Assessment of flamelet manifolds for turbulent flame-wall interactions in Large-Eddy Simulations

A turbulent side-wall quenching (SWQ) flame in a fully developed channel flow is studied using Large-Eddy Simulation (LES) with a tabulated chemistry approach. Three different flamelet manifolds with increasing levels of complexity are applied: the Flamelet-Generated Manifold (FGM) considering varying enthalpy levels, the Quenching Flamelet-Generated Manifold (QFM), and the recently proposed Quenching Flamelet-Generated Manifold with Exhaust Gas Recirculation (QFM-EGR), with the purpose being to assess their capability to predict turbulent flame-wall interactions (FWIs), which are highly relevant to numerical simulations of real devices such as gas turbines and internal combustion engines. The accuracy of the three manifolds is evaluated and compared a posteriori, using the data from a previously published flame-resolved simulation with detailed chemistry for reference. For LES with the FGM, the main characteristics such as the mean flow field, temperature, and major species can be captured well, while notable deviations from the reference results are observed for the near-wall region, especially for pollutant species such as \ce{CO}. In accordance with the findings from laminar FWI, improvement is also observed in the simulation with QFM under turbulent flow conditions. Although LES with the QFM-EGR shows a similar performance in the prediction of mean quantities as LES with QFM, it presents significantly better agreement with the reference data regarding instantaneous thermo-chemical states near the quenching point. This indicates the necessity to take into account the mixing effects in the flamelet manifold to correctly capture the flame-vortex interaction near the flame tip in turbulent configurations. Based on the findings from this study, suitable flamelet manifolds can be chosen depending on the aspects of interest in practical applications

Numerical Study of Quenching Distances for Side-Wall Quenching Using Detailed Diffusion and Chemistry

The numerical investigation of quenching distances in laminar flows is mainly concerned with two setups: head-on quenching (HOQ) and side-wall quenching (SWQ). While most of the numerical work has been conducted for HOQ with good agreement between simulation and experiment, far less analysis has been done for SWQ. Most of the SWQ simulations used simplified diffusion models or reduced chemistry and achieved reasonable agreement with experiments. However, it has been found that quenching distances for the SWQ setup differ from experimental results if detailed diffusion models and chemical reaction mechanisms are employed. Side-wall quenching is investigated numerically in this work with steady-state 2D and 3D simulations of an experimental flame setup. The simulations fully resolve the flame and employ detailed reaction mechanisms as well as molecular diffusion models. The goal is to provide data for the sensitivity of numerical quenching distances to different parameters. Quenching distances are determined based on different markers: chemiluminescent species, temperature and OH iso-surface. The quenching distances and heat fluxes at the cold wall from simulations and measurements agree well qualitatively. However, quenching distances from the simulations are lower than those from the experiments by a constant factor, which is the same for both methane and propane flames and also for a wide range of equivalence ratios and different markers. A systematic study of different influencing factors is performed: Changing the reaction mechanism in the simulation has little impact on the quenching distance, which has been tested with over 20 different reaction mechanisms. Detailed diffusion models like the mixture-averaged diffusion model and multi-component diffusion model with and without Soret effect yield the same quenching distances. By assuming a unity Lewis number, however, quenching distances increase significantly and have better agreement with measurements. This was validated by two different numerical codes (OpenFOAM and FASTEST) and also by 1D head-on quenching simulations (HOQ). Superimposing a fluctuation on the inlet velocity in the simulation also increases the quenching distance on average compared to the reference steady-state case. The inlet velocity profile, temperature boundary condition of the rod and radiation have a negligible effect. Finally, three dimensional simulations are necessary in order to obtain the correct velocity field in the SWQ computations. This however has only a negligible effect on quenching distances

Protocol for a randomized controlled trial on risk adapted damage control orthopedic surgery of femur shaft fractures in multiple trauma patients

<p>Abstract</p> <p>Background</p> <p>Fractures of the long bones and femur fractures in particular are common in multiple trauma patients, but the optimal management of femur fractures in these patients is not yet resolved. Although there is a trend towards the concept of "Damage Control Orthopedics" (DCO) in the management of multiple trauma patients with long bone fractures as reflected by a significant increase in primary external fixation of femur fractures, current literature is insufficient. Thus, in the era of "evidence-based medicine", there is the need for a more specific, clarifying trial.</p> <p>Methods/Design</p> <p>The trial is designed as a randomized controlled open-label multicenter study. Multiple trauma patients with femur shaft fractures and a calculated probability of death between 20 and 60% will be randomized to either temporary fracture fixation with fixateur externe and defined secondary definitive treatment (DCO) or primary reamed nailing (early total care). The primary objective is to reduce the extent of organ failure as measured by the maximum sepsis-related organ failure assessment (SOFA) score.</p> <p>Discussion</p> <p>The Damage Control Study is the first to evaluate the risk adapted damage control orthopedic surgery concept of femur shaft fractures in multiple trauma patients in a randomized controlled design. The trial investigates the differences in clinical outcome of two currently accepted different ways of treating multiple trauma patients with femoral shaft fractures. This study will help to answer the question whether the "early total care" or the „damage control” concept is associated with better outcome.</p> <p>Trial registration</p> <p>Current Controlled Trials ISRCTN10321620</p

Turbulent Flame-Wall interaction visualization

Visualization of turbulent Side-wall quenching for the use in presentations or public relations. If you use the figures please give approriate credit to the author.v1.

Numerical simulation of single rising bubbles influenced by soluble surfactant in the spherical and ellipsoidal regime

Surfactants are surface active agents that accumulate at fluid interfaces and influence interfacial properties, e.g. the surface tension. For single rising bubbles, even a small amount of surfactant causes Marangoni forces that influence the bubble rise significantly. In this work, Direct Numerical Simulations (DNS) with an Arbitrary Lagrangian-Eulerian (ALE) Interface-Tracking method are performed. The use of a subgrid-scale model enables the simulation of realistic time and length scales and the comparison with experiments. The resolution requirements close to the interface are examined using 2D simulations to reduce the computational costs further. Then, 3D simulations of single rising bubbles under the influence of Triton-X100 are carried out, investigating different bubble diameters and initial surfactant bulk concentrations. The 3D simulations provide new insights into the transition from a helical motion into a zig-zag motion, which can only be observed in the presence of a surfactant. Additionally, the reciprocal influence of the local surfactant distribution on the interface and the vortex structures for path-unstable bubbles are analysed. Finally, the local surfactant distribution on the interface is modelled using a data-driven approach. The model is based on the DNS data obtained from the 3D simulations and is in good agreement with the validation data. In future work, the derived model can be used to improve existing simplified models for the simulation of bubbly flows under the influence of surfactant

Patch-based sparse reconstruction of material BTFs

We propose a simple and efficient method to reconstruct materials’ bidirectional texture functions (BTFs) from angularly sparse measurements. The key observation is that materials of similar types exhibit both similar surface structure and reflectance properties. We exploit this by manually clustering an existing database of fully measured material BTFs and fitting a linear model to each of the clusters. The models are computed not on per-texel data but on small spatial BTF patches we call apparent BTFs. Sparse reconstruction can then be performed by solving a linear least-squares problem without any regularization, using a per-cluster sampling strategy derived from the models. We demonstrate that our method is capable of faithfully reconstructing fully resolved BTFs from sparse measurements for a wide range of materials

Acquiring Bidirectional Texture Functions for Large-Scale Material Samples

Most current acquisition setups for bidirectional texture functions (BTFs) are incapable of capturing large-scale material samples. We propose a method based on controlled texture synthesis to produce BTFs of appealing visual quality for such materials. Our approach uses as input data a complete measurement of a small fraction of the sample, together with few images of the large-scale structure controlling the synthesis process. We evaluate the applicability of our approach by reconstructing sparsified ground truth data and investigate the consequences of choosing different kinds and numbers of constraint images