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The mean and instantaneous flow separation of two different three-dimensional asymmetric diffusers is analysed using the data of large-eddy simulations. The geometry of both diffusers under investigation is based on the experimental configuration of Cherry et al. (Int J Heat Fluid Flow 29(3):803–811, 2008). The two diffusers feature similar area ratios of $$\mathrm{AR}=4.8$$ AR = 4.8 and $$\mathrm{AR}=4.5$$ AR = 4.5 while exhibiting differing asymmetric expansion ratios of $$\mathrm{AER}=4.5$$ AER = 4.5 or $$\mathrm{AER}=2.0$$ AER = 2.0 , respectively. The Reynolds number based on the averaged inlet velocity and height of the inlet duct is approximately $${\textit{Re}}=10{,}000$$ Re = 10 , 000 . The time-averaged flow in both diffusers in terms of streamwise velocity profiles or the size and location of the mean backflow region are validated using experimental data. In general good agreement of simulated results with the experimental data is found. Further quantification of the flow separation behaviour and unsteadiness using the backflow coefficient reveals the volume portion in which the instantaneous reversal flow evolves. This new approach investigates the cumulative fractional volume occupied by the instantaneous backflow throughout the simulation, a power density spectra analysis of their time series reveals the periodicity of the growth and reduction phases of the flow separation within the diffusers. The dominating turbulent events responsible for the formation of the energy-containing motions including ejection and sweep are examined using the quadrant analysis at various locations. Finally, isourfaces of the Q-criterion visualise the instantaneous flow and the origin and fate of coherent structures in both diffusers.

The Moderate or Intense Low-oxygen Dilution (MILD) combustion is a promising technique to reduce pollutant emissions of combustion processes, especially nitrogen oxides. This combustion mode involves Turbulence-Chemistry Interaction (TCI), which constitutes a challenge in terms of numerical simulation since it must be properly captured. Up to now, the TCI has been modelled and the corresponding models generally involve coefficients, leading to epistemic uncertainties and, therefore, to different numerical results depending on the used model. The study presented in this paper aims to assess the relevance of performing Large Eddy Simulation of a typical Jet-in-Hot-Coflow flame, simulating diluted combustion, assuming that the TCI is directly resolved, given the grid and the chemical kinetics resolutions. Avoiding TCI modelling allows for lower numerical uncertainties. However, simulations without TCI modelling will normally fail for other types of flames and for higher Reynolds-numbers, so that such simulations can normally only be conducted using TCI modelling. Here, the simulations are performed using Finite Rate Chemistry without TCI model on the Adelaide Jet-in-Hot-Coflow flame. First, the proposed methodology was experimentally validated, highlighting that the obtained reacting results are consistent in terms of temperature and mass fractions with the measurements. Additionally, the results obtained with the “TCI-resolved” assumption are compared to the results obtained using a classical TCI model, the Partially Stirred Reactor model. Moreover, the validity of the approach, consisting in directly resolving the TCI, is assessed based on an analysis of the local Damkohler number A large part of the mesh cells presents a very low Damkohler number, confirming that TCI modelling is not required for the burner under consideration.

International audience; The isothermal mixing of a heavy and a light liquid of different physical properties is numerically investigated by means of Large Eddy Simulations. The validation is based on experimental data held in a system reproducing various components of a pressurized water nuclear reactor, during a scenario of cold water injection at a low Atwood number of 0.05. The flow has two distinct stages: first a buoyancy-driven phase is characterized by a fluid front development in the cold leg and gives rise to Kelvin-Helmholtz whorls under the action of density changes. Then, the heavy liquid discharges into the downcomer filled with light liquid, which causes a turbulent mixing. These phenomena are analyzed through a single-phase approach where the density of the working fluid is either variable or modeled by the Boussinesq approximation. The influence of grid refinement is deeply examined, which shows that the mesh convergence is well achieved for the main flow quantities, unlike the low-magnitude spanwise components. Overall, the numerical solutions are found to reproduce the experimental measurements with a fair accuracy for both physical models used. These latter exhibit similar trends, due to the small density difference under consideration. The predictions in the downcomer appear to be more challenging owing to a strongest turbulence than in the cold leg, some flow features being not properly captured. However, the experimental data in the downcomer are found to be incomplete and somewhat dubious for a strict validation of the numerical simulations. Lastly, the flow distribution in the dowcomer is investigated, providing further insight on the mixing process.

The paper focuses on a global instability phenomenon in counter-current round jets issuing from co-axial nozzles. Three different configurations that differ in a way of the counter flow generation are investigated. Besides typical configurations used in experimental and numerical research performed so far, in which suction applied in an annular nozzle is a driving force for the counterflow, a novel set-up is proposed where the annular nozzle is oriented in the opposite direction and placed above the main one. Such a configuration eliminates the suction of fluid from the main jet, which in previous research was found to have a destructive impact on the occurrence of the global flow instability. The research is performed using a large eddy simulation (LES) method and the computations are carried out applying a high-order numerical code, the accuracy of which has been proven in previous works and also in the present research through comparisons with available experimental data. The research is complemented by the linear stability analysis which supports the LES results and formulated conclusions. In agreement with a number of the previous works it has been shown that the global modes can be triggered only when the velocity ratio (I) between the main jet velocity and the velocity of the jet issuing from the annular nozzle is above a certain threshold level ($$I_{\text {cr}}$$Icr). It has been shown that in the classical configurations of the co-axial nozzles the range of$$I\ge I_{\text {cr}}$$I≥Icrfor which the global instability phenomenon exists is very narrow and it disappears for larger velocity ratios. Reasons for that have been identified through detailed scrutiny of instantaneous flow pictures. In the new set-up of the nozzles the global instability persists for a significantly wider range ofI. It has been shown that$$I_{\text {cr}}$$Icrdepends on both the momentum thickness of the mixing layer formed between the counter-current streams and the applied configuration of the nozzles. The LES results univocally showed that the latter factor decides on the type of the instability mode (Mode I or Mode II) that emerges in the flow, as it directly influences on a length of the region where the counter-current streams are parallel allowing the growth of short or long wave disturbances characteristic for Mode I and Mode II, respectively.

The present paper deals with the large-eddy simulation (LES) of a zero-pressure-gradient smooth flat-plate boundary layer undergoing bypass transition due to the presence of freestream turbulence of intensity around $$3.0\%$$ . Transition has been achieved by the introduction of synthetic turbulence into the freestream in a manner designed to invoke a behaviour closely resembling that of the ERCOFTAC T3A experiment. We make use of the spectral-element dynamic model (SEDM), implemented within the framework of a high-order spectral-difference solver, to carry out LES upon several sets of computational grids. Furthermore, comparisons against several other well known models such as the WALE and SIGMA model, are performed and presented in addition to implicit-LES (ILES). Our results show that the SEDM inputs relatively minor levels of dissipation into the pre-transitional region. This allows the fluctuating stresses to grow unhindered, until the critical amplitude, required for transition to occur, is reached. Furthermore, within the transitional region, the eddy-viscosity is dominant near the wall and not at the interface between the freestream and boundary layer where the breakdown mechanism predominates. The rapid grid convergence properties of the SEDM, demonstrated in this work, make them well-suited for use in conjunction with other high-order schemes.

A computational investigation of three configurations of the Delft Spray in Hot-diluted Co-flow (DSHC) is presented. The selected burner comprises a hollow cone pressure swirl atomiser, injecting an ethanol spray, located in the centre of a hot co-flow generator, with the conditions studied corresponding to Moderate or Intense Low-oxygen Dilution (MILD) combustion. The simulations are performed in the context of Large Eddy Simulation (LES) in combination with a transport equation for the joint probability density function (pdf) of the scalars, solved using the Eulerian stochastic field method. The liquid phase is simulated by the use of a Lagrangian point particle approach, where the sub-grid-scale interactions are modelled with a stochastic approach. Droplet breakup is represented by a simple primary breakup model in combination with a stochastic secondary breakup formulation. The approach requires only a minimal knowledge of the fuel injector and avoids the need to specify droplet size and velocity distributions at the injection point. The method produces satisfactory agreement with the experimental data and the velocity fields of the gas and liquid phase both averaged and ‘size-class by size-class’ are well depicted. Two widely accepted evaporation models, utilising a phase equilibrium assumption, are used to investigate the influence of evaporation on the evolution of the liquid phase and the effects on the flame. An analysis on the dynamics of stabilisation sheds light on the importance of droplet size in the three spray flames; different size droplets play different roles in the stabilisation of the flames.

The turbulent counter-flow flame (TCF) has proven to be a useful benchmark to study turbulence-chemistry interactions, however, the widely observed bulk flow fluctuations and their influence on the flame stability remain unclear. In the present work, premixed TCFs are studied numerically using a Large Eddy Simulation (LES) method. A transported probability density function (pdf) approach is adopted to simulate the sub-grid scale (sgs) turbulence-chemistry interactions. A solution to the joint sgs-pdf evolution equation for each of the relative scalars is obtained by the stochastic fields method. The chemistry is represented using a simplified chemical reaction mechanism containing 15 reaction steps and 19 species. This work compares results with two meshing strategies, with the domain inside nozzles included and excluded respectively. A conditional statistical approach is applied to filter out the large scale motions of the flame. With the use of digital turbulence, the velocity field in the flame region is well reproduced. The processes of local extinction and re-ignition are successfully captured and analysed together with the strain rate field, and local extinctions are found correlated to the turbulent structures in the reactant stream. The predicted probability of localised extinction is in good agreement with the measurements, and the influence of flame stoichiometry are also successfully reproduced. Overall, the current results serve to demonstrate the capability of the LES-pdf method in the study of the premixed opposed jet turbulent flames.

Abstract The development of gasoline spray at ultra-high injection pressures was analyzed using Large-Eddy simulation (LES). Two different nozzle hole geometries, divergent and convergent shape, were considered to inject the fuel at injection pressures ranging from 200 to 1500 bar inside a constant volume spray chamber maintained at atmospheric conditions. The discrete droplet phase was treated using a Lagrangian formulation together with the standard spray sub-models. The numerical results were calibrated by reproducing experimentally observed liquid penetration length and efforts were made to understand the influence of ultra-high injection pressures on the spray development. The calibrated model was then used to investigate the impact of ultra-high injection pressures on mean droplet size and droplet size distribution. In addition, the spray-induced large-scale eddies and entrainment rate were evaluated at different ultra-high injection pressures. Overall, simulation results showed a good agreement with available measurement data. At ultra-high injection pressures mean droplet sizes were significantly reduced and comprised very high velocities. Integral length scales of spray-induced turbulence and air entrainment rate into the spray were larger at higher injection pressure compared to lower ones. Graphic abstract

The aim of this paper is to analyze the self-ignition of a jet fame in hot vitiated cross fow using Large Eddy Simulation with analytically reduced chemistry. A rich premixed ethylene-air mixture at 300 K is injected into a hot vitiated crossfow at 1500 K. The simulated reacting fow steady-state was validated against experiments in previous publications and the focus of the present work is the transient self-ignition of the jet. It is shown that spontaneous ignition occurs at very lean mixture fractions in the form of reacting patches in the windward jet mixing layer. These patches grow, laterally wrap the jet and extend into the recirculation region. Chemical explosive mode analysis is performed to identify the chemically active regions that are precursors of the patches undergoing spontaneous ignition. It is shown that the self-ignition occurs at very lean fuel concentrations regions, which are leaner and hotter than the most reactive mixture fraction of the jet and crossfow. This is explained by the fact that the scalar dissipation is signifcantly lower in these very lean regions. Ultimately, the peak heat release moves toward the richer regions and an autoignition cascade governs the steady state fame anchoring., Flow, Turbulence and Combustion, 106, ISSN:1386-6184, ISSN:1573-1987

Accurate prediction of nitrogen oxides (NOX) emissions is of great importance in combustion simulations. This work proposes an improved model to predict NO distributions in turbulent flames, based on the large eddy simulation approach and flamelet-progress-variable turbulence combustion model. A separate table for NO reaction rate is constructed to account for both unsteady and nonadiabatic effects on NO production. Different from the existing models, the nonadiabatic effect is considered by specifying different enthalpy defects by a scaling factor in the flamelet calculations. Additionally, it is observed that the transient variation of NO reaction rate with NO concentration exhibits two linear stages. As a result, only three flamelet databases are included to describe the NO reaction rate in a transient process, thereby simplifying the flamelet table while maintaining model accuracy. The proposed NO prediction model is validated in large eddy simulations of turbulent combustion, including the classic Sandia D and the Sydney swirling SM1 flames. The calculated NO variations show good agreement with experimental data, demonstrating the applicability and accuracy of the proposed model.

A new explicit algebraic wall law for the Large Eddy Simulation of flows with adverse pressure gradient is proposed. This new wall law, referred as adverse pressure gradient power law (APGPL), is developed starting from the power-law of Werner and Wengle (Turbulent Shear Flows, vol 8, Springer, New York, pp 155–168, 1993) in order to mimic an implicit non-equilibrium log-law based on Afzal’s law (Afzal, IUTAM Symposium on Asymptotic Methods for Turbulent Shear Flows at High Reynolds Numbers, Kluwer Academic Publishers, Bochum, pp 95–118, 1996). No iterative method is needed for the evaluation of the wall shear stress from the APGPL contrary to the majority of models available in the literature. The APGPL model relies on the definition of three modes: the equilibrium power-law is used in regions of no or favourable pressure gradient, the APGPL is used in regions of adverse pressure gradient, and no wall model is used in separated flow regions. This model is assessed via Large Eddy Simulations of flows involving adverse pressure gradient and boundary layer separation using the Lattice Boltzmann Method on uniform nested grids. The flow around a clean and iced NACA23012 airfoil at Reynolds number $$Re = 1.88 \times 10^6$$ and the flow over the LAGOON landing gear at $$Re = 1.59 \times 10^6$$ are considered. Results are found in good agreement with those obtained by the non-equilibrium log-law and experimental and numerical data available in the literature.

In recent years Computational Fluid Dynamics (CFD) has become a widespread practice in industry. The growing need to simulate off-design conditions, characterized by massively separated flows, strongly promoted research on models and methods to improve the computational efficiency and to bring the practice of Scale Resolving Simulations (SRS), like the Large Eddy Simulation (LES), to an industrial level. Among the possible approaches to the SRS, an appealing choice is to perform Implicit LES (ILES) via a high-order Discontinuous Galerkin (DG) method, where the favourable numerical dissipation of the space discretization scheme plays directly the role of a subgrid-scale model. To reduce the large CPU time for ILES, implicit time integrators, that allows for larger time steps than explicit schemes, can be considered. The main drawbacks of implicit time integration in a DG framework are represented by the large memory footprint, the large CPU time for the operator assembly and the difficulty to design highly scalable preconditioners for the linear solvers. In this paper, which aims to significantly reduce the memory requirement and CPU time without spoiling the high-order accuracy of the method, we rely on a p-adaptive algorithm suited for the ILES of turbulent flows and an efficient matrix-free iterative linear solver based on a cheap p-multigrid preconditioner and a Flexible GMRES method. The performance and accuracy of the method have been assessed by considering the following test cases: (1) the T3L test case of the ERCOFTAC suite, a rounded leading edge flat plate at $${\mathrm{Re}}_D=3450$$ ; (2) the flow past a sphere at $$\mathrm{Re}_D=300$$ ; (3) the flow past a circular cylinder at $$\mathrm{Re}_D=3900$$ .

Eddy viscosity-based and regularization-based subgrid-scale (SGS) models are quantitatively assessed for large eddy simulations (LES) of stratified Taylor–Green vortex (TGV). LES calculations using the dynamic Smagorinsky, Leray- $$\alpha$$ , LANS- $$\alpha$$ and Clark- $$\alpha$$ models are conducted using a pseudo-spectral formulation on a $$128^{3}$$ grid and using $$\alpha =\frac{1}{160}$$ (for all the regularization models) for Froude number, $$Fr_0=\frac{{\mathcal {U}}}{{{\mathcal {N}}}{{\mathcal {L}}}}=1$$ , and Reynolds number, $$Re_{0}= \frac{\mathcal {U L} }{\nu }=1600$$ . Results are compared in detail with in-house direct numerical simulation (DNS) calculations. Validation of the formulation is carried out through grid-dependent studies using grids of $$64^3$$ , $$128^3$$ and $$256^3$$ , $$\alpha$$ (regularization parameter)-dependent studies using $$\alpha$$ values of $$\frac{1}{40}, \frac{1}{80}, \frac{1}{160}$$ and $$\frac{1}{320}$$ , and comparisons with previously published results Remmler and Hickel, 2012, of the same problem. Various quantities including turbulent kinetic energy (tke), turbulent potential energy (tpe), Q-criteria based vortical structures, potential & total energy spectra, and horizontal & vertical energy fluxes in the respective directions, are analyzed to understand the capability of the various LES models in predicting the development of stratified turbulence. Results showed that all the SGS model predictions were very similar to each other with Smagorinsky and Leray displaying the lowest and highest errors, respectively, in comparison to DNS. The effect of $$Fr_0$$ was also studied through additional LES calculations at $$Fr_0=0.5$$ and 0.16, again using the dynamic Smagorinsky and regularization models and comparing various parameters including tke, tpe and energy spectra to DNS. Decay rates of tke and tpe decreased and turbulence was significantly suppressed with decrease in $$Fr_0$$ . At these lower $$Fr_0$$ , all the SGS model predictions were almost identical to each other and compared extremely well with the DNS results.

The downward shift of the mean velocity profile in the logarithmic region, known as roughness function, $$\Delta U^+$$ , is the major macroscopic effect of roughness in wall bounded flows. This speed decrease, which is strictly linked to the friction Reynolds number and the geometrical properties which define the roughness pattern such as roughness height, density, shape parameters, has been deeply investigated in the past decades. Among the geometrical parameters, the effective slope (ES) seems to be suitable to estimate the roughness function at fixed friction Reynolds number, Re $$_{\tau }$$ . In the present work, the effects of several geometrical parameters on the roughness function, in both transitional and fully rough regimes, are investigated by means of large eddy simulation of channel flows characterized by different wall-roughness textures at different values of Re $$_{\tau }$$ up to 1000. The roughness geometry is generated by the superimposition of sinusoidal functions with random amplitudes and it is exactly resolved in the simulations. A total number of 10 cases are solved. With the aim to find a universal correlation between the roughness geometry and the induced roughness function, we analyzed the effect of more than a single geometrical parameter, including the effective slope, which takes into account both the roughness height and its texture. Based on data obtained from our simulations and a number of data points from the literature, a correlation between the ES and the root mean square of the roughness oscillation, as well as between ES and the mean absolute deviation of the roughness, satisfactorily predicts the roughness function.

Physiologically, sinus ventilation is a critical aspect of good functionality for human respiratory function, the understanding of which is still unclear. In this study we develop a method to measure sinus ventilation. Spatial and temporal features of the sinus recirculation are provided through dynamic mode decomposition (DMD). We associate the recirculation feature on the sinus epithelial surface through the wall shear-stress to the 3D airflow features corresponding to the sinus. Turbulent airflow simulations using a Large Eddy Simulation model were conducted in a patient who receives an endoscopic sinus surgery of the ethmoid + sphenoid sinuses. We analyze the flow rate through the ostium and the average wall shear-stress on the sinus epithelial surface. Based on the results, we then use a dynamic mode decomposition method on the sinus, which accurately measures the recirculation in the cavity. Drug delivery in the paranasal sinuses is challenging; therefore it is essential to understand well the airflow in this region. We suggest that DMD is a powerful method to analyze and understand this complex pathophysiology problem. This method has to be considered for the rhinologists as a milestone.

Temporal evolution of an ignition kernel in a spark ignited turbulent hydrogen–air mixing layer is studied using the large eddy simulation approach with an implicit treatment of the reaction source terms. The applied numerical code is based on a high-order compact difference approximation combined with a weighted essentially non-oscillatory scheme, which provide an accurate resolution of the small scale phenomena. The spark is modelled by an energy deposition model coupled with the enthalpy equation. Since the fuel and oxidiser streams move in the opposite directions the flow is dominated by shear stresses and vortical structures. The ignition follows three different scenarios depending on the spark location. An interaction between the developing flame kernel and large turbulent structure is analysed starting from the energy transfer up to a fully reacting state. The size and shape of the flames are correlated with the initial ignition scenario. A 3D visualisation of the transient position of the flame kernel shows its different spatio-temporal behaviour. It is observed that the flame can stay close to the initial position and spread equally in all directions or it can move far from the initial location and follow the evolving flow field. In the latter scenario two separate sub-stages are convincingly identified. Concerning the ignition probability ($$P_{ign}$$ P ign ), when the spark is located in the immediate vicinity of the mixing layer, the $$P_{ign}$$ P ign on the fuel-rich side is higher. On the other hand, when the ignition occurs far from the mixing layer, the $$P_{ign}$$ P ign is significantly larger on the lean side. Unlike the local composition the impact of the strain rate of the velocity field was found to have very limited impact on the $$P_{ign}$$ P ign .

Due to the continuous increase in available computing power, the Large Eddy Simulation (LES) of two-phase flows started to receive more attention in recent years. Well-established models from single-phase flows are often used to close the sub-grid scale convective momentum transport and recently some modifications have been suggested to account for the jump of density and viscosity at the interface of multi-phase flows. However, additional unclosed terms in multi-phase flows, which are absent in single-phase flows, often remain ignored. This paper focuses on the crucial gaps in literature, namely the modeling of volume fraction advection and surface tension effects on sub-grid level. An a-priori analysis has been conducted for this purpose, i.e. the Direct Numerical Simulation of an academic two-phase flow configuration (single wobbling bubble in a turbulent background flow) has been explicitly filtered (corresponding to implicit filtering in actual LES) for varying filter width and the corresponding sub-grid terms have been compared to potentially suitable model expressions. Besides other approaches, adequately formulated models based on the scale similarity principle emerged to be promising candidates for both sub-grid volume fraction advection as well as sub-grid surface tension effects. In this context, special attention has to be paid to the secondary filter. Owing to the nature of the quasi-singular surface tension term, surface-weighted filtering may be more appropriate and robust than standard volume filtering.

In Lagrangian stochastic collision models, afictitiousparticle is generated to act as a collision partner, with a velocity correlated to the velocity of therealcolliding particle. However, most often, the fluid velocity seen by this fictitious particles is not accounted for in the generation of the fictitious particle velocity, leading to a de-correlation between the fictitious particle velocity and the local fluid velocity, which, after collision, leads to an unrealistic de-correlation of the real particle velocity and the fluid velocity as seen by the particle. This de-correlation, in turn, causes a spurious decrease of the particle kinetic energy, even though the collisions are assumed perfectly elastic. In this paper, we propose a new model in which the generated fictitious particle velocity is correctly correlated to both the real particle velocity and the local fluid velocity at the particle, hence preventing the spurious loss of the total particle kinetic energy. The model is suitable for small inertial particles. Two algorithms for integrating the collision frequency are also compared to each other. The models are validated using large eddy simulation (LES) of mono-dispersed particle-laden stationary homogeneous isotropic turbulence. Simulations are conducted with spherical particles with different turbulent Stokes number,$$St_t = [0.75 - 5.8]$$Stt=[0.75-5.8], and volume fractions,$$\alpha _p = [0.014 - 0.044]$$αp=[0.014-0.044], and are compared to the results of the LES using a deterministic discrete particle simulation model.

The present work investigates the formation process and early stage evolution of turbulent swirling vortex rings, by using planar Particle Image Velocimetry (PIV) and Large Eddy Simulation (LES). Vortex rings are produced in a piston-nozzle arrangement with swirl generated by 3D-printed axial swirlers in experiments. Idealised solid-body rotation is applied in LES to evaluate the effect of nozzle exit velocity profile in experiments. The Reynolds number (Re) based on the nozzle diameter D and the slug velocity U0 in the nozzle is 20,000. The swirl number S generated ranges from 0 (zero-swirl vortex ring) and 1.1, covering the two critical swirl numbers previously identified in a swirling jet. Both PIV and LES results show that the formation number F decreases linearly as S increases, with the maximum F ≈ 2.6 at S = 0 (produced by the swirler with straight vanes) and minimum F = 1.9 at S = 1.1. The corresponding maximum attainable circulation in the nozzle axis parallel plane also diminishes with increasing S. Evolution of compact rings produced by a stroke ratio L/D = 1.5 reveals that circulation decay rate is largely proportional to S. The trajectory of the vortex core in the axial direction, hence the ring axial propagation velocity, decreases as S, while that in the radial direction and the radial propagation velocity, increase with S. An empirical scaling function is proposed to scale these variables.

The high-pressure turbine blades are the components of the aero-engines which are the most exposed to extreme thermal conditions. To alleviate this issue, the blades are equipped with cooling systems to ensure long-term operation. However, the accurate prediction of the blade temperature and the design of the cooling system in an industrial context still remains a major challenge. Potential improvement is foreseen with Large-Eddy Simulation (LES) which is well suited to predict turbulent flows in such complex systems. Nonetheless, performing LES of a real cooled high-pressure turbine still remains expensive. To alleviate the issues of CPU cost, a cooling model recently developed in the context of combustion chamber liners is assessed in the context of blade cooling. This model was initially designed to mimic coolant jets injected at the wall surface and does not require to mesh the cooling pipes leading to a significant reduction in the CPU cost. The applicability of the model is here evaluated on the cooled Nozzle Guide Vanes (NGV) of the Full Aerothermal Combustor Turbine interactiOns Research (FACTOR) test rig. To do so, a hole modeled LES using the cooling model is compared to a hole meshed LES. Results show that both simulations yield very similar results confirming the capability of the approach to predict the adiabatic film effectiveness. Advanced post-processing and analyses of the coolant mass fraction profiles show that the turbulent mixing between the coolant and hot flows is however reduced with the model. This finding is confirmed by the turbulent map levels which are lower in the modeled approach. Potential improvements are hence proposed to increase the accuracy of such methods.

The development of better laser-based experimental methods and the fast rise in computer power has created an unprecedented shift in turbulent combustion research. The range of species and quantities measured and the advent of kHz-level planar diagnostics are now providing great insights in important phenomena and applications such as local and global extinction, pollutants, and spray combustion that were hitherto unavailable. In simulations, the shift to LES allows better representation of the turbulent flow in complex geometries, but despite the fact that the grid size is smaller than in RANS, the push towards realistic conditions and the need to include more detailed chemistry that includes very fast species and thin reaction zones emphasize the necessity of a sub-grid turbulent combustion model. The paper discusses examples from current research with experiments and modelling that focus on flame transients (self-excited oscillations, local extinction), sprays, soot emissions, and on practical applications. These demonstrate how current models are being validated by experimental data and the concerted efforts the community is taking to promote the modelling tools to industry. In addition, the various coordinated International Workshops on non-premixed, premixed, and spray flames, and on soot are discussed and some of their target flames are explored. These comprise flames that are relatively simple to describe from a fluid mechanics perspective but contain difficult-to-model combustion problems such as extinction, pollutants and multi-mode reaction zones. Recently, swirl spray flames, which are more representative of industrial devices, have been added to the target flames. Typically, good agreement is found with LES and some combustion models such as the progress variable - mixture fraction flamelet model, the Conditional Moment Closure, and the Transported PDF method, but predicting soot emissions and the condition of complete extinction in complex geometries is still elusive.

The upper layer of the ocean participates directly in the exchange of momentum, heat and moisture with the atmosphere. We consider three examples of upper-ocean flow and heat transfer in the present contribution. These examples range from the canonical problem of a stratified shear layer to the surface boundary layer driven by wind and a diurnally varying heat flux to deep cycle turbulence in the Equatorial UnderCurrents (EUC). These problems illustrate stratified shear flow turbulence, wind-driven entrainment in a stratified, rotating fluid, and the communication of surface forcing to subsurface currents in the upper ocean. We discuss the three cases by including new simulations as well as some of our previous work. Direct numerical simulation (DNS) is our tool for the canonical shear layer and, for the other problems, our tool is large eddy simulation (LES) which is increasingly being used to examine turbulent transport and mixing in the ocean. We discuss how buoyancy and rotation affects the spatial structure and temporal evolution of turbulent fluxes, and thereby the distribution of surface inputs of momentum and heat in the upper ocean.

The development of supersonic combustion engines (scramjet-type) presents several challenges. Numerical simulations of scramjet engine combustion have become an attractive alternative to experimental investigations. However, supersonic combustion simulations still have several challenges: such as adequate modelling of shock/boundary layer and turbulence/chemistry interactions. The present work presents two large-eddy simulation probability density function (LES-PDF) novel formulations developed for high-speed applications. One is a conservative joint-scalar approach, where the joint-probability for reactive scalars and energy is solved, while the other is a joint velocity-scalar. The LES-PDF transport equations are solved using the Eulerian stochastic fields technique implemented in a density-based compressible solver. The performance of the models is verified through the simulations of two and three-dimensional supersonic reacting mixing layers and compared against DNS data from the literature. The results show that the joint-scalar formulation is accurate and robust, while the joint velocity-scalar closures require further development.

High-order, wall-resolved large eddy simulations (LES) using the spectral element method (SEM) were performed to investigate the gas-exchange process inside a laboratory-scale internal combustion engine (ICE) and study the in-cylinder flow evolution. Using a stabilizing filter, over 30 engine cycles were simulated to generate data for statistical analysis, which demonstrated good agreement in the mean and root mean-squared (rms) phase-averaged velocity fields across three different filter parameter/resolution combinations. The large scale flow motion was characterized during each stage of the engine cycle. Tumble ratio profiles indicate peak values during the intake stroke which decay during compression and are almost non-existent thereafter. The tumble breakdown process is quantified by investigating the evolution of the mean and turbulent kinetic energy over the full cycle, and its effect on the evolution of the momentum and thermal boundary layers is discussed. Algorithmic advances to the computational fluid dynamics (CFD) solver Nek5000, employed in the current study, resulted in significant reduction in the wall-time needed for the simulation of each cycle for mesh resolutions of at least an order of magnitude higher than the current state-of-the-art.

This study quantifies the flow characteristics of an opposed facing fluidic oscillator pair that was arranged in a back-to-back configuration. The objective was to obtain two synchronized oscillating sweeping jets using a shared feedback channel, incorporated in a no moving parts design. The oscillation frequency and the sweep angle of the fluidic oscillator exiting jets were obtained with hot-wire measurements and water visualizations, respectively. The dynamics of the fluidic flow characteristics were assessed using the compressible Unsteady Reynolds-Averaged Navier-Stokes and the Large Eddy Simulation methods, that were validated against the hot-wire measurements. Five different Reynolds number (Re) flow conditions were considered and the proposed setup was capable of producing a synchronized oscillation in the range 101 to 422 Hz, and with a sweep angle up to 80°. Water flow visualization and hot-wire measurement identified two synchronized sweeping exiting jets, which was observed from the opposed facing oscillator pair. The oscillation frequency of the exiting jets was found to be a linearly function w.r.t. Re, which resulted in a Strouhal number in the order of 0.01. It was quantified that the pressure variation and the accompanying flow momentum were balanced and in-phase between both halves of the fluidic oscillator, which was established by means of the shared feedback channel. The opposed facing oscillator pair design provides two synchronized exiting sweeping jets, which widens the application range of the single feedback type fluidic oscillator.

International audience; A detailed computational study was performed for the case of a wall-bounded pin-fin-array in a staggered arrangement, representative of industrial configurations designed to enhance heat transfer. In order to evaluate the level of turbulence modelling necessary to accurately reproduce the flow physics at three (3) different Reynolds numbers (3, 000, 10, 000 and 30, 000), four models were selected: two eddy-viscosity URANS models (k-ω-SST and φ-model), an Elliptic Blending Reynolds-stress model (EB-RSM) and Large Eddy Simulation (LES). Global comparisons for the pressure loss coefficients and average Nusselt numbers were performed with available experimental data which are relevant for industrial applications. Further detailed comparisons of the velocity fields, turbulence quantities and local Nusselt numbers revealed that the correct prediction of the characteristics of the flow is closely related to the ability of the turbulence model to reproduce the large-scale unsteadiness in the wake of the pins, which is at the origin of the intense mixing of momentum and heat. Eddy-viscosity-based turbulence models have difficulties to develop such an unsteadiness, in particular around the first few rows of pins, which leads to a severe underestimation of the Nusselt number. In contrast, LES and EB-RSM are able to predict the unsteady motion of the flow and heat transfer in a satisfactory manner.

The behaviour of a homogeneously charged Turbulent Jet Ignition (TJI) system, fueled with iso-octane was investigated numerically using Large Eddy Simulation (LES) and Reynolds Averaged Navier-Stokes (RANS) turbulence models. This study was an attempt to capture the start of autoignition in a lean charge TJI system, numerically, and, validate the results with experimental pressure measurements and OH chemilominescence images recorded during high speed imaging. Experiments were performed in an optically acessible Rapid Compression Machine (RCM) and the effect of auxiliary fuel injection in the prechamber and ignition distribution through various orrifices was investigated in depth through jet induced autoignition analysis. Heat release and pressure trace analysis were completed to capture the onset of autoignition, as well as comparing LES and RANS capabilities in this regard. Results showed that enhanced prechamber fueling leads to an increase in combustion stability while reducing the ignition delay. It was determined that keeping the prechamber mixture near stoichiometric is essential in order to have a more powerful turbulent jet discharged into the main chamber and to enhance ultra-lean main chamber combustion. The predicted flame propagation speed in the lean iso-octane mixtures was found to be slower than that observed in the experimental measurements. Results showed that the LES turbulence model is capable of predicting the start of autoignition more accurately and enhances the accuracy of the calculated peak pressure and burn rates relative to the RANS model. Two detailed iso-octane mechanisms were tested and based on the ignition delay comparisons, the LLNL-v3 mechanism was used in the current study.

International audience; The single element GCH 4 /GOx rocket combustion chamber developed at the Technische Universität München has been computed using Large Eddy Simulation. The aim of this work is to analyze the flow and combustion features at high pressure, with a particular focus on the prediction of wall heat flux, a key point for the development of reusable engines. The impact of the flow and flame, as well as of the model used, on thermal loads is investigated. Longitudinal distribution of wall heat flux, as well as chamber pressure, have been plotted against experimental data, showing a good agreement. The link between the heat released by the flame, the heat losses and the chamber pressure has been explained by performing an energetic balance of the combustion chamber. A thermally chained numerical simulation of the combustor structure has been used to validate the hypothesis used in the LES.

The exact reconstruction of statistical quantities from filtered data is a fundamental question in turbulence research. Filtered data can be either data from a Large Eddy Simulation (LES) or it can be experimental data with insufficient resolution to capture the finest scales. Usually it is assumed that averages of filtered and unfiltered quantities are the same and as a consequence the Reynolds stress or turbulent scalar flux is traditionally split in a resolved part and a mean subfilter part. However, this decomposition holds only true for homogeneous flows without mean gradients as demonstrated recently (Klein and Germano [1]). In this work an exact relation between filtered, respectively Favre filtered, and unfiltered turbulent fluxes will be derived and tested in the context of turbulent premixed combustion, where heat release gives locally rise to strong gradients and consequently the failure of the standard assumption is expected. For demonstration of the problem, filtered data will be considered from a purely mathematical point of view by applying a convolution operation to data representing solutions to the Navier-Stokes Equations. Results will be discussed for the subfilter stress, the turbulent scalar flux and the subfilter scalar variance.

International audience; Large eddy simulations are carried out for flows in a channel with streamwise-periodic constrictions, a well-documented benchmark case to study turbulent flow separation from a curved surface. Resolution criteria such as wall units are restricted to attached flows and enhanced criteria, such as energy spectra or two-point correlations, are used to evaluate the effective scale separation in the present large eddy simulations. A detailed analysis of the separation above the hill crest and of the early shear layer development shows that the delicate flow details in this region may be hardly resolved on coarse grids already at Re = 10595, possibly leading to a non monotonic convergence with mesh refinement. The intricate coupling between numerical and modeling errors is studied by means of various discretization schemes and subgrid models. It is shown that numerical schemes maximizing the resolution capabilities are a key ingredient for obtaining high-quality solutions while using a reduced number of grid points. On this respect, the introduction of a sharp enough filter is an essential condition for separating accurately the resolved scales from the subfilter scales and for removing ill-resolved structures. The high-resolution approach is seen to provide solutions in very good overall agreement with the available experimental data for a range of Reynolds numbers (up to 37000) without need for significant grid refinement.

Over the past two decades high-fidelity simulations have become feasible for most main gas path turbomachinery components. This paper introduces the key challenges of simulating and modelling turbomachinery flows and presents an overview of possible simulation strategies. A comprehensive overview is given of which particular design challenges have to date been addressed by high-fidelity simulations, with a particular focus on high-pressure and centrifugal compressors, and high-pressure and low-pressure turbines. Recommendations are provided for how quality and accuracy can be ensured for high-fidelity simulations, using direct numerical simulations of a low-pressure turbine as a case study. It is argued that industrial impact from high-fidelity simulations can be achieved in two ways, either by conducting design-of-experiment-like studies that can provide designers with insight into certain physical mechanisms and phenomena, or by helping mature and improve lower-order models. The latter is discussed with particular emphasis on recent advances in machine learning for model assessment and improvement, and the potential of one selected data-driven approach is demonstrated on predictions of wake mixing for low-pressure and high-pressure turbines.

International audience; Discontinuity of the dissipation rate associated with the temperature variance at the fluid-solid interface is analyzed in a turbulent channel flow at a Reynolds number, based on the friction velocity of 395 and a Prandtl number of 0.71. The analysis is performed with a wall-resolved Large Eddy Simulation and the results are used to derive a regression for the dissipation rate discontinuity, which depends only on the fluid-solid thermal diffusivity and conductivity ratios. Wall-resolved Large Eddy Simulations at a higher Reynolds number and a higher Prandtl number are used to investigate the validity of two correlations derived from the regression for the selected thermal properties ratios. The present results are obtained with the open-source Computational Fluid Dynamics solver Code Saturne, and use the fully conservative fluid-solid thermal coupling capability introduced by the authors in version 5.0.

In this paper, the new idea of application of a curved rotor disk in rotor stator systems is presented and analyzed by using the large eddy simulation technique. The geometry of the examined rotor-stator system consists of a stationary flat disk (stator), a rotating curved disk (rotor) and a stationary enclosing cylinder (shroud). A hole in the center of stator allows the flow to enter the cavity, and the clearance between the shroud and rotor enables the flow to exit the cavity. Employing elliptical bumps with different geometrical parameters on the rotor disk (creating a curvature on the rotor), the rotating curved disk is parametrized. Three cavity cases (one with a flat rotor disk, another with the maximum outflow total pressure, and the third with the highest mass flow rate) are selected for LES analysis and more detailed investigation of flow and turbulence structures. The Favre-filtered governing equations for LES analysis of compressible turbulent flows are solved for all three cases. Radial and circumferential flow velocities as well as shear and normal Reynolds stresses in different cavity regions are studied. The flow in a rotor-stator cavity is simultaneously affected by the inlet flow, rotor rotation, and the bump on rotor disk. Creating a bump on rotor disk causes increase of both the radial pressure gradient and the mass flow rate of fluid that enters the rotor-stator cavity.

The physical mechanism leading to flame local extinction remains a key issue to be further understood. An analysis of large eddy simulation (LES) data with presumed probability density function (PDF) based closure (Chen et al., 2020, Combust. Flame, vol. 212, pp. 415) indicated the presence of localised breaks of the flame front along the stoichiometric line. These observations and their relation to local quenching of burning fluid particles, together with the possible physical mechanisms and conditions allowing their appearance in LES with a simple flamelet model, are investigated in this work using a combined Lagrangian-Eulerian analysis. The Sidney/Sandia piloted jet flames with compositionally inhomogeneous inlet and increasing bulk speeds, amounting to respectively 70 and 90% of the experimental blow-off velocity, are used for this analysis. Passive flow tracers are first seeded in the inlet streams and tracked for their lifetime. The critical scenario observed in the Lagrangian analysis, i.e., burning particles crossing extinction holes on the stoichiometric iso-surface, is then investigated using the Eulerian control-volume approach. For the 70% blow-off case the observed flame front breaks/extinction holes are due to cold and inhomogeneous reactants that are cast onto the stoichiometric iso-surface by large vortices initiated in the jet/pilot shear layer. In this case an extinction hole forms only when the strain effect is accompanied by strong subgrid mixing. This mechanism is captured by the unstrained flamelets model due to the ability of the LES to resolve large-scale strain and considers the SGS mixture fraction variance weakening effect on the reaction rate through the flamelet manifold. Only at 90% blow-off speed the expected limitation of the underlying combustion model assumption become apparent, where the amount of local extinctions predicted by the LES is underestimated compared to the experiment. In this case flame front breaks are still observed in the LES and are caused by a stronger vortex/strain interaction yet without the aid of mixture fraction variance. The reasons for these different behaviours and their implications from a physical and modelling point of view are discussed in this study.

The first-order, single-conditioned sub-grid Conditional Moment Closure (CMC) model for Large Eddy Simulation (LES) is applied to simulate a globally lean swirling methane flame in a gas turbine model combustor that has been studied experimentally. The time-averaged velocity, mixture fraction, temperature, major species and OH mass fractions, and the heat release rate are predicted well for most locations. A transient lift-off and flashback of the flame root due to localized extinction near the burner exit is observed that is qualitatively consistent with the experimental measurements. The time-averaged temperature is over-predicted very close to the fuel injection point, while it is accurately reproduced downstream. Comparisons of instantaneous conditionally-filtered temperature in mixture fraction space shows that the LES/CMC reproduces the large scatter of the experimental data points in temperature‒mixture fraction plane that span the full range unburnt to fully burnt, but to a smaller extent, suggesting a minor under-prediction of local extinction or the inaccuracy of the present first-order, coarse-grid CMC formulation to capture locally premixed flame propagation behaviour. Periodic variation of the heat release rate is observed with a frequency close to that of the measured Precessing Vortex Core (PVC). In general, the current LES/CMC model captures most features of this high-mixing-rate nominally non-premixed swirl flame in a gas turbine model combustor in agreement with experimental measurements.

The Stochastic Fields approach is an effective way to implement transported Probability Density Function modelling into Large Eddy Simulation of turbulent combustion. In premixed turbulent combustion however, thin flame-like structures arise in the solution of the Stochastic Fields equations that require grid spacing much finer than the filter scale used for the Large Eddys Simulation. The conventional approach of using grid spacing equal to the filter scale yields substantial numerical error, whereas using grid spacing much finer than the filter length scale is computationally-unaffordable for most industrially-relevant combustion systems. A Partially-Thickened Stochastic Fields approach is developed in this study in order to provide physically accurate and numerically-converged solutions of the Stochastic Fields equations with reduced compute time. The Partially-Thickened Stochastic Fields formulation bridges between the conventional Stochastic Fields and conventional Thickened-Flame approaches depending on the numerical grid spacing utilised, and converges towards Direct Numerical Simulation in the limit of full-resolution. One-dimensional Stochastic Fields simulations of freely-propagating turbulent premixed flames are used in order to obtain criteria for the thickening factor required, as a function of relevant physical and numerical parameters, and to obtain a model for an efficiency function that accounts for the loss of resolved flame surface area caused by applying the thickening transformation to the Stochastic Fields equations. The Thickened Stochastic Fields formulation is tested by performing LES of a laboratory Bunsen flame, demonstrating that the method leads to numerically-converged simulations that agree with results of conventional Stochastic Fields simulations using orders of magnitude more grid points. The present development therefore facilitates the accurate application of the Stochastic Fields approach to industrially-relevant combustion systems.

The flow field in a complete one-stage axial-flow turbine with 30 stator and 62 rotor blades is investigated by large-eddy simulation (LES). To solve the compressible Navier-Stokes equations, a massively parallelized finite-volume flow solver based on an efficient Cartesian cut-cell/level-set approach, which ensures a strict conservation of mass, momentum and energy, is used. This numerical method contains two adaptive Cartesian meshes, one mesh to track the embedded surface boundaries and a second mesh to resolve the fluid domain and to solve the conservation equations. The overall approach allows large scale simulations of turbomachinery applications with multiple relatively moving boundaries in a single frame of reference. The relative motion of the geometries is described by a kinematic motion level-set interface method. The focus of the numerical analysis is on the flow inside the rim seal between the stator and the rotor disks. Full $360^{\circ }$ computations of the turbine stage are performed for two rim seal configurations. First, the impact of the mesh resolution on the LES results is analyzed for the single lip rim seal configuration. Second, the LES results are compared to experimental data, followed by a detailed analysis of the unsteady flow field. For the single lip rim seal configuration, two modes unrelated to the rotor frequency and its harmonics are identified inside the rotor-stator wheel space, where the first more dominant mode shows a major impact on the ingress of the hot gas into the rotor-stator wheel space. The second mode is a counter-rotating mode which results from the interaction of the first mode with the flow field downstream of the stator blades. Third, at the same operating condition a modified configuration with a double lip rim seal is investigated and compared to the reference configuration to demonstrate the impact of the rim seal geometry on the overall flow field. The additional lip on the rotor disk damps the aforementioned modes and reduces the ingress of the hot gas resulting in an increase of the cooling effectiveness inside the rotor-stator wheel space, which is in a good agreement with the experimental results.

In this work a recently presented combustion chamber that is specifically designed for the investigation of gas-assisted coal combustion and the validation of models is simulated under reactive conditions for the first time. In the configuration coal combustion is assisted and stabilized by a methane flame. In the course of the investigation, the configuration’s complexity is increased successively. Results of the isothermal single-phase flow are discussed first. Subsequently, reproducibility of the single-phase methane flame by means of the applied modeling approach is evaluated. In a further step, coal particles having the same thermal power as the methane flame are injected into the configuration. Particle histories, the conversion of the coal particles as well as its retroactive effect on the gas phase are investigated. Experimental results based on laser diagnostics are provided for all operating points and used for comparison with numerical results. Gas phase velocity fields for all operating points are available. In order to identify the reaction in the reactive single-phase case planar laser induced fluorescence of the OH-radical (OH-PLIF) was applied. Overall good agreement between numerical and experimental results could be obtained. In the Large Eddy Simulation (LES) a Flamelet Generated Manifold (FGM) based model is utilized. The four-dimensional manifold is spanned by two mixture fractions, a reaction progress variable and the enthalpy on which the gas phase chemistry gets mapped onto. Thereby, the model accounts for both, volatiles reaction and char conversion. Furthermore, finite rate chemistry effects as well as non-adiabatic physics are considered.

We present large-eddy simulation (LES) of a high-pressure gas jet that is injecting into a quiescent inert environment. The injection is through a nozzle with a diameter of 1.35 mm. Four injection strategies are considered in which the results of a single continuous injection case are compared with those of double injection cases with different injection splitting timing. In all double injection cases, the injection pulsing interval is kept the same, and the total injected mass is equal to that of the single injection case. On the other hand, the splitting timing is varied to investigate the effects of various injection splitting strategies on the mixture formation and the penetration length of the jet. Results show that the jet penetration length is not so sensitive to the splitting timing whereas the mixing quality can significantly change as a result of shifting the onset of injection splitting toward the end of injection. Especially, it is found that by adopting a post-injection strategy where a single injection splits into the main injection and late small injection near the end of injection period the mixing between the injected gas and ambient air is significantly improved. This trend is not as obvious when the injection splitting timing shifts toward the beginning or even in the middle of injection period. The increase of entrainment in the tail of each injection is one of the underlying physics in the mixing improvement in double injection cases. In addition to that, splitting a single injection into two smaller injections increases the surrounding area of the jet and also stretches it along the axial direction. It can potentially increase the mixing of injected gas with the ambient air.

Direct Numerical Simulation (DNS) and Large Eddy Simulation (LES) of spatially inhomogeneous flows strongly depend on turbulent inflow boundary conditions. Realistic coherent structures need to be prescribed to avoid the immediate damping of random velocity fluctuations. A new turbulent inflow data generation method based on an auxiliary simulation of forced turbulence in a box is presented. The new methodology combines the flexibility of the synthetic turbulence generation with the accuracy of precursor simulation methods. In contrast to most auxiliary simulations, the new approach provides full control over the turbulence properties and computational costs remain reasonable. The lack of physical information and artificiality attested with pseudo-turbulence methods is overcome since the inflow data stems from a solution of the Navier-Stokes equations. The generated velocity fluctuations are by construction divergence-free and exhibit the non-Gaussian characteristics of turbulence. The generated inflow data is applied to the simulation of multiphase primary breakup.

Turbulent flow over variably-shaped rough walls, characterized by either a regular or a random arrangement of axisymmetric roughness elements in an open channel flow configuration, is investigated computationally within a VLES (Very Large Eddy Simulation) framework by utilizing a volumetric forcing-based roughness model. The prime objective of the present work is to assess the roughness model’s capability to predict mean velocities and turbulent intensities in conjunction with this recently formulated hybrid LES/RANS (Reynolds-Averaged Navier-Stokes) model. The friction velocity-based Reynolds number is in the range Reτ = 460 − 500. A non-dimensional drag function accounting for the shape of the roughness elements is introduced and evaluated based on the results of complementary direct numerical simulations (DNS). The dynamics of the residual motion of the presently adopted VLES methodology is described by an appropriately modified elliptic-relaxation-based ζ − f ( $\zeta =\overline {v^{2}}/k$ ) RANS model.

Fuel efficiency improvement and harmful emission reduction are the paramount driving forces for development of gas turbine combustors. Lean-burn combustors can accomplish these goals, but require specific flow topologies to overcome their sensitivity to combustion instabilities. Large Eddy Simulations (LES) can accurately capture these complex and intrinsically unsteady flow fields, but estimating the appropriate numerical resolution and subgrid model(s) still remain challenges. This paper discusses the prediction of non-reacting flow fields in the DLR gas turbine model combustor using LES. Several important features of modern gas turbine combustors are present in this model combustor: multiple air swirlers and recirculation zones for flame stabilisation. Good overall agreement is obtained between LES outcomes and experimental results, both in terms of time-averaged and temporal RMS values. Findings of this study include a strong dependence of the opening angle of the swirling jet inside the combustion chamber on the subgrid viscosity, which acts mainly through the air mass flow split between the two swirlers in the DLR model combustor. This paper illustrates the ability of LES to obtain accurate flow field predictions in complex gas turbine combustors making use of open-source software and computational resources available to industry.

Motivated by the sizable increase of available computing resources, large-eddy simulation of complex turbulent flow is becoming increasingly popular. The underlying filtering operation of this approach enables to represent only large-scale motions. However, the small-scale fluctuations and their effects on the resolved flow field require additional modeling. As a consequence, the assumptions made in the closure formulations become potential sources of incertitude that can impact the quantities of interest. The objective of this work is to introduce a framework for the systematic estimation of structural uncertainty in large-eddy simulation closures. In particular, the methodology proposed is independent of the initial model form, computationally efficient, and suitable to general flow solvers. The approach is based on introducing controlled perturbations to the turbulent stress tensor in terms of magnitude, shape and orientation, such that propagation of their effects can be assessed. The framework is rigorously described, and physically plausible bounds for the perturbations are proposed. As a means to test its performance, a comprehensive set of numerical experiments are reported for which physical interpretation of the deviations in the quantities of interest are discussed.

International audience; The selective non-catalytic reduction (SNCR) process used to transform nitrogen monoxide molecules into environmentally friendly gases is reviewed with its numerical modeling. The fundamentals of SNCR in terms of chemistry and flow physics are first discussed, to then examine how they impact on the design and optimization of furnaces and incinerators relying on this technology. The various options in operation today are presented for coal-fired or waste-to-energy power plants, biomass and CO boilers. In complement, specific applications using additive components are discussed along with the amine reclaimer waste approach. The methodology and the challenges of computational fluid dynamics applied to SNCR systems are reviewed, before discussing emerging simulation techniques, as large eddy simulation (LES) of such complex systems. The modeling of the multicomponent evaporation of the liquid reagent injected for transforming NOx, as urea, and the modeling of the chemistry in the gaseous phase, are addressed for the numerical simulation of SNCR. Finally, reduced-order modeling of SNCR is discussed and perspectives are drawn for the control, in situ, of SNCR systems.

Low swirl flame characteristics under external flow excitations are numerically investigated using large eddy simulations with a dynamically thickened flame combustion model. A finite volume scheme on a Cartesian grid with a dynamic one equation eddy viscosity subgrid scale model is used for large eddy simulations. The excitations are imposed on inlet velocity profiles by a sinusoidal forcing function over a wide range of amplitudes and frequencies. Present investigation shows that although, the swirling motion of the low swirl flame is not intense enough to induce a recirculation zone in ensemble averaged results, external flow excitations increase the local swirl number upstream of the flame front. Such increase in the local swirl number is at maximum value when the low swirl flame is excited at the dominant frequency of the flow field, which in turn induces a vortex breakdown and hence a central recirculation zone. The strength and size of the time averaged recirculation zone depend on both the amplitude and frequency of the excitations. Moreover, phase-locked results indicate that external flow excitations induce local swirl fluctuations ahead of the flame front which alter the strength of the recirculation zone at different phase angles of the excitations.

International audience; This review presents the state of the art of hybrid RANS/LES modeling for thesimulation of turbulent flows. After recalling the modeling used in RANS and LES methodologies,we propose in a first step a theoretical formalism developed in the spectral spacethat allows to unify the RANS and LES methods from a physical standpoint. In a secondstep, we discuss the principle of the hybrid RANS/LES methods capable of representing aRANS-type behavior in the vicinity of a solid boundary and an LES-type behavior far awayfrom the wall boundary. Then, we analyze the principal hybrid RANS/LES methods usuallyused to perform numerical simulation of turbulent flows encountered in engineering applications.In particular, we investigate the very large eddy simulation (VLES), the detachededdy simulation (DES), the partially integrated transport modeling (PITM) method, thepartially averaged Navier-Stokes (PANS) method, and the scale adaptive simulation (SAS)from a physical point of view. Finally, we establish the connection between these methodsand more precisely, the link between PITM and PANS as well as DES and PITMshowing that these methods that have been built by different ways, practical or theoreticalmanners have common points of comparison. It is the opinion of the author to considerthat the most appropriate method for a particular application will depend on the expectationsof the engineer and the computational resources the user is prepared to expend on theproblem.; Cet article d’expertise présente l’état de l’art de la modélisation hybride RANS/LES pour la simulation des écoulements turbulents. Après avoir rappelé la modélisation des méthodologies RANS et LES, nous proposons dans une première étape un formalisme théorique développée dans l’espace spectral qui permet d’unifier les méthodes RANS et LES sur la base d’arguments physiques. Dans une seconde étape, nous étudions les principes des méthodes hybrides RANS et LES, capables de reproduire un comportement de type RANS à proximité des zones de paroi et de type LES plus au loin des parois. Nous analysons ensuite les principales méthodes RANS/LES utilisées pour simuler les écoulements turbulents rencontrées dans les applications d’ingénieur. En particulier, nous abordons les méthodes “very large eddy simulation ”(VLES), “detached eddy simulation ”(DES), “partially integrated transport modeling ”(PITM), “partially averaged Navier-Stokes ”(PANS), et “scale adaptive simulation ”(SAS). Finalement, nous établissons les connexions entre ces différentes méthodes et plus précisement le lien entre les méthodes PITM et PANS ainsi que le lien entre les méthodes DES et PITM, montrant que ces méthodes qui ont été construites par différentes voies, pratique ou théorique, ont des points communs de comparaison. C’est l’opinion de l’auteur de considérer que la méthode la plus approprée pour une application donnée dépendra des attentes de l’ingénieur et des ressources informatiques que l’utilisateur est pret à consacrer sur le problème.

An efficient Cartesian cut-cell/level-set method based on a multiple grid approach to simulate turbulent turbomachinery flows is presented. The finite-volume approach in an unstructured hierarchical Cartesian setup with a sharp representation of the complex moving boundaries embedded into the computational domain, which are described by multiple level-sets, ensures a strict conservation of mass, momentum, and energy. Furthermore, an efficient kinematic motion level-set interface method for the rotation of embedded boundaries described by multiple level-set fields on a computational domain distributed over several processors is introduced. This method allows the simulation of multiple boundaries rotating relatively to each other in a fixed frame of reference. To demonstrate the efficiency of the numerical method and the quality of the computed findings the generic test problem of a rotating cylinder surrounded by a stationary hull and the flow over a ducted rotating axial fan with a stationary turbulence generating grid at the inflow are simulated. The computational results of the axial fan show a good agreement with the experimental data.

Large Eddy Simulations (LES) of a swirl-stabilized natural gas-air flame in a laboratory gas turbine combustor is performed using six different LES combustion models to provide a head-to-head comparative study. More specifically, six finite rate chemistry models, including the thickened flame model, the partially stirred reactor model, the approximate deconvolution model and the stochastic fields model have been studied. The LES predictions are compared against experimental data including velocity, temperature and major species concentrations measured using Particle Image Velocimetry (PIV), OH Planar Laser-Induced Fluorescence (OH-PLIF), OH chemiluminescence imaging and one-dimensional laser Raman scattering. Based on previous results a skeletal methane-air reaction mechanism based on the well-known Smooke and Giovangigli mechanism was used in this work. Two computational grids of about 7 and 56 million cells, respectively, are used to quantify the influence of grid resolution. The overall flow and flame structures appear similar for all LES combustion models studied and agree well with experimental still and video images. Takeno flame index and chemical explosives mode analysis suggest that the flame is premixed and resides within the thin reaction zone. The LES results show good agreement with the experimental data for the axial velocity, temperature and major species, but differences due to the choice of LES combustion model are observed and discussed. Furthermore, the intrinsic flame structure and the flame dynamics are similarly predicted by all LES combustion models examined. Within this range of models, there is no strong case for deciding which model performs the best.

In this work, ignition process in a turbulent shear-less methane-air mixing layer is numerically investigated. A compressible large eddy simulation method with Smagorinsky sub-grid scale model is used to solve the flow field. Also, a thickened flame combustion model and DRM-19 reduced mechanism are used to compute species distribution and the heat release. Non-reacting mean and RMS axial velocity profiles and mean mixture fraction are validated against experimental data. Instantaneous mixture fraction contours show that the large bursts penetrate from the fuel stream into that of the oxidizer and vice versa and a random behaviour in the cross-stream direction. Flame kernel initiation, growth and propagation are analysed and compared with the experimental data. The ignition results show that the flame is not stable and blow-off occurs, but a more detailed investigation shows that local and short time flame stabilization exist during blow-off. During these local stabilization, heat release increased at the upstream edge of the flame. Most_upstream flame edge scalar analysis shows that the methane mass fraction has a dominant role in the local flame stabilization. OH, HO2, CH2O and heat release contours demonstration reveal that HO2 and CH2O mass fraction as well as the heat release reach a maximum on the border of the flame, but the maximum OH concentration is located in the middle of flame kernel.

The objective of the analysis presented in this work is to investigate the distribution of the anisotropy invariants of the in-cylinder flow to examine the characteristics of the turbulence during an engine cycle and its interactions with the in-cylinder coherent structures. For this purpose, both the results of Large Eddy Simulations of the flow in an internal combustion engine under motored conditions and measurements obtained in the same configuration have been employed. Investigations include an analysis of the in-cylinder flow properties in the engine’s cross plane by means of first and second order statistical moments. Additionally, the behaviour of the flows anisotropy tensor has been analysed. It has been observed that during the intake stroke the in-cylinder turbulence shows a rather anisotropic structure, whereas during the compression stroke it tends to be more isotropic. Furthermore, the degree of anisotropy increases again in correspondence of vortex breakdown, during which particularly high velocity fluctuations have been identified also by experimental investigation.