The Experts below are selected from a list of 14073 Experts worldwide ranked by ideXlab platform
Francis Bernardeau - One of the best experts on this subject based on the ideXlab platform.
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numerical complexity of the Joint nulled weak lensing probability distribution function
arXiv: Cosmology and Nongalactic Astrophysics, 2021Co-Authors: Alexandre Barthelemy, Sandrine Codis, Francis Bernardeau, Cora UhlemannAbstract:In the context of tomographic cosmic shear surveys, there exists a nulling transformation of weak lensing observations (also called BNT transform) that allows us to simplify the correlation structure of tomographic cosmic shear observations, as well as to build observables that depend only on a localised range of redshifts and thus independent from the low-redshift/small-scale modes. This procedure renders possible accurate, and from-first-principles, predictions of the convergence and aperture mass one-point distributions (Pdf). We here explore other consequences of this transformation on the (reduced) numerical complexity of the estimation of the Joint Pdf between nulled bins and demonstrate how to use these results to make theoretical predictions.
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two is better than one Joint statistics of density and velocity in concentric spheres as a cosmological probe
Monthly Notices of the Royal Astronomical Society, 2017Co-Authors: Cora Uhlemann, Christophe Pichon, Sandrine Codis, Francis Bernardeau, Oliver HahnAbstract:The analytical formalism to obtain the probability distribution functions (Pdfs) of spherically-averaged cosmic densities and velocity divergences in the mildly non-linear regime is presented. A large-deviation principle is applied to those cosmic fields assuming their most likely dynamics in spheres is set by the spherical collapse model. We validate our analytical results using state-of-the-art dark matter simulations with a phase-space resolved velocity field finding a 2% percent level agreement for a wide range of velocity divergences and densities in the mildly nonlinear regime (~10Mpc/h at redshift zero), usually inaccessible to perturbation theory. From the Joint Pdf of densities and velocity divergences measured in two concentric spheres, we extract with the same accuracy velocity profiles and conditional velocity Pdf subject to a given over/under-density which are of interest to understand the non-linear evolution of velocity flows. Both Pdfs are used to build a simple but accurate maximum likelihood estimators for the redshift evolution of the variance of both the density and velocity divergence fields, which have smaller relative errors than their sample variances when non-linearities appear. Given the dependence of the velocity divergence on the growth rate, there is a significant gain in using the full knowledge of both Pdfs to derive constraints on the equation of state of dark energy. Thanks to the insensitivity of the velocity divergence to bias, its Pdf can be used to obtain unbiased constraints on the growth of structures ($\sigma_8$,f) or it can be combined with the galaxy density Pdf to extract bias parameters.
N Swaminathan - One of the best experts on this subject based on the ideXlab platform.
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large eddy simulation of flame edge evolution in a spark ignited methane air jet
Proceedings of the Combustion Institute, 2017Co-Authors: Zhi X Chen, Shaohong Ruan, N SwaminathanAbstract:Abstract The unsteady evolution of lifted methane–air jet flames following spark ignition is computed using Large Eddy Simulation (LES). A presumed Joint Probability Density Function (Pdf) approach is used for the sub-grid combustion modelling accounting for both premixed and non-premixed mode contributions. Two flames, one with high and another with low jet velocities are investigated and the computed temporal variation of flame leading point agrees quite well with the measured data for both the transient evolution and final lift-off height. The Joint Pdf of the axial and radial stabilisation locations shows that these locations are correlated with the jet exit velocity. The flame leading point evolution in the three-dimensional physical space is visualised using its trajectory, starting from the ignition location to the final lift-off height. A spiral -shaped path is observed for both velocity cases showing different flame propagation behaviours at different heights from the jet exit. These observations are explained on a physical basis.
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simulation of turbulent lifted methane jet flames effects of air dilution and transient flame propagation
Combustion and Flame, 2015Co-Authors: Zhi X Chen, Shaohong Ruan, N SwaminathanAbstract:Abstract Turbulent lifted methane jet flames with various air-dilution levels and a range of inlet velocities are simulated. A partially premixed combustion model based on premixed flamelets with presumed Joint Probability Density Function (Pdf) is used. The Joint Pdf is obtained using a copula to include the statistical correlation between mixture fraction, Z , and progress variable, c . The non-premixed combustion effect is included using a simple algebraic model. Both steady and unsteady RANS simulations are performed. The steady simulations show that the computed lift-off heights agree well with measured values for a wide range of jet velocities and air-dilution level. Both of the Z – c correlation and non-premixed combustion effects are found to be important to get the correct lift-off height. Their individual and combined effects are analysed systematically. The unsteady RANS results indicate that multi-stage flame development, namely the initial expansion, flame brush development, its propagation and final stabilisation, is captured reasonably well in simulations. The various stages of temporal evolution of the flame brush edge is captured well and the agreement with experimental measurements is good.
Matthias Ihme - One of the best experts on this subject based on the ideXlab platform.
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An SMLD Joint Pdf Model for Turbulent Non-Premixed Combustion Using the Flamelet Progress-Variable Approach
Flow Turbulence and Combustion, 2015Co-Authors: Alessandro Coclite, G Pascazio, Pietro De Palma, Luigi Cutrone, Matthias IhmeAbstract:This paper provides an improved flamelet/progress variable (FPV) model for the simulation of turbulent combustion, employing the statistically most likely distribution (SMLD) approach for the Joint probability density function (Pdf) of the mixture fraction, Z , and of the progress parameter, Λ. Steady-state FPV models are built presuming the functional shape of the Joint Pdf of Z and Λ in order to evaluate Favre-averages of thermodynamic quantities. The mixture fraction is widely assumed to behave as a passive scalar with a mono-modal behaviour modelled by a β -distribution. Moreover, under the hypothesis that Z and Λ are statistically independent, the Joint Pdf coincides with the product of the two marginal Pdfs. In this work we discuss these two constitutive hypotheses. The proposed model evaluates the most probable Joint distribution of Z and Λ, relaxing some crucial assumption on their statistical behaviour. This provides a more general model in the context of FPV approach and an effective tool to verify the adequateness of widely used hypotheses. The model is validated versus experimental data of well-known test cases, namely, the Sandia flames. The results are also compared with those obtained by the standard FPV approach, analysing the role of the Pdf functional form on turbulent combustion simulations.
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an smld Joint Pdf model for turbulent non premixed combustion using the flamelet progress variable approach
arXiv: Fluid Dynamics, 2015Co-Authors: Alessandro Coclite, G Pascazio, Pietro De Palma, Luigi Cutrone, Matthias IhmeAbstract:This paper provides an improved flamelet/progress variable (FPV) model for the simulation of turbulent combustion, employing the statistically most likely distribution (SMLD) approach for the Joint probability density function (Pdf) of the mixture fraction, Z, and of the progress parameter, {\Lambda} . Steady-state FPV models are built presuming the func- tional shape of the Joint Pdf of Z and {\Lambda} in order to evaluate Favre-averages of thermody- namic quantities. The mixture fraction is widely assumed to behave as a passive scalar with a mono-modal behaviour modelled by a \b{eta} -distribution. Moreover, under the hypothesis that Z and {\Lambda} are statistically independent, the Joint Pdf coincides with the product of the two marginal Pdfs. In this work we discuss these two constitutive hypotheses. The proposed model evaluates the most probable Joint distribution of Z and {\Lambda} , relaxing some crucial as- sumption on their statistical behaviour. This provides a more general model in the context of FPV approach and an effective tool to verify the adequateness of widely used hypotheses. The model is validated versus experimental data of well-known test cases, namely, the San- dia flames. The results are also compared with those obtained by the standard FPV approach, analysing the role of the Pdf functional form on turbulent combustion simulations.
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prediction of local extinction and re ignition effects in non premixed turbulent combustion using a flamelet progress variable approach
Proceedings of the Combustion Institute, 2005Co-Authors: Matthias Ihme, Heinz PitschAbstract: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.
Cora Uhlemann - One of the best experts on this subject based on the ideXlab platform.
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numerical complexity of the Joint nulled weak lensing probability distribution function
arXiv: Cosmology and Nongalactic Astrophysics, 2021Co-Authors: Alexandre Barthelemy, Sandrine Codis, Francis Bernardeau, Cora UhlemannAbstract:In the context of tomographic cosmic shear surveys, there exists a nulling transformation of weak lensing observations (also called BNT transform) that allows us to simplify the correlation structure of tomographic cosmic shear observations, as well as to build observables that depend only on a localised range of redshifts and thus independent from the low-redshift/small-scale modes. This procedure renders possible accurate, and from-first-principles, predictions of the convergence and aperture mass one-point distributions (Pdf). We here explore other consequences of this transformation on the (reduced) numerical complexity of the estimation of the Joint Pdf between nulled bins and demonstrate how to use these results to make theoretical predictions.
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two is better than one Joint statistics of density and velocity in concentric spheres as a cosmological probe
Monthly Notices of the Royal Astronomical Society, 2017Co-Authors: Cora Uhlemann, Christophe Pichon, Sandrine Codis, Francis Bernardeau, Oliver HahnAbstract:The analytical formalism to obtain the probability distribution functions (Pdfs) of spherically-averaged cosmic densities and velocity divergences in the mildly non-linear regime is presented. A large-deviation principle is applied to those cosmic fields assuming their most likely dynamics in spheres is set by the spherical collapse model. We validate our analytical results using state-of-the-art dark matter simulations with a phase-space resolved velocity field finding a 2% percent level agreement for a wide range of velocity divergences and densities in the mildly nonlinear regime (~10Mpc/h at redshift zero), usually inaccessible to perturbation theory. From the Joint Pdf of densities and velocity divergences measured in two concentric spheres, we extract with the same accuracy velocity profiles and conditional velocity Pdf subject to a given over/under-density which are of interest to understand the non-linear evolution of velocity flows. Both Pdfs are used to build a simple but accurate maximum likelihood estimators for the redshift evolution of the variance of both the density and velocity divergence fields, which have smaller relative errors than their sample variances when non-linearities appear. Given the dependence of the velocity divergence on the growth rate, there is a significant gain in using the full knowledge of both Pdfs to derive constraints on the equation of state of dark energy. Thanks to the insensitivity of the velocity divergence to bias, its Pdf can be used to obtain unbiased constraints on the growth of structures ($\sigma_8$,f) or it can be combined with the galaxy density Pdf to extract bias parameters.
Zhi X Chen - One of the best experts on this subject based on the ideXlab platform.
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large eddy simulation of flame edge evolution in a spark ignited methane air jet
Proceedings of the Combustion Institute, 2017Co-Authors: Zhi X Chen, Shaohong Ruan, N SwaminathanAbstract:Abstract The unsteady evolution of lifted methane–air jet flames following spark ignition is computed using Large Eddy Simulation (LES). A presumed Joint Probability Density Function (Pdf) approach is used for the sub-grid combustion modelling accounting for both premixed and non-premixed mode contributions. Two flames, one with high and another with low jet velocities are investigated and the computed temporal variation of flame leading point agrees quite well with the measured data for both the transient evolution and final lift-off height. The Joint Pdf of the axial and radial stabilisation locations shows that these locations are correlated with the jet exit velocity. The flame leading point evolution in the three-dimensional physical space is visualised using its trajectory, starting from the ignition location to the final lift-off height. A spiral -shaped path is observed for both velocity cases showing different flame propagation behaviours at different heights from the jet exit. These observations are explained on a physical basis.
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simulation of turbulent lifted methane jet flames effects of air dilution and transient flame propagation
Combustion and Flame, 2015Co-Authors: Zhi X Chen, Shaohong Ruan, N SwaminathanAbstract:Abstract Turbulent lifted methane jet flames with various air-dilution levels and a range of inlet velocities are simulated. A partially premixed combustion model based on premixed flamelets with presumed Joint Probability Density Function (Pdf) is used. The Joint Pdf is obtained using a copula to include the statistical correlation between mixture fraction, Z , and progress variable, c . The non-premixed combustion effect is included using a simple algebraic model. Both steady and unsteady RANS simulations are performed. The steady simulations show that the computed lift-off heights agree well with measured values for a wide range of jet velocities and air-dilution level. Both of the Z – c correlation and non-premixed combustion effects are found to be important to get the correct lift-off height. Their individual and combined effects are analysed systematically. The unsteady RANS results indicate that multi-stage flame development, namely the initial expansion, flame brush development, its propagation and final stabilisation, is captured reasonably well in simulations. The various stages of temporal evolution of the flame brush edge is captured well and the agreement with experimental measurements is good.
