Motivated by increasing open access of direct numerical simulation (DNS) and experimental data, we discuss potentially overlooked phenomenological issues related to the formulation of a leading-order reconstruction of the law of the wall (LoW) in fully developed turbulent channel and pipe flows of constant fluid properties. Most of these issues relate to seemingly asymptotic constants or scalings evidenced in the available DNS data. These allow the derivation of a classical layer-based fit for the LoW of the mean velocity profile (MVP) in a clean and straightforward way. For low-to-moderate Reynolds number flows, the inherent empiricism due to incomplete similarity is handled by the specification of additional data-based correlations. As an example, the centerline velocity is required for the full specification of the MVP-LoW. We also discuss a novel layer-based fit for the wall-normal profiles of the turbulence kinetic energy (TKE) and the TKE-dissipation rate. The latter fits are mostly derived from DNS data, although we offer a series of novel insights, and we derive an overlooked scaling of the turbulence frequency which is related to our MVP-LoW. We consider the layer-wise LoW to be of relevance for the understanding of wall-bounded turbulence. Its closed and explicit form relying on very simple linear or quasi-linear fits renders it appealing for implementation in coarsely resolved Reynolds-Averaged Navier-Stokes (RANS) models for turbulent pipe and channel flows.
In this study, we clarified the characteristics of flame propagation during combustion and the mechanism of laminarization resulting from the attenuation of turbulence structures in the flow field under the influence of the flame. We performed three-dimensional simulations of the wall turbulence in the combustion of premixed natural gas and premixed hydrogen. We adopted a direct numerical simulation method for compressible flow with combustion using a reaction model. Premixed natural gas and premixed hydrogen combustion have different laminarization mechanisms in turbulent combustion fields. The heat of the flame affects the velocity streaks and vortices of the turbulence structures differently, and changes in the wall shear stress and wall heat flux exhibit different tendencies. Depending on the thermal Péclet number, defined using the maximum local intensity of turbulence as a parameter for classifying the characteristics of the laminarization mechanism of the flow field, the wall shear stress may change before the wall heat flux, or both may change simultaneously, under the influence of combustion. The aforementioned results provide insights into the prediction and control of turbulent combustion for future applications.
The use of hydrogen to enrich or replace hydrocarbon-based flames represents an appealing perspective to reduce the emission of pollutants in gas turbine combustors. This implementation is, however, not straightforward, as the properties of hydrogen consistently differ from those of the most widely used fuels, resulting in a change in the combustion stability of the mixture. This work investigates the non-linear thermoacoustic stability of a simple Rijke tube containing a perfectly premixed, laminar, conical flame fed by either a pure methane-based mixture or a hydrogen-enriched one. The mean heat release rate and the geometry of the system were kept constant for both mixtures. Flame dynamics were modelled using a Level Set Method, from which a Flame Describing Function was derived for each mixture and coupled with a one-dimensional acoustic model within a low-order thermoacoustic framework to asses the stability of the system. The hydrogen-enriched flame was found to exhibit a weaker non-linear response than the methane-based flame, in agreement with previous observations reported in the literature. This behaviour was attributed to the higher unstretched laminar flame speed of the hydrogen-enriched mixture, which enhances the resilience of the flame front to develop geometric non-linearities following an acoustic perturbation, reducing its tendency towards heat release rate saturation. As a consequence, the hydrogen-enriched system generated larger thermoacoustic oscillations and ultimately experienced flashback, whereas the methane-based configuration remained bounded by more consistent non-linear saturation effects.
The mitigation of nitrogen oxide ( NO_x ) emissions remains a key challenge for the use of ammonia as a carbon-free energy carrier. In contrast to hydrogen, ammonia combustion is inherently prone to fuel- NO_x formation due to the nitrogen content of the fuel, limiting the effectiveness of purely temperature-based mitigation strategies. In this work, liquid water addition is investigated as a potential emission control approach for premixed ammonia/air flames using high-fidelity numerical simulations. A hybrid Eulerian–Lagrangian framework is employed to resolve the interaction between a spherically expanding quasi-laminar ammonia flame and evaporating water droplets. The influence of droplet sizes on flame propagation, flame thickness, and emission behavior is examined under stoichiometric conditions. Water addition is found to reduce the burning rate per unit area and increase flame thickness through combined thermal and dilution effects, while having a limited impact on flame surface wrinkling. Both small and large droplets lead to a significant reduction in NO formation, with large droplets producing localized regions of strong suppression and small droplets yielding a more spatially uniform reduction. Analysis of intermediate nitrogen species indicates a minor contribution of the N_2O pathway under the present conditions, while thermal and fuel- NO_x pathways remain sensitive to water-induced temperature changes. These results provide physical insight into the coupling between water evaporation, flame structure, and NO_x formation in premixed ammonia flames.
Developing accurate and robust combustion models is challenging, and their validation can be equally demanding. Even when suitable experimental or numerical datasets are available, setting up and performing validation simulations often requires substantial resources. As a result, most validation studies focus on a single dataset from experiments or numerical simulations, which is generally insufficient for a comprehensive model assessment. Systematic model evaluation, therefore, benefits from a comprehensive, standardized, and easy-to-use validation framework containing multiple validation datasets. This paper introduces a validation framework for combustion models and demonstrates its application to a state-of-the-art flamelet model for large-eddy simulations. The framework deploys the open-source OpenFOAM toolbox to provide an openly accessible simulation standard allowing straightforward integration of newly developed models and facilitating the adoption by the research community. It includes eight validation cases based on both experimental data and direct numerical simulations, and is designed to be readily expandable by the community to accommodate an arbitrarily large number of additional cases. For all cases, the reference data, together with the required configurations and mesh files, are provided. The framework facilitates the systematic assessment of combustion models by automatically performing simulations for all cases and providing comparisons between model predictions and reference data. As a demonstration, a flamelet-based model for lean premixed hydrogen combustion is implemented in OpenFOAM and evaluated within the proposed framework, providing insights into the model’s capabilities and limitations across different flow and combustion conditions.
Reynolds stress budgets computed from a direct numerical simulation of the spanwise-periodic flow over a Gaussian bump are analyzed to gain insight into turbulence modification under different pressure-gradient regimes in three subregions. The four nonzero Reynolds stress components are defined in the local orthogonal coordinate system at a given location on the bump surface. The first subregion starts upstream of the bump under a mild adverse pressure gradient (APG) that becomes progressively stronger toward the bump. The second subregion succeeds the first subregion near the bump foot and contains a strong favorable pressure gradient (FPG) until very near the bump apex. The third subregion is the strong APG section that begins near the apex and lasts until the flow separation point. The mild/strong APG reduces the mean shear rate near the wall, which has a dampening effect on turbulence production while the strong FPG leads to an order of magnitude increase in near-wall turbulence production as a result of the much-enhanced mean shear rate. The mean shear rate of the buffer zone between inner and outer parts of the decelerated flow past the apex also gives way to enhanced production. The turbulent kinetic energy (TKE) gets redistributed internally among the normal stress components via the pressure-strain correlation. For the shear stress, pressure strain is generally a consuming term. The reduction in near-wall TKE production in the APG sections leads to decreases in pressure-strain amplitudes for all normal stresses. For the shear stress, a similar correlation between production and pressure strain also exists in the mild APG section, but a more intricate behavior is found in the strong APG section. In the strong FPG region, the much-amplified near-wall production of turbulence leads to substantial enhancements of the pressure-strain correlation for all components. As the flow goes through different pressure-gradient regimes, the decreasing/increasing deficits or surpluses between energizing (production and/or pressure strain) and consuming (dissipation and/or pressure strain) terms near the wall are balanced by the relevant spatially-redistributive terms that proportionally either diminish or amplify in magnitude. The budget surpluses resulting from the much-amplified near-wall turbulence production in the strong FPG section lead to the formation of an internal layer beneath the accelerated boundary layer, where new near-wall peaks emerge for the wall-normal and shear stresses as the original peaks of the other components strengthen substantially. The paper describes the rather complex flow dynamics that eventually leads to boundary-layer separation.
This work presents a flame resolved direct numerical simulation (DNS) and three Large Eddy Simulations (LES) of the bluff-body stabilized turbulent premixed H_2 -air flame experimentally studied at NTNU. The DNS are compared to experimental data, and the velocity fields, heat release rate, and OH distributions, show good overall agreement. An Optimal Estimator analysis is conducted in order to identify the set of progress variables most suited to represent the thermochemical states present in the DNS on a tabulated manifold. The resulting progress variable set is used to construct a tabulated manifold through 1D unstretched laminar flamelets, which is combined with a consistent FTACLES formulation to conduct three Large Eddy Simulations (LES) of the same burner. The three LES, differing in the subgrid wrinkling model considered, show a good agreement with the DNS velocity fields, but fail in reproducing correctly the heat release rate and the Y_OH distribution, while consistently overpredicting the temperature values. These findings, combined with the analysis of the joint Probability Density Functions of the LES, suggest that the inclusion of a wrinkling factor is not enough for the correct reproduction of the flame with the adopted formulation, and strained flamelets should be included in the generated manifold.
The present manuscript investigates the influence of velocity, vorticity and temperature in a hydrogen combustor operating under Moderate or Low Intense oxygen Dilution conditions, focusing on the NO formation. The study was conducted on a database obtained from Large Eddy Simulations and an approach based on Proper Orthogonal Decomposition was used to study the most relevant coherent structures. It was shown that the dominant structures are primarily associated with air (oxidizer) and hydrogen (fuel) inlet jets and the vortex region that forms at the outlet. Modal decomposition of these specific areas identifies vortex wandering and pulsation modes. Whereas, the two jets exhibit a bending motion, tending to interact and coalesce within the vortex. Moreover, the temporal analysis of the POD coefficients shows a strong mutual connection between velocity, temperature and NO formation. It is also shown that vortex dynamics is closely related to jet modes and, consequently, strongly affects both heat transfer processes and NO formation.
This study investigates the physical mechanism of laminar-to-turbulent transition in natural convection by developing algebraic Local-Correlation-based Transition Models (LCTMs) through a physics-based, data-driven framework combining Bayesian optimisation and Symbolic Regression. Accurate transition prediction is critical for high-Rayleigh-number natural convection applications, where standard RANS models and existing transition correlations, calibrated for flows without buoyancy effects, fail to capture the transition onset and boundary layer development. We employ DNS data from differentially heated rectangular cavities (Rayleigh number (Ra) = 10^10 to 10^11 ) to calibrate model parameters by Bayesian optimisation, then use Symbolic Regression with buoyancy and transition-specific invariants to discover explicit algebraic expressions that generalise across flow conditions. Two models are developed: ML-1, optimised for high Rayleigh-number accuracy, and ML-2, designed for broader generalisation across laminar and turbulent regimes. Validation against a natural convection vertical boundary layer, tall rectangular cavities, and a square cavity demonstrates substantial improvements over baseline RANS models, with the data-driven LCTMs accurately predicting transition location, Nusselt number, and velocity and temperature profiles, showing promising capability for enhancing natural convection predictions while maintaining RANS efficiency.
Hydrogen-assisted combustion has emerged as a promising pathway for extending ultra-lean operating limits in spark-ignition engines; however, the physical mechanisms governing combustion stabilization under extreme dilution remain insufficiently understood. In particular, the existence of critical hydrogen concentrations separating partial combustion enhancement from fundamentally reorganized ultra-lean combustion behavior has not been systematically investigated. The present study experimentally examines hydrogen-assisted ultra-lean combustion in a dual-fuel spark-ignition engine over a wide range of air excess ratios (λ = 1.0–2.2) and hydrogen energy shares (0–40
This study performed a high-fidelity wall-resolved large-eddy simulations to investigate the impact of active flow control using an unsteady suction on an Ahmed body model with a 25^∘ slant angle. A key goal of this work is the validation of the baseline flow, which yields a drag coefficient in close agreement with the ERCOFTAC benchmark experimental results. A comprehensive analysis is conducted to characterize the influence of the suction on flow topology and turbulence dynamics across the separation, reverse-flow, reattachment, and wake-recovery regions. Flow modifications are examined using qualitative visualizations of mean velocity and pressure coefficient, together with quantitative analyses of first- and higher-order turbulence statistics, including resolved Reynolds stresses. The results reveal that suction control re-energizes the boundary layer near the slant leading edge, modifies the so-called large-scale C-pillar vortices and substantially reduces the recirculation zone. Furthermore, deeper insight is obtained through single probability density functions of resolved velocity fluctuation at multiple downstream locations. In controlled case, the analysis reveals a marked reduction in flow intermittency over the slant surface and in the near-wake.
Stringent emission regulations and the demand for improved fuel economy have driven advancements in spark-ignition engines. While direct injection offers better fuel economy and lean-burn combustion, its high cost and packaging complexity limit its adoption in small-bore engines for two- and three-wheelers. Port fuel injection remains preferred for its simpler design and compatibility with modern control strategies. However, during cold start, limited droplet evaporation and persistent wall films hinder combustible mixture formation, leading to misfires and increased unburned hydrocarbon emissions. Additionally, tumble development is restricted by an anticlockwise vortex beneath the intake valve, causing early decay during compression. To examine the influence of in-cylinder flow and injection timing on fuel–air mixture distribution across multiple planes, a computational fluid dynamics framework was developed and validated against experimental data for spray tip penetration (with and without wall impingement) and mixture distribution from planar laser-induced fluorescence. Results show that, in closed-valve injection, vaporized fuel is entrained by intake air and evenly distributed by tumble motion, producing a homogeneous mixture prior to combustion. In contrast, during open-valve injection, the spray enters asymmetrically through the intake valve and interacts with an already developed tumble vortex, confining vapor to one side and sustaining a stratified mixture throughout compression due to weak swirl motion. In addition, a statistical analysis based on the probability density function of the equivalence ratio was evaluated to assess mixture homogeneity across the sectional regions of the in-cylinder domain.
The present study investigates the agglomeration of nanoparticles under the joint influence of Brownian motion and turbulent shear using direct numerical simulations. Under these conditions, it is still unclear how different turbulence parameters affect the agglomeration dynamics and particle morphology. Our computations resolve the trajectories of individual agglomerates, enabling a direct, model-free description of their fractal morphology. Specifically, we find that both the morphology and agglomeration rate are largely insensitive to the Reynolds number. In contrast, smaller Kolmogorov scales lead to an accelerated agglomeration process. The smallest turbulent eddies also influence agglomerate morphology, with more compact structures forming at smaller Kolmogorov scales. Additionally, we demonstrate that the agglomeration dynamics observed in detailed simulations can be reproduced using a population balance model that incorporates a recently proposed expression for the agglomerate collision frequency.
The estimation of flame acceleration induced by thermo-diffusive and hydrodynamic instabilities is a key step toward developing accurate models for lean hydrogen combustion. One way to estimate this acceleration is to perform Direct Numerical Simulations (DNS) of laminar unstable flames. However, previous studies have shown that the 3D nature of instabilities significantly enhances the flame propagation speed compared to the 2D predictions. Systematically performing 3D simulations of laminar unstable flames remains out of reach due to prohibitive computational costs. In this work, a series of 2D and 3D DNS under various conditions are performed to establish a link between 2D and 3D flame wrinkling and stretch factors. Models are proposed that accurately estimate the 3D quantities from their 2D counterparts, leading in particular to an improved prediction of the 3D flame consumption speed. The robustness of these models is assessed using simulations performed with another chemical mechanism, and using published data obtained with another CFD solver. The prediction of the wrinkling and stretch factors is found to be highly robust. Overall, the resulting 3D flame consumption speed predictions are very accurate. These findings pave the way for improved subgrid-scale models of flame instabilities in RANS and LES frameworks.
This study evaluates the capability of a Remeshed Vortex Method (RVM) to accurately simulate complex wall-bounded turbulent flows by performing both Direct Numerical Simulations (DNS) and Large Eddy Simulations (LES) of the flow over periodic hills. The RVM stems from Lagrangian Vortex methods, discretizing the flow fields on numerical particles, where a remeshing procedure is applied to redistribute the particles onto a Cartesian grid in order to control the interparticle distance. In DNS, different algorithmic configurations inherent to the RVM’s remeshing procedure are explored. The RVM shows good accuracy and computational efficiency in terms of mesh resolution and time steps compared to the DNS results reported in literature, despite the limitation of using a uniform Cartesian grid. In the LES framework, first a systematic analysis of the directional splitting sequence in the RVM algorithm for anisotropic flows allows us to select a priori the optimal direction ordering in the operators splitting procedure. Secondly, different subgrid-scale models are selected and assessed, including some models previously evaluated and calibrated in a HIT configuration. Among these, the Variational Multiscale (VMS) eddy-viscosity models and the Spectral Vanishing Viscosity (SVV) approaches, which introduce dissipation only at the smallest resolved vorticity scales, are found to be the most suitable for use with the RVM. In particular, the calibrated VMS Smagorinsky model shows the best performance, confirming our previous findings on the robustness of the model calibration.
Atmospheric Pressure Plasma Jet (APPJ) accelerating following the fluid flow is responsible for the modification of the gas flow dynamics. This study investigates the gas flow dynamics in the presence of non-thermal atmospheric pressure plasma jets (APPJs), focusing on the Greek letter lambda shape transition region between laminar and vortex flow regimes. Using Schlieren imaging, we analyzed flow characteristics at different gas flow rates (2 slm, 3 slm, and 4 slm), identified distinct flow regions and the onset of vortex. A small to moderate flow instability around plasma jet is observed. Key parameters, including Reynolds number, electron density, electron drift velocity ( 105 m/s) and electrohydrodynamic (EHD) forces, are evaluated experimentally to understand their roles in flow instability. Electron and gas temperature estimated from emission data are 3500 K and 350 K, respectively. The plasma jet extends past the 50
When jet-based flow control, such as stall control and fluidic thrust vectoring, is employed, jet nozzles are commonly installed near a convex-curved surface. Numerous researchers have investigated the most fundamental curved-wall continuous jet. For synthetic jets, although numerous applied studies exist, fundamental investigations on cylindrical walls remain scarce. Synthetic jets have zero net mass flux; the vortex pair generated during the expulsion phase dominates the suction phase and establishes a time-averaged flow downstream. This study clarifies how a cylindrical surface influences the vortex pair and the resulting mean flow. We performed flow visualization using Particle Image Velocimetry and surface-pressure measurements for a synthetic jet issuing tangentially over a cylindrical surface. We examined the effects of stroke length (the distance fluid particle travels at the representative velocity during one oscillation period) and radius of curvature on the flow. The boundary layer on the wall organized into discrete vortices linked to the dimensionless stroke length, and both the time-averaged pressure distribution and separation point on the cylindrical surface depended on it. Moreover, the vortex street and time-averaged-flow characteristics can be parameterized using a nominal dimensionless stroke based on the radius of curvature rather than the slot width typically used as the representative length.
