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Heinz Pitsch - One of the best experts on this subject based on the ideXlab platform.

  • prediction of extinction and reignition in nonpremixed turbulent flames using a flamelet progress variable model 2 application in les of sandia flames d and e
    Combustion and Flame, 2008
    Co-Authors: Matthias Ihme, Heinz Pitsch
    Abstract:

    An extension of the flamelet/progress variable (FPV) model for the prediction of extinction and reignition is applied in large-eddy simulation (LES) of flames D and E of the Sandia piloted turbulent jet flame series. This model employs a presumed probability density function (Pdf), in which the Marginal Pdf of a reactive scalar is modeled by a statistically most likely distribution. This provides two advantages. First of all, the shape of the distribution depends on chemical and mixing time-scale information, and second, an arbitrary number of moments can be enforced. This model was analyzed in an a priori study in the first part of this work. In the present LES application, the first two moments of mixture fraction and reaction progress variable are used to constrain the shape of the presumed Pdf. Transport equations for these quantities are solved, and models for the residual scalar dissipation rates, which appear as unclosed terms in the equations for the scalar variances, are provided. Statistical flow field quantities for axial velocity, mixture fraction, and temperature, obtained from the extended FPV model, are in good agreement with experimental data. Mixture-fraction-conditioned data, conditional Pdfs, and burning indices are computed and compared with the delta-function flamelet closure model, which employs a Dirac distribution as a model for the Marginal Pdf of the reaction progress parameter. The latter model considerably underpredicts the amount of local extinction, which shows that the consideration of second-moment information in the presumed Pdf of the reaction progress parameter is important for the accurate prediction of extinction and reignition. Mixture-fraction-conditioned results obtained from the extended FPV model are in good agreement with experimental data; however, the overprediction of the consumption of fuel and oxidizer on the fuel-rich side results in an overprediction of minor species. The predictions for the conditional Pdfs and burning indices are in good agreement with measurements.

  • prediction of local extinction and re ignition effects in non premixed turbulent combustion using a flamelet progress variable approach
    Proceedings of the Combustion Institute, 2005
    Co-Authors: Matthias Ihme, Heinz Pitsch
    Abstract:

    The flamelet/progress variable approach (FPVA) has been proposed by Pierce and Moin as a model for turbulent non-premixed combustion in large-eddy simulation. The filtered chemical source term in this model appears in unclosed form, and is modeled by a presumed probability density function (Pdf) for the joint Pdf of the mixture fraction Z and a flamelet parameter λ. While the Marginal Pdf of Z can be reasonably approximated by a beta distribution, a model for the conditional Pdf of the flamelet parameter needs to be developed. Further, the ability of FPVA to predict extinction and re-ignition has also not been assessed. In this paper, we address these aspects of the model using the DNS database of Sripakagorn et al. It is first shown that the steady flamelet assumption in the context of FPVA leads to good predictions even for high levels of local extinction. Three different models for the conditional Pdf of the flamelet parameter are tested in an a priori sense. Results obtained using a delta function to model the conditional Pdf of λ lead to an overprediction of the mean temperature, even with only moderate extinction levels. It is shown that if the conditional Pdf of λ is modeled by a beta distribution conditioned on Z, then FPVA can predict extinction and re-ignition effects, and good agreement between the model and DNS data for the mean temperature is observed.

Matthias Ihme - One of the best experts on this subject based on the ideXlab platform.

  • prediction of extinction and reignition in nonpremixed turbulent flames using a flamelet progress variable model 2 application in les of sandia flames d and e
    Combustion and Flame, 2008
    Co-Authors: Matthias Ihme, Heinz Pitsch
    Abstract:

    An extension of the flamelet/progress variable (FPV) model for the prediction of extinction and reignition is applied in large-eddy simulation (LES) of flames D and E of the Sandia piloted turbulent jet flame series. This model employs a presumed probability density function (Pdf), in which the Marginal Pdf of a reactive scalar is modeled by a statistically most likely distribution. This provides two advantages. First of all, the shape of the distribution depends on chemical and mixing time-scale information, and second, an arbitrary number of moments can be enforced. This model was analyzed in an a priori study in the first part of this work. In the present LES application, the first two moments of mixture fraction and reaction progress variable are used to constrain the shape of the presumed Pdf. Transport equations for these quantities are solved, and models for the residual scalar dissipation rates, which appear as unclosed terms in the equations for the scalar variances, are provided. Statistical flow field quantities for axial velocity, mixture fraction, and temperature, obtained from the extended FPV model, are in good agreement with experimental data. Mixture-fraction-conditioned data, conditional Pdfs, and burning indices are computed and compared with the delta-function flamelet closure model, which employs a Dirac distribution as a model for the Marginal Pdf of the reaction progress parameter. The latter model considerably underpredicts the amount of local extinction, which shows that the consideration of second-moment information in the presumed Pdf of the reaction progress parameter is important for the accurate prediction of extinction and reignition. Mixture-fraction-conditioned results obtained from the extended FPV model are in good agreement with experimental data; however, the overprediction of the consumption of fuel and oxidizer on the fuel-rich side results in an overprediction of minor species. The predictions for the conditional Pdfs and burning indices are in good agreement with measurements.

  • prediction of local extinction and re ignition effects in non premixed turbulent combustion using a flamelet progress variable approach
    Proceedings of the Combustion Institute, 2005
    Co-Authors: Matthias Ihme, Heinz Pitsch
    Abstract:

    The flamelet/progress variable approach (FPVA) has been proposed by Pierce and Moin as a model for turbulent non-premixed combustion in large-eddy simulation. The filtered chemical source term in this model appears in unclosed form, and is modeled by a presumed probability density function (Pdf) for the joint Pdf of the mixture fraction Z and a flamelet parameter λ. While the Marginal Pdf of Z can be reasonably approximated by a beta distribution, a model for the conditional Pdf of the flamelet parameter needs to be developed. Further, the ability of FPVA to predict extinction and re-ignition has also not been assessed. In this paper, we address these aspects of the model using the DNS database of Sripakagorn et al. It is first shown that the steady flamelet assumption in the context of FPVA leads to good predictions even for high levels of local extinction. Three different models for the conditional Pdf of the flamelet parameter are tested in an a priori sense. Results obtained using a delta function to model the conditional Pdf of λ lead to an overprediction of the mean temperature, even with only moderate extinction levels. It is shown that if the conditional Pdf of λ is modeled by a beta distribution conditioned on Z, then FPVA can predict extinction and re-ignition effects, and good agreement between the model and DNS data for the mean temperature is observed.

Ender Ayanoglu - One of the best experts on this subject based on the ideXlab platform.

  • an upper bound to the Marginal Pdf of the ordered eigenvalues of wishart matrices
    arXiv: Information Theory, 2012
    Co-Authors: Hong Ju Park, Ender Ayanoglu
    Abstract:

    Diversity analysis of a number of Multiple-Input Multiple-Output (MIMO) applications requires the calculation of the expectation of a function whose variables are the ordered multiple eigenvalues of a Wishart matrix. In order to carry out this calculation, we need the Marginal Pdf of an arbitrary subset of the ordered eigenvalues. In this letter, we derive an upper bound to the Marginal Pdf of the eigenvalues. The derivation is based on the multiple integration of the well-known joint Pdf, which is very complicated due to the exponential factors of the joint Pdf. We suggest an alternative function that provides simpler calculation of the multiple integration. As a result, the Marginal Pdf is shown to be bounded by a multivariate polynomial with a given degree. After a standard bounding procedure in a Pairwise Error Probability (PEP) analysis, by applying the Marginal Pdf to the calculation of the expectation, the diversity order for a number of MIMO systems can be obtained in a simple manner. Simulation results that support the analysis are presented.

  • an upper bound to the Marginal Pdf of the ordered eigenvalues of wishart matrices and its application to mimo diversity analysis
    International Conference on Communications, 2010
    Co-Authors: Hong Ju Park, Ender Ayanoglu
    Abstract:

    Diversity analysis of a number of multiple-input multiple-output (MIMO) applications requires the calculation of the expectation of a function whose variables are the ordered multiple eigenvalues of the Wishart matrices. To solve this, we need the Marginal Pdf of an arbitrary subset of the ordered eigenvalues. The Marginal Pdf shown in the literature is useful in numerical analysis, but not beneficial to diversity analysis. In this paper, we derive an upper bound to the Marginal Pdf of the eigenvalues. The derivation is based on the multiple integration of the well-known joint Pdf, which is very complicated due to the exponential factors of the joint Pdf. We suggest an alternative function that provides simpler calculation of the multiple integration. As a result, the Marginal Pdf is shown to consist of a multivariate polynomial with a given degree. By applying the Marginal Pdf to the calculation of the expectation, the diversity order for a number of MIMO systems can be calculated. Simulation results that support the analysis are presented.

Josette Bellan - One of the best experts on this subject based on the ideXlab platform.

  • a priori evaluation of the double conditioned conditional source term estimation model for high pressure heptane turbulent combustion using dns data obtained with one step chemistry
    Combustion and Flame, 2020
    Co-Authors: Kendal W Bushe, C B Devaud, Josette Bellan
    Abstract:

    Abstract An evaluation of the submodels constituting the Double-Conditioned Source-term Estimation (DCSE) method for utilization in Large Eddy Simulation (LES) is presented which makes use of a single realization of a turbulent high-pressure reactive-flow database obtained from Direct Numerical Simulation (DNS). A filtered and coarsened DNS (FCDNS) field is first created to mimic a real LES field, and the FCDNS values for thermodynamic variables (temperature, density and species mass fractions) are used as inputs to the DCSE model to predict the FCDNS reaction rates. It is found that there are significant errors in the predictions of the filtered reaction rate compared to the template. These errors are attributed to three important aspects of the model. First, the conditional-filtered values of the thermodynamic variables are found by an inversion of an integral equation, but this inversion requires the definition of spatial ensembles in which it must be performed; these ensembles are a subset of the entire spatial domain containing the LES volumes. A drastic reduction in the number of ensembles from the number of LES volumes (i.e. the largest possible number of such ensembles) deteriorates the model, whereas a further and smaller such reduction has a small effect. Second, the procedure for the integral inversion introduces problems related to the method selected to regularize the inversion. Third, by far the largest source of error was found to be due to the modelling of the joint probability density function of the conditioning variables: assuming a β-Pdf for the Marginal Pdf of the reaction progress variable is shown to be a particularly poor choice, as is the assumption that the conditioning variables are statistically independent.

Nedunchezhian Swaminathan - One of the best experts on this subject based on the ideXlab platform.

  • Strained flamelets for turbulent premixed flames, I: Formulation and planar flame results
    Combustion and Flame, 2010
    Co-Authors: Hemanth Kolla, Nedunchezhian Swaminathan
    Abstract:

    Abstract A strained flamelet model is proposed for turbulent premixed flames using scalar dissipation rate as a parameter. The scalar dissipation rate of reaction progress variable is a suitable quantity to describe the flamelet structure since it is governed by convection–diffusion-reaction balance and it is defined at every location in the flamelets, which are represented by laminar flames in reactant-to-product opposed flow configuration. The mean reaction rate is obtained by using the flamelets reaction rate and the joint Pdf of the progress variable and its dissipation rate. The Marginal Pdf of the progress variable is presumed to be β -Pdf and the Pdf of the conditional dissipation rate is taken to be log-normal. The conditional mean dissipation rate is obtained from modelled mean dissipation rate. This reaction rate closure is assessed using RANS calculations of statistically planar flames in the corrugated flamelets and thin reaction zones regimes. The flame speeds calculated using this closure are close to the experimental data of Abdel-Gayed et al. (1987) [ 27 ] for flames in both the regimes. Comparisons with other reaction rate closures showed the benefits of the strained flamelets approach.