Dimensional Decomposition of Turbulent Reacting Flows

A dimensional-decomposition approach, decomposing 3D into 3×1D for turbulent (reacting) flows are motivated, discussed and investigated. In the three-dimensional Linear Eddy Model (LEM3D), three orthogonally intersecting arrays of 1D domains are coupled to capture the 3D characteristics of fluid flows. The currently used recouplings for LEM3D are enlightened and thoroughly discussed. A study of the flame front of a freely propagating laminar premixed flame shows that the flame stabilizes at the upstream face of the initial solution when both the advective and auxiliary recouplings are activated. Furthermore, results from LEM3D simulations of a vitiated co-flow burner are re-visited providing a more detailed discussion of the noted early mixing and reaction of the hydrogen fuel of the burner. The main conclusion of the present work is that the auxiliary coupling based on rotations of the 3D control volumes introduces very large gradients in the near-field geometry of jets, leading to a significant amount of artificial diffusion and locally increased burning rates. This implies that applications of LEM3D should be restricted to sub-regions where high-resolution treatment of scalar mixing and reaction is of particular interest.

Effects of Entrainment and Mixing on the Wegener–Bergeron–Findeisen Process

The growth of ice crystals at the expense of water droplets, the Wegener–Bergeron–Findeisen (WBF) process, is of major importance for the production of precipitation in mixed-phase clouds. The effects of entrainment and mixing on WBF, however, are not well understood, and small-scale inhomogeneities in the thermodynamic and hydrometeor fields are typically neglected in current models. By applying the linear eddy model, a millimeter-resolution representation of turbulent deformation and molecular diffusion, we investigate these small-scale effects on WBF. While we show that entrainment is accelerating WBF by contributing to the evaporation of liquid droplets, entrainment may also cause aforementioned inhomogeneities, particularly regions filled with exclusively ice or liquid hydrometeors, which tend to decelerate WBF if the ice crystal concentration exceeds 100 L−1. At lower ice crystal concentrations, even weak turbulence can homogenize hydrometeor and thermodynamic fields sufficiently fast so as to not affect WBF. Independent of the ice crystal concentration, it is shown that a fully resolved entrainment and mixing process may delay the nucleation of entrained aerosols to ice crystals, thereby delaying the uptake of water vapor by the ice phase, further slowing down WBF. All in all, this study indicates that, under specific conditions, small-scale inhomogeneities associated with entrainment and mixing counteract the accelerated WBF in entraining clouds. However, further research is required to assess the importance of the newly discovered processes more broadly in fully coupled, evolving mixed-phase cloud systems.

Entrainment and Mixing in Stratocumulus: Effects of a New Explicit Subgrid-Scale Scheme for Large-Eddy Simulations with Particle-Based Microphysics

The entrainment and mixing of free-tropospheric air is an essential component of the observed microphysical structure of stratocumulus clouds. Since the relevant scales involved in this process are usually smaller than the grid spacing of typical large-eddy simulations (LESs), their correct representation is difficult. To adequately accommodate these small-scale processes, we apply a recently developed approach that explicitly simulates LES subgrid-scale (SGS) turbulence fluctuation of supersaturation using the one-dimensional linear eddy model. As a result of reduced numerical diffusion and the ability to explicitly represent the SGS distribution of liquid water and supersaturation, entrainment rates tend to be lower in the new approach compared to simulations without it. Furthermore, cloud holes comprising free-tropospheric air with negligible liquid water are shown to persist longer in the stratocumulus deck. Their mixing with the cloud is shown to be more sensitive to the microphysical composition of the cloud as a result of the explicitly resolved inhomogeneous mixing, which is also confirmed analytically. Moreover, inhomogeneous mixing is shown to decrease the droplet concentration and to increase droplet growth significantly, in contrast to previous studies. All in all, the simulations presented can be seen as a first step to bridge the gap between ultra-high-resolution direct numerical simulation and LES, allowing an appropriate representation of small-scale mixing processes, together with the large-scale dynamics of a stratocumulus system.

Inhomogeneous Mixing in Lagrangian Cloud Models: Effects on the Production of Precipitation Embryos

Although small-scale turbulent mixing at cloud edge has substantial effects on the microphysics of clouds, most models do not represent these processes explicitly, or parameterize them rather crudely. This study presents a first use of the linear eddy model (LEM) to represent unresolved turbulent mixing at the subgrid scale (SGS) of large-eddy simulations (LESs) with a coupled Lagrangian cloud model (LCM). The method utilizes Lagrangian particles to provide the trajectory of air masses within LES grid boxes, while the LEM is used to redistribute these air masses among the Lagrangian particles based on the local features of turbulence, allowing for the appropriate representation of inhomogeneous to homogeneous SGS mixing. The new approach has the salutary effect of mitigating spurious supersaturations. At low turbulence intensities, as found in the early stages of an idealized bubble cloud simulation, cloud-edge SGS mixing tends to be inhomogeneous and the new approach is shown to be essential for the production of raindrop embryos. At higher turbulence intensities, as found in a field of shallow cumulus, SGS mixing tends to be more homogeneous and the new approach does not significantly alter the results, indicating that a grid spacing of 20 m may be sufficient to resolve all relevant scales of inhomogeneous mixing. In both cases, droplet in-cloud residence times are important for the production of precipitation embryos in the absence of small-scale inhomogeneous mixing, either naturally due to strong turbulence or artificially as a result of coarse resolution or by not using the LEM as an SGS model.

Investigation of turbulence–chemistry interactions in a heavy-duty diesel engine with a representative interactive linear eddy model

Simulations of a heavy-duty diesel engine operated at high-load and low-load conditions were compared to each other, and experimental data in order to evaluate the influence of turbulence–chemistry interactions on heat release, pressure development, flame structure, and temperature development are quantified. A recently developed new combustion model for turbulent diffusion flames called representative interactive linear eddy model which features turbulence–chemistry interaction was compared to a well-stirred reactor model which neglects the influence of turbulent fluctuations on the mean reaction rate. All other aspects regarding the spray combustion simulation like spray break-up, chemical mechanism, and boundary conditions within the combustion chamber were kept the same in both simulations. In this article, representative interactive linear eddy model is extended with a progress variable, which enables the model to account for a flame lift-off and split injection, when it is used for diffusion combustion. In addition, the extended version of representative interactive linear eddy model offers the potential to treat partially premixed and premixed combustion as well. The well-stirred reactor model was tuned to match the experimental results, thus computed pressure and apparent heat release are in close agreement with the experimental data. Representative interactive linear eddy model was not tuned specifically for the case and thus the computed results for pressure and heat release are in reasonable agreement with experimental data. The computational results show that the interaction of the turbulent flow field and the chemistry reduce the peak temperatures and broaden up the turbulent flame structure. Since this is the first study of a real combustion engine (metal engine) with the newly developed model, representative interactive linear eddy model appears as a promising candidate for predictions of spray combustion in engines, especially in combustion regimes where turbulence–chemistry interaction plays an even more important role like, example given, in low-temperature combustion or combustion with local extinction and re-ignition.