Transport Budget Research Articles

Particle Image Velocimetry And Scaling Of Mach 0.3 And 1.25 Axisymmetric Turbulent Jets

The influence of compressibility on the scaling of the Reynolds stress transport (RST) budgets is investigated for Mach 0.3 and Mach 1.25 free axisymmetric turbulent jets using particle image velocimetry (PIV). It is proposed that the RST equations can be shown in self-preserving form scaled by the local streamwise evolving spreading rate, b’. It is shown that b’ is a function of the local convective Mach number, M_c. Compressibility effects on the suppression of the Reynolds stresses and higher turbulence moments are detailed. From self-preservation principles, the budgets scaled by powers of b’. In this form, most terms in the budgets are scaled appropriately indicating self-preservation. Terms which are not properly scaled are isolated then which allows for insight on effects of compressibility on turbulence.

Embedded direct numerical simulation of ignition kernel evolution and flame initiation in dual-fuel spray assisted combustion

The flame initiation process in dual-fuel spray assisted combustion is presently not fully understood. Here, diesel spray assisted combustion of premixed methane/oxidizer/EGR is explored in the post-ignition phase by scale-resolved simulations. The modified dual-fuel ECN Spray A forms the baseline configuration. An extensive local grid refinement (approaching DNS limit) around one of the first high-temperature ignition kernels is carried out in order to examine the validity of hypothesized flame initiation and deflagration. A high quality LES is used to solve the spray dynamics, while the embedded quasi-DNS (eq-DNS) region offers detailed information on the ignition kernel evolution. The finite-rate chemistry is directly integrated, utilizing 54 species and 269 reactions. Local combustion modes are investigated for the ignition kernel development toward spontaneous ignition and premixed flame propagation using various approaches, including the reaction front displacement speed, energy transport budget, and chemical explosive mode analysis. Furthermore, a new criterion based on reaction flux analysis is introduced, which is compatible with dual-fuel combustion. The spatial and temporal scales associated with the ambient methane consumption and consequent flame initiation are characterized. For the first time in dual-fuel spray assisted simulations, numerical evidence is provided on the initiation of premixed flames, and the corresponding timescale is reported. Particularly, there is a transient mixed-mode combustion phase of approximately 0.2 ms after the spray second stage ignition wherein extinction, ignition fronts, and quasi-deflagrative structures co-exist. After such a transient period, the combustion mode becomes essentially deflagrative. Finally, interactions between turbulence and premixed flame front are characterized mostly in the corrugated regime.

Analysis of improved digital filter inflow generation methods for compressible turbulent boundary layers

We propose several enhancements to improve the accuracy and performance of the digital filter turbulent inflow generation technique and assess their efficacy in the context of wall-resolved large-eddy simulations of a compressible turbulent boundary layer. Improvements of accuracy include a more realistic correlation function for the transversal directions, target length scales that vary with wall-distance, and a counter-intuitive approach that involves the suppression of streamwise velocity fluctuations at the inflow. For improving the computational performance, we propose to generate the inflow data in parallel in single precision and at a prescribed time interval based on the turbulence time scale, and not at every time-step of the simulation. Based on the results of 7 wall-resolved large-eddy simulations, we find that the new correlation functions and the considered performance improvements are beneficial and therefore desired. Suppressing streamwise velocity fluctuations at the inflow leads to the fastest relaxation of the pressure fluctuations; however, this approach increases the adaptation length defined in terms of compliance with the von Kármán integral equation. The adaptation length can be shortened by artificially increasing the wall-normal Reynolds stresses, thereby preserving the desired turbulence kinetic energy level. A detailed inspection of the Reynolds stress transport budgets reveals that the observed spurious spatial transients are largely driven by pressure-related terms. For instance, increased values of u′p′¯ are found throughout the computational domain when a physical Reynolds stress distribution is prescribed at the inflow. Therefore, efforts to enhance digital filter techniques should aim at modeling pressure fluctuations as well as their correlation with the velocity components.

The Processes of Aggradation and Incision in the Channels in the Terek River Basin, the North Caucasus: The Hydrological Fluvial Archives of the Recent Past

The rates of incision and aggradation in the channels in the Terek River basin (North Caucasus) for the last 50–85 years were estimated at 18 gauging stations. The stage–discharge method (annual low water stages at the same discharges) was applied. The stability of the Terek River channel was recorded on the tectonically subsiding Tersko–Kuma Lowland. On the subsiding Kabardian Plain, channel aggradation up to 14 mm a−1 was registered. The rapid (~32 mm a−1) incision of the Terek River occurs within the antecedent valley of the rising Sunzha Ridge, causing regressive erosion and incision (~25 mm a−1) of rivers on the Ossetian Plain, despite its tectonic subsiding. The rivers in the uplifting mountains of the North Caucasus transport the sediments delivered from slopes as climatically controlled debris flows. Aggradation and incision here alternate without a visible overall trend. The rates of modern channel bed deformations are 10 to 100 times higher than the mean rates of tectonic movements. The main effect of tectonics is the changes in river channel slopes, which cause changes in the bed load transport budget and channel bed deformation. Human-made constructions induce rapid deformations in the channels but have a local effect.

A numerical study on Langmuir circulations and coherent vortical structures beneath surface waves

Langmuir circulations (LCs) arise through the interaction between the Lagrangian drift of the surface waves and the wind-driven shear layer. Quasi-streamwise vortices (QSVs) also form in the turbulent shear layer next to a flat surface. Both vortical structures manifest themselves by inducing wind-aligned streaks on the surface. In this study, numerical simulations of a stress-driven turbulent shear layer bounded by monochromatic surface waves are conducted to reveal the vortical structures of LCs and QSVs, and their interactions. The LC structure is educed from conditional averaging guided by the signatures of predominant streaks obtained from empirical mode decomposition; the width of the averaged LC pair is found to be comparable to the most unstable wavelength of the Craik–Leibovich equation. Coherent vortical structures (CVSs) are identified using a detection criterion based on local analysis of the velocity-gradient tensor and their topological geometry; QSVs accumulated beneath the windward surface are found to dominate the distribution. Employing the variable-interval spatial average to the identified QSVs further reveals that QSVs tend to form in the edge vicinity of the surface streaks induced by the LCs. The transport budgets of streamwise enstrophy are examined to reveal the interaction. It is found that QSVs perturb the streaks resulting in a localized streamwise gradient of the spanwise velocity, that is, vertical vorticity. The vertical shear tilts the vertical vorticity, therefore enhancing streamwise enstrophy production and the formation of QSVs. The results highlight the differences in the CVSs between the Langmuir turbulence and the wall turbulence.

Numerical study of combustion mode and diffusive transport underlying fuel ignitions in lean stratified dimethyl ether-air premixture at various turbulence intensities

This paper presents a systematic numerical investigation of the auto-ignition phenomenon in stratified dimethyl ether (DME)/air turbulent mixtures. The study was conducted at an initial pressure of 30 atm, mean temperature of 650 K, and equivalence ratio of 0.45, with varying Damköhler numbers (Da) ranging from 0.01 to 1.1. The simulations indicated that the transition point between two combustion modes, deflagration wave and spontaneous ignition, occurred at a Da value of 0.06. The chemical explosive mode analysis identified the leading role of diffusive transport in both modes. A diffusion index (DI) was defined to quantify contributions of each diffusive sub-process. The DI results revealed that heat conduction and oxygen diffusion are the most important processes for the low-temperature ignition, followed by DME, CH2O, and HOCH2OCO with decreasing importance. Further DI analysis of the detailed local flame structure demonstrated that heat, DME, O2, and H2O2 diffusive transports have moderate importance for depressing turbulent propagation at local extinction modes. In conclusion, the proposed DI method has the potential to be a more efficient and rigorous substitution for sensitivity analysis and transport budget analysis.

Transport infrastructure development issues affecting the socio-economic situation in Arctic

The object of the research in this article is the transport infrastructure of the regions that are part of the Arctic zone of the Russian Federation. The authors aim to study the main socioeconomic problems that exist in the region due to the underdevelopment of transport infrastructure, as well as ways to solve them. To fulfil their research objectives, the authors used a comparative analysis method, a correlation analysis method, and a legal analysis. The study identified the specifics of the construction of transport infrastructure in the far north regions. The length of the road and rail network in the Arctic has been analysed compared with the regions of Central Russia, and the factors that limit the development of transport infrastructure have been identified. The strong dependence of freight traffic volumes on population size was revealed using the example of the Murmansk Oblast, and the main problems affecting the demographic situation in the region were identified. The problem of low passenger flow in air transportation was considered. The authors suggested the following ways of solving the problems: increasing the number of contracts PPP in their classic form, reducing the number of concession agreements, creating a transport system project based on the example of the US Interstate Highway System in the Russian Arctic, drafting a unified transport budget for all territories that make up the Arctic zone, creating a single executive authority for the Arctic zone to distribute the budget evenly.

Unsteady self-similarity of jet fluid age and mass fraction

Scalar mixing is investigated in a decelerating turbulent round jet using direct numerical simulation. The main focus is to determine and model any new self-similar states in the unsteady flow as well as predict the centerline evolution of relevant flow quantities. The mass fraction of jet fluid and the fluid residence time, measured by the mass-weighted stream age of the jet fluid, both exhibit self-similar radial profiles in statistically stationary turbulent jets. Upon stopping the inflow, a deceleration wave passes through the jet, behind which a new self-similar state is observed for the two scalar variables. The self-similar state during the jet deceleration is different from that in the statistically stationary jet. Contrary to the steady-state behavior, the jet fluid mass fraction exhibits a linear increase with downstream distance in the decelerating jet, whereas the centerline mass-weighted stream age of jet fluid remains proportional to downstream distance. Scalar transport budget analysis shows that the radial transport term increases for both scalars, and that the contribution of the streamwise transport term of jet mass fraction changes sign between steady and unsteady cases.

Circulation and overturning in the eastern North Atlantic subpolar gyre

This study describes new transport estimates of the North Atlantic Current in the Iceland Basin, and uses these results along with other contemporaneous measurements to determine mass and overturning budgets for the eastern North Atlantic subpolar gyre. As part of the Overturning in the Subpolar North Atlantic Program (OSNAP), estimates of the North Atlantic Current are determined using three full-depth dynamic height moorings spanning the Iceland Basin and are supplemented by Argo and satellite altimetry data. Along with historical estimates of the exchanges over the Iceland-Scotland Ridge, additional OSNAP results from the Rockall Trough and Rockall-Hatton Bank regions are used to calculate transport budgets in different density layers over a broad portion of the eastern subpolar gyre. Results show that 13–14 Sv of the North Atlantic Current (σθ < 27.8 kg m−3) flow northward into the middle of the Iceland Basin through a primary baroclinic flow near 23.5°W and a secondary quasi-barotropic flow near 26°W. Together with the observed northward flow in the Rockall-Hatton area, we conclude that 19–20 Sv of the upper limb of the Atlantic Meridional Overturning Circulation (σθ < 27.56 kg m−3) flows into the region where nearly 40 % of it (7.3 Sv) is converted into the lower limb primarily through progressive water mass modification from atmospheric cooling. This accounts for nearly half of the strength of Atlantic Meridional Overturning Circulation defined by the full OSNAP array extending across the basin from Greenland to Scotland.

Spectral Analysis on Transport Budgets of Turbulent Heat Fluxes in Plane Couette Turbulence

In recent years, scale-by-scale energy transport in wall turbulence has been intensively studied, and the complex spatial and interscale transfer of turbulent energy has been investigated. As the enhancement of heat transfer is one of the most important aspects of turbulence from an engineering perspective, it is also important to study how turbulent heat fluxes are transported in space and in scale by nonlinear multi-scale interactions in wall turbulence as well as turbulent energy. In the present study, the spectral transport budgets of turbulent heat fluxes are investigated based on direct numerical simulation data of a turbulent plane Couette flow with a passive scalar heat transfer. The transport budgets of spanwise spectra of temperature fluctuation and velocity-temperature correlations are investigated in detail in comparison to those of the corresponding Reynolds stress spectra. The similarity and difference between those scale-by-scale transports are discussed, with a particular focus on the roles of interscale transport and spatial turbulent diffusion. As a result, it is found that the spectral transport of the temperature-related statistics is quite similar to those of the Reynolds stresses, and in particular, the inverse interscale transfer is commonly observed throughout the channel in both transport of the Reynolds shear stress and wall-normal turbulent heat flux.

Spectral analysis on dissimilarity between turbulent momentum and heat transfers in plane Couette turbulence

Nonlinear interactions between different scales in turbulence result in both interscale and spatial transport of turbulent energy, and their role in the turbulent heat transfer mechanism is also of practical importance from an engineering viewpoint. In this study, we investigate a turbulent plane Couette flow with passive-scalar heat transfer at the Prandtl number of 0.71 to discuss the similarity/difference between scale interactions in velocity and temperature fields. The constant-temperature-difference boundary condition is used so that the mean velocity and temperature profiles are similar, and then, the roles of interscale and spatial transports are compared for the spectral transport budgets of turbulent energies and temperature-related statistics. We show that turbulent heat transfer occurs at relatively small streamwise length scales compared to momentum transfer, although molecular diffusion is more significant in the temperature field as the Prandtl number is less than 1. Detailed analysis on the transport budgets of temperature-related spectra shows that scale interactions in the temperature field supply more energy to small scales than those in the velocity field. This significant temperature cascade causes more energetic temperature fluctuation at small scales, resulting in the spectral dissimilarity between turbulent heat and momentum transfers.

Insights into multidimensional transport flux from vertical observation and numerical simulation in two cities in North China

This study represents the first quantitative evaluation of pollution transport budget within the boundary layer of typical cities in the Beijing-Tianjin-Hebei (BTH) region from the perspective of horizontal and vertical exchanges and further discusses the impact of the atmospheric boundary layer (ABL)-free troposphere (FT) exchange on concentration of fine particulate matter (PM2.5) within the ABL during heavy pollution. From the perspective of the transport flux balance relationship, differences in pollution transport characteristics between the two cities is mainly reflected in the ABL-FT exchange effect. The FT mainly flowed into the ABL in BJ, while in SJZ, the outflow from the ABL to the FT was more intense. Combined with an analysis of vertical wind profile distribution, BJ was found to be more susceptible to the influence of northwest cold high prevailing in winter, while sinking of strong cold air allowed the FT flowing into the ABL influence the vertical exchange over BJ. In addition, we selected a typical pollution event for targeted analysis to understand mechanistic details of the influence of ABL-FT exchange on the pollution event. These results showed that ABL-FT interaction played an important role in PM2.5 concentration within the ABL during heavy pollution. Especially in the early stage of heavy pollution, FT transport contributed as much as 82.74% of PM2.5 within the ABL. These findings are significant for improving our understanding of pollution transport characteristics within the boundary layer and the effect of ABL-FT exchange on air quality.

Microplastics in agricultural soils from a semi-arid region and their transport by wind erosion

Despite the importance of agricultural soils, little is known about the fate of microplastics (MPs) in this environment. In the present study, MPs have been determined in soils and wind-eroded sediments from two vegetable-growing fields in the Fars province of Iran, one using plastic mulch for water retention (Field 1) and the other using wastewater for irrigation (Field 2). MPs were heterogeneously distributed in the surface (0–5 cm) and subsurface (5–15 cm) soils of both fields, with a maximum concentration overall of about 1.1 MP g−1 and no significant differences in concentrations between either fields or depths. Fibres represented the principal shape of MPs, but spherules, presumably from wastewater, also made a significant (∼25%) contribution to MPs in Field 2. Analysis of selected samples by Raman spectroscopy and scanning electron microscopy revealed that polyethylene terephthalate (PET) and nylon were the most abundant polymers and that MPs exhibited varying degrees of weathering. Concentrations of MPs in this study are within the range reported previously for agricultural soils, although the absence of PET observed in earlier studies is attributed to the use of insufficiently dense solutions to isolate plastics. Deployment of a portable wind tunnel revealed threshold wind velocities for soil erosion of up to 7 and 12 m s−1 and MP erosion rates up to about 0.4 and 1.1 MP m−2 s−1 for Fields 1 and 2, respectively. Erosion rates are considerably greater than published depositional rates for MPs and suggest that agricultural soils act as both a temporary sink and dynamic secondary source of MPs that should be considered in risk assessments and global transport budgets.

Analysis of turbulence generation and dissipation in shear layers of methane–oxygen diffusion flames using direct numerical simulations

A turbulent methane–oxygen diffusion flame is studied using a direct numerical simulation setup. The operating regime and turbulence characteristics are chosen to resemble those of a modern methane rocket combustor. Local flame characteristics and dimensionless numbers are defined and evaluated, and their relationship with the turbulent kinetic energy transport budget is studied. Positive net turbulence generation is observed in the reaction shear layer. It is found that the underlying mechanisms for these results are similar to those encountered in premixed flames, with pressure terms acting as the primary turbulent kinetic energy sources. Models for predicting turbulent transport through mean pressure gradients, fluctuating pressure gradients, and turbulent flux of turbulent kinetic energy are adapted. The accuracy of the proposed formulations is assessed, and the involved challenges are discussed.

Local Equilibrium and Discretization Effects on the Prediction of Scalar Dissipation Rates in Large Eddy Simulation of Turbulent Non-Premixed Combustion

ABSTRACT The effects of discretization and local equilibrium on the prediction of scalar dissipation rates are investigated by performing large-eddy simulation (LES) of a turbulent-piloted jet diffusion flame, Sandia Flame D. As a way to reduce the discretization errors on the prediction of scalar dissipation, an approach to use a higher-order finite difference scheme only for the evaluation of scalar gradients in a scalar dissipation model, while keeping the overall accuracy of the numerical method at second order, is investigated. Results indicate that the fourth-order scheme improves the accuracy with negligible addition to computational cost. With the improved evaluation of scalar gradients, the local equilibrium assumption, which is used in the algebraic model, is investigated by solving the subgrid-scale variance transport equation, where a model coefficient in a scaling-based relaxation model for subgrid-scale scalar dissipation is varied. It is found that the conditional mean scalar dissipation rates are insensitive to the model coefficient. This insensitivity is attributed to the prevalence of local equilibrium. A rapid decay of imbalance between production and dissipation of subgrid-scale scalar fluctuations is seen in both the predicted scalar dissipation and the transport budget in the variance equation. It is argued that since, at high Reynolds numbers, the transport of the scalar fluctuations occurs over a timescale larger than the subgrid-scale mixing timescale, its contribution to the evolution of subgrid-scale variance is statistically insignificant except near the nozzle exit where different streams begin to mix and scalar layers evolve rapidly. Consequently, the conditional (and unconditional) mean scalar dissipation rates in LES tend to depend solely on the production of subgrid-scale scalar fluctuations in most regions of turbulent jet flames, when the modeling is based on the Kolmogorov scaling.

North to South Variations in the Suspended Sediment Transport Budget within Large Siberian River Deltas Revealed by Remote Sensing Data

This study presents detailed suspended sediment budget for the four Siberian river deltas, representing contrasting conditions between Northern and Southern environments. Two of the studied rivers empty their water and sediments into the marine located in the permafrost zone in the Arctic region (Lena and Kolyma), and the other two (Selenga and Upper Angara) flow into Lake Baikal located in the steppe and forest-steppe zone of Southern Siberia. For the first time, these poorly monitored areas are analyzed in terms of the long-term and seasonal changes of spatial patterns of suspended sediment concentrations (SSC) over distributaries systems. Remote sensing reflectance is derived from continuous time series of Landsat images and calibrated with the onsite field measurements of SSC. Seasonal variability of suspended sediment changes over deltas was captured for the period from 1989 to 2020. We identify significant variability in the sedimentation processes between different deltas, which is explained by particularities of deltas networks and geomorphology and the existence of specific drivers—continuous permafrost impact in the North and abundant aquatic vegetation and wetland-dominated areas in the South. The study emphasizes that differences exist between Northern and Southern deltas regarding suspended sediments transport conditions. Mostly retention of suspended sediment is observed for Southern deltas due to sediment storage at submerged banks and marshlands located in the backwater zone of the delta during high discharges. In the Northern (arctic) deltas due to permafrost impacts (melting of the permafrost), the absence of sub-aquatic banks and river to ocean interactions of suspended sediment transport is mostly increased downwards, predominantly under higher discharges and along main distributary channels. These results shine light on the geochemical functions of the deltas and patterns of sequestering various metals bound to river sediments.

Structure and propagation speed of autoignition-assisted flames of jet fuels

The flame structure and propagation speed of autoignition-assisted fuel-lean and fuel-rich flames of jet fuels are numerically and analytically investigated at elevated pressures and temperatures. The results show that while the propagation speed increases dramatically for large residence time, for fuels with strong low temperature chemistry (LTC), the propagation speed may first increase slightly at relatively small residence time. The species evolution in time shows that with the increase of propagation speed, the trajectories for autoignition-assisted flames first rapidly and then gradually converge to the autoignition trajectory. Furthermore, transport budget analysis shows that as the propagation speed increases, the balance in the high temperature reaction zone evolves from reaction-diffusion balance to reaction-convection balance, demonstrating the transition from flame propagation to autoignition. An analytical model, which only needs to calculate the corresponding 0D autoignition process, is proposed to predict the autoignition-assisted flame speed. The good qualitative agreement with the 1D simulation result indicates that the autoignition assistance on the propagation speed mainly results from the temperature rise ahead of the preheat zone. Finally, different scaling correlations for the normalized flame propagation speed Sl/Sl0 are compared, showing a universal scaling for n-heptane, n-dodecane, and Jet-A over a wide range of pressures (5–35 atm), temperatures (600–1015 K), and equivalence ratios (0.5–1.2) with the normalized maximum mole fractions of CO.

Flame edge dynamics in counterflow nonpremixed flames of CH4/He versus air at low strain rates: An experimental and numerical study

The characteristics of the flame structure, stabilization, and extinction of counterflow nonpremixed flames of CH4/He versus air at low strain rates are investigated by performing a series of experiments and two-dimensional (2-D) numerical simulations. By adopting an experimental methodology using He curtain flow, we can locate the flames near the center of the counterflow burner and measure the critical He mole fraction in the fuel stream, XHe,cr, for flame extinction at very-low strain rates. XHe,cr obtained from 2-D numerical simulations in normal and zero gravity show a good agreement with those from the experiments, which substantiates that the experimental methodology can effectively reduce the buoyancy effect at low strain rates. It is found from various steady and unsteady 2-D numerical simulations that the dynamics of flame edge plays a critical role in determining the flame stabilization and extinction, and the edge flame is stabilized at a location where negative edge flame propagation speed, Se, balances positive local flow velocity, Ue. The transport budget analysis reveals that despite the negative Se by the diffusive loss of heat and radicals, the edge flame can survive by the help of the convective gain of heat and radicals from the trailing diffusion flame. It is also found that the counterflow flame can survive the increase of He mole fraction in the fuel stream, XHe, by shrinking its flame length since the local chemical reaction at the flame edge is enhanced with decreasing the flame length. However, as XHe exceeds XHe,cr, a slight inward movement of the edge flame induces a large magnitude of negative Se compared to positive Ue such that the counterflow flame is totally extinguished by the shrinkage of the outer edge flame toward the flame center.

Regulating mobility-on-demand services: Tri-level model and Bayesian optimization solution approach

The goal of this paper is to develop a modeling framework that captures the inter-decision dynamics between mobility service providers (MSPs) and travelers that can be used to optimize and analyze policies/regulations related to MSPs. To meet this goal, the paper proposes a tri-level mathematical programming model with a public-sector decision maker (i.e. a policymaker/regulator) at the highest level, the MSP in the middle level, and travelers at the lowest level. The public-sector decision maker aims to maximize social welfare via implementing regulations, policies, plans, transit service designs, etc. The MSP aims to maximize profit by adjusting its service designs. Travelers aim to maximize utility by changing their modes and routes. The travelers’ decisions depend on the regulator and MSP’s decisions while the MSP decisions themselves depend on the regulator’s decisions. To solve the tri-level mathematical program, the study employs Bayesian optimization (BO) within a simulation–optimization solution approach. At the lowest level, the solution approach includes an agent-based transportation system simulation model to capture travelers’ behavior subject to specific decisions made by the regulator and MSP. At the middle and highest levels, the solution approach employs BO for the MSP to maximize profit and for the regulator to maximize social welfare. The agent-based transportation simulation model includes a mode choice model, a road network, a transit network, and an MSP providing automated mobility-on-demand (AMOD) service with shared rides. The modeling and solution approaches are applied to Munich, Germany in order to validate the model. The case study investigates the tolls and parking costs the city administration should set, as well as changes in the public transport budget and a limitation of the AMOD fleet size. Best policy settings are derived for two social welfare definitions, in both of which the AMOD fleet size is not regulated as the shared-ride AMOD service provides significant value to travelers in Munich.

Direct numerical simulation of a spatially developing n-dodecane jet flame under Spray A thermochemical conditions: Flame structure and stabilisation mechanism

We present results from a three-dimensional (3D) direct numerical simulation (DNS) of a spatially developing n-dodecane round jet flame. The thermochemical conditions (i.e. pressure, temperature and oxidiser composition) correspond to those of Spray A, an experimental target flame of the Engine Combustion Network (ECN). To make the DNS computationally tractable, we consider a gas jet with a reduced Reynolds number of 17,000 and a shorter lifted length of 11 times the jet diameter. The flame structure and stabilisation mechanism of the statistically steady jet flame are discussed. Overall the flame structure is similar to that identified from experimental observations, i.e. a region of high formaldehyde (CH2O) concentration, which marks the low-temperature chemistry (LTC), is present upstream of the main flame and persists downstream in the central region of the jet where the mixture is rich. A high-temperature (HTC) nonpremixed flame marked by OH radicals shrouds the jet. Ignition kernels are observed upstream of the flame base, some of which are convected downstream and join with the main flame. At the flame base, a three-branch structure is observed, namely an LTC branch upstream of the flame, a rich HTC branch attached to the flame base and a trailing nonpremixed flame anchored at the stoichiometric mixture fraction. A detailed analysis of the flame stabilisation mechanism is reported, employing structural comparisons to two-dimensional (2D) reference cases, transport budgets, and flame speeds. It is concluded from these analyses that the flame is stabilised principally by flame propagation. The role of autoignition kernels is also analysed and found to be secondary.