Transition Length Research Articles

Critical regime of gravity currents flowing in non-rectangular channels with density stratification

We present theoretical and experimental analyses of the critical condition where the inertial–buoyancy or viscous–buoyancy regime is preserved in a uniform-density gravity current (which propagates over a horizontal plane) of time-variable volume ${\mathcal{V}}=qt^{\unicode[STIX]{x1D6FF}}$ in a power-law cross-section (with width described by $f(z)=bz^{\unicode[STIX]{x1D6FC}}$, where $z$ is the vertical coordinate, $b$ and $q$ are positive real numbers, and $\unicode[STIX]{x1D6FC}$ and $\unicode[STIX]{x1D6FF}$ are non-negative real numbers) occupied by homogeneous or linearly stratified ambient fluid. The magnitude of the ambient stratification is represented by the parameter $S$, with $S=0$ and $S=1$ describing the homogeneous and maximum stratification cases respectively. Earlier theoretical and experimental results valid for a rectangular cross-section ($\unicode[STIX]{x1D6FC}=0$) and uniform ambient fluid are generalized here to a power-law cross-section and stratified ambient. Novel time scalings, obtained for inertial and viscous regimes, allow a derivation of the critical flow parameter $\unicode[STIX]{x1D6FF}_{c}$ and the corresponding propagation rate as $Kt^{\unicode[STIX]{x1D6FD}_{c}}$ as a function of the problem parameters. Estimates of the transition length between the inertial and viscous regimes are also derived. A series of experiments conducted in a semicircular cross-section ($\unicode[STIX]{x1D6FC}=1/2$) validate the critical values $\unicode[STIX]{x1D6FF}_{c}=2$ and $\unicode[STIX]{x1D6FF}_{c}=9/4$ for the two cases $S=0$ and $1$. The ratio between the inertial and viscous forces is determined by an effective Reynolds number proportional to $q$ at some power. The threshold value of this number, which enables a determination of the regime of the current (inertial–buoyancy or viscous–buoyancy) in critical conditions, is determined experimentally for both $S=0$ and $S=1$. We conclude that a very significant generalization of the insights and results from two-dimensional (rectangular cross-section channel) gravity currents to power-law cross-sections is available.

The transition from linear to-parabolic growth of Cu3Si phase in Cu/a-Si system

The solid state reaction between Cu and a-Si films was investigated at 150–200°C by depth profiling with secondary neutral mass spectrometry. Intermixing was observed leading to the formation of a homogeneous Cu3Si layer at the interface. The growth of the crystalline silicide follows a parabolic law at 165°C and 200°C. At 150°C a transition from linear to parabolic kinetics is observed. Combining with our previous experimental results showing linear kinetics at 135°C [Acta Materialia, 61 (2013) 7173–7179], the temperature dependence of the linear and parabolic coefficients as well as the transition length can be estimated.

The Effect of a Dissipative Ladle Shroud on Mixing in Tundish: Mathematical and Experimental Modelling

AbstractThe effect of a dissipative ladle shroud (DLS) on mixing in tundish was investigated, compared with that of a conventional ladle shroud (CLS) using mathematical and physical modelling. The tracer profiles of mathematical results, achieved using large eddy simulation, were validated by physical observations employing high-speed cinephotography. The design of a DLS dramatically changed the flow patterns and contributed the intermixing of fluid elements inside the ladle shroud. The vortex flow encouraged the turbulent mixing and was verified by tracking of physical tracer dispersion inside the DLS. Residence Time Distribution (RTD) curves were obtained in two different sized tundishes to examine the mixing behaviours. The findings indicated that the DLS benefited the tundish mixing in terms of increasing active volume. The effect seemed to be more remarkable in the smaller tundish. The DLS gave rise to a more plug-like flow pattern inside the tundish, showing potential to shorten the transition length during grade change.

Introduction of Standardized Tobacco Packaging During a 12-Month Transition Period: Findings From Small Retailers in the United Kingdom.

Factory-made cigarettes (FMC) and roll-your-own (RYO) tobacco have had to be produced in standardized packaging since May 20, 2016 in the United Kingdom, with a minimum pack size of 20 sticks for FMC and 30 g for RYO. Manufacturers and retailers were given a 12-month transition period. An observational study was conducted using monthly Electronic Point of Sale data from 500 small retailers in England, Scotland, and Wales, between May 2016 and May 2017. The 20 top selling tobacco products (15 FMC, 5 RYO) were monitored to observe when standardized packs were first introduced, the proportion of retailers selling each fully branded and standardized product, and the average number of monitored fully branded and standardized products sold by each retailer. The number of unique tobacco-related product codes sold by each retailer was also recorded each month. Eighteen of the fully branded products continued to be sold throughout the transition period and no standardized variants were sold in the first 5 months. It was not until month eleven that the average number of standardized products sold by retailers exceeded the fully branded products. The average number of unique tobacco-related product codes sold by each retailer decreased by a third over the transition period. Tobacco companies used the transition period to delay the removal of fully branded products and gradually introduce standardized variants. This staggered introduction may have mitigated some of the immediate intended effects of the legislation by desensitizing consumers to new pack designs. Evaluation research from countries which have introduced standardized packaging for tobacco products is key to help inform future implementation. This is the first study to monitor the transition from fully branded to standardized products using real-time retail data. The findings demonstrate that tobacco companies delayed the introduction of standardized products and removal of fully branded packaging. Countries seeking to introduce standardized packaging should consider what length of transition is allowed, as the protracted 12-month period in the United Kingdom appeared longer than needed to transition stockholding and may have mitigated immediate intended effects by desensitizing consumers to new pack designs.

FEATURES OF VIDEO PULSE PARAMETERS SIMULATION FOR STUDYING SPECTRA OF SECONDARY ELECTROMAGNETIC RADIATION

Features of video-pulse parameters simulation for studying spectra of secondary electromagnetic radiation are described on the basis of the Monte-Carlo statistical test method. It is established that the key parameters in terms of the possibility of information leakage are: the length of the smooth transition of the signal between the linear parts and the length of the approximation of the linear component of the pulse front, which determine the shape of the signal. Using a probabilistic approach, a method for determining the density distribution of the probability of electric field intensity at the boundary of the controlled zone is described to assess the technical protection of information.

Dependence of square cylinder wake on Reynolds number

The wake of a square cylinder is investigated for Reynolds number Re < 107. Two-dimensional (2D) laminar simulation and three-dimensional (3D) large-eddy simulation are conducted at Re ≤ 1.0 × 103, while experiments of hotwire, particle image velocimetry, and force measurements are carried out at a higher Re range of 1.0 × 103 < Re < 4.5 × 104. Furthermore, data covering a wide Re range, from 100 to 107, in the literature are comprehensively collected for discussion and comparison purposes. The dependence on Re of the recirculation bubble size or vortex formation length, wake width, shear-layer transition, time-mean drag force, and Strouhal number is discussed in detail, revealing five flow regimes, each having distinct variations of the above parameters. With increasing Re, while the streamwise recirculation size enlarges at Re < 50 (steady flow regime), the vortex formation length reduces at 50 < Re < 1.6 × 102 (laminar flow regime), remains unchanged at 1.6 × 102 < Re < 2.2 × 102 (2D-to-3D transition flow regime), and decreases at 2.2 × 102 < Re < 1 × 103 (shear layer transition I regime), approaching asymptotically a constant at Re > 1.0 × 103 (shear layer transition II regime). Meanwhile, the wake width decreases with Re in the laminar flow regime, grows in 2D-to-3D transition and shear layer transition I regimes, and levels off in the shear layer transition II regime. The narrowest wake width is identified in the 2D-to-3D transition flow regime, corresponding to a minimum time-mean drag force and a largest Strouhal number. With increasing Re, the shear-layer transition length rapidly declines in the shear layer transition I regime where the transition occurs downstream of the trailing corner of the cylinder. On the other hand, it slowly tapers off in the shear layer transition II regime where the transition takes place upstream of the trailing corner. An extensive comparison is made between the dependence on Re of a circular cylinder wake and a square cylinder wake, with their distinct natures highlighted.

Length scale effect on Mach reflection of cellular detonations

An experimental study of the Mach reflection of cellular detonations over the wedge is reported in this paper. Throughout the experiments, high-quality smoked foil is obtained to record cell pattern variation. The initial pressures are varied to yield cellular detonations with varied transverse wave spacing (or cell size). This paper focuses on length scales controlling the deviation and recovery of self-similarity in an unsteady Mach reflection process. The results show that the frozen limit and equilibrium limit both exist for all the mixture compositions. The Mach reflection undergoes a frozen condition in the near field, and then goes through a transition process before asymptotically approaching an equilibrium state in the far field. The cell size variation in the Mach stem region also confirms the transient process. Although the final triple-point trajectory angle in the equilibrium state disagrees with classical three-shock theories, it is in agreement with the reactive three-shock theory when the wedge angle is less than 30°. The triple-point trajectory angle asymptotically approaches zero as the wedge angle increases to approximately 50°, indicating the critical wedge angle from Mach to regular reflection. The transition length associated with the equilibrium limit is found to be dependent on the mixture composition and has the same order of a hydrodynamic thickness, which is approximately a few cell lengths. This means that the hydrodynamic thickness is the characteristic length scale that most significantly dominates the Mach reflection process of cellular detonations.

An experimental analysis of the defect sensitivity of solid foams

Single edge notched bending (SENB) and single edge notched tensile (SENT) fracture experiments were conducted to study the influence of the defect size on the global fracture behavior of three different brittle low-density PVC solid foams. It was found that for sufficiently small defects (initial cracks), the continued fracture process was through breakage of cell edges located at random positions far away from the defect while the global fracture load was fairly constant and hence unaffected by the initial defects. At defects larger than about four cells, however, the continued cell edge fractures were localized to the near vicinity of the defect, resulting in a decreasing global fracture load in accordance with classical linear elastic fracture mechanics theory. Hence a size of about four cells is considered to be a critical microstructure transition length, meaning that the foam is unaffected for defects smaller than this transition length from a fracture point of view.

Design of elliptical-core mode-selective photonic lanterns with six modes for MIMO-free mode division multiplexing systems.

Elliptical-core few mode fiber (EC-FMF) is used in a mode division multiplexing (MDM) transmission system to release multiple-input-multiple-output (MIMO) digital-signal-processing, which reduces the cost and the complexity of the receiver. However, EC-FMF does not match with conventional multiplexers/de-multiplexers (MUXs/DeMUXs) such as a photonic lantern, leading to extra mode coupling loss and crosstalk. We design elliptical-core mode-selective photonic lanterns (EC-MSPLs) with six modes, which can match well with EC-FMF in MIMO-free MDM systems. Simulation of the EC-MSPL using the beam propagation method was demonstrated employing a combination of either step-index or graded-index fibers with six different sizes of cores, and the taper transition length of 8cm or 4cm. Through numerical simulations and optimizations, both types of photonic lanterns can realize low loss transmission and low crosstalk of below -20.0 dB for all modes.

A Study on Elliptical Cross-Sectioned Spiral Equal-Channel Extrusion of AA7075: FE Simulation and RSM Approach

Elliptical cross-sectioned spiral equal-channel extrusion is newly developed sever plastic deformation technique that can be used for processing of cylindrical components. In the present study, numerical and experimental investigations were made to identify optimal setting of process parameters i.e. torsion angle, elliptical rotation transitional length and elliptical length ratio attaining minimum punch force and maximum effective strain. Firstly, the process was numerically simulated by using DEFORM software. Then, 15 series of simulation runs was implemented based on central composite design incorporating three-factors and three-levels. Response surface methodology was used here to correlate relationship between factors and responses. In order to find optimum solution regarding maximum strain and minimum punch force, desirability approach was used. The optimization results showed that for achieving maximum strain (that is most desirable during severe plastic deformation), settings of 120° torsion angle, 9 mm channel length and 1.45 elliptical length ratio was required. From the results obtained through experiments, it was found that performing three-passes ECSEE under optimal combination of factors resulted in about 30% enhancement in values of ultimate tensile strength and hardness.

Modeling of Laminar-Turbulent Transition in Boundary Layers and Rough Turbine Blades

A local, intermittency-function-based transition model was developed for the prediction of laminar-turbulent transitional flows with freestream turbulence intensity Tu at low (Tu < 1%), moderate (1% < Tu < 3%), and high Tu > 3% levels, and roughness effects in a broad range of industrial applications such as turbine and helicopter rotor blades, and in nature. There are many mechanisms (natural or bypass) that lead to transition. Surface roughness due to harsh working conditions could have great influence on transition. Accurately predicting both the onset location and length of transition has been persistently difficult. The current model is coupled with the k–ω Reynolds-averaged Navier–Stokes (RANS) model, that can be used for general computational fluid dynamics (CFD) purpose. It was validated on the ERCOFTAC experimental zero-pressure-gradient smooth flat plate boundary layer with both low and high leading-edge freestream turbulence intensities. Skin friction profiles agree well with the experimental data. The model was then tested on ERCOFTAC experimental flat plate boundary layer with favorable/adverse pressure gradients cases, periodic wakes, and flows over Stripf's turbine blades with roughness from hydraulically smooth to fully rough. The predicted skin friction and heat transfer properties by the current model agree well with the published experimental and numerical data.

Isolated Main Pancreatic Duct Dilatation: CT Differentiation Between Benign and Malignant Causes.

The purpose of this study is to retrospectively evaluate the differential CT features of isolated benign and malignant main pancreatic duct (MPD) dilatation and to investigate whether the diagnostic performance of radiologists can be improved with knowledge of these differential CT features. Forty-one patients who had isolated MPD dilatation without any visible mass on CT from January 2000 to October 2016 were retrospectively enrolled in the study. Two radiologists reviewed CT images in consensus for the location, shape (smooth vs abrupt), length of transition, dilated pancreatic duct (PD) diameter, presence of duct penetrating sign, parenchymal atrophy, attenuation difference, associated pancreatitis, calcification, PD or common bile duct (CBD) enhancement, and perilesional cyst. The chi-square test, Fisher exact test, and t test were used to find the differential CT features of benign and malignant MPD dilatation. Two successive review sessions for differentiation between the two disease entities were then independently performed by three other reviewers with differing expertise, with the use of a 5-point confidence scale. The first session provided no information for differentiation; however, reviewers were aware of the results of univariate analyses in the second session. The diagnostic performance of the radiologists was evaluated using a pairwise comparison of ROC curves. A total of 19 benign and 22 malignant MPD dilatations were identified. In patients with benign MPD dilatation, transition areas were frequently located in the head (57.9% [11/19] vs 13.6% [3/22], p = 0.003) and showed significantly shorter (< 6.1 mm) (78.9% [15/19] vs 9.1% [2/22], p < 0.0001) and smooth transition (89.5% [17/19] vs 9.1% [2/22], p < 0.0001). Duct penetrating sign was exclusively observed in patients with benign MPD dilatation (73.7% [14/19] vs 0% [0/22], p < 0.0001). In contrast, malignant MPD dilatation frequently was accompanied by attenuation difference (63.6% [14/22] vs 10.5% [2/19], p = 0.001) and associated PD or CBD enhancement (36.4% [8/22] vs 0% [0/19], p = 0.003). The AUC values of three reviewers significantly increased from 0.653, 0.587, and 0.884 to 0.864, 0.964, and 0.908, respectively, with knowledge of significant CT features (p = 0.013, p < 0.0001, and p = 0.701, respectively). Distal, long (≥ 6.1 mm), and abrupt transition, the absence of duct penetrating sign, and the presence of attenuation difference and PD or CBD enhancement were highly suggestive CT findings for differentiation of malignant from benign MPD dilatation. The diagnostic performance of radiologists with regard to differentiation was significantly improved with knowledge of these highly suggestive CT criteria.

Effect of Lactic Acid Bacteria-Fermented Mulberry Leaf Extract on the Improvement of Intestinal Function in Rats.

This study examined the laxative effects of mulberry leaf extract (MLE) fermented by lactic acid bacteria (LAB), which contains high levels of polyphenolic and flavonoid compounds, against loperamide-induced constipation in rats. Sprague-Dawley rats were divided into a normal group (N) and three experimental groups; loperamide treated group (C), loperamide and LAB-fermented MLE 300 mg/kg treated group (MLEL), and loperamide and LAB-fermented MLE 600 mg/kg treated group (MLEH). After 33 d, fecal pellet amount, fecal weight, water content of fecal, gastrointestinal transit time and length, and serum lipid profiles were measured. Constipation was induced via subcutaneous injection of loperamide (2.0 mg/kg b.w., twice a day) for the final 5 d of the experiment. After loperamide administration, the LAB-fermented MLE groups showed a significantly increase in the fecal pellets number, wet weight, and water content in rats compared with the C group. Moreover, increases in the intestinal length and viable Lactobacillus numbers in the feces were observed in the LAB-fermented MLE groups. The intestinal transit time was shorter in the LAB-fermented MLE groups than in the C group. In addition, the LAB-fermented MLE groups showed a significant decrease in triglyceride and total cholesterol levels and an increase in HDL-cholesterol level. These results indicated that oral administration of LAB-fermented MLE shows laxative effect in loperamide-induced constipated rats.

Economic and efficient method of design of a flumed canal fall

Innumerable falls are to be provided in irrigation canals where ground slope exceeds the permissible bed slope of a canal. In the conventional method of design, fluming ratio is fixed arbitrarily irrespective of inflow Froude’s number. Long lengths of inlet and outlet transitions are provided to prevent flow separation. Transition and dissipation structures are kept separate resulting in high costs. Hydraulic performance of the conventional fall structure is also not so satisfactory. Analytical and experimental studies were conducted by the author to find an efficient and economic method of design of falls. Optimum fluming ratio and optimum length of transitions are found both for economy as well as efficiency. An efficient and economic stilling basin with rapidly diverging side walls and adversely sloping floor, which act simultaneously as energy dissipater and transition, has been recommended. An example has been worked out to illustrate the design procedure of the proposed canal fall.

On the capability of the Thick Level Set (TLS) damage model to fit experimental data of size and shape effects

Size and shape effects are important issues in predicting the global response of concrete structures. Small-scale tests performed in laboratory to determine the material properties are not enough to simulate large-scale structures. Many models are used to extrapolate small scale results to large scale simulations, but only few are able to recover size and shape effects. Recently a model of graded damage (TLS) has been proposed and comparison with cohesive zone models shows that this new model contains a new degree of freedom, the length of transition between totally damaged material and undamaged zone (ie the process zone size). In this paper, the capability of the model (TLS) to represent size and shape effects for two recently published experimental campaigns is studied.

Properties of recycled aggregate concrete with different water control methods

The paper studies the properties of Recycled Aggregate Concrete (RAC) with four different water control methods: the control group, pre-wetting with the same total water amount group, pre-wetting with the same mixing water amount group and adding extra mixing water group. Macro properties of concrete materials, i.e., compressive strength, drying shrinkage and chloride penetrability, and micro properties of Interfacial Transition Zone (ITZ), i.e., length and elastic modulus, are tested. The results show that RAC with a pre-wetting process has lower compressive strength and a higher Cl− diffusion coefficient, but a smaller shrinkage value compared to RAC with the same total water amount and no pre-wetting process. Adding extra mixing water process weakens the properties of RAC. The comprehensive elastic modulus is defined as an integrated interface parameter, and it has a good correlation with the macro properties like compressive strength and diffusion coefficient of Cl− of RAC.

Transverse bulk solid behaviour during discharge from troughed belt conveyors

Guidelines for the calculation of bulk solid material cross sectional dimensions and the influence of the belt conveyor transition length on the inclination of the trajectory at discharge are well established. However, not a great deal of research has been conducted on the influence of bulk solid material properties and conveyor belt transition geometry on the bulk solid material cross section at discharge. As such, assessment of cross section break-up associated with cohesive materials and transverse spreading of free flowing materials is missing. Conversely, the majority of discharge trajectory analysis techniques focus on analysis in a single vertical plane along the length of the belt.This paper presents an analysis of high speed conveyor discharge trajectories in three dimensions, taking into account transverse spreading of free flowing materials and shearing, or cross section discontinuity, exhibited by cohesive materials. Transverse bulk solid material behaviour and trajectory discharge is evaluated using a combination of experimental laboratory tests, a continuum mechanics approach incorporating CAD and Discrete Element Modelling (DEM). The work presented shows that bulk solid material behaviour at discharge is directly influenced by material characteristics and interactions resulting from the geometry of the belt conveyor transition zone.

Estimating Turbulent Spot Initiation Rates from Transition Lengths in Hypersonic Boundary Layers

Experimental evidence indicates that boundary layers in hypersonic flows transition from being laminar to turbulent via the mechanism of formation and propagation of turbulent spots. Turbulent spots have been detected using either surface heat transfer gauge signals or pressure traces, or they have been visualized using high-speed schlieren techniques. The extent of the transition zone is strongly dependent on the rate of initiation of turbulent spots and spot propagation parameters. However, it is not possible at present to predict the turbulent spot initiation rates in hypersonic transitional flows. As a first step toward developing an effective predictive model for the purpose, a methodology is developed to enable estimation of the spot initiation rate and intermittency distribution from experimental heat transfer data. The methodology is validated by comparison with experimental data where both the heat transfer and intermittency were measured. The methodology is then used in a predictive mode on three experimental datasets, and it is found to give good agreement. This enables prediction of other important variables as well, including skin friction and a wide variety of other boundary-layer properties in the transition zone at hypersonic Mach numbers.

Optimized temperature perturbation method to generate turbulent inflow conditions for LES/DNS simulations

Abstract The temperature perturbation method (TPM) is used as an alternative technique to the classical velocity perturbations to generate inflow turbulence for LES or DNS simulations. The TPM consists in seeding the flow with random temperature perturbations which, through a buoyancy triggered mechanism, will induce the creation of turbulent structures. Naturally, this implies that the physical buoyancy effects need to be sufficiently strong, which has limited up to now its application to atmospheric flows. The proposed modification makes it applicable to a wider variety of wall-bounded flows, that would involve much smaller length scales than in the atmosphere. In this novel approach, a perturbation zone is defined at the entrance of the domain, along which an artificial local Richardson number is applied so that buoyancy effects are sufficiently strong to create turbulence. The TPM is implemented in the incompressible solver of OpenFOAM v2.3 with buoyancy effects and tested on a plane channel flow at R e τ = 395 . Prior to testing, periodic channel flow results are obtained to represent the reference fully developed turbulent state. The perturbation zone is divided into a fully active zone, where buoyancy is maximum, and a transition zone, needed to insure a smooth progression back to zero buoyancy. Several perturbation functions are applied to modulate these artificial effects, in order to identify the function that most efficiently pre-mixes the flow, thus preventing stratification effects from slowing down the flow recovery. Thereafter, the transition length and the artificial local Richardson number are optimized, based on wall shear stress recovery. Moreover, the effect of the perturbation frequency on not only the flow solution but also the energy spectra is examined in order to prevent the technique from contaminating the energy content. The flow development is compared to the synthetic turbulence generator method of Xie and Castro [1] in terms of the recovery distances of both the wall shear stress and the Reynolds stresses. The optimized method appears to be efficient with a flow that reaches equilibrium and a good quality energy spectra after about 15 δ . Although this final length remains similar to that obtained by competing methods, the optimized TPM benefits from a greater flexibility since only first-order statistics are required as input. Therefore, it can be applied without prior knowledge of second order moments or integral length scales, making it directly applicable to a wide variety of flows.

A Study of Topological Quantum Phase Transition and Majorana Localization Length for the Interacting Helical Liquid System

We consider a helical spin liquid system which shows majorana fermion modes at the edge. The interaction between the quasiparticles in this system induces phase transition, Majorana-Ising transition. We comply the density matrix renormalization group method to study this phase transition for the entire regime of the parameter space. We observe the presence of topological quantum phase transition for repulsive interaction, however this phase is more stable for the attractive interaction. The length scale dependent study shows many new and important results and we show explicitly that the major contribution to the excitation comes from the edge of the system when the system is in the topological state. We also show the dependence of Majorana localization length for various values of chemical potential.