A wildland fire model with data assimilation

A wildfire model is formulated based on balance equations for energy and fuel, where the fuel loss due to combustion corresponds to the fuel reaction rate. The resulting coupled partial differential equations have coefficients that can be approximated from prior measurements of wildfires. An ensemble Kalman filter technique with regularization is then used to assimilate temperatures measured at selected points into running wildfire simulations. The assimilation technique is able to modify the simulations to track the measurements correctly even if the simulations were started with an erroneous ignition location that is quite far away from the correct one.Comment: 35 pages, 12 figures; minor revision January 2008. Original version available from http://www-math.cudenver.edu/ccm/report

Data Driven Forecast for Fire

Being able to forecast the evolution of a fire is essential for fire safety design and fire response strategies. Despite advances in understanding fire dynamics and improvements in computational capability, the ability to predict the evolution of a fire remains limited due to large uncertainties associated to multiple scales and the non-linearities. The data-driven approach provides a viable technique from models corrected by observations. However, the complicated coupling between gaseous and condensed phases has, in the past, limited proper prediction with a positive leading time. This work proposes and investigates a series of approaches to data-driven hybrid modelling that integrate analytical and numerical descriptions to address the coupling effects. The data-driven hybrid model is developed for different scenarios covering various complexity and scales. Different approaches are evaluated to reflect the dominant physics; nevertheless, they are structured by differentiating the condensed and gas phases. The initial scenario corresponds to one-dimensional convective-diffusive droplet combustion in micro and normal gravity. Then, concurrent flame spread in micro and normal gravity where a two-dimensional boundary layer combustion approach is implemented. Finally, the Malveira fire test represents a large-scale, three-dimensional, travelling fire. Coefficients assimilated with their experimental observations are used to alter analytical formulations describing the gas and condensed phases. By separating the phases, the data-driven hybrid model can forecast various types of variables while reducing processing resources. Convergence of the assimilated coefficients is used as an indicator for an appropriate representation of the model and therefore is suitable for predictions. The proposed methodology still requires ongoing research, however. This work provides evidence for specific approaches and of areas where additional attention is necessary. It has become apparent that to adequately predict real-scale fire, it is necessary for more sophisticated explanations of heat and mass transfer and descriptions of the interactions between fire and its environment

Data driven forecast of droplet combustion

The characteristics of a diffusion flame resulting from the gasification of a condensed fuel are predicted from the synthesis of simple models and data. Combustion of a droplet in microgravity is used as a canonical configuration to illustrate the methodology. The simplicity of the spherical configuration and the detail of the measurements make the available experimental data ideal for this study. The approach followed combines the classical analytical solution first proposed by Spalding to describe the condensed phase gasification with a numerical method that describes the gas phase. Available data on flame geometry and regression rates are used to initialize the model and produce adequate predictions of the time evolution of all relevant variables. The method was shown to make proper predictions under numerous configurations and with very small computational cost

Numerical study of substrate assimilation by a microorganism exposed to fluctuating concentration

In most modelling works on bioreactors, the substrate assimilation is computed from the volume average concentration. The possible occurrence of a competition between the transport of substrate towards the cell and the assimilation at the cell level is generally overlooked. In order to examine the consequences of such a competition, a diffusion equation for the substrate is coupled with a specific boundary condition defining the up take rate at the cell liquid interface. Two assimilation laws are investigated, whereas the concentration far from the cell is varied in order to mimic concentration fluctuations. Both steady and unsteady conditions are investigated. The actual uptake rate computed from the interfacial concentration is compared to the time-averaged uptake rate based on the mean far-field concentration. Whatever the assimilation law, it is found that the uptake rate can be correlated to the mean far-field concentration, but the actual values of the parameters are affected in case of transport limitation. Moreover, the structure of the far-field signal influences the substrate assimilation by the microorganism, and the mean interfacial uptake rate depends on the ratio between the characteristic time of the signal and the diffusional time scale, as well as on the amplitude of the fluctuations around the mean far-field concentration in substrate. The present work enlightens some experimental results and helps in understanding the differences between the concentration measured and that present in the microenvironment of the cells

Modeling and Simulation of Thermo-Fluid-Electrochemical Ion Flow in Biological Channels

In this article we address the study of ion charge transport in the biological channels separating the intra and extracellular regions of a cell. The focus of the investigation is devoted to including thermal driving forces in the well-known velocity-extended Poisson-Nernst-Planck (vPNP) electrodiffusion model. Two extensions of the vPNP system are proposed: the velocity-extended Thermo-Hydrodynamic model (vTHD) and the velocity-extended Electro-Thermal model (vET). Both formulations are based on the principles of conservation of mass, momentum and energy, and collapse into the vPNP model under thermodynamical equilibrium conditions. Upon introducing a suitable one-dimensional geometrical representation of the channel, we discuss appropriate boundary conditions that depend only on effectively accessible measurable quantities. Then, we describe the novel models, the solution map used to iteratively solve them, and the mixed-hybrid flux-conservative stabilized finite element scheme used to discretize the linearized equations. Finally, we successfully apply our computational algorithms to the simulation of two different realistic biological channels: 1) the Gramicidin-A channel considered in~\cite{JeromeBPJ}; and 2) the bipolar nanofluidic diode considered in~\cite{Siwy7}

Large Eddy Simulations of gaseous flames in gas turbine combustion chambers

Recent developments in numerical schemes, turbulent combustion models and the regular increase of computing power allow Large Eddy Simulation (LES) to be applied to real industrial burners. In this paper, two types of LES in complex geometry combustors and of specific interest for aeronautical gas turbine burners are reviewed: (1) laboratory-scale combustors, without compressor or turbine, in which advanced measurements are possible and (2) combustion chambers of existing engines operated in realistic operating conditions. Laboratory-scale burners are designed to assess modeling and funda- mental flow aspects in controlled configurations. They are necessary to gauge LES strategies and identify potential limitations. In specific circumstances, they even offer near model-free or DNS-like LES computations. LES in real engines illustrate the potential of the approach in the context of industrial burners but are more difficult to validate due to the limited set of available measurements. Usual approaches for turbulence and combustion sub-grid models including chemistry modeling are first recalled. Limiting cases and range of validity of the models are specifically recalled before a discussion on the numerical breakthrough which have allowed LES to be applied to these complex cases. Specific issues linked to real gas turbine chambers are discussed: multi-perforation, complex acoustic impedances at inlet and outlet, annular chambers.. Examples are provided for mean flow predictions (velocity, temperature and species) as well as unsteady mechanisms (quenching, ignition, combustion instabil- ities). Finally, potential perspectives are proposed to further improve the use of LES for real gas turbine combustor designs

Validation of flacs code for risk analysis of hydrocarbon pool fires

Fires have been object of study over the last decades due to their destructive power. Fire’s hazardous nature and its ability to inflict damage to property, the environment and people, has produced a need to understand how it works in every aspect. Currently, the main focus is to estimate the fire characteristics and main effects, in order to accurately design emergency plans and prevention measures. Due to the needs previously stated, fires have been studied and analyzed mainly from an experimental point of view. However, experimental data is arduous and extremely expensive to obtain due to the amount of resources needed. Additionally, small-scale models, which are generally easier to be undertaken, cannot be extrapolated to full-scale models. Considering this, semi-empirical methods were developed, but can only be applied to simple scenarios and they cannot fully model them. To achieve complete models of fires, CFD (Computational Fluid Dynamics) modeling has been recently used as a way to achieve a cheaper and easier method to study the fire development of full-scale fires in a wide range of conditions. Nevertheless, CFD models require a huge validation effort before they could be widely applied. The main objective of this thesis is to analyze the performance and if possible validate the CFD code FLACS-Fire v10.5 (Flame Accelerator Simulator) for pool-fires. FLACS is a Computational Fluid Dynamic (CFD) program, which solves the compressible conservation equations for mass, momentum, enthalpy, and mixture fraction using a finite volume method. To model a fire it is necessary to include, among others, processes that involve submodels for: turbulence, combustion, thermal radiation, and soot generation. It is of utmost importance, while developing fire models, to validate them against experimental data in pursuance of being able to conclude whether the simulation is valid or not, and to determine the inherent error in comparison with reality. This process consists in a replication of the experimental setup in the CFD, in this case FLACS, and compare it with experimental data previously available. In the present work, gasoline and diesel fuel experimental pool fires were modeled with FLACS-Fire v10.5 code. Simulations considered different pool fire experiments with diameters ranging from 1.5 to 4 meters. In addition, simulations were run with the Eddy Dissipation Concept (EDC) as combustion model; with the κ-ε model as turbulence model; and with the Discrete Transfer Model (DTM) as the radiation model. The predicted results of temperature’s evolution at different heights, burning rate, and thermal radiation were compared with experimental measurements. The results for gasoline and diesel pool fires indicate that FLACS-Fire v10.5 is able to model pool fires. Pool model 3 (PM3) was able to run all simulations, and Pool Model 1 (PM1) does not perform well with pool diameters higher than 1.5m. Predicted values of the proposed parameters are in a fair concordance with experimentally obtained values. Temperatures measured at the centerline of the flame are in most cases overestimated. Burning rates are well approximated with small and large pool fires (0.15 kg/s-0.5kg/s) but largely over predicted in gasoline pool fires of medium size. Thermal radiation is also forecasted with values larger than their experimental counterparts. Chapter 1, contains a brief introduction to the master thesis. It gives a general understanding of the importance of pool fires in the industry. It also gives a global introduction to Computational Fluid Dynamics (CFD), and its relevancy in the study of accidents, especially in the case of pool fires. Chapter 2, consists of a theoretical background of fire phenomena and the combustion process, with a special focus on pool fires. First, a brief and simple explanation of the combustion process is given. Then, an introduction to heat transfer is provided, in order to show the essentials of how thermal energy is transferred and how it affects pool fires. Finally, an introduction to pool fires characteristics and their mechanisms is given, with an emphasis on the zones composing the fire as well as its main features. Chapter 3, mainly covers the existing work concerning the ongoing topic. It covers authors who have worked with pool fires, especially in the validation of FLACS-Fire; as well as others who gather experimental data. Chapter 4, comprises the crucial elements in fire modeling using FLACS-Fire v10.5. Principally, it contains the submodels FLACS uses for: fluid flow, turbulence, radiation, combustion, soot formation, and pool modeling. This chapter shows a theoretical understanding and the basis from which the simulations are later performed. Chapter 5, is constituted by a detailed explanation of the experimental data used in the present thesis. Instrumentation used in the experiments is thoroughly analyzed, as well as the fuels used and the experiments performed. Chapter 6, includes the simulations performed in the present thesis, as well as, a comprehensive analysis of the data obtained. Initial simulations studying various variables such as grid, radiation model and pool model are studied. Final simulations are also evaluated, which especial emphasis on the discrepancies with the experimental data

Data Assimilation for Wildland Fires: Ensemble Kalman filters in coupled atmosphere-surface models

Two wildland fire models are described, one based on reaction-diffusion-convection partial differential equations, and one based on semi-empirical fire spread by the level let method. The level set method model is coupled with the Weather Research and Forecasting (WRF) atmospheric model. The regularized and the morphing ensemble Kalman filter are used for data assimilation.Comment: Minor revision, except description of the model expanded. 29 pages, 9 figures, 53 reference

Differential growth of wrinkled biofilms

Biofilms are antibiotic-resistant bacterial aggregates that grow on moist surfaces and can trigger hospital-acquired infections. They provide a classical example in biology where the dynamics of cellular communities may be observed and studied. Gene expression regulates cell division and differentiation, which affect the biofilm architecture. Mechanical and chemical processes shape the resulting structure. We gain insight into the interplay between cellular and mechanical processes during biofilm development on air-agar interfaces by means of a hybrid model. Cellular behavior is governed by stochastic rules informed by a cascade of concentration fields for nutrients, waste and autoinducers. Cellular differentiation and death alter the structure and the mechanical properties of the biofilm, which is deformed according to Foppl-Von Karman equations informed by cellular processes and the interaction with the substratum. Stiffness gradients due to growth and swelling produce wrinkle branching. We are able to reproduce wrinkled structures often formed by biofilms on air-agar interfaces, as well as spatial distributions of differentiated cells commonly observed with B. subtilis.Comment: 19 pages, 13 figure