Transition Length Research Articles

PIV Measurement of Separation Bubble on an Airfoil at Low Reynolds Numbers

A wind tunnel experiment was conducted to measure the flow over a suction side of a DAE51 airfoil at low Reynolds numbers using particle image velocimetry (PIV). Five chord Reynolds numbers ranging from 39,000 to 118,000 were investigated at angles of attack ranging from 0° to 10° at an interval of 1°. The locations of laminar separation, transition onset, transition end, and reattachment were identified by examining the measured averaged and RMS velocity data. The locations determined from the PIV data were in very good agreement with the features of measured surface pressure distribution. A large amplification of the RMS velocity leading to transition of a separated shear layer was observed centered at the inflection point of the averaged velocity profile, which indicates the important role of inflectional instability on the transition onset. The behaviors of those locations with respect to the Reynolds number and angle of attack were analyzed. For a fixed Reynolds number, the size of the separation bubble decreases quickly, and the bubble moves upstream as the angle of attack increases. The transition onset location moves upstream with increasing Reynolds number and angle of attack. The transition length becomes shorter as the Reynolds number increases. The lowest angle of attack at which a separation bubble is formed on the airfoil surface depends significantly on the Reynolds number. The movement of the separation point with respect to the Reynolds number indicates a different tendency depending on the presence of the separation bubble.

Metal-insulator phase transition in a non-Hermitian Aubry-André-Harper model

Non-Hermitian extensions of the Anderson and Aubry-Andr\'e-Harper models are attracting a considerable interest as platforms to study localization phenomena, metal-insulator and topological phase transitions in disordered non-Hermitian systems. Most of available studies, however, resort to numerical results, while few analytical and rigorous results are available owing to the extraordinary complexity of the underlying problem. Here we consider a parity-time ($\mathcal{PT}$) symmetric extension of the Aubry-Andr\'e-Harper model, undergoing a topological metal-insulator phase transition, and provide rigorous analytical results of energy spectrum, symmetry breaking phase transition and localization length. In particular, by extending to the non-Hermitian realm the Thouless$^{\prime}$s result relating localization length and density of states, we derive an analytical form of the localization length in the insulating phase, showing that -- like in the Hermitian Aubry-Andr\'e-Harper model-- the localization length is independent of energy.

Graph-theoretical analysis for energy landscape reveals the organization of state transitions in the resting-state human cerebral cortex.

The resting-state brain is often considered a nonlinear dynamic system transitioning among multiple coexisting stable states. Despite the increasing number of studies on the multistability of the brain system, the processes of state transitions have rarely been systematically explored. Thus, we investigated the state transition processes of the human cerebral cortex system at rest by introducing a graph-theoretical analysis of the state transition network. The energy landscape analysis of brain state occurrences, estimated using the pairwise maximum entropy model for resting-state fMRI data, identified multiple local minima, some of which mediate multi-step transitions toward the global minimum. The state transition among local minima is clustered into two groups according to state transition rates and most inter-group state transitions were mediated by a hub transition state. The distance to the hub transition state determined the path length of the inter-group transition. The cortical system appeared to have redundancy in inter-group transitions when the hub transition state was removed. Such a hub-like organization of transition processes disappeared when the connectivity of the cortical system was altered from the resting-state configuration. In the state transition, the default mode network acts as a transition hub, while coactivation of the prefrontal cortex and default mode network is captured as the global minimum. In summary, the resting-state cerebral cortex has a well-organized architecture of state transitions among stable states, when evaluated by a graph-theoretical analysis of the nonlinear state transition network of the brain.

Plasma flow equilibria in 2D cylindrically symmetric expanding magnetic field

A steady-state plasma flow exiting through an expanding magnetic field is studied by means of 2D particle in cell numerical simulations. The effects of change in the localized plasma source region dimension and the associated plasma transit length in an upstream, uniform-magnetic-field region were examined, simulating cases with different axial lengths of the dielectric plasma source region, distinct from the location of physical expansion. Axial potential profiles at various radial locations show development of a stepwise axial potential drop, producing plasma (ion) acceleration in the corresponding regions. For a narrow source region with a long pre-expansion flow region, nonmonotonic potential variation is recovered where ion phase-space scatters show the trapped region and signatures of chaotic ion trajectories and possible pre-expansion ion heating. Considering a relevance of the studied flow equilibria to the thrust generation schemes for space propulsion, a formal estimate of thrust values associated with the plasma outflow is also done for the cases simulated.

Topological edge states in the Su-Schrieffer-Heeger model

Su-Schrieffer-Heeger (SSH) model on two-dimensional square lattice exhibits a topological phase transition, which is related to the Zak phase determined by bulk band topology. The strong modulation of electron hopping causes nontrivial charge polarization even in the presence of inversion symmetry. The energy band structures and topological edge states have been calculated numerically in previous studies. Here, however, full energy spectrum and explicit form of wave functions for two-dimensional bulk and one-dimensional ribbon geometries of SSH model are analytically derived using wave mechanics approach. Explicit analytic representations of wave functions provide the information of parity for each subband, localization length and critical point of topological phase transition in SSH ribbon. It is also shown that the dimensional crossover of topological transition point for SSH model from one to two-dimension.

Liner buckling during reeling of lined pipe

An effective way of protecting offshore lines that carry corrosive hydrocarbons, is by mechanically lining carbon steel pipes with a thin layer of corrosion resistant alloy. A disadvantage of this product, is that when bent to curvatures such as those experienced during reeling the liner can buckle. The sensitivity of this instability to the main problem parameters was previously studied under pure bending. In the present study the problem is examined under the more complex loading experienced as it is wound onto a reel while under some back tension. A large-scale custom numerical model of the winding process of a previously expanded lined pipe is presented capable of dealing with the geometric, material and contact nonlinearities of the problem. The model is used to establish the deformation history experienced by the carrier and liner pipes during winding, the causes of liner instability, and its sensitivity to the main problem parameters. During reeling, a transition length a few diameters long bends to an increasingly larger curvature until it comes into contact with the reel. Back tension controls the length and curvature of this transition zone safeguarding the line against local buckling. Modest levels of tension are shown to also delay the onset of liner wrinkling. The more complex loading history experienced by the lined pipe as it approaches the reel changes the prevalent buckling mode but its sensitivity to initial geometric imperfections reported under pure bending remains. The liner instability becomes more prevalent as the diameter increases but at the same time smaller diameter lines could be reelable. The recent trend of winding lined pipes under small levels of internal pressure is confirmed to be an effective means of avoiding liner instability.

Identification of coherent flow structures and experimental analysis of the hydroacoustic emission of a hubless propeller

The progress in drive technology and the increasing needs for compact ship propulsion systems has brought a new class of hubless propellers into the field of operation. So far, no detailed analyses have been done concerning the unsteady flow structures which are important for the hydroacoustic emission. The objective of this paper is to analyze the unsteady flow topology and the related hydroacoustic emission of a hubless propeller with numerical and experimental methods. In order to capture the unsteady wake flow structures, a stress-blended eddy simulation (SBES) has been done. Experimental investigations in a circulating water channel provided the validation data in terms of integral quantities like thrust, wall pressure fluctuation and the hydroacoustic noise emission. The wake flow can be characterized by a multi-helix vortex system which develops from the blade root and tip region as well as from the trailing edge. The transition length of the helixes is larger compared to hub propellers in conjunction with an accelerated dissipation. The application of the proper orthogonal decomposition (POD) onto the unsteady wall pressure fields obtained from the SBES simulation was capable to assign different flow structures to the measured hydroacoustic emissions.

Transition radiation in a piecewise-linear and infinite one-dimensional structure\u2013a Laplace transform method

Transition zones in railway tracks are areas with considerable variation of track properties (i.e., foundation stiffness), encountered near structures such as bridges and tunnels. Due to strong amplification of the response, transition zones require frequent maintenance. To better understand the underlying degradation mechanisms, a one-dimensional model is formulated, consisting of an infinite Euler–Bernoulli beam resting on a locally inhomogeneous and nonlinear Winkler foundation, subjected to a moving load. The nonlinearity is characterized by a piecewise-linear stiffness, and the system thus behaves piecewise linearly. Therefore, the solution can be obtained by sequentially applying the Laplace transform combined with the Finite Difference Method for the spatial discretization and derived non-reflective boundary conditions. Results show that the plastic deformation is a consequence of constructive interference of the excited waves and the response to the load’s deadweight, particularly for the soft-to-stiff transition. The plastic deformation area decreases quasi-monotonically with increasing transition length, and for super-critical velocities, small transition lengths and/or large stiffness dissimilarities, parts of the foundation experience plastic deformation even in the second loading–unloading cycle. Furthermore, the nonlinearity causes the maximum energy associated with the waves radiated forward and the maximum energy input not to occur for the smallest transition length, contrary to findings in corresponding linear systems. Moreover, the energy input drastically increases for the second passage of the moving load, making it a possible indicator of the damage in the supporting structure. The novelty of the current work lies in the computationally efficient solution method for an infinite system which locally exhibits nonlinear behaviour and in the study into the influence of the foundation’s nonlinear behaviour on the generated waves (i.e., transition radiation), and on the resulting plastic deformation. The model presented here can be used for the preliminary design of transition zones in railway tracks.

Identifying Optimum Taper Lengths for Zipper Merging Applications using Real Data and Microscopic Simulation

Motorists lack of understanding on the proper way to maneuver through lane closures during congested periods cause driver confusion. This confusion directly and indirectly creates inconsistent flow patterns, forced merges, travel time delays, and crashes. Engineers and developers have tried to improve the merge systems used in construction zones to reduce driver frustration, improve travel time, and increase safety. Encouraging drivers to use the zipper merge approach has been assumed by some to target these issues. When implemented, drivers jointly merge together in an alternating fashion at two-to-one lane closures/reductions. There is a difference in opinion between traffic officials concerning the taper length required to efficiently accommodate these types of merging patterns – particularly those that occur near construction sites. Current practice uses the taper design guideline presented in the MUTCD. However, some believe this unique approach to merging at lane reductions should be accompanied by a shorter/longer taper. This study simulated 192 scenarios consisting of eight different percent truck compositions, six different transition lengths, and four different traffic volumes in VISSIM. The simulation models were calibrated with field data taken while a zipper merge configuration was in operation on a freeway. The main objective was to identify the optimum transition length when placing a zipper merge configuration because it visually and physically promoted alternating merging maneuvers. The results indicated none of the six tested taper lengths had a clear advantage over the other under multiple traffic volumes and truck percentages. Although statistically equal, operational differences in response to taper lengths were present and became more pronounced as volumes and truck percentages increased.

Advancement and Application of the Bezgin Method to Estimate Effects of Stiffness Variations along Railways on Wheel Forces

The need for an analytical method that one can apply manually to estimate dynamic impact forces on railway tracks that occur because of varying track stiffness or track profile initiated a study to develop an analytical method named as the Bezgin Method. The advancement of this method presented in this paper includes an extension of a set of equations developed and introduced by the first author earlier as the Bezgin Equations using the proposed method and development of a new equation. In addition to track stiffness taken into consideration in the equations introduced earlier, the Extended Bezgin Equations presented in this paper take into account bogie stiffness, wheel spring stiffness, Hertzian contact stiffness, and a factor for damping. The new equation takes into account the effect of vertical wheel acceleration as a train transitions to a stiffer structure or transitions along an ascending track profile. The paper unites and applies these equations to estimate wheel forces that develop along stiffness transition zones by considering an array of train speeds for an array of track stiffness ratios and representative values for track profile deviations along the transitions. Final section of the paper includes elaborate finite element analyses of structural track models that involve transitions of soil supported ballasted railway tracks with concrete based ballasted tracks along various transition lengths and compares their estimates for dynamic impact force factors with those estimated by the Extended Bezgin Equations. The paper concludes with a discussion of the potential uses, benefits, and the value of the Bezgin Method for railway engineering.

The improvement and substitution effect of transportation infrastructure on air quality: An empirical evidence from China's rail transit construction

A large number of studies on road construction and rail transit have not taken the improvement effect and the substitution effect into account. Using the quarterly data from 2013 to 2016 of 28 cities with metro opened in China, this research firstly compares the improvement effect of road reconstruction with the substitution effect of rail transit construction on air quality. Empirical results show that urban rail construction exerts a greater marginal impact on improving air quality than urban road reconstruction. Then, we employ operation length of urban rail transit as a long-term proxy variable and the length of rail built in season as a short-term proxy variable of rail transit construction to make a further discussion. In general, rail transit has an air pollution-reducing effect in the long run, while the construction of rail transit has a negative short-term effect on air quality. Based on above findings, governments should take measures to conduct convenient and efficient urban transportation network, and guide the construction of rail transit to multi-level development.

Uncertainty and sensitivity analysis of flow parameters for transition models on hypersonic flows

The impossibility to reproduce the experimental conditions, which is difficult to perfectly control and inevitably fluctuate, raises the uncertainties in numerical simulation. The objective of this work is to quantify the uncertainty and sensitivity of hypersonic transition modeling due to uncertainty in the flow parameters. Two representative transition models, e.g. γ-Reθ and k-ω-γ, are considered. And five flow parameters including freesteam Mach number, pressure, temperature, turbulence intensity and wall temperature are selected as aleatory uncertain variables uniformly distributing among the bound. Then the stochastic expansions based on point-collocation non-intrusive polynomial chaos (NIPC) method is utilized to represent and propagate the uncertainties in the output quantities of interests (QoIs) for hypersonic flow over the flat plate and the straight cone. Benefiting from the efficiency of the NIPC method, accurate uncertainty results are obtained with only 42 evaluations for each problem. Furthermore, Sobol indices are utilized to provide a relative ranking of each input uncertainty to the overall uncertainty in the output. The uncertainty results quantitatively verify that the transition zone is great sensitive to changes in flow parameters. Besides, for both the heat flux and skin friction coefficient, the importance ranking of the flow parameters and their trends are roughly resemble between the γ-Reθ and k-ω-γ model. This signifies that their sensitivity with respect to the input parameters is mainly associated with flow features rather than transition models. However, the contrary is the case for uncertainty in transition onset and length whose sensitivity to uncertainties of input parameters depends strongly on transition models. The present investigation shows that the freestream pressure is the greatest contributor to uncertainty in most cases. The freestream turbulence intensity, whose effect can be nearly negligible in heat flux and Cf, plays a predominant role on transition length.

High-speed running maglev trains interacting with elastic transitional viaducts

This paper presents a general framework for the dynamic analysis of high-speed maglev trains running on elastic transitional viaducts, a complicated problem that lacks an efficient analysis tool. Unlike the straight and circular tracks, both the curve radius (CR) and height difference (HD) between the outer and inner rails vary along the transitional track. To start, the motion of each vehicle of the train running over a transitional track is expressed by a set of trajectory coordinates, with their orientations given in the Euler angles as functions of the CR and HD. The elastic transitional viaduct is simulated by finite elements in the global coordinates. For the maglev trains, the electromagnet force-air gap model is used to account for the interaction and coupling effect between the moving vehicles and supporting viaduct. By applying the proposed framework to the Shanghai maglev line (SML), the dynamic responses of the maglev vehicles running on the transitional viaduct are studied and compared with those on the straight and circular viaducts. Moreover, the effects of transitional track length and cant deficiency on the coupled system are investigated. The results show that for vehicles running on the transitional track, the levitation forces and vehicle’s angular velocity are highly related to the rate of change of HD of the track. The length of the transitional viaduct affects significantly all the vehicle’s responses. The increase of deficient cant angle causes a sharp increase in the responses of the transitional viaduct in both the radial and rotational directions.

Measurement of Transitional Surface Roughness Effects on Flat-Plate Boundary Layer Transition

An experimental study has been conducted to investigate the effects of transitionally rough surface on the flat-plate boundary layer transition. Transitional boundary layers with three different flat plates (ks+ = 0.07 ∼ 0.19, 2.71 ∼ 7.05, and 13.65 ∼ 41.09) have been measured with a single-sensor hot-wire probe. All of the measurements have been conducted under zero pressure gradient (ZPG) at the fixed Reynolds number (ReL) and freestream turbulence intensity (Tu) of 3.05 × 106 and 0.2%. Transitionally, rough surface does not affect the sigmoidal distribution of turbulence intermittency model; but induces earlier transition onset and shortens the transition length. For all surfaces, streamwise turbulence intensity profiles with similar values of turbulence intermittency are similar for the transition length less than 60%. Therefore, mean velocity profiles with the similar values of turbulence intermittency are similar regardless of surface conditions. However, downstream of 60% of the transition length, mean velocity defect increases as the surface roughness increases. Enhanced diffusion of turbulent kinetic energy from the near wall (y/δ < 0.1) to the outer part (y/δ ≈ 0.4) of the boundary layer due to the surface roughness is responsible for the increased momentum deficit.

Kinesin-8 and Dis1/TOG collaborate to limit spindle elongation from prophase to anaphase A for proper chromosome segregation in fission yeast.

ABSTRACTHigh-fidelity chromosome segregation relies on proper microtubule regulation. Kinesin-8 has been shown to destabilise microtubules to reduce metaphase spindle length and chromosome movements in multiple species. XMAP215/chTOG polymerases catalyse microtubule growth for spindle assembly, elongation and kinetochore-microtubule attachment. Understanding of their biochemical activity has advanced, but little work directly addresses the functionality and interplay of these conserved factors. We utilised the synthetic lethality of fission yeast kinesin-8 (Klp5-Klp6) and XMAP215/chTOG (Dis1) to study their individual and overlapping roles. We found that the non-motor kinesin-8 tailbox is essential for mitotic function; mutation compromises plus-end-directed processivity. Klp5-Klp6 induces catastrophes to control microtubule length and, surprisingly, Dis1 collaborates with kinesin-8 to slow spindle elongation. Together, they enforce a maximum spindle length for a viable metaphase–anaphase transition and limit elongation during anaphase A to prevent lagging chromatids. Our work provides mechanistic insight into how kinesin-8 negatively regulates microtubules and how this functionally overlaps with Dis1 and highlights the importance of spindle length control in mitosis.

Numerical study on effect of the turbulence initial conditions on transition flow over 2D airfoil.

The performance of the airfoil is strongly dependent on the development of the boundary layer on the surface and therefore an accurate prediction of the laminar to turbulent transition onset is essential. A grid independence study is performed, turbulence variables values have been changed frequently, regularly, and carefully so that they cover the entire range of acceptable values reported by previous researches. Effects of turbulent variating at far stream on turbulent boundary layer structure and on transition stage characteristics at moderate Reynolds number have been studied over a full range of angles of attack of NACA0012. numerical results have been post-processed, analyzed and found that far stream turbulence variables have a significant effect on transition characteristic, their effects on skin friction is limited to small extent along wing surface where transition take place, increasing turbulence intensity or eddy viscosity ratio at far boundary shifts transition onset towards leading edge and increase transition length.

Acoustic properties of Vietnamese initial consonants

In acoustic phonetic research, phonetic data is needed to prove authenticity. The acoustic phonetic analysis method is valid for verifying previous phonetic hypotheses. Thereby, lay the foundations of science to reinforce the notion of phonetic or phonetic study. The formant frequencies F1, F2, F3 are considered as the basis for measuring vowels. According to consonants, the length of VOT, formant transitions, antiformants, and locus frequencies will be noted. In this article, we present the basis to measure Vietnamese initial consonants such as: voiced consonants, voiceless consonants, stop consonants, fricative consonants, nasal consonants. The voiced consonants will have a voice bar and voiceless consonants will have no voice bar. Fricative consonants always have higher frequencies than stop consonants. Based on the spectral image of a consonant, we can determine the articulation of consonants. The acoustic properties of the nasal consonant and lateral consonant are nearly identical to the acoustic properties of the vowels, because in the construction of these consonants, the vocal cords are more vibrating.

The role of internal migration in accessing a first job: A case study of Uganda

AbstractDoes experiencing internal migration hasten access to the labour market? This article studies the gap in length of transition to a first job between internal migrant and non‐migrant youth in Uganda. According to the specific context of this developing country, three transition starting points are considered: date of birth, minimum legal working age and school exit. Extended Cox proportional hazard models suggest that migrants experience shorter transitions. However, when excluding child labourers or measuring school‐to‐work transitions, significant gaps disappear. Decomposition of transition length gaps reveals the importance of observable and unobservable factors related in particular to area of origin, gender and access to education.

Characterizing the school-to-work transitions of young men and women in Tunisia

This paper investigates the characteristics and determinants of the transition of young men and women from education (dropping out) to work in Tunisia, using data from the Panel Study of Tunisia Labor Market 2014 (TLMPS 2014). One of the main findings is that 17 per cent of the 15 to 35 years old population remains in transition. One - third had completed the transition. The remaining (55 per cent) had not started transition. The transited category is strongly male-dominated while young females face a particularly difficult time. Youths who were still in transition were largely unemployed. Econometric method (Kaplan-Meier or product limit estimator) is used to analyze the determinants of unemployment duration. Results point to differences between men and women in the difficulty of getting a decent job. They confirm that individual characteristics (gender, area of residence and level of education…) influence the length of transition from school to the labor market. Indeed, we found the following. First, for most youth, the school-to-work transition is not long to a first job, but it takes on average more five years to complete the transition to a stable or satisfactory job. One third of young males and females found a first job through contact with employers. Second, transition period decreases substantially as education level increases. Third, the rural – urban gap continues to be significant. Fourth, the probability of longer unemployment is found to be considerably higher among women and lower education. The study offers some recommendations in terms of economic policy on the insertion of young people.

Study of macro-bending biconical tapered plastic optical fiber for relative humidity sensing

PurposeThe purpose of this paper is to propose and optimize plastic optical fiber (POF) probe with macro-bending biconical tapered structure for the relative humidity (RH) sensing.Design/methodology/approachIn this study, the principle is the evanescent wave power modulated by the ambient humidity. The probe is fabricated by using fused biconical taper and heat-setting method and then coated with a fluorescent moisture-sensitive film.FindingsThe probe’s sensing performance can be optimized by changing the probe’s curvature radius, biconical tapered transition length and taper waist diameter. The result shows that the sensitivity of the probe is up to 1.60 and 3.40 mV/ per cent, respectively, at low humidity (10-45 per cent) and high humidity (45-90 per cent). Also, this probe has good linearity, repeatability, photostability and long-term stability.Practical implicationsThe proposed probe can improve the sensitivity and linearity of RH sensing without complex devices, which is necessary for mass production, remote measurement and convenient operation.Originality/valuePOF probe with macro-bending biconical tapered structure is investigated in this paper, which is proved to be effective in improving the sensitivity and linearity.