Asymmetric Phenomena Research Articles

Polarization: A Universal Driving Force for Energy, Environment, and Electronics

AbstractThe sustainable future relies on the synergistic development of energy, environmental, and electronic systems, founded on the development of functional materials by exploring their quantum mechanisms. Effective control over the distribution and behavior of charges within these materials, a basic quantum attribute, is crucial in dictating their physical, chemical, and electronic properties. At the core of charge manipulation lies “polarization”—a ubiquitous phenomenon marked by separating positive and negative charges. This review thoroughly examines polarization techniques, spotlighting their transformative role in catalysis, energy storage, solar cells, and electronics. Starting with the foundational mechanisms underlying various forms of polarization, including piezoelectric, ferroelectric, and pyroelectric effects, the perspective is expanded to cover any asymmetric phenomena that generate internal fields, such as heterostructures and doping. Afterward, the critical role of polarization across various applications, including charge separation, surface chemistry modification, and energy band alignment, is highlighted. Special emphasis is placed on the synergy between polarization and material properties, demonstrating how this interplay is pivotal in overcoming existing technological limitations and unlocking new functionalities. Through a comprehensive analysis, a holistic roadmap is offered for harnessing polarization across the broad spectrum of applications, thus finding sustainable solutions for future energy, environment, and electronics.

Superposition of standing waves induced ultra high-resolution atom localization

Abstract In this study, we have theoretically demonstrated ultra-high resolution two-dimensional atom localization within a three-level $\lambda$-type atomic medium via superposition of asymmetric and symmetric standing wave fields. Our analysis provides understanding into the precise spatial localization of atomic positions at the atomic level, utilizing advanced theoretical approaches and principles of quantum mechanics. The dynamical behavior of a three-level atomic system is thoroughly analyzed using the density matrix formalism within the realm of quantum mechanics. A theoretical approach is constructed to describe the interaction between the system and external fields, specifically a control field and a probe field. The absorption spectrum of the probe field is thoroughly examined to clarify the spatial
localization of the atom within the proposed configuration. We have theoretically investigated that symmetric and asymmetric superposition phenomena significantly influence the localized peaks within a two-dimensional spatial domain. Specifically, the emergence of one and two sharp localized peaks has been observed within a region of one wavelength domain. We observed notable influences of the intensity of the control field, probe field detuning, and decay rates on atom localization. Ultimately, we have achieved an unprecedented level of ultra-high resolution and precision in localizing an atom within an area smaller than λ/35×λ/35. These findings hold promise for potential applications in domains such as Bose-Einstein condensation, nanolithography, laser cooling, trapping of neutral atoms, and the measurement of center-of-mass wave-functions.

Suppression of the Petermann noise factor in a chiral optical mode converter without encircling an exceptional point

The engineering of exceptional points (EP) in gain-loss-assisted optical systems has recently garnered considerable attention for the development of unconventional photonic components. Moreover, the substantial noise associated with EPs poses a significant challenge to realizing their full potential in applications. There is a gap in the exploration of asymmetric mode conversion phenomena in the close proximity of the dynamical encirclement around an EP. In response, we propose the design of a fabrication-feasible dual-mode optical planar waveguide. Non-Hermiticity, in terms of a customized gain-loss profile, is introduced to modulate the interaction between the coupled modes. Contrary to previous assumptions, we establish that the dynamical encirclement of an EP is not a prerequisite for achieving asymmetric mode conversion. Furthermore, we calculate the Petermann factor (K) near the EP and demonstrate a significant two orders of magnitude reduction in the Petermann noise factor when we do not dynamically encircle the EP. Hence, we propose a new scheme to reduce the inherent noise.

Symmetries and Asymmetries in Movement

Symmetric and asymmetric phenomena are prevalent in linguistics. Recently, two famous approaches have been put forward to analyze the symmetry and asymmetry in grammar: Kayne s Linear Correspondence Axiom (LCA)[1]and Ackema and Neeleman s human parser[2]. Both of them prove that there is only leftward movement in human s spoken languages. Kayne s theory proves it from a linguistically syntactic perspective while Ackema and Neeleman testify the leftward movement through a mechanic approach. Though these two analyses are quite popular with linguists, exceptions still exist in many languages. In this paper, I will apply two linguists analysis on CPs movement to show that rightward-movement explanation for moved CPs is better than leftward movement. Then, the traditional point of view that human grammar is asymmetric because of the unique leftward movement will be proved to be wrong. From the demonstration of Cp s rightward movement, we will see our grammar is symmetric because both leftward movement and rightward movement exist.

Stellar Escape from Globular Clusters. II. Clusters May Eat Their Own Tails

We apply for the first time orbit-averaged Monte Carlo star cluster simulations to study tidal tail and stellar stream formation from globular clusters (GCs), assuming a circular orbit in a time-independent spherical Galactic potential. Treating energetically unbound bodies—potential escapers (PEs)—as collisionless enables this fast but spherically symmetric method to capture asymmetric extratidal phenomena with exquisite detail. Reproducing stream features such as epicyclic overdensities, we show how returning tidal tails can form after the stream fully circumnavigates the Galaxy, enhancing the stream's velocity dispersion by several kilometers per second in our ideal case. While a truly clumpy, asymmetric, and evolving Galactic potential would greatly diffuse such tails, they warrant scrutiny as potentially excellent constraints on the Galaxy’s history and substructure. Reexamining the escape timescale Δt of PEs, we find new behavior related to chaotic scattering in the three-body problem; the Δt distribution features sharp plateaus corresponding to distinct locally smooth patches of the chaotic saddle separating the phase-space basins of escape. We study for the first time Δt in an evolving cluster, finding that Δt∼(EJ−0.1,EJ−0.4) for PEs with (low, high) Jacobi energy E J, flatter than for a static cluster ( EJ−2 ). Accounting for cluster mass loss and internal evolution lowers the median Δt from ∼10 Gyr to ≲100 Myr. We finally outline potential improvements to escape in the Monte Carlo method intended to enable the first large grids of tidal tail/stellar stream models from full GC simulations and detailed comparison to stream observations.

3D numerical simulation on thermal hydraulic characteristics in integrated sodium pool of PHENIX reactor during dissymmetric test

Temperature distribution of internals under different transient events is important for the design and safety analysis of pool-type sodium-cooled fast reactor (SFR). It is influenced by overall flow and thermal stratification phenomena in the multi-scale sodium pool. Due to the difficulty to perform experiment, three-dimensional (3D) numerical method is usually adopted for obtaining the temperature distribution and analyzing complex phenomena in pool-type SFRs. It needs to be further validated using the published benchmark, such as PHENIX Dissymmetric Test. The simplified model in system code and 3D model of local area in current study cannot capture the complex coupling effect of cold and hot pool during dissymmetric test. In this paper, an integrated 3D model of PHENIX reactor primary system both including cold and hot pools is established. The steady and transient simulation with 1800 s of PHENIX Dissymmetric Test process are carried out. The key parameters of simulation are compared with the available experimental and predicted data, which achieves good agreement. Overall flow and asymmetric temperature distribution phenomena in cold pool, hot pool and local area of IHX inlet are simulated. There is a temperature difference between different sides of IHX inlet during a period after scram, with the maximum value of 80 ℃ approximately at 90 s. This validated numerical method can be further used for the analysis of thermal hydraulic characteristics in other pool-type SFR. The obtained temperature distribution of internals can provide the reference for the stress assess of structures in the engineering design.

Streamwise dispersion of soluble matter in solvent flowing through a tube

For the dispersion of soluble matter in solvent flowing through a tube as investigated originally by G.I. Taylor, a streamwise dispersion theory is developed from a Lagrangian perspective for the whole process with multi-scale effects. By means of a convected coordinate system to decouple convection from diffusion, a diffusion-type governing equation is presented to reflect superposable diffusion processes with a multi-scale time-dependent anisotropic diffusivity tensor. A short-time benchmark, complementing the existing Taylor–Aris solution, is obtained to reveal novel statistical and physical features of mean concentration for an initial phase with isotropic molecular diffusion. For long times, effective streamwise diffusion prevails asymptotically corresponding to the overall enhanced diffusion in Taylor's classical theory. By inverse integral expansions of local concentration moments, a general streamwise dispersion model is devised to match the short- and long-time asymptotic solutions. Analytical solutions are provided for most typical cases of point and area sources in a Poiseuille tube flow, predicting persistent long tails and skewed platforms. The theoretical findings are substantiated through Monte Carlo simulations, from the initial release to the Taylor dispersion regime. Asymmetries of concentration distribution in a circular tube are certified as originated from (a) initial non-uniformity, (b) unidirectional flow convection, and (c) non-penetration boundary effect. Peculiar peaks in the concentration cloud, enhanced streamwise dispersivity and asymmetric collective phenomena of concentration distributions are illustrated heuristically and characterised to depict the non-equilibrium dispersion. The streamwise perspective could advance our understanding of macro-transport processes of both passive solutes and active suspensions.

Three-dimensional numerical study on reducing hypersonic blunt body drag and aeroheating with spike-aerodisk-bleed air channel

Shock drag and aeroheating are major problems faced by hypersonic vehicles. This study proposes a novel three-dimensional model combining spike-aerodisk with a bleed air channel featuring multiple lateral nozzles for drag and aeroheating reduction. The high-temperature and high-pressure air at the stagnation point of the aerodisk is introduced into the channel and ejected laterally through multiple nozzles to modify the flow field structure and reduce drag and aeroheating. In contrast to previous research employing a two-dimensional axisymmetric assumption, our innovation considers the structure of the exit nozzles and the spacing between nozzles. Due to multiple lateral nozzles, the connection thickness between nozzles can lead to asymmetric flow phenomena. Three-dimensional flow field simulation has been employed with the Reynolds-averaged Navier-Stokes (RANS) equations coupled with the shear stress transport (SST) k-ω turbulence model. The effects of the convergence half-angle of the channel inlet, the divergent angle of the nozzle, and the nozzle position on modifying the characteristics of the flow field are studied. Increasing the convergence half-angle of the channel inlet can further push away the reattachment shock, but when it exceeds 60°, the drag reduction performance degrades. Reducing the divergent angle of the nozzle and increasing the number of nozzles can reduce the drag coefficient and the peak value of the Stanton number. However, when the divergent angle is reduced to 10°, the performance of reducing drag and aeroheating is weakened. In the range of research parameters, when the convergence half-angle is 60°, the divergent angle is 15°, and the nozzles are located in the middle of the spike, compared with the plain spike-aerodisk model, the drag coefficient and peak Stanton number of the combined model are further reduced by 17.8 % and 34.8 %, respectively. Our findings confirm that a multi-nozzle can effectively reduce drag and aeroheating while the channel structure is viable in practice.

Mode transition mechanisms in semi-passive flapping-wing propulsion or energy extraction

To facilitate the smooth transition of operational modes in self-sustaining micro-thrusters, this study focuses on a device employing semi-passive flapping wings. Transient numerical methods are used to analyze the variation of flap energy acquisition and propulsion characteristics with pitching frequency, and the relationship between the dominant vortex structure and the force characteristics is analyzed by using the integral momentum theorem and the dynamic mode decomposition (DMD) method. The results indicate that, under the investigated operational conditions, with an increase in the pitching frequency, distinct wake evolution characteristics were observed. In the energy harvesting operational regime, the wake patterns manifest as 2P + mS, 2S + mS, and mS types. During the transitional phase from energy harvesting to propulsion, the wake patterns shift from 2S + mS to 2S transitional types, eventually leading to the manifestation of a reverse Karman vortex street (2S RBVK). In the propulsion operational regime, the wake patterns consist of a reverse Karman vortex street and asymmetric reverse Karman vortex street phenomena. Simultaneously, it was observed that the transition of flapping-wing performance from energy harvesting to propulsion conditions delays behind the transformation of vortex street structures. This delay is attributed to the necessity for the flapping-wing device to overcome its own resistance before generating a net effective propulsive force. The contributions of unsteady wake to thrust primarily encompass vortex thrust, thrust due to localized fluid acceleration, induced momentum force, and induced pressure force. As the pitching frequency increases, the influence of wake vortices on propulsion also intensifies. The contribution of wake vortices to flapping-wing propulsion is determined by the spatial distribution of Lamb vectors and localized fluid accelerations. The conclusions drawn from the dynamic modal analysis and reconstructed flow field analysis of wake vortices align with the findings of the investigation of wake vortices based on the integral momentum theorem.

Asymmetric Flow Phenomena Affecting the Characterization of the Control Plant of an Altitude Test Facility for Aircraft Engines

As an indispensable part of the engine manufacturer supply chain, the eco-efficiency of altitude test facility (ATF) operations must improve. Automation is a key enabler in this context since it not only increases precision and reproducibility but also allows for reducing the test time and energy consumption. A suitable controller and reliable validation are crucial to ensure the stability and appropriate response of the control system. Both aspects necessitate a thorough understanding of the control plant, expressed in a numerical model. These models have to be suitable for control system design and validation while covering asymmetric flow phenomena that occur in the pipe system and detailing the nonlinear system dynamics to a high degree of accuracy. One-dimensional network models, state-space models, highly resolving numerical models, and data-driven models are relevant applications for this task. We compare the results of state-of-the-art one-dimensional network models which mainly imply symmetric flow conditions with those of three-dimensional Reynolds-averaged-Navier–Stokes (RANS) simulations which cover asymmetric flow phenomena. The findings show that the assumptions of idealized, axis-symmetric flow in the one-dimensional flow elements do not hold true for the complex flows in an altitude test facility. In a second step, we have compared the results of one-dimensional simulations with in-service measurements taken during ATF test campaigns. Deviations were observed, which become explainable based on the insights gained from the comparison with the RANS simulation. The findings reveal that the one-dimensional simulation-based approach is insufficient to adequately reflect the plant and subsequently for validation due to the observed asymmetric flow phenomena. To overcome this limitation, the application of an empirical first-order transfer function using system identification methods is proposed. Its applicability is successfully demonstrated for the exhaust gas section of the ATF. Subsequently, essential criteria for the design of a suitable control concept for the outlet condition are derived.

Electro-momentum coupling tailored in piezoelectric metamaterials with resonant shunts

Local microstructural heterogeneities of elastic metamaterials give rise to non-local macroscopic cross coupling between stress–strain and momentum–velocity, known as Willis coupling. Recent advances have revealed that symmetry breaking in piezoelectric metamaterials introduces an additional macroscopic cross coupling effect, termed electro-momentum coupling, linking electrical stimulus and momentum and enabling the emergence of exotic wave phenomena characteristic of Willis materials. The electro-momentum coupling provides an extra degree of freedom for controlling elastic wave propagation in piezoelectric composites through external electrical stimuli. In this study, we present how to tune the electro-momentum coupling arising in 1D periodic piezoelectric metamaterials with broken inversion symmetry through shunting the inherent capacitance of the individual piezoelectric layers with a resistor and an inductor in series forming a resistor–inductor–capacitor circuit. Guided by the effective elastodynamic theory and homogenization method for piezoelectric metamaterials, we derived a closed-form expression of the electro-momentum coupling in shunted piezoelectric metamaterials. Moreover, we demonstrate the ability to tailor the electro-momentum coupling coefficient and control the amplitudes and phases of the forward and backward propagating waves, yielding tunable asymmetric wave responses. The results of our study hold promising implications for applications involving asymmetric wave phenomena and programmable metamaterials.

On the inadequacy of rate-dependent models in simulating asymmetric rate-independent hysteretic phenomena

Aim of this paper is to establish the limits of the common approach according to which, in oscillating systems, dissipative phenomena, often having an asymmetric rate-independent nature, are simulated by means of equivalent rate-dependent models relating forces and velocities. To this end we compare the time evolution of the generalized work and energy components of a Single Degree of Freedom (SDoF) hysteretic mechanical system including both conservative and non-conservative forces; in addition the generalized work of the non-conservative forces is related to the generalized internal energy of the system. To achieve such a result, we provide (closed-form) expression(s) to compute the path-dependent work performed by generalized rate-dependent (rate-independent) hysteretic forces, simulated by using the Seleemah and Constantinou (Asymmetric Bilinear) model. For the latter, we also provide a closed-form expression for the variation in internal energy associated with the generalized rate-independent work when a complete cycle of generalized displacement is applied. Such results are exploited to describe the behavior of symmetric, and asymmetric rate-independent hysteretic systems under loads of arbitrary nature, e.g., free vibration, harmonic load, and random load. Finally, the inadequacy of rate-dependent models in simulating asymmetric rate-independent hysteretic behaviors is assessed by showing that calibrating the former model according to the methodology usually suggested in the literature yields inaccurate results in terms of displacement and dissipated energy of the system.

Study on Temperature Field Distribution Law and Mechanical Properties of Hydraulic Tunnel-Surrounding Rock under the Action of Large Temperature Differences

This study aims to explore the temperature field and mechanical characteristics of hydraulic tunnels during operation in an area with large temperature differences throughout a year. By relying on the measured displacement field change results of a water transmission tunnel in Xinjiang, a finite element calculation model applicable to the hydraulic tunnel-surrounding rock under the action of large temperature differences was established. Then, the FEM model was used to analyze the temperature field distribution and stress field variation of hydraulic tunnel-surrounding rock under the effect of large temperature differences. The results showed that, under the action of large temperature differences, the displacement of the cave top was the largest, followed by the cave bottom, and finally the cave waist. The obtained displacement field data using the calculation model in this study were basically consistent with the on-site measured data, and the displacement field change law was the same, indicating that the established finite element calculation model is reliable. When the ambient temperature was below zero, the freezing front gradually expanded to the depth of the surrounding rock along the tunnel radial direction, while the frozen rock at the intersection of the lining and the surrounding rock melted first when the ambient temperature was above zero. The stress in various parts of the surrounding rock of the hydraulic tunnel increased continuously. To be specific, the growth rate of the tensile stress at the cave bottom, the tensile stress at the cave top, and the compressive stress at the cave waist of the tunnel reached 0.052 MPa/year, 0.053 MPa/year, and 0.096 MPa/year, respectively, in the first year, but the increasing rate subsequently tended to slow down continuously. Therefore, in actual projects, attention should be focused on the effect of large temperature differences on the stress growth of tunnel-surrounding rocks at an early stage, especially the growth of compressive stress at the cave waist. Notably, there are a large number of symmetrical and asymmetric phenomena related to engineering in this study.

Natural convective heat transfer and fluid flow in a porous medium filled corrugated enclosure: Effect of discrete heat sources

AbstractIn this work, we investigate the two‐dimensional unsteady natural convective fluid flow problem in a porous‐corrugated enclosure with a fixed sinusoidal heated upper wall. The corrugations of the enclosure are discretely heated while vertical walls are maintained isothermally cold. Subject to where the heat sources are located, five different cases are taken into consideration. The vorticity–streamfunction equations are discretized using a transformation‐free higher order compact approach, and the hybrid BiCGSTAB technique is used to solve the system of algebraic equations that derives from the numerical discretization. To validate our findings, we first compare them to previously published numerical and experimental data. The numerically simulated outcomes are then examined over a variety of essential parameters, such as the Darcy (10−5 ≤ Da ≤ 10−1), Rayleigh (103 ≤ Ra ≤ 106), and Prandtl (0.1 ≤ Pr ≤ 10) numbers. Symmetric and asymmetric fluid flow phenomena are observed. Asymmetric flow phenomenon can be caused by miscible or non‐miscible movements of lighter fluids by heavier fluids, or almost exclusively by nonuniform buoyancy‐driven forces caused by density variations that have arisen because of variations in fluid temperature. The averaged Nusselt value for Case 1 and Case 5 exhibits the highest percentage ratio. The thermal boundary layer is strongly affected by compression, dispersion, suppression, the zone of stratification, and the outweighing of isotherms. The simulated results are visualized by stream functions, isotherms, local and averaged Nusselt number plots.

Stability and chaotic dynamic analysis of Li-doped fullerene-IRMOF composite materials for hydrogen storage

Isoreticular metal organic framework (IRMOF) material is a type of ideal hydrogen storage material for its porosity and large surface area. Lithium doping and fullerene impregnation can enhance the hydrogen storage capacity of IRMOF efficiently. In this paper, the nonlinear dynamic characteristics of Li-doped fullerene-IRMOF composite material are qualitatively discussed in theory, which determine its thermal stability. A type of Li-doped fullerene-IRMOF composite material is proposed for hydrogen storage, and a new constitutive model is proposed to describe the hysteretic property of IRMOF. The system's nonlinear dynamic model is developed, and the fast-slow dynamic analysis method is introduced to interpret not only the system's jumping phenomena between the multi-pulse homoclinic and heteroclinic orbits but also its chaotic dynamics. The nonlinear dynamic responses of the system under deterministic and stochastic excitation are obtained respectively, and symmetrical and asymmetric orbital fusion phenomena have been discovered. The results of theoretical analysis and numerical simulation both show that the system has novel nonlinear dynamic characteristics such as double periodic motion, multi-pulse jumping orbits, and chaos; multi-pulse jumping phenomena are caused by the stiffness difference between fullerene and IRMOF, which form a fast-slow system; symmetrical fusion and asymmetrical fusion of system's orbits occur under different parameters. Safe, efficient and rapid hydrogen storage and release can be achieved through controlling the system's parameters to adjust the dynamic behavior of the system, which will be a development direction in the application of hydrogen energy.

A comparative and epistemic approach to Anglophone and Latin American postcolonial theory and criticism

Postcolonial criticism is one of the critical derivations of recent times. Nevertheless, not many works have undertaken studies based upon comparative analyses grounded on its episteme(s). The present study demonstrates that there is more than one episteme in postcolonial criticism. To prove it, the analysis considers key points in the evolution of the ideology and criticism of Anglophone and Latin American postcolonial realms. It first covers the evolution of the Anglophone postcolonial ideology and criticism. After that, it deals with Latin American postcolonial criticism. It concludes by establishing a comparison between both. It leads us to see that the postcolonial Anglophone and Latin American epistemes are asymmetric phenomena that share common features and mutual benefits. Additionally and on equal terms, the present study shows that the post- and de- prefixes linked to the “colonial” term and applied to the Anglophone and Latin American realms, respectively, are indicators that mark an ideological turn in the conception of both terms. Despite the shared plurality and multiplicity of the postcolonial world, the apparent wish for the release and unleashing from the matrix of power, and the paradoxical “sameness convergence” into globalization, these indicators are different, as in fact they show different degrees of epistemic disobedience and social dissidence.

MULBER: Effective Android Malware Clustering Using Evolutionary Feature Selection and Mahalanobis Distance Metric

Symmetric and asymmetric patterns are fascinating phenomena that show a level of co-existence in mobile application behavior analyses. For example, static phenomena, such as information sharing through collaboration with known apps, is a good example of a symmetric model of communication, and app collusion, where apps collaborate dynamically with unknown malware apps, is an example of a serious threat with an asymmetric pattern. The symmetric nature of app collaboration can become vulnerable when a vulnerability called PendingIntent is exchanged during Inter-Component Communication (ICC). The PendingIntent (PI) vulnerability enables a flexible software model, where the PendingIntent creator app can temporarily share its own permissions and identity with the PendingIntent receiving app. The PendingIntent vulnerability does not require approval from the device user or Android OS to share the permissions and identity with other apps. This is called a PI leak, which can lead to malware attacks such as privilege escalation and component hijacking attacks. This vulnerability in the symmetric behavior of an application without validating an app’s privileges dynamically leads to the asymmetric phenomena that can damage the robustness of an entire system. In this paper, we propose MULBER, a lightweight machine learning method for the detection of Android malware communications that enables a cybersecurity system to analyze multiple patterns and learn from them to help prevent similar attacks and respond to changing behavior. MULBER can help cybersecurity teams to be more proactive in preventing dynamic PI-based communication threats and responding to active attacks in real time. MULBER performs a static binary analysis on the APK file and gathers approximately 10,755 features, reducing it to 42 key features by grouping the permissions under the above-mentioned four categories. Finally, MULBER learns from these multivariate features using evolutionary feature selection and the Mahalanobis distance metric and classifies them as either benign or malware apps. In an evaluation of 22,638 malware samples from recent Android APK malware databases such as Drebin and CICMalDroid-2020, MULBER outperformed others by clustering applications based on the Mahalanobis distance metric and detected 95.69% of malware with few false alarms and the explanations provided for each detection revealed the relevant properties of the detected malware.

Sub‐Saharan Africa's debt‐financed growth: How sustainable and inclusive?

AbstractSub‐Saharan African (SSA) countries recorded impressive economic growth in the last two decades. The question however is, how inclusive and sustainable is this growth? With this in mind, this study examined the sustainability and inclusiveness of economic growth in a panel of 44 SSA countries, over a period of 38 years, while taking into account the diversity of the continent's institutional quality, income growth and resource endowment. The study adopts the innovative nonlinear fiscal reaction function and the dynamic panel threshold model to account for potential asymmetric phenomena in public debt series. The study also adopts Driscoll and Kraay's estimator to account for cross‐section dependency and cross‐country heterogeneity. The result shows that the recent increase in the economic growth rate in SSA is not sustainable and inclusive. This is tenable since if economic growth is debt induced, more money will be spent on servicing public debt, thus depriving governments of funds for critical intervention programs. Lastly, the study found a public debt/gross domestic product ratio threshold of 34% beyond which public debt impairs growth inclusiveness across SSA. The research and policy implications are discussed.

A New Interpretation of Hot Gas Ingress Through Turbine Rim Seals Influenced by Mainstream Annulus Swirl

Abstract Rim seals are fitted at the periphery of the stator and rotor disks to reduce the adverse effects of hot gas ingress on highly stressed turbine components limited by temperature. Ingress is induced by rotational effects such as disk pumping, as well as by asymmetric pressure-driven unsteady phenomena. These influences superpose to form a complex flow-physics problem that is a challenge for computational fluid dynamics. Engine designers typically use practical low-order models that require empirical validation and correlating parameters. This paper identifies the swirl ratio in the mainstream annulus as a dominant characterizing parameter to predict ingress. This is a new interpretation that is supported by extending a low-order model based on turbulent transport using an effective eddy mixing length based on the difference in swirl between the annulus and seal clearance. Experimental measurements were made using a 1.5-stage turbine rig at low Reynolds number. The influence of annulus swirl ratio was investigated over a range of flow conditions and two rim-seal geometries, with the ingress quantified using CO2 tracer concentration in the sealing flow. The concentration data were complemented by measurements in the annulus using a five-hole aerodynamic probe.

How to explain the direction of time

Reichenbach explains temporally asymmetric phenomena by appeal to entropy and ‘branch structure’. He explains why the entropic gradients of isolated subsystems are oriented towards the future and not the past, and why we have records of the past and not the future, by appeal to the fact that the universe is currently on a long entropic upgrade with subsystems that branch off and become quasi-isolated. Reichenbach’s approach has been criticised for relying too closely on entropy. The more popular approach nowadays is to appeal instead to a particular low-entropy initial state—Albert’s ‘Past Hypothesis’. I’ll argue that this neglect of Reichenbach’s approach is unwarranted. A Reichenbachian account has important advantages over Albert’s: it correctly identifies the minimal temporally asymmetric posit needed to derive key temporally asymmetries and it offers a more adequate account of the record asymmetry. While a Reichenbachian account needs to be supplemented, it provides the right foundations for explaining temporally asymmetric phenomena and what we might ultimately mean by ‘the direction of time’.