Transition Length Research Articles

Thermo-Hydraulic Performance of Spring Tape Inserts in a Circular Channel in Transitional Flow Regime

In this work, forced convection experiments were carried out to investigate the variation of heat transfer and pressure drop inside a uniformly heated circular duct fitted with full-length spring tape inserts of varying spring ratios in a transition flow regime. Air was used as the working fluid medium, and experiments were conducted for three constant heat fluxes of 2, 3, and 4 kW-m−2 with Reynolds numbers varied from 502 to 10936 to cover all three flow regimes. For a plain channel heated with 2 kW/m2 of heat flux, the transition begins at a Reynolds number of 2897 and terminates at a Reynolds number of 3783 which brings the transition Reynolds number width of 886. For the same heat flux, a channel with spring tape having a spring ratio of 3, the transition begins at Reynolds number 661 and ends at 1734 while for the case of spring ratio 5, the transition begins at 1305 and ends at 2226. For constant heat flux conditions, a decrease in the spring ratio results in the early onset of transition and which in turn increased the transition length. Predictive Nusselt number and friction factor correlations for different flow regimes have also been presented.

Fracture behavior analysis of X80 pipelines welded joints with unequal wall thickness

Pipeline transportation is one of the main methods for transporting oil and natural gas worldwide. The structural safety of welded joints of large diameter pipes with cracks and unequal wall thickness has become a research focus due to discontinuous geometrical structure and cracks in weld seams. In this paper, the tensile experiment and analysis of specimens with unequal wall thickness across welding seam based on the digital image correlation technology are carried out. Moreover, the influence of transition length and transition form on fracture behavior of welded joints of unequal wall thickness pipeline is obtained. In addition, a three-dimensional finite element model of welded joint of unequal wall thickness pipe with oblique line transition form and circular arc transition form is established. and the accuracy of the full-scale pipeline finite element model modeling method is verified by small-scale experiment data. On this basis, the influence of geometrical parameters of the transition section and wall thickness ratio, pipe diameter, and weld strength matching coefficient on the crack driving force of welded joints with unequal wall thickness is obtained. From the perspective of strain-based design, a method for obtaining the minimum transition length section and avoiding girth weld fracture is proposed. The research results guide the design and safety evaluation of welded joints with unequal wall thickness.

Assessing passive scalar dynamics in bubble-induced turbulence using direct numerical simulations

By using direct numerical simulations (DNS) of bubbly flows with passive scalars, we show a transition in the scalar spectra from a $k^{-5/3}$ to a $k^{-3}$ scaling with the wavenumber $k$ , in contrast with those of single-phase isotropic turbulence. For cases with a mean scalar gradient in the horizontal direction, the scalar spectrum decays faster than $k^{-3}$ at high wavenumbers. While the $k^{-3}$ scaling is well established in the bubbly flow velocity spectrum, the scalar spectrum behaviour is not fully understood. We find that the transition length scale of the scalar spectra is comparable to or below the bubble diameter and decreases with the molecular diffusivity of the scalar in the liquid. We use DNS to compute the scalar spectrum budget and show that the scalar fluctuations are produced by the mean scalar gradient at length scales above the bubble diameter, contrary to the velocity fluctuations. At length scales below the bubble diameter, the net scalar transfer scales as $k^{-1}$ inducing the $k^{-3}$ scaling of the scalar spectra. This finding is consistent with the hypothesis proposed by Dung et al. (J. Fluid Mech., vol. 958, 2023, p. A5) about the physical mechanism behind the $k^{-3}$ scaling. We also show dependencies of the bubble suspension's convective scalar diffusivity on the gas volume fraction and molecular diffusivity that differ based on the direction of the mean scalar gradient. For a mean scalar gradient in the vertical direction, we find and qualitatively explain a significant effect of the molecular diffusivity in the gas on the convective scalar diffusivity.

Heat-Transfer Gauge Arrays for Identifying Spot Initiation in Hypersonic Transitional Boundary Layers

The initiation and growth of turbulent spots in transitional hypersonic boundary layers can influence the transition length and the distribution of heat transfer and skin friction through the transitional region. There is debate in the literature about whether spots initiate in a small band at the start of transition or throughout the transitional region. This paper presents a new design of arrays of thin-film heat transfer specifically designed to detect whether spots initiate in a small band. The gauges were arranged in two spanwise rows at different streamwise locations and were tested on a 7° blunt cone in the T4 Stalker Tube. It is shown that such instrumentation is capable of identifying that some spots initiate between the rows when other spots have already formed upstream of the first row. This demonstrates that the breakdown of laminar boundary layers is not concentrated in a region of small streamwise extent in the current hypersonic flows.

Association Between Intravenous to Oral Opioid Transition Time and Length of Hospital Stay After Posterior Spinal Fusion for Adolescent Idiopathic Scoliosis.

Transitioning from intravenous (IV) to oral opioids after posterior spinal fusion (PSF) for adolescent idiopathic scoliosis (AIS) is necessary during the postoperative course. However, few studies have assessed the effects of longer transition times on hospital length of stay (LOS). This study investigated the impact of longer IV to oral opioid transition times on LOS after PSF for AIS. The medical records of 129 adolescents (10-18 years old) with AIS undergoing multilevel PSF at a major academic institution from 2013 to 2020 were reviewed. Patients were categorized by IV to oral opioid transition time: normal (≤2 days) vs prolonged (≥3 days). Patient demographics, comorbidities, deformity characteristics, intraoperative variables, postoperative complications, and LOS were assessed. Multivariate analyses were used to determine odds ratios for risk-adjusted extended LOS. Of the 129 study patients, 29.5% (n = 38) had prolonged IV to oral transitions. Demographics and comorbidities were similar between the cohorts. The major curve degree (P = 0.762) and median (interquartile range) levels fused (P = 0.447) were similar between cohorts, but procedure time was significantly longer in the prolonged cohort (normal: 6.6 ± 1.2 hours vs prolonged: 7.2 ± 1.3 hours, P = 0.009). Postoperative complication rates were similar between the cohorts. Patients with prolonged transitions had significantly longer LOS (normal: 4.6 ± 1.3 days vs prolonged: 5.1 ± 0.8 days, P < 0.001) but similar discharge disposition (P = 0.722) and 30-day readmission rates (P > 0.99). On univariate analysis, transition time was significantly associated with extended LOS (OR: 2.0, 95% CI [0.9, 4.6], P = 0.014), but this assocation was not significant on multivariate analysis (adjusted OR: 2.1, 95% CI [1.3, 4.8], P = 0.062). Longer postoperative IV to oral opioid transitions after PSF for AIS may have implications for hospital LOS.

The wideband tunability of double Gunn diodes

Gunn diodes use the Ridley–Watkins–Hilsum effect to generate low noise millimeter wave signals for communication and sensing applications. However, Monte Carlo calculations indicate that the velocity–field characteristics of compensated GaAs contain a second peak due to phonon scattering. A novel device that leverages this double Ridley–Watkins–Hilsum effect to achieve ultra-wideband tunability of low-noise millimeter waves is proposed in this article. The second region of negative differential mobility is gradually introduced in seven mobility profiles, which are each simulated in eight device lengths. It is found that a 2.5 μm Gunn diode with a compensation ratio of 0.6 operating at 160 K has a 700% increase in frequency tunability. An analysis of the domain formation indicates that the increase in tunability is due to significant distortions in domain shape. Gunn diodes with transit lengths of 2 μm and smaller show a 100% increase in tunability due to modulations in the effective transit length and absorption rate. These simulations indicate that compensated GaAs could provide linear, ultra-wideband frequency tunability.

Sensitivity Analysis of Pipe–Soil Interaction Influencing Factors under Frost Heaving

The mechanism of pipe–soil interaction under frost heaving is complicated due to many factors affecting the pipe–soil system. In order to analyze the sensitivity of various pipe–soil interaction influencing factors and highlight the relationship between the factors and the pipe’s mechanical characteristics during frost heaving, a pipe–soil interaction model based on a semi-infinite elastic frozen soil foundation is developed. Besides, the mechanical indices characterizing the influence factors and their change law are emphatically explored. The results show that the pipe stress changes most obviously at the transition region between the frost-heaving and non-frost-heaving regions. The equivalent stress increases nonlinearly with the increase of foundation coefficient, linearly with the increase of frost heave and elastic modulus of pipe, and decreases nonlinearly with the increase of transition length and pipe wall thickness. The peak stress of the pipe increases linearly with the increase of temperature difference. Moreover, the maximum allowable frost heave deformation decreases nonlinearly with the increase of oil pressure. This study helps provide theoretical reference for the adjustment, control, and prediction of stress and deformation in the design of buried pipelines under frost heaving.

Optimization process of spin-coated black phosphorus/polydimethylsiloxane composite on microfiber as saturable absorber for ultrafast photonics

Numerous saturable absorber (SA) fabrication methods have been proposed and employed in the development of ultrafast laser systems, particularly for microfiber-based SAs decorated with various 2D materials. However, the study on microfiber-SA with different transition lengths is still inadequate. This paper focuses on the design and fabrication method of black phosphorus/polydimethylsiloxane composite microfiber-based SA for ultrashort pulse generation. With optimized microfiber-SA dimensions of 30 mm (up/down taper), 1 mm (waist length) and 10 µm (waist diameter), black phosphorus/polydimethylsiloxane composite was spin-coated on the microfiber at 4000 rpm for 6 min. The integration of the microfiber-SA in an erbium-doped fiber laser exhibited pulse duration of 724 fs at center wavelength of 1560.18 nm. This work provides a guideline for the design of microfiber-based SA using spin-coating technique to maximize its laser performances.

Convective mass deposition near a topologically singular transition

The leading-edge of a substrate undergoing convective mass deposition is a region of significant local deposition rate compared to the mass deposition at the downstream Leveque concentration boundary layer. The local increase in mass deposition is due to an intrinsic topological transition at the leading edge, a transition that is usually in the chemistry or geometry of the target surface for deposition. We study two leading-edge cases for model convective electrodeposition: a flat and a corner/step transitions between the inert wall and active cathode. We find that mass deposition at the leading-edge is faster than at the boundary layer and is connected to the Pećlet number differently. Its rate is correlated with the transition length and decays downstream to match the deposition rate at the boundary layer.

Chain length and amino acid composition dependence of coil-globule transition in intrinsically disordered proteins.

A fundamental question related to intrinsically disordered proteins (IDPs) is whether they undergo a coil-to-globule transition with changes in cosolvent conditions. IDPs are hetero-polymers and finite in size. The chain length and amino acid composition could lead to the varying extent of compaction in IDPs and the coil-to-globule transition (theta condition). To probe the theta condition in polypeptides, we studied the behavior of polyleucine and polyglutamine with six different chain lengths (N = 16 to 512) in various concentrations (0 to 8 M) of Urea (denaturant) and TMAO (osmolyte) using coarse-grained models and computer simulations. The Flory scaling exponent of polyleucine (polyglutamine) undergoes a transition from approximately 0.6 (0.6) to 0.3 (0.5) with increasing N in water. We observed that the theta condition for both the IDPs strongly depends on N and varies as N−0.5. Depending on N, polyglutamine (polyleucine) chains reach the theta condition at different [TMAO] ([TMAO] or [Urea]), confirming that in addition to chain length, the amino acid composition also influences the theta solvent condition for the IDPs. Moreover, the theta point for smaller chain lengths (N = 16 to 64) calculated using the structure factor mimicking the small-angle X-ray scattering (SAXS) experiments and using pair distances mimicking fluorescence resonance energy transfer (FRET) experiments are in disagreement. The theta condition computed using the structure factor and pair distances converge to the same point only at large N. As the theta condition is correlated with the critical condition of liquid-liquid phase separation (LLPS) for IDPs, we predict that LLPS propensity of IDPs will increase with N.

Microscopic fatigue crack propagation model for polycrystalline alloys

In the present work, using the crystal plasticity finite element method in combination with the extended finite element method, a microscopic polycrystalline fatigue crack propagation (MPFCP) model based on the total cumulative shear strain was developed. MPFCP processes were reproduced well by the MPFCP model. The MPFCP behaviors of micro-single edge notched tensile (M−SENT) and micro-central cracked tension (M-CCT) model specimens under different constraint conditions were analyzed using the new model. Furthermore, the effects of grain orientation on the MPFCP rate and path were discussed. It was found that with the increase of constraints, the number of cycles required for the same MPFCP length increased for both M−SENT and M-CCT model specimens. A crack length of a = 100 μm was the transition length from the microstructurally small crack stage to the physically small crack stage. The diversity of grain orientation distribution had a significant influence on the MPFCP path and rate within each grain.

Estimating suburban preferences for integrated public transit and ridesourcing services through a revealed-preference/stated-preference survey

Low-density areas like suburbs can be challenging to serve with fixed-route transit. Demand responsive transit has been used to service these areas with mixed results, but suburban travellers may respond to ridesourcing integrated with transit more positively because of the improved booking and dispatching interface offered by ridesourcing. This paper explores how suburban residents perceive transit-integrated ridesourcing, both for cost and time attributes of the system and in comparison to other travel modes in the area (cycling, auto, taxi or Uber, and transit). A Hierarchical Bayes mixed logit model was estimated using the results of a revealed-preference/stated-preference survey, revealing that transit-integrated ridesourcing had similar preferences to fixed-route transit. Across time and cost variables, respondents were more sensitive to walk time, transfer time, the number of transfers, parking cost, and taxi or Uber fare, and were minimally impacted by changes in the IVTT deviation of their trip times. The change in utility for the length of transit or transit-integrated ridesourcing in-vehicle travel time versus auto in-vehicle travel time was found to have a strongly linear relationship, while other attributes exhibited non-linearity to different degrees, particularly walk time, transfer time, transit-integrated ridesourcing fare, and taxi or Uber fare. Respondents who were female, younger, taking home-based school trips, or had lower household incomes were more open to transit-integrated ridesourcing and transit than other demographic groups. The likelihood of taking each of the five travel modes was assessed after COVID-19, finding generally minimal to negative impact for transit-integrated ridesourcing. Insights into vehicle and transit pass ownership, existing travel alternatives, and a prior transit-integrated ridesourcing pilot in the area are presented.

Overlap classification-based and kinematically coordinated corner rounding using double asymmetrical transitions for five-axis short-segmented tool path

For traditional corner rounding methods that adopt a pair of symmetrical and asymmetrical transition curves to round the corners in tool tip position and tool orientation, there are still two critical issues remain unsolved. One is the overlaps between adjacent transition curves have to be eliminated specifically to avoid the excessive approximation of the transition curves to the corners, the other is the asynchronous motions between tool tip position and tool orientation on the transition curves need to be addressed in addition to achieving smoothness at the junctions between transition curves and linear segments. For solving these two issues, this paper proposes the overlap classification-based and kinematically coordinated double asymmetrical transitions to round the corners. In this method, the overlaps between adjacent transition curves in tool tip position and tool orientation are first classified. Then, parameter synchronization is derived analytically and a targeted overlap elimination strategy is also proposed, so that the transition lengths without forcing the transition curves over-approach the corners are determined. Furthermore, to alleviate the asynchronous motions, the kinematics coordination between tool tip position and tool orientation is introduced, which is utilized to further evaluate the transition lengths to improve the kinematics performance of rotary axes. Finally, the computer simulations and machining experiment both demonstrate that our method can improve the feedrate and achieve the low acceleration and jerk of rotary axes indeed.

Seeking for Optimal Excited States in Photoinduced Electron-Transfer Processes─The Case Study of Brooker's Merocyanine.

Material design enters an era in which control of electrons in atoms, molecules, and materials is an essential property to be predicted and thoroughly understood in view of discovering new compounds with properties optimized toward specific optical/optoelectronic applications. π-electronic delocalization and charge separation/recombination enter notably into the set of features that are highly desirable to tailor. Diverse domains are particularly relying on photoinduced electron-transfer (PET), including fields of paramount importance such as energy production through light-harvesting, efficient chemoreceptive sensors, or organic field-effect transistors. In view of completing the arsenal of strategies in this area, we selected Brooker's merocyanine─a typical [D-π-A] compound─as the case study and examined from time-dependent density functional theory the opportunity offered by selected excited states to reach a suited manipulation of the charge transfer (CT) extent. In addition to the consideration of diagnostic tools able to spot the charge amount (i.e., magnitude of electron fraction) transferred upon excitation (qCT), the spatial extent associated with such an electronic transition or CT length (DCT), as well as the corresponding variation in dipole moment between the ground and the excited states (μCT), further analysis of the excitation process was undertaken. The advantage of going beyond the above-mentioned molecular indicators─which can be considered as PET global indices─was explored on the basis of a partitioning of the electron density. Relevant insight was gained on the relation these global indices have with the evolution of (local) features characterizing either chemical bond or electron delocalization upon vertical excitations.

Investigation on optimum lengths of railway clothoid transition curves based on passenger ride comfort

Transition curves are considered as inevitable parts of railways. As they tend to bend the train’s course of movement, the passengers ride comfort and safety are strongly challenged. Although many investigations have performed on the dynamic behavior characteristics of transition curves, only a few studies have focused on the optimum transition length based on the ride comfort index. In response to this need, a vehicle-track dynamic model was developed. The results obtained from model were compared with those available in the literature to prove the validity of the model. Through a parametric study, the dynamic responses of a vehicle passing through the transition curves was studied using the model developed in this study. The ride comfort index was obtained using the dynamic response of the vehicle. The lateral jerk, vertical acceleration, and rolling velocity were considered as the dynamic response of the vehicle. The optimum transition curve length for various configurations of curve radius and super-elevation magnitudes were investigated based on the obtained ride comfort.

Incoherent light in tapered graded-index fibre: A study of transmission and modal noise

We investigated the impact of taper length on light transmission through tapered graded-index fibres. We tested commercial fibres from Thorlabs and a custom graded-index fibre using both coherent and incoherent light sources. Our experimental results show optimum performance for taper transition lengths of 25 mm, although our simulations suggest further improvement may be possible for even shorter transition lengths. We also measured the modal noise power fluctuations caused by bending the fibre. Here, we observe that the custom fibre tapers have the highest transmission but suffer from the most modal noise. Accordingly, we find that the commercial graded-index fibre tapers promise practical usage as a beam mode-field converter, as they have lower power fluctuations but retain relatively high transmission if compared to commercial small core step-index fibre.

Motion In-Betweening via Two-Stage Transformers

We present a deep learning-based framework to synthesize motion in-betweening in a two-stage manner. Given some context frames and a target frame, the system can generate plausible transitions with variable lengths in a non-autoregressive fashion. The framework consists of two Transformer Encoder-based networks operating in two stages: in the first stage a Context Transformer is designed to generate rough transitions based on the context and in the second stage a Detail Transformer is employed to refine motion details. Compared to existing Transformer-based methods which either use a complete Transformer Encoder-Decoder architecture or additional 1D convolutions to generate motion transitions, our framework achieves superior performance with less trainable parameters by only leveraging the Transformer Encoder and masked self-attention mechanism. To enhance the generalization of our transformer-based framework, we further introduce Keyframe Positional Encoding and Learned Relative Positional Encoding to make our method robust in synthesizing longer transitions exceeding the maximum transition length during training. Our framework is also artist-friendly by supporting full and partial pose constraints within the transition, giving artists fine control over the synthesized results. We benchmark our framework on the LAFAN1 dataset, and experiments show that our method outperforms the current state-of-the-art methods at a large margin (an average of 16% for normal-length sequences and 55% for excessive-length sequences). Our method trains faster than the RNN-based method and achieves a four-time speedup during inference. We implement our framework into a production-ready tool inside an animation authoring software and conduct a pilot study to validate the practical value of our method.

Computational fluid dynamics-based parametric study on the performance of solar air heater channel.

Computational fluid dynamics (CFD) plays a prominent role in the design and development of solar air heaters. The previous investigations have lagged in using a radiation model for the solar heat input; instead, most of the researchers simulated a constant heat flux model. Moreover, an extensive study on the geometrical and boundary conditions like confinement and transition length, suction, and blowing effects has not been studied. The present investigation deals with the aforementioned effects on the flow and heat transfer characteristics of the SAH channel, which is designed for residential space heating. The finite volume-based solver Ansys Fluent is used for finding the field variables. The confinement height is varied from 25 to 150mm, and the transition length is varied from 250 to 1000mm. The suction and blowing effect is investigated by changing the flow direction across the channel. Even though the temperature rise is less significant with respect to confinement height and transition length, the effective efficiency increases with decreasing channel height and increasing transition length. In general, blowing of air across the channel gives better performance than suction. When comparing them, the influence is less in temperature rise and more in pressure drop for the channel height of 25mm, whereas the channel height of 150mm has better influence in temperature rise and less influence in pressure drop.

Experimental Study on Mechanical Properties of Half-Grouted Sleeve Connections with Grouted Defects

Prefabricated concrete structures are driving the development of green buildings, and the connection between prefabricated components is the main factor affecting the safety performance of these structures. Grouted sleeve technology can effectively improve the safety performance of precast component connections. In the process of grouting operation, the grouted sleeves are affected by the construction environment and often have various defects. In this work, to study the influence of defects on the mechanical properties of half-grouted steel sleeve connections, 33 specimens (10 groups of defective specimens (three in each group) and 1 standard group) were prepared and subjected to uniaxial tensile tests. The failure modes, load–displacement curves, stress distribution, and other mechanical properties of the specimens were studied. Sleeves with different defects were simulated, and the simulation results were compared with the experimental results. The experimental results showed that the failure modes are rebar fracture and rebar pull-out. In the strengthening stage, the specimens exhibited a large slip. The critical length for failure mode transition was 2.5 d (defect length). The middle defects and uniform defects had the most unfavorable effect on the ultimate bearing capacity of the specimens. The stress transfer was blocked in specimens with end and middle defects. The numerical simulation results were consistent with the experimental results, thus verifying the accuracy and feasibility of the simulation method for practical applications.

Ignition limit and shock-to-detonation transition mode of n-heptane/air mixture in high-speed wedge flows

In this work, oblique detonation of n-heptane/air mixture in high-speed wedge flows is simulated by solving the reactive Euler equations with a two-dimensional (2D) configuration. This is a first attempt to model complicated hydrocarbon fuel oblique detonation waves (ODWs) with a detailed chemistry (44 species and 112 reactions). Effects of freestream equivalence ratios and velocities are considered, and the abrupt and smooth transition from oblique shock to detonation are predicted. Ignition limit, ODW characteristics, and predictability of the transition mode are discussed. Firstly, homogeneous constant-volume ignition calculations are performed for both fuel-lean and stoichiometric mixtures. The results show that the ignition delay generally increases with the wedge angle. However, a negative wedge angle dependence is observed, due to the negative temperature coefficient effects. The wedge angle range for successful ignition of n-heptane/air mixtures decreases when the wedge length is reduced. From two-dimensional simulations of stationary ODWs, the initiation length generally decreases with the freestream equivalence ratio, but the transition length exhibits weakly non-monotonic dependence. Smooth ODW typically occurs for lean conditions (equivalence ratio < 0.4). The interactions between shock/compression waves and chemical reaction inside the induction zone are also studied with the chemical explosive mode analysis. Moreover, the predictability of the shock-to-detonation transition mode is explored through quantifying the relation between ignition delay and chemical excitation time. It is demonstrated that the ignition delay (the elapsed time of the heat release rate, HRR, reaches the maximum) increases, but the excitation time (the time duration from the instant of 5% maximum HRR to that of the maximum) decreases with the freestream equivalence ratio for the three studied oncoming flow velocities. Smaller excitation time corresponds to stronger pressure waves from the ignition location behind the oblique shock. When the ratio of excitation time to ignition delay is high (e.g., > 0.5 for n-C7H16, > 0.3 for C2H2 and > 0.2 for H2, based on the existing data compilation in this work), smooth transition is more likely to occur.