sensing characteristics of hematite and barium oxide doped hematite films towards ozone and nitrogen dioxide

Abstract Hematite (α-Fe2O3) and barium oxide doped hematite (BaO-Fe 2 O 3 ) thin films were investigated as ozone (O 3 ) and nitrogen dioxide (NO 2 ) sensing materials. Fe 2 O 3 and BaO-Fe 2 O 3 films were deposited by radio- frequency sputtering using pure Fe 2 O 3 , and 1-2% BaO doped Fe 2 O 3 targets. The 700 °C (1 hour) annealed films showed significant responses to O 3 at temperatures ranging from 150 °C to 300 °C. Although, hematite is an n-type semiconductor, the Fe 2 O 3 and BaO-Fe 2 O 3 films exhibit p-type behavior to O 3 and n- type behavior to NO 2 at the studied concentration ranges in this work. The response to oxidizing gases is not strictly an increase in resistance due to a conversion from n-type to p-type depending on gas concentrations. This effect is more visible with increasing Ba concentration

Local shell-to-shell energy transfer via nonlocal Interactions in fluid turbulence

In this paper we analytically compute the strength of nonlinear interactions in a triad, and the energy exchanges between wavenumber shells in incompressible fluid turbulence. The computation has been done using first-order perturbative field theory. In three dimension, magnitude of triad interactions is large for nonlocal triads, and small for local triads. However, the shell-to-shell energy transfer rate is found to be local and forward. This result is due to the fact that the nonlocal triads occupy much less Fourier space volume than the local ones. The analytical results on three-dimensional shell-to-shell energy transfer match with their numerical counterparts. In two-dimensional turbulence, the energy transfer rates to the near-by shells are forward, but to the distant shells are backward; the cumulative effect is an inverse cascade of energy.Comment: 10 pages, Revtex

Pseudo-spectral methods applied to hydrodynamic and magnetohydrodynamic turbulence

In our everyday life, turbulence is an omnipresent phenomenon and yet remains poorly understood. Its random and chaotic nature makes it a subject almost impossible to treat from the mathematical point of view and, at present, thereis no real prospect of a simple analytic theory. Scientists have therefore regarded the numerical simulation as an alternative to compute the relevant properties of turbulent flows. In this context, our thesis aims at developing and using accurate computational methods, namely pseudo-spectral methods, for studying hydrodynamic (1st part) and magnetohydrodynamic (2nd part) turbulence.In the hydrodynamic part, Chapter I introduces the governing equations of fluid mechanics as well as the main issues related to the numerical study of turbulent flows. In particular, the Direct Numerical Simulations (DNS) of turbulence, in which accurate numerical solutions of the Navier-Stokes equations are obtained, are shown to be limited to moderately turbulent flows.Chapter II introduces the Large Eddy Simulation (LES) technique which aims at simulating highly turbulent flows and which is based on a separation of scales.In practice, it consists of simulating the large - resolved - scales of the flow explicitly while modelling the small - unresolved - scales. Two different approaches for modelling the kinetic energy of the unresolved scales are proposed and their respective advantages and drawbacks are discussed.Chapter III is devoted the study of the mixing-layer using both DNS and LES. It consists of an inhomogeneous turbulent flow which has been studied experimentally and for which well-documented measurements are available. A highly accurate DNS mimicking the same experiment has been produced. It allows to study the inhomogeneity and anisotropy properties of this flow. Also, LES of the same flow, using different models, have been evaluated. In Chapter IV, we explore a pseudo-spectral method to investigate turbulence in a pipe. In this case, the method has to take into account two additional difficulties: i) the presence of the boundary and ii) the axis singularity. We detail how to circumvent these issues.The second part of the thesis is devoted to magnetohydrodynamic (MHD) turbulence. It concerns phenomena where electrically conducting flows interact with electromagnetism and for which governing equations are derived in Chapter V. In Chapter VI, a detailed analysis of the energy transfers between the magnetic and velocity fields is performed thanks to a high resolution database of homogeneous MHD turbulence. It provides some insights to understand the physics of the nonlinear interactions and is also a valuable diagnostic in the framework of LES modelling. Finally, the inhomogeneous configuration studied in Chapter III has been extended to MHD. Several statistics related to the kinetic and magnetic energies are measured and LES of this flow are performed and presented in Chapter VII.Doctorat en sciences, Spécialisation physiqueinfo:eu-repo/semantics/nonPublishe

Pseudo-spectral methods applied to hydrodynamic and magnetohydrodynamic turbulence

In our everyday life, turbulence is an omnipresent phenomenon and yet remains poorly understood. Its random and chaotic nature makes it a subject almost impossible to treat from the mathematical point of view and, at present, thereis no real prospect of a simple analytic theory. Scientists have therefore regarded the numerical simulation as an alternative to compute the relevant properties of turbulent flows. In this context, our thesis aims at developing and using accurate computational methods, namely pseudo-spectral methods, for studying hydrodynamic (1st part) and magnetohydrodynamic (2nd part) turbulence.In the hydrodynamic part, Chapter I introduces the governing equations of fluid mechanics as well as the main issues related to the numerical study of turbulent flows. In particular, the Direct Numerical Simulations (DNS) of turbulence, in which accurate numerical solutions of the Navier-Stokes equations are obtained, are shown to be limited to moderately turbulent flows.Chapter II introduces the Large Eddy Simulation (LES) technique which aims at simulating highly turbulent flows and which is based on a separation of scales.In practice, it consists of simulating the large - resolved - scales of the flow explicitly while modelling the small - unresolved - scales. Two different approaches for modelling the kinetic energy of the unresolved scales are proposed and their respective advantages and drawbacks are discussed.Chapter III is devoted the study of the mixing-layer using both DNS and LES. It consists of an inhomogeneous turbulent flow which has been studied experimentally and for which well-documented measurements are available. A highly accurate DNS mimicking the same experiment has been produced. It allows to study the inhomogeneity and anisotropy properties of this flow. Also, LES of the same flow, using different models, have been evaluated. In Chapter IV, we explore a pseudo-spectral method to investigate turbulence in a pipe. In this case, the method has to take into account two additional difficulties: i) the presence of the boundary and ii) the axis singularity. We detail how to circumvent these issues.The second part of the thesis is devoted to magnetohydrodynamic (MHD) turbulence. It concerns phenomena where electrically conducting flows interact with electromagnetism and for which governing equations are derived in Chapter V. In Chapter VI, a detailed analysis of the energy transfers between the magnetic and velocity fields is performed thanks to a high resolution database of homogeneous MHD turbulence. It provides some insights to understand the physics of the nonlinear interactions and is also a valuable diagnostic in the framework of LES modelling. Finally, the inhomogeneous configuration studied in Chapter III has been extended to MHD. Several statistics related to the kinetic and magnetic energies are measured and LES of this flow are performed and presented in Chapter VII.Doctorat en sciences, Spécialisation physiqueinfo:eu-repo/semantics/nonPublishe

Numerical simulations of turbulent systems in hydrodynamics and magnetohydrodynamics

info:eu-repo/semantics/publishe

Large-eddy simulation without filter

An large-eddy simulation (LES) formalism based on sampling operators instead of filters is developed. The major advantage of this approach is that sampling operators commute with the product and their application to nonlinear terms is not at the origin of any closure problem. In absence of filters that smooth out the small scale structures in the flow, the discretization errors in the LES are expected to be important. They must be modelled. The possible confusion between modelling and discretization errors is however avoided since these two effects are identical in the present formalism. A generalized dynamic procedure is proposed for sampling-based LES which allows for model parameter optimization and does not require a detailed analysis of the discretization error. In addition to its interesting mathematical properties for LES, the velocity obtained by a spatial sampling is much closer to experimental probe data than the filtered velocity field. © 2004 Elsevier Inc. All rights reserved.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

A dynamic subgrid-scale model based on the turbulent kinetic energy

info:eu-repo/semantics/publishe

Modelling of MHD turbulence

SCOPUS: ar.kinfo:eu-repo/semantics/publishe

Large-eddy simulation of a shear-free magnetohydrodynamic mixing layer

info:eu-repo/semantics/publishe