@misc{MenonOevermannKerstein, author = {Menon, Abhilash M. and Oevermann, Michael and Kerstein, Alan R.}, title = {A super-grid approach for LES combustion closure using the linear eddy model}, series = {Combustion Theory and Modelling}, volume = {28}, journal = {Combustion Theory and Modelling}, number = {1}, publisher = {Informa UK Limited}, issn = {1364-7830}, doi = {10.1080/13647830.2023.2260351}, pages = {99 -- 126}, abstract = {LES-LEM is a simulation approach for turbulent combustion in which the stochastic Linear Eddy Model (LEM) is used for sub-grid mixing and combustion closure in Large-Eddy Simulation (LES). LEM resolves, along a one-dimensional line, all spatial and temporal scales, provides on-the-fly local turbulent flame statistics, captures finite rate chemistry effects and directly incorporates turbulence-chemistry interaction. However, the approach is computationally expensive as it requires advancing an LEM-line in each LES cell. This paper introduces a novel turbulent combustion closure model for LES using LEM to address this issue. It involves coarse-graining the LES mesh to generate a coarse- level 'super-grid' comprised of cell-clusters. Each cell-cluster, instead of each LES cell, then contains a single LEM domain. This domain advances the combined advection-reaction-diffusion solution and also provides suitably conditioned statistics for thermochemical scalars such as species mass fractions. Local LES-filtered thermochemical states are then obtained by probability-density-function (PDF) weighted integration of binned conditionally averaged scalars, akin to standard presumed PDF approaches for reactive LES but with physics-based determination of the full thermochemical state for particular values of the conditioning variables. The proposed method is termed 'super-grid LEM' or 'SG-LEM'. The paper describes LEM reaction-diffusion advancement, the LEM representation of turbulent advection, a novel splicing algorithm (a key feature of LES-LEM) formulated for the super-grid approach, a wall treatment, and a thermochemical LES closure procedure. To validate the proposed model, a pressure-based solver was developed using the OpenFOAM library and tested on a premixed ethylene flame stabilised over a backward facing step, a setup for which some DNS data is available. SG-LEM provides high resolution flame structures, temperature and mass fractions suitable for LES thermochemical closure. Additionally, it provides reaction-rate data at the coarse level, a unique feature compared to other mapping-type closure methods. Quantitative comparisons are made between the proposed model and time-averaged DNS data, focussing on velocity, temperature and species mass fraction. Results show good agreement downstream of the step. Furthermore, comparison with an equivalent Partially-Stirred Reactor (PaSR) simulation demonstrates the superior predictive capability of SG-LEM. Additionally, the paper briefly examines the sensitivity of the model to coarse-graining parameters and finally, explores computational efficiency highlighting the substantial speedup achieved when compared to the standard LES-LEM approach with potentially significant speedup relative to PaSR closure for the intensely turbulent regimes of principal interest.}, language = {en} } @article{DoubianiKersteinOevermann, author = {Doubiani, Nidal and Kerstein, Alan R. and Oevermann, Michael}, title = {A pressure-coupled Representative Interactive Linear Eddy Model (RILEM) for engine simulations}, series = {Fuel}, volume = {355 (2024)}, journal = {Fuel}, publisher = {Elsevier BV}, issn = {0016-2361}, doi = {10.1016/j.fuel.2023.129423}, pages = {1 -- 15}, abstract = {The Representative Linear Eddy Model (RILEM) was introduced by Lackmann et al. (2018) as an alternative modeling approach to simulate turbulent non-premixed combustion in engines. The model utilizes a RANS approach for turbulence and the Linear Eddy Model (LEM) with a presumed probability density function (PDF) approach for combustion closure. A distinct feature of RILEM is its potential to handle arbitrary combustion regimes and the consideration of complex physical phenomena such as differential diffusion effects. The original version of RILEM implemented a volume-based coupling between LEM and the flow solver. This work presents a new variant of RILEM, i.e., Multiple Representative Interactive Linear Eddy Model (MRILEM) based on a pressure-based coupling, to overcome some deficiencies of the original RILEM, namely statistical fidelity. Due to the introduced pressure coupling, the effects of heat losses (wall heat fluxes, latent heat of evaporation) on combustion are intrinsically included via the pressure trace. Furthermore, we introduce a new step function PDF for the progress variable defined by its mean value only. Issues with an incomplete solution space for mixture fraction and progress variable due to the stochastic nature of LEM are remedied with a PDF scaling technique, aided by a novel parameterization of the progress-variable PDF The new variant of RILEM is evaluated using part- and full-load cases of a heavy-duty metal engine. The impact of utilizing multiple LEM lines on the completeness of the solution space and its influence on the distribution of scalar values in the CFD domain was demonstrated. Results for pressure trace, flame structure, and CO emissions are analyzed and compared with simulations using the Multi-Zone Well-Mixed Model (MZWM) model and experiments. While pressure traces agree well among the different models and experiments, noteworthy differences are observed between the models regarding CO emissions and temperature. Effects of turbulence chemistry interaction were noticed when comparing MRILEM to the results of the MZWM simulation, namely flame brush and species mass fraction distribution.}, language = {en} } @misc{DoubianiOevermann, author = {Doubiani, Nidal and Oevermann, Michael}, title = {Multiple representative interactive linear eddy model: Investigation of turbulence chemistry interaction and evaluation of progress variable definition and PDFs}, series = {Fuel}, volume = {381}, journal = {Fuel}, publisher = {Elsevier BV}, issn = {0016-2361}, doi = {10.1016/j.fuel.2024.133445}, pages = {1 -- 15}, abstract = {Improving the predictions of unsteady effects in combustion processes requires novel combustion models that include turbulence chemistry interaction effects. The Multiple Representative Interactive Linear Eddy Model (MRILEM) is an improved version of the previous RILEM variant. MRILEM utilizes a pressure coupling instead of a volume constraint to intrinsically include heat effects into the LEM line with no supplementary modeling. In addition, it advances multiple LEM lines in parallel to improve statistical fidelity. The pressure coupling of MRILEM generates a coupling effect between the LEM lines that assists in communicating the combustion process between the lines. The "Spray-B" engine of the Engine Combustion Network (ECN) was simulated using MRILEM. While the original RILEM variation employs a straightforward Dirac δ-peak for the progress variable, a realistic PDF requires this function to extend over the entire space. The introduced MRILEM compares the utilization of two progress variable PDFs, namely a step function defined based on the mean and a β-PDF generated from the progress variable mean and variance. The progress variable variance was calculated based on the Pierce and Moin formulation with a RANS adaptation based on the integral length scale. In addition, two definitions of the progress variable are investigated, namely O2 and h298. A tabulation method is introduced for RILEM to reduce the computational time by advancing pre-generated LEM solution matrices constructed in mixture fraction Z and progress variable c spaces. The different variants of the model, i.e., MRILEM-βZ-Stepc, MRILEM-βZ-βc, TRILEM-β-Stepc, and TRILEM-βZ-βc were compared against experiments based on heat release rate, ignition delay, flame lift-off, and computational time.}, language = {en} } @misc{DoubianiOevermannLucchinietal., author = {Doubiani, Nidal and Oevermann, Michael and Lucchini, Tommaso and Zhou, Qiyan}, title = {Numerical investigation of ducted fuel injection using Multiple Representative Interactive Linear Eddy Model}, series = {International Journal of Engine Research}, volume = {2025}, journal = {International Journal of Engine Research}, publisher = {SAGE Publications}, issn = {1468-0874}, doi = {10.1177/14680874241298046}, abstract = {Fuel-air mixing in non-premixed and partially premixed combustion has a major influence on soot emissions. Improving the mixing quality of the premixed region in non-premixed combustion upstream of the auto-ignition zone aids substantially in reducing soot emissions. The Ducted Fuel Injection (DFI) concept is based on injecting fuel inside a metal cylinder inside the combustion chamber at a certain distance from the injection nozzle exit hole. Although the concept is straightforward, recent studies have shown that implementing DFI in compression ignition combustion chambers dramatically affects soot mitigation. DFI improves air entrainment by a suction effect when fuel travels inside the duct, which increases the turbulence level at the inlet and enhances the fuel-air mixing quality. Additionally, DFI delays ignition by preventing the formation of stoichiometric regions at the spray cone's outer region. This work utilizes a recently presented combustion model titled the Multiple Representative Interactive Linear Eddy Model (MRILEM) to simulate DFI in a high-pressure, high-temperature constant volume chamber. Several simulations are realized for two different ambient temperatures using an n-dodecane mechanism, where several parameters, such as ignition delay, lift-off length, and flame structure, are assessed. Simulation results are compared to experimental data from the literature and simulation results obtained with the Multi-Zone Well Mixed (MZWM) model simulation results. Results show that by enforcing the duct on the LEM line and modifying the turbulence implementation strategy, MRILEM shows overall realistic predictions for DFI cases and reasonable quantitative results for lift-off length and ignition delay compared to the MZWM model.}, language = {en} } @misc{OezguenoğluMouokueOevermannetal., author = {{\"O}zg{\"u}noğlu, Mehmet and Mouokue, Gerard and Oevermann, Michael and Bensow, Rickard E.}, title = {Numerical investigation of cavitation erosion in high-pressure fuel injector in the presence of surface deviations}, series = {Fuel}, volume = {386}, journal = {Fuel}, publisher = {Elsevier BV}, issn = {0016-2361}, doi = {10.1016/j.fuel.2024.134174}, pages = {1 -- 21}, abstract = {This study investigates cavitation-induced erosion in high-pressure fuel injectors using numerical simulations, focusing on the effects of surface deviations, turbulence modeling, and a refined approach for the erosion assessment. The proposed erosion model combines advanced erosion indicators to enhance predictive accuracy while addressing limitations in existing methodologies. Cavitation dynamics are simulated with the modified Zwart-Gerber-Belamri model, employing Reynolds-averaged Navier-Stokes (RANS) and Large Eddy Simulation (LES) approaches. Numerical results for a high-lift needle position are validated against experimental data, providing insights into erosion behavior in industrial heavy-duty injectors. Both Computer-Aided Design (CAD) and Tomography Scan (TS) models are used to evaluate the impact of surface deviations on erosion patterns. Results reveal that incorporating surface deviations reduces the vapor volume and alters the erosion patterns. LES simulations exhibit enhanced sensitivity to the surface deviations, capturing finer turbulence structures and local pressure fluctuations, whereas RANS provides reasonable accuracy with lower computational cost.}, language = {en} } @misc{MenonKersteinOevermann, author = {Menon, Abhilash M. and Kerstein, Alan and Oevermann, Michael}, title = {Investigation of coarse-graining parameters for super-grid LEM closure applied to LES of practical bluff-body flames}, series = {Combustion Theory and Modelling}, volume = {2024}, journal = {Combustion Theory and Modelling}, publisher = {Taylor\&Francis}, issn = {1364-7830}, doi = {10.1080/13647830.2024.2428156}, pages = {1 -- 22}, abstract = {Large Eddy Simulation (LES) coupled with the Linear Eddy Model (LEM) provides a robust method for studying turbulent combustion, but it is computationally expensive due to the need for highly resolved sub-grid LEM domains. These domains simulate sub-grid stirring through stochastic rearrangements of scalar fields, while large-scale transport is modelled using a Lagrangian 'splicing' scheme. To address the computational cost of LES-LEM, a super-grid (SG) framework for LEM closure was developed by the authors (Comb. Theor. Model. 28, 2024), which uses coarse-graining, on-the-fly chemistry tabulation and a presumed PDF approach to reconstruct thermochemical fields at LES resolution. This study applies SG-LEM to a challenging setup, Case 1 of the Volvo Validation Rig, which involves a bluff-body-stabilised turbulent premixed propane-air flame, as a stress test to identify limitations that were not revealed by the previous application, in particular that of the coarse-graining parameters used to generate the super-grid. The intent is to yield a more realistically constrained assessment of the current capabilities of the method, and insight into possible ways for improving it. Four simulations were conducted using three SG cluster sizes. The finest resolution was tested with a global 2-step mechanism, showing good agreement with experimental data for temperature and velocity, particularly near the bluff body. The two larger cluster sizes used a 66-step skeletal mechanism for more detailed chemical closure but led to unphysical quenching due to splicing inaccuracies. To mitigate these issues, two novel additions were introduced: an intra-cluster-stirring routine and a method to control SG cluster shapes to reduce numerical dissipation. These methods improved flame stability with coarser SG clusters and more detailed mechanisms. Comparison with experiments showed good agreement for temperature and velocity, though elevated CO levels were observed in the recirculation region. Potential methods for further improving SG-LEM's capabilities are discussed.}, language = {en} }