Papers by Mikael Mortensen
Chemical Engineering Science, 2004
An implementation of the conditional moment closure (CMC) of Bilger (Phy. Fluids A 5 (1993) 436) ... more An implementation of the conditional moment closure (CMC) of Bilger (Phy. Fluids A 5 (1993) 436) and Klimenko (Fluid Dyn. 25 (1990) 327) has been carried out in the commercial CFD software FLUENT. A new implementation method is suggested that uses the CMC-equations to calculate an unconditional reaction rate that is subsequently inserted in a Reynolds averaged species equation. The objectives are to ensure conservation of reacting species by elimination of the spurious term that usually contaminates a CMC solution and to obtain convergence on a coarser grid than is usually required in CMC. The implementation has been successfully carried out for two sets of well-known mixing sensitive reaction pairs (parallel competitive and competitive consecutive) that both have one instantaneous and one fast irreversible reaction. Calculations with di erent discretizations of the CMC-equations in mixture fraction space have been performed and compared to experiments, full/ÿnite-mode PDF models and the interpolation model of Baldyga. It is shown that the suggested implementation can achieve su cient numerical accuracy with a much coarser mixture fraction space discretization than standard CMC implementations, which allows for more e cient CFD calculations. ?
Combustion and Flame, 2007
The double scalar mixing layer ͑DSML͒ is a canonical problem for studying the mixing of multiple ... more The double scalar mixing layer ͑DSML͒ is a canonical problem for studying the mixing of multiple streams and, with reaction, combustion of the partially premixed type. In a DSML, a third stream consisting of a premixture of the reactants is introduced in between the pure fuel and air streams of the classic twin-feed or binary mixing problem. The well-known presumed probability density function ͑PDF͒, such as the -PDF, can adequately model passive scalar mixing for the binary mixing problem on which state-of-the-art turbulent combustion models such as conditional moment closure and flamelet approaches rely. However, the -PDF model, now a standard in CFD simulation, cannot describe turbulent mixing involving multiple streams; e.g., the asymmetric three-stream mixing characterizing the DSML. In this paper, direct numerical simulations of the DSML are performed to make available a high-fidelity database for developing more general, fine-scale mixing models required to compute turbulent combustion problems of practical engineering interest, which usually involve mixing between multiple streams. In this first part of two investigations, nonreacting numerical experiments are presented with emphasis on the nontrivial distributions of the passive scalar and its dissipation rate. Mapping closure modeling is applied to describe the PDFs and conditional dissipation rates of a single mixture fraction.
Physics of Fluids, 2006
The double scalar mixing layer (DSML) is a canonical problem for studying the mixing of multiple ... more The double scalar mixing layer (DSML) is a canonical problem for studying the mixing of multiple streams and, with reaction, combustion of the partially premixed type. In a DSML, a third stream consisting of a premixture of the reactants is introduced in between the pure fuel and air streams of the classic twin-feed or binary mixing problem. The well-known presumed probability density function (PDF), such as the β-PDF, can adequately model passive scalar mixing for the binary mixing problem on which state-of-the-art turbulent combustion models such as conditional moment closure and flamelet approaches rely. However, the β-PDF model, now a standard in CFD simulation, cannot describe turbulent mixing involving multiple streams; e.g., the asymmetric three-stream mixing characterizing the DSML. In this paper, direct numerical simulations of the DSML are performed to make available a high-fidelity database for developing more general, fine-scale mixing models required to compute turbulent combustion problems of practical engineering interest, which usually involve mixing between multiple streams. In this first part of two investigations, nonreacting numerical experiments are presented with emphasis on the nontrivial distributions of the passive scalar and its dissipation rate. Mapping closure modeling is applied to describe the PDFs and conditional dissipation rates of a single mixture fraction.
Advances in Water Resources, 2011
Finding an appropriate turbulence model for a given flow case usually calls for extensive experim... more Finding an appropriate turbulence model for a given flow case usually calls for extensive experimentation with both models and numerical solution methods. This work presents the design and implementation of a flexible, programmable software framework for assisting with numerical experiments in computational turbulence. The framework targets Reynolds-averaged Navier-Stokes models, discretized by finite element methods. The novel implementation makes use of Python and the FEniCS package, the combination of which leads to compact and reusable code, where model- and solver-specific code resemble closely the mathematical formulation of equations and algorithms. The presented ideas and programming techniques are also applicable to other fields that involve systems of nonlinear partial differential equations. We demonstrate the framework in two applications and investigate the impact of various linearizations on the convergence properties of nonlinear solvers for a Reynolds-averaged Navier-Stokes model.
Conditional velocity statistics in the double scalar mixing layer - A mapping closure approach
Combustion Theory and Modelling, 2008
In this work we use 3D direct numerical simulations (DNS) to investigate the average velocity con... more In this work we use 3D direct numerical simulations (DNS) to investigate the average velocity conditioned on a conserved scalar in a double scalar mixing layer (DSML). The DSML is a canonical multistream flow designed as a model problem for the extensively studied piloted diffusion flames. The conditional mean velocity appears as an unclosed term in advanced Eulerian models of
Advances in Water Resources, 2011
Finding an appropriate turbulence model for a given flow case usually calls for extensive experim... more Finding an appropriate turbulence model for a given flow case usually calls for extensive experimentation with both models and numerical solution methods. This work presents the design and implementation of a flexible, programmable software framework for assisting with numerical experiments in computational turbulence. The framework targets Reynolds-averaged Navier-Stokes models, discretized by finite element methods. The novel implementation makes use of Python and the FEniCS package, the combination of which leads to compact and reusable code, where model-and solver-specific code resemble closely the mathematical formulation of equations and algorithms. The presented ideas and programming techniques are also applicable to other fields that involve systems of nonlinear partial differential equations. We demonstrate the framework in two applications and investigate the impact of various linearizations on the convergence properties of nonlinear solvers for a Reynolds-averaged Navier-Stokes model.
Combustion and Flame, 2007
The double scalar mixing layer ͑DSML͒ is a canonical problem for studying the mixing of multiple ... more The double scalar mixing layer ͑DSML͒ is a canonical problem for studying the mixing of multiple streams and, with reaction, combustion of the partially premixed type. In a DSML, a third stream consisting of a premixture of the reactants is introduced in between the pure fuel and air streams of the classic twin-feed or binary mixing problem. The well-known presumed probability density function ͑PDF͒, such as the -PDF, can adequately model passive scalar mixing for the binary mixing problem on which state-of-the-art turbulent combustion models such as conditional moment closure and flamelet approaches rely. However, the -PDF model, now a standard in CFD simulation, cannot describe turbulent mixing involving multiple streams; e.g., the asymmetric three-stream mixing characterizing the DSML. In this paper, direct numerical simulations of the DSML are performed to make available a high-fidelity database for developing more general, fine-scale mixing models required to compute turbulent combustion problems of practical engineering interest, which usually involve mixing between multiple streams. In this first part of two investigations, nonreacting numerical experiments are presented with emphasis on the nontrivial distributions of the passive scalar and its dissipation rate. Mapping closure modeling is applied to describe the PDFs and conditional dissipation rates of a single mixture fraction.
Journal of Biomechanics, 2011
In experiments turbulence has previously been shown to occur in intracranial aneurysms. The effec... more In experiments turbulence has previously been shown to occur in intracranial aneurysms. The effects of turbulence induced oscillatory wall stresses could be of great importance in understanding aneurysm rupture. To investigate the effects of turbulence on blood flow in an intracranial aneurysm, we performed a high resolution computational fluid dynamics (CFD) simulation in a patient specific middle cerebral artery (MCA) aneurysm using a realistic, pulsatile inflow velocity. The flow showed transition to turbulence just after peak systole, before relaminarization occurred during diastole. The turbulent structures greatly affected both the frequency of change of wall shear stress (WSS) direction and WSS magnitude, which reached a maximum value of 41.5 Pa. The recorded frequencies were predominantly in the range of 1-500 Hz. The current study confirms, through properly resolved CFD simulations that turbulence can occur in intracranial aneurysms.
Fuel and Energy Abstracts, 2011
Finding an appropriate turbulence model for a given flow case usually calls for extensive experim... more Finding an appropriate turbulence model for a given flow case usually calls for extensive experimentation with both models and numerical solution methods. This work presents the design and implementation of a flexible, programmable software framework for assisting with numerical experiments in computational turbulence. The framework targets Reynolds-averaged Navier-Stokes models, discretized by finite element methods. The novel implementation makes use of Python and the FEniCS package, the combination of which leads to compact and reusable code, where model-and solver-specific code resemble closely the mathematical formulation of equations and algorithms. The presented ideas and programming techniques are also applicable to other fields that involve systems of nonlinear partial differential equations. We demonstrate the framework in two applications and investigate the impact of various linearizations on the convergence properties of nonlinear solvers for a Reynolds-averaged Navier-Stokes model.
Presumed Mapping Functions for Eulerian Modelling of Turbulent Mixing
Flow Turbulence and Combustion, 2006
The mapping closure of Chen et al. [Phys. Rev. Lett., 63, 1989] is a transported probability dens... more The mapping closure of Chen et al. [Phys. Rev. Lett., 63, 1989] is a transported probability density function (PDF) method that has proven very efficient for modelling of turbulent mixing in homogeneous turbulence. By utilizing a Gaussian reference field, the solution to the mapping function (in homogeneous turbulence) can be found analytically for a range of initial conditions common for turbulent combustion applications, e.g. for binary or trinary mixing. The purpose of this paper is to investigate the possibility of making this solution a presumed mapping function (PMF) for inhomogeneous flows. The PMF in turn will imply a presumed mixture fraction PDF that can be used for a wide range of models in turbulent combustion, e.g. flamelet models, the conditional moment closure (CMC) or large eddy simulations. The true novelty of the paper, though, is in the derivation of highly efficient, closed algebraic expressions for several existing models of conditional statistics, e.g. for the conditional scalar dissipation/diffusion rate or the conditional mean velocity. The closed form expressions nearly eliminates the overhead computational cost that usually is associated with nonlinear models for conditional statistics. In this respect it is argued that the PMF is particularly well suited for CMC that relies heavily on manipulations of the PDF for consistency. The accuracy of the PMF approach is shown with comparison to DNS of a single scalar mixing layer to be better than for the β-PDF. Not only in the shape of the PDF itself, but also for all conditional statistics models computed from the PDF.
Experimental and Numerical Investigations of a Jet Mixing in a Multifunctional Channel Reactor
Chemical Engineering Research & Design, 2004
Mixing of a Jet in a Pipe
Chemical Engineering Research & Design, 2004
Page 1. MIXING OF A JET IN A PIPE Mikael Mortensen, Wojciech Orciuch, Mounir Bouaifi, Bengt Ander... more Page 1. MIXING OF A JET IN A PIPE Mikael Mortensen, Wojciech Orciuch, Mounir Bouaifi, Bengt Andersson Chalmers University of Technology, Departement of Chemical Reaction Engineering, 412 96 Gothenburg, Sweden CFD, PDF, PLIF, scalar mixing ...
Uploads
Papers by Mikael Mortensen