila optimizer algorithm is used to find the optimal modal number K and penalty factor α of variational modal decomposition. According to the grey relational degree, an inherent modal function (IMF) containing rich fault features is selected to reduce the noise of the original signal. Further, a double-coupled Duffing oscillator equation is constructed, and a quantitative criterion for chaotic oscillator phase transition based on the standard deviation weighted-average Euclidean distance is proposed. Based on the reverse detection method, the sideband growth of the most sensitive IMF after standardization is detected to determine the health state of the planetary gearbox. Detecting the fault characteristic frequency in the most sensitive IMF after normalization and Hilbert transform processing is based on the forward detection method in order to determine the fault type of the planetary gearbox. The effectiveness of the proposed method is verified by simulation and experiment. The results show that the proposed method successfully identifies the weak fault characteristics of the planetary gearbox, which fully shows that this method has an excellent diagnostic effect for planetary gearbox faults with rich frequency fault characteristics, and provides a new method for the diagnosis and identification of weak faults in planetary gearboxes in engineering practice.

A molecular dynamics study of evaporation of multicomponent stationary and moving fuel droplets in multicomponent ambient gases under supercritical conditions

The evaporation of a six-component fuel droplet under supercritical conditions is investigated using molecular dynamics (MD) simulations. The focus here is on effects of multicomponent ambient gases and the relative motion between the droplet and the ambient. The ambient pressure ranges from 8 MPa to 36 MPa and the ambient temperature ranges from 750 K to 3600 K. In the lower range of the temperature and pressure, the average displacement increment (ADI) per fuel atom gradually increases with time and the classic evaporation is observed. In the higher range of the temperature and pressure, the ADI profile has a unimodal distribution with time and the diffusive mixing between the droplet and the ambient gases dominates. Based on the ADI profile of fuel atoms, a criterion (τ0.9P) for mode transition from evaporation to diffusion is proposed. Among the ambient gases investigated, the mode transition is the most difficult in the nitrogen ambient but the easiest in combustion exhaust gases. For multicomponent fuel droplets close to or in diffusion mode, with higher relative velocities, the relative difference between evaporation rates for light/heavy fuel components is reduced. This study demonstrates that supercritical conditions alone are insufficient for mode transition of evaporation.

Transport-Based Transition Prediction for the Common Research Model Natural Laminar Flow Configuration

This paper presents the application of a new -based one-equation transition transport model to the Common Research Model Natural Laminar Flow Configuration (CRM-NLF), which was the central test case of the 1st AIAA CFD Transition Modeling and Prediction Workshop. The new model uses a simplified formulation of the Arnal, Habiballah, and Delcourt transition criterion that is stability based. The model is formulated in a grid-point-local manner and is coupled to the Menter shear stress transport eddy-viscosity turbulence model. The flow conditions applied are exactly those that were reported from the National Transonic Facility wind tunnel test campaign. The comparisons of the results yielded by the new model in terms of the transition fronts along the wing span show a very good match with the transition fronts derived from the temperature-sensitive paint images taken during the wind tunnel test and with computational result from an -based Two--factor strategy using critical -factors for Tollmien–Schlichting and crossflow instabilities and applying a local linear stability code.

Dimensionless Analysis of the Effects of Junction Angle on the Gas-Liquid Two-Phase Flow Transition and the Scaling Law of the Microbubble Generation Characteristics in Y-Junctions

Gas-liquid two-phase flow patterns and gas slug hydrodynamics were experimentally studied in three Y-junctions with different junction angles of 60°, 90° and 120°. Microbubbles were generated in the sodium alginate aqueous solution with the surfactant Tween20. Four main flow patterns were observed, i.e., stratified flow, annular flow, dispersed bubble flow and slug bubble flow. The formation mechanism of the bubble flow was explained by a force analysis, which was based on the dimensionless analysis regarding Capillary number, Weber number and Euler number. The transition criteria of the gas-liquid two-phase flow patterns was set up by these three dimensionless numbers. Additionally, the characteristics of the slug bubble were investigated, which made a scaling criterion for eliminating the influence of the angle factor become possible. A new scaling law (validity range within 2.88 < Re1 < 14.38, 0.0068 < We1 < 0.1723) was proposed to predict the bubble size and it showed a good agreement with the experimental results.

Transitional model for rarefaction-driven Rayleigh−Taylor mixing on the diffuse interface

Large eddy simulations are performed to explore transitional characteristics with parametric variations under initial conditions for rarefaction-driven Rayleigh–Taylor mixing flows and to develop a transitional model for the prediction of transition onset. Three-dimensional, multi-mode perturbations imposed on the diffuse interface are excited by a rarefaction wave, resulting in a large but non-constant acceleration. Initial conditions investigated here profoundly impact characteristic transition times through the variation in the net contributions between the advection and pressure components dominated in the laminar regimes, making the spatial transition criteria sensitive and questionable as well as the temporal transition criteria insensitive and feasible. Therefore, a new transition model originating from empirical correlations and ignoring physical details is calibrated and tested in a series of numerical realizations, and it is certified that the modeling framework is sufficiently flexible, accurate, and robust and can be a promising strategy for handling complex transitional phenomena.

Molecular dynamics investigation of the vaporization characteristics of n-alkane blended fuels under different ambient conditions

Molecular dynamics (MD) simulation is a powerful tool to reveal the microscopic characteristics of supercritical transitions. However, the accuracy of MD depends strongly on the potential model that describes the interaction forces between atoms. In this study, four commonly used potential models for long-chain n-alkanes in MD simulations are evaluated, and a hybrid model is introduced. The vaporization and phase-transition characteristics of n-alkane blended fuels with different mole fractions are then explored under a wide variety of ambient conditions by using the hybrid model. Compared to the commonly used potentials, the hybrid model shows higher accuracy for predicting the thermodynamic and transport properties. In subcritical environments, vaporization belongs to typical two-phase evaporation with a sharp gas–liquid interface. The preferential evaporation of the light-end component is obvious, and the evaporation rate of the heavy-end component is maximized after the light-end component is consumed. Under supercritical conditions, the interface dissolves rapidly, the evaporation rates for both the light- and heavy-end components increase simultaneously, and both components coexist throughout the evaporation process. Based on the maximum potential energy and evaporation rate, a new criterion for the supercritical transition is proposed. The dimensionless transition time, which reflects the proportion of the sub/supercritical stage within the lifetime, is nearly independent of the ambient temperature and fuel composition; instead, it mainly depends on the ambient pressure. Finally, an empirical formula is obtained by curve-fitting to describe the variation in the dimensionless transition time with ambient pressure.

Modeling and validation of three‐dimensional sliding‐rocking rigid block subjected to earthquake excitation

AbstractA freestanding rigid block subjected to earthquake excitation is a problem of significant interest to the earthquake engineering community because it has many direct applications in the seismic performance evaluation of both structural and non‐structural components. Previous studies on this classical problem usually assume planar rocking is the only motion that can be activated. However, this is clearly not true when friction coefficient is not sufficiently large or the earthquake excitation is bi‐directional. To address this knowledge gap, this paper presents a three‐dimensional sliding‐rocking rigid body model suitable for dynamic analysis. Important aspects for this model, which include the formulation of nonlinear equations of motion, transitional criteria for different motion modes, as well as the approach to handle the impact, are described. Validation of the proposed model consists of two parts. The first part is the comparison with the numerical prediction from a different model available in the literature. The second part is the comparison with the experimental study from a series of shake table tests using vision‐based measurement. These comparisons suggest the proposed model provide relatively good accuracy. Finally, a case study of overturning probability of a non‐structural component shows that using a three‐dimensional modeling approach can give substantially different result compared to a two‐dimensional rocking model, which highlights the necessity of adopting such model.

Effect of Oil Viscosity on Flow Pattern Transition of Upward Gas-Oil Two-Phase Flow in Vertical Concentric Annulus

Summary Gas-oil two-phase flow usually occurs during gas influx into the wellbore annulus of deepwater drilling with oil-based drilling fluid or gas-oil production from the wellbore annulus. A flow pattern transition often is used in well control or in production parameter design. Related studies primarily have focused on gas-water two-phase flow, which is significantly different from gas-viscosity oil two-phase flow. We conducted experiments on a facility that included a flow section that was 13 000 mm long, with a 60-mm inner pipe and a 100-mm outer pipe in the annulus. A range of oil viscosities from 16 to 39 mPa∙s has been studied. The superficial gas and oil velocities varied from 0.55 to 17.077 m/s and from 0 to 0.414 m/s, respectively. A flow pattern was identified based on visualized analysis using electrical capacitance volume tomography (ECVT) and void faction wave analysis. Flow pattern maps of different viscosities were plotted based on measured data. Transition criteria based on Froude number and Archimedes number were established. The new model was compared with gas-water two-phase flow transition models and validated with experimental data. The effect of viscosity on flow pattern transition was revealed. The transition boundary of the bubble to slug flow and slug to churn flow both moved in the direction of smaller superficial gas velocity with an increase in oil viscosity. There may exist a critical viscosity value when churn flow transited to annular flow. When the viscosity was lower than this critical point, the above result was the same for churn to annular flow when the superficial oil velocity was low. With an increase in the superficial oil velocity, however, the boundary gradually changed to high superficial gas velocity as the oil viscosity increased. When the viscosity was larger than the critical point, the oil viscosity had a slight influence on the transition.

Predictors of Weight Gain in Under Five Children With Severe Acute Malnutrition: An Analysis of the Icddr, B Hospital Dataset

ObjectivesChildren admitted to hospital with severe acute malnutrition (SAM) and acute illness can be challenging to nutritionally rehabilitate. There is limited understanding on predictors of weight gain during hospitalization in this vulnerable population. This work aimed to predict the weight gain in children using anthropometric, biochemical, clinical, and socio-demographic variables.MethodsWe included 5,044 children aged 0–59 months with SAM hospitalized in the Dhaka Hospital at icddr, b between 2011 and 2019. Surveillance data was collected during hospitalization and analyzed retrospectively. The 15% weight gain from hospital admission to discharge was considered as outcome because it is recommended as the transition criteria from facility to community-based management. We trained a Random Forest classifier to identify the best set of predictors of a 15% weight gain. A total of 78 features were considered. The developed diagnostic model was validated based on the area under the curve (AUC) between the true positive and the false positive rates.ResultsThe classification of data based on the outcome (weight gain > 15%) created unbalanced classes, a larger group with < 15% changes in weight and a very small group with > 15% weight gain. To balance this data disparity, we finally included 263 children in this analysis. A model including 197 children (75% of the dataset) was identified in the training dataset, while the rest were used as a test dataset. Validation in the test dataset revealed an AUC of 69.05% when considering all 78 predictors. Among the top predictors were mid-upper arm circumference at admission, family income and breastfeeding duration.ConclusionsThis analysis revealed the role of socio-economic status as well as the importance of breastfeeding practices in attaining 15% weight gain from hospital admission to discharge in under five children treated for SAM. This finding has important implications for future work regarding childhood feeding practices and community-based detection of children with SAM.Funding SourcesJoannah and Brian Lawson Center for Child Nutrition, Ontario Graduate Scholarship, Canadian Institutes of Health Research Healthy Cities Research Initiative, and icddr, b, Dhaka, Bangladesh.

Prognostic Accuracy of DSM-5 Attenuated Psychotic Symptoms in Adolescents: Prospective Real-World 5-year Cohort Study

IntroductionThere is limited research in adolescent at risk for psychosis. The new criteria of Attenuated Psychosis Syndrome (APS) of Diagnostic and Statistical Manual of Mental Disorders- 5 (DSM-5) have not been validated.ObjectivesThe aims of this study were to: 1) characterize adolescent’s profile with APS (DSM-5 APS) compared to adolescents with early onset psychosis (EOP) and with other psychiatric disorders (non-APS); 2) to estimate their long-term risk of transition to psychosis and prognostic accuracy of DSM-5 APS.Methods243 adolescents, aged 12-17, were included (October 2012- July 2019) and dived in three sub-groups (110 DSM-5 APS, 31 EOP, 102 non-APS). All underwent a comprehensive assessment evaluating: sociodemographic characteristics, family and personal history of any DSM-5 psychiatric disorders, psychopathological assessment and level of functioning. An annual follow-up evaluation was carried out (up to 7 years) including a clinical interview to investigate DSM-5 criteria for transition to psychosis.ResultsDSM-5 APS adolescents had on average higher comorbid disorders (2.3) and intermediate psychopathological and functioning profile between non-APS/EOP. The cumulative risk of transition at 1,2,3, 4-5 years was 13%, 17%, 24.2%, 26.8% and 26.8% in DSM-5 APS group, 0%, 0%, 3.2%, 3.2% and 3.2% in the non-APS. The 5-year prognostic accuracy of the DSM-5 APS in adolescent was adequate (Area Under the Curve=0.77) with high sensitivity (91.3%) and suboptimal specificity (63.2%).ConclusionsThe DSM-5 APS diagnosis can be used to detect help-seeking adolescents at risk of psychosis and predict their long-term outcomes, leading the way to new preventive approaches.DisclosureThe authors declare that they do not have a significant financial interest, consultancy or other relationship with products, manufacturer(s) of products or providers of services related to this abstrac.

Finite deformation continuum model for mechanically induced phase transition in transition metal dichalcogenide monolayers

Tuning the electronic properties through phase engineering of two-dimensional transition metal dichalcogenides (TMDCs) is promising for their applications in electronic devices, energy conversion, and so on. Here, we establish a phase-field continuum mechanics model that accounts for both the finite deformation and mechanically induced phase transition of monolayer molybdenum disulfide (MoS2) and molybdenum ditelluride (MoTe2). Informed by first-principle calculations, our model can accurately describe not only the nonlinear mechanical behavior but also the phase transition criteria and processes driven by the mechanical energy. Applied to the nanoindentation tests on MoS2 drum specimens, the model reproduces well the force vs. depth curves and provides rich details on how the phase transition initiates and develops near the contact region. Further, based on the mechanical instability analysis, our study suggests that MoS2 monolayers under indentation tests would fail prematurely due to the mechanical instability in shear mode before the phase transition completes. In comparison, MoTe2 can finish the phase transition without worrying about mechanical instability, as observed in the indentation experiments. The model and methodology developed herein would serve as a powerful tool to guide the phase engineering of two-dimensional TMDCs.

A Galilean Invariant Variable–Based RANS Closure Model for Bypass and Laminar Separation Bubble–Induced Transition

A one-equation Reynolds-averaged Navier-Stokes (RANS) closure model has been established for bypass transition and laminar separation bubble (LSB)–induced transition. A new local indicator is proposed to describe the influence of turbulence intensities and pressure gradients, which makes the model Galilean invariant. Based on this new indicator, a novel and efficient transition criterion is formulated. For LSB-induced transition, an empirical correlation is developed to modify the intermittency factor and control the size of separation bubbles. Several flow cases, including flat plates with various pressure gradients, flat plates with LSBs, semicircular leading-edge flat plates, National Advisory Committee for Aeronautics (NACA) 0018 airfoil, and SD7003 airfoil, are employed for the model verification. The predictions show good agreement with the experimental data and large-eddy simulation data for different inlet conditions, which indicates the increment of free-stream turbulence intensity leads to the reduction of the size of laminar separation bubbles.

Just transition principles and criteria for food systems and beyond

In this article, we propose a framework of principles and criteria for just transitions in food systems. Climate mitigation activities are urgently needed in food systems, but can have damaging social, environmental, economic, and health impacts. Consequently, food system transitions can cause significant side effects across and beyond food systems, aggravating existing inequalities and unsustainabilities, causing new ones, or hampering equal engagement in the transition itself. Thus, justice questions stand at the core of assessing decarbonization pathways and policies and must link to other sectors as well: Who bears the costs and who enjoys the benefits of the transitions? Can transitions be inclusive, leaving no one behind? We examine the establishment and purpose of general principles and food system-specific criteria for just transition, present the framework – standards for judging, evaluating, and deliberating justice in food system transitions – and reflect upon the uses of the framework and future developments.

Theoretical analysis of bubble nucleation in liquid film boiling

Many industrial applications such as electronic cooling and thermal desalination have adopted the heat dissipation technologies of capillary evaporation and liquid film boiling, with bubble nucleation being the transition criterion between them. In this paper, a theoretical model for bubble nucleation in liquid film boiling is developed, which mainly considers three aspects: transient heat conduction inside the liquid film supported with porous wick, evaporation at the liquid-vapor interface, and superheat requirement for the stable existence of a bubble nucleus. The criterion for the onset of nucleate boiling (ONB) in liquid film boiling is established, and size range of active microcavities for bubble nucleation is obtained, as well as comparisons being made with that during pool boiling and flow boiling. The results indicate that the size range of active microcavities in liquid film boiling is narrower with the minimum size being several times larger, and bubble nucleation in liquid film boiling is much more difficult. Although superhydrophilic surface with small contact angle is disadvantageous to bubble nucleation, it will decrease the minimum size of effective microcavities radius within the size scope of a few micrometers, which is beneficial for ONB in liquid film boiling. Moreover, critical thickness to trigger ONB in liquid film boiling is found to decrease with increasing contact angle, augmenting superheat and strengthening effective thermal conductivity of the wick, and its theoretical prediction is validated with experimental data in the literature. Our model offers a new avenue for understanding the ONB in liquid film boiling, and therefore can serve as a guideline for future enhancement design.

A continuum damage model for prediction of crack initiation life of pitting corrosion and fatigue

Corrosion fatigue damage can lead to degradation of engineering structures and components. As a major stage of the total corrosion fatigue life, the crack initiation of pitting corrosion and fatigue is studied. A continuum damage mechanics model for predicting the crack initiation life of pitting corrosion fatigue is presented based on a pure fatigue damage model and a fracture-mechanics-based pit-to-crack transition criterion. The synergistic effect of pitting corrosion and fatigue are quantified as a time-varying amplification of the fatigue stress field. The proposed model is validated by corrosion fatigue test data of two different metallic materials. The predicted curves of corrosion fatigue crack initiation life are in good agreement with the test data. The model demonstrates a good prediction of corrosion fatigue crack initiation life, i.e., corrosion fatigue life is always lower than fatigue life, and the lower the stress, the more significant the life reduction.

Hypermassive quark cores

ABSTRACT Using a quantum hadrodynamics (QHD) and MIT-based models, we construct hybrid stars within the Maxwell criteria of hadron-quark phase transition. We are able to produce a hybrid star with maximum mass of 2.15 M⊙. Furthermore, a 2.03-M⊙ star with a quark core corresponding to more than $80{{\ \rm per\ cent}}$ of both, its total mass and radius, is also possible.

Thermal characteristic and spatial morphology between electrode and phase changing ice during de-icing process of dielectric barrier discharge and critical behavior of the surface charge density

Dielectric barrier discharge (DBD) plasma recently has been demonstrated as an effective in situ heating methodology in anti-ice and de-ice applications of the aircraft. However, the thermal stability of this special heat source during the interactions between the plasma and the ice block may pose serious concerns. In this paper, it is found that when the ice blocks of different characteristic locations covered the DBD device with alternative current (AC) power, two different kinds of interaction processes can be resulted: First, the concentration of the heat flow through the local enhancement of the streamers between the electrode and the ice, long-lasting for the glaze ice ended with the partial evaporation of the melted ice block with larger separations, or short-lived intermittent for the rime ice until the boundary is beyond the reach. Second, sparks, very likely to be generated between the electrode and the ice block when the separation and electric field meet the transition conditions, resulting in the fast evaporation of the melted ice block and the severe aging or on-sight burning of the devices, even at the operation voltage safe for the AC-DBD plasma without ice blocks. It is emphasized that the coexistence or the number of the ice blocks of different separations does not alter such behavior. Supported by the calculation of the streamer-to-spark transition criterion, and affected by ice phase change, it is argued that the de-icing AC-DBD plasma is a strong nonlinear heat source, resulting from the surface charge accumulation, melting of the ice blocks, and the enhancement of ionization collision frequency due to the mixing of the evaporated water molecules. A new critical condition, the critical surface charge density was proposed for the initiation of spark, ρsc: Densities higher than ρsc will lead to the catastrophic spark, while the lower will lead to the selective concentration of the heat flow for the faster de-icing process. Due to the random distribution nature of the electrode-to-ice separations during the real-world practice of an AC-DBD heat source for de-icing, the existence of ρsc may be under further scrutiny and considered as a crucial factor for the stable operation of the plasma heat sources for de-icing.

A new impact model for the flexible rocking oscillator

AbstractMotivated by the rocking motion of non‐rigid structures during earthquakes, this paper investigates the rocking behaviour of laterally flexible oscillators. Previous studies in this area utilised ad hoc assumptions to consider transitions between different motion phases at impact. Some models explored direct transitions between two rocking phases assuming that elastic translational velocities remain the same, while others assumed the dissipation of all vertical momentum after a rocking phase, leading to a full contact phase upon impact. In this paper, an improved impact model is proposed, where consistent mechanical principles are used to determine the phase transition criteria. The model departs from the principle that the spring and damper elements of the oscillator cannot transfer horizontal impulses over infinitesimally small durations. Alongside other mechanical constraints, this principle yields a series of momentum equations, which are solved to determine the post‐impact velocities when transiting from one rocking phase to another. Through these equations, it is shown that a transition to full contact implies a specific locus of the vertical impulse from the support medium. Analytical and numerical investigations are then conducted to comparatively evaluate the new impact model. It is demonstrated that the new impact model yields equivalent results to established rigid rocking impact models for effectively rigid structures and resolves long‐standing issues associated with excessive energy dissipation observed in previous models. Time histories of free rocking motion and stability analyses under pulse excitations are used to illustrate and generalise the findings for a range of geometries.

Observation of the jet transition at a single vertical nozzle under pool scrubbing conditions

Submerged gas injection and the subsequent hydrodynamic behavior are phenomena that significantly affect the pool scrubbing efficiency at the injection zone. The Weber number jet transition criteria which have been normally used in the pool scrubbing research could not cover the wide range of the flow rate conditions including high-speed gas jet. In this study, the submerged air injection experiment was conducted at a 5 mm vertical nozzle. Air injection behavior was visually observed with a high-speed camera and analyzed using the shadowgraphy method. The local void fraction along the centerline of the pool was measured by using an optical fiber probe. From the measurement results, a jet transition criteria was suggested based on the nominal Mach number conditions. The effect of flow regime transition on the pool scrubbing efficiency has been confirmed through decontamination factor (DF) experiments. The suggested transition criteria matched with the DF experiment results.