Casimir forces can appear between intrusions placed in different media driven by several fluctuat... more Casimir forces can appear between intrusions placed in different media driven by several fluctuation mechanisms, either in equilibrium or out of it. Herein, we develop a general formalism to obtain such forces from the dynamical equations of the fluctuating medium, the statistical properties of the driving noise, and the boundary conditions of the intrusions (which simulate the interaction between the intrusions and the medium). As a result, an explicit formula for the Casimir force over the intrusions is derived. This formalism contains the thermal Casimir effect as a particular limit and generalizes the study of the Casimir effect to such systems through their dynamical equations, with no appeal to their Hamiltonian, if any exists. In particular, we study the Casimir force between two infinite parallel plates with Dirichlet or Neumann boundary conditions, immersed in several media with finite correlation lengths (reaction--diffusion system, liquid crystals, and two coupled fields with non-Hermitian evolution equations). The driving Gaussian noises have vanishing or finite spatial or temporal correlation lengths; in the first case, equilibrium is reobtained and finite correlations produce nonequilibrium dynamics. The results obtained show that, generally, nonequilibrium dynamics leads to Casimir forces, whereas Casimir forces are obtained in equilibrium dynamics if the stress tensor is anisotropic.
The Brazilian Institute of Radiation Protection and Dosimetry (IRD/CNEN) carried out quality assu... more The Brazilian Institute of Radiation Protection and Dosimetry (IRD/CNEN) carried out quality assurance regulatory audits in Brazilian radiotherapy facilities from 1995 to 2007. In this work, the set of data collected from 195 radiotherapy facilities that use high-energy photon beams are analyzed. They include results from audits in linear electron accelerators and/or Co-60 units. The inspectors of IRD/CNEN performed the dosimetry of high-energy radiotherapy photon beams according to the IAEA dosimetry protocols TRS 277 and TRS 398, and the values of measurements were compared to stated values. Other aspects of radiological protection were checked during on-site audits such as calibration certification of clinical dosimeters and portable monitors, existence and use of check source, use of barometer and thermometer, individual dose registry and training of staff. It was verified that no check source was available in 38% of the visited facilities; the training of personnel was not adequate in 9% of the facilities and the registry of accumulated individual doses was not being done in 6% of the facilities. Measurements of absorbed dose have indicated deviations in the range ± 3% for 67.6% of the cobalt-60 units and 79.6% of medical linear accelerators; 18.5% of Co-60 irradiators and 9.6% of linear accelerators presented deviations in the range 3% < δ ≤ 5%. Finally, 13.9% of Co-60 facilities and 10.8% of linear accelerator facilities presented dosimetry deviations above 5%. The effort in dosimetric quality control performed by IRD/CNEN audits has yielded positive changes that make radiation treatment facilities more reliable.
An initially homogeneous freely evolving fluid of inelastic hard spheres develops inhomogeneities... more An initially homogeneous freely evolving fluid of inelastic hard spheres develops inhomogeneities in the flow field u(r, t) (vortices) and in the density field n(r, t) (clusters), driven by unstable fluctuations, δa = {δn, δu}. Their spatial correlations, δa(r, t)δa(r ′ , t) , as measured in molecular dynamics simulations, exhibit long range correlations; the mean vortex diameter grows as ξ(t) ∝ √ ln t; there occur transitions to macroscopic shearing states, etc. The Cahn-Hilliard theory of spinodal decomposition offers a qualitative understanding and quantitative estimates of the observed phenomena. When intrinsic length scales are of the order of the system size, effects of physical boundaries and periodic boundaries (finite size effects in simulations) are important.
An initially homogeneous freely evolving fluid of inelastic hard spheres develops inhomogeneities... more An initially homogeneous freely evolving fluid of inelastic hard spheres develops inhomogeneities in the flow field u(r, t) (vortices) and in the density field n(r, t) (clusters), driven by unstable fluctuations, δa = {δn, δu}. Their spatial correlations, δa(r, t)δa(r ′ , t) , as measured in molecular dynamics simulations, exhibit long range correlations; the mean vortex diameter grows as ξ(t) ∝ √ ln t; there occur transitions to macroscopic shearing states, etc. The Cahn-Hilliard theory of spinodal decomposition offers a qualitative understanding and quantitative estimates of the observed phenomena. When intrinsic length scales are of the order of the system size, effects of physical boundaries and periodic boundaries (finite size effects in simulations) are important.
For a freely evolving granular fluid, the buildup of spatial correlations in density and flow fie... more For a freely evolving granular fluid, the buildup of spatial correlations in density and flow field is described using fluctuating hydrodynamics. The theory for incompressible flows is extended to the general, compressible case, including longitudinal velocity and density fluctuations, and yields qualitatively different results for long range correlations. The structure factor of density fluctuations shows a maximum at finite wavenumber, shifting in time to smaller wavenumbers and corresponding to a growing correlation length. It agrees well with two-dimensional molecular dynamics simulations.
For a freely evolving granular fluid, the buildup of spatial correlations in density and flow fie... more For a freely evolving granular fluid, the buildup of spatial correlations in density and flow field is described using fluctuating hydrodynamics. The theory for incompressible flows is extended to the general, compressible case, including longitudinal velocity and density fluctuations, and yields qualitatively different results for long range correlations. The structure factor of density fluctuations shows a maximum at finite wavenumber, shifting in time to smaller wavenumbers and corresponding to a growing correlation length. It agrees well with two-dimensional molecular dynamics simulations.
Replacement of cond-mat/0002383. Re-analysis of computer simulations, new figures and modificatio... more Replacement of cond-mat/0002383. Re-analysis of computer simulations, new figures and modification of conclusions.
Replacement of cond-mat/0002383. Re-analysis of computer simulations, new figures and modificatio... more Replacement of cond-mat/0002383. Re-analysis of computer simulations, new figures and modification of conclusions.
Spatial fluctuations in dissipative systems, such as rapid granular flows, behave very differentl... more Spatial fluctuations in dissipative systems, such as rapid granular flows, behave very differently from those in elastic fluids. Fluctuations in the flow field drive the linear and nonlinear instability in the density field (clustering), while vortex structures appear and grow through the mechanism of noise reduction. The dynamics of the flow field on the largest space and time scales is described by diffusion equations with different diffusivities for the transverse and longitudinal flow fields. The results are obtained from analytic and simulation methods.
Spatial fluctuations in dissipative systems, such as rapid granular flows, behave very differentl... more Spatial fluctuations in dissipative systems, such as rapid granular flows, behave very differently from those in elastic fluids. Fluctuations in the flow field drive the linear and nonlinear instability in the density field (clustering), while vortex structures appear and grow through the mechanism of noise reduction. The dynamics of the flow field on the largest space and time scales is described by diffusion equations with different diffusivities for the transverse and longitudinal flow fields. The results are obtained from analytic and simulation methods.
Revista Brasileira de Engenharia Agrícola e Ambiental, 2001
Go to AGRIS search. Revista Brasileira de Engenharia Agrícola e Ambiental (2001/05). Desempenho d... more Go to AGRIS search. Revista Brasileira de Engenharia Agrícola e Ambiental (2001/05). Desempenho do Distrito de Irrigação Senador Nilo Coelho. ...
Revista Brasileira de Engenharia Agrícola e Ambiental, 2001
Go to AGRIS search. Revista Brasileira de Engenharia Agrícola e Ambiental (2001/05). Desempenho d... more Go to AGRIS search. Revista Brasileira de Engenharia Agrícola e Ambiental (2001/05). Desempenho do Distrito de Irrigação Senador Nilo Coelho. ...
Revista Brasileira de Engenharia Agrícola e Ambiental, 2001
Go to AGRIS search. Revista Brasileira de Engenharia Agrícola e Ambiental (2001/05). Desempenho d... more Go to AGRIS search. Revista Brasileira de Engenharia Agrícola e Ambiental (2001/05). Desempenho do Distrito de Irrigação Senador Nilo Coelho. ...
International Journal of Modern Physics: Conference Series, 2012
Recently, a method based on stochastic quantization has been proposed to compute the Casimir forc... more Recently, a method based on stochastic quantization has been proposed to compute the Casimir force and its fluctuations in arbitrary geometries. It relies on the spectral decomposition of the Laplacian operator in the given geometry. Both quantum and thermal fluctuations are considered. Here we use such method to compute the Casimir force on the plates of a finite piston of arbitrary cross section. Asymptotic expressions valid at low and high temperatures and short and long distances are obtained. The case of a piston with triangular cross section is analyzed in detail. The regularization of the divergent stress tensor is described.
International Journal of Modern Physics: Conference Series, 2012
Recently, a method based on stochastic quantization has been proposed to compute the Casimir forc... more Recently, a method based on stochastic quantization has been proposed to compute the Casimir force and its fluctuations in arbitrary geometries. It relies on the spectral decomposition of the Laplacian operator in the given geometry. Both quantum and thermal fluctuations are considered. Here we use such method to compute the Casimir force on the plates of a finite piston of arbitrary cross section. Asymptotic expressions valid at low and high temperatures and short and long distances are obtained. The case of a piston with triangular cross section is analyzed in detail. The regularization of the divergent stress tensor is described.
Casimir forces can appear between intrusions placed in different media driven by several fluctuat... more Casimir forces can appear between intrusions placed in different media driven by several fluctuation mechanisms, either in equilibrium or out of it. Herein, we develop a general formalism to obtain such forces from the dynamical equations of the fluctuating medium, the statistical properties of the driving noise, and the boundary conditions of the intrusions (which simulate the interaction between the intrusions and the medium). As a result, an explicit formula for the Casimir force over the intrusions is derived. This formalism contains the thermal Casimir effect as a particular limit and generalizes the study of the Casimir effect to such systems through their dynamical equations, with no appeal to their Hamiltonian, if any exists. In particular, we study the Casimir force between two infinite parallel plates with Dirichlet or Neumann boundary conditions, immersed in several media with finite correlation lengths (reaction--diffusion system, liquid crystals, and two coupled fields with non-Hermitian evolution equations). The driving Gaussian noises have vanishing or finite spatial or temporal correlation lengths; in the first case, equilibrium is reobtained and finite correlations produce nonequilibrium dynamics. The results obtained show that, generally, nonequilibrium dynamics leads to Casimir forces, whereas Casimir forces are obtained in equilibrium dynamics if the stress tensor is anisotropic.
The Brazilian Institute of Radiation Protection and Dosimetry (IRD/CNEN) carried out quality assu... more The Brazilian Institute of Radiation Protection and Dosimetry (IRD/CNEN) carried out quality assurance regulatory audits in Brazilian radiotherapy facilities from 1995 to 2007. In this work, the set of data collected from 195 radiotherapy facilities that use high-energy photon beams are analyzed. They include results from audits in linear electron accelerators and/or Co-60 units. The inspectors of IRD/CNEN performed the dosimetry of high-energy radiotherapy photon beams according to the IAEA dosimetry protocols TRS 277 and TRS 398, and the values of measurements were compared to stated values. Other aspects of radiological protection were checked during on-site audits such as calibration certification of clinical dosimeters and portable monitors, existence and use of check source, use of barometer and thermometer, individual dose registry and training of staff. It was verified that no check source was available in 38% of the visited facilities; the training of personnel was not adequate in 9% of the facilities and the registry of accumulated individual doses was not being done in 6% of the facilities. Measurements of absorbed dose have indicated deviations in the range ± 3% for 67.6% of the cobalt-60 units and 79.6% of medical linear accelerators; 18.5% of Co-60 irradiators and 9.6% of linear accelerators presented deviations in the range 3% < δ ≤ 5%. Finally, 13.9% of Co-60 facilities and 10.8% of linear accelerator facilities presented dosimetry deviations above 5%. The effort in dosimetric quality control performed by IRD/CNEN audits has yielded positive changes that make radiation treatment facilities more reliable.
An initially homogeneous freely evolving fluid of inelastic hard spheres develops inhomogeneities... more An initially homogeneous freely evolving fluid of inelastic hard spheres develops inhomogeneities in the flow field u(r, t) (vortices) and in the density field n(r, t) (clusters), driven by unstable fluctuations, δa = {δn, δu}. Their spatial correlations, δa(r, t)δa(r ′ , t) , as measured in molecular dynamics simulations, exhibit long range correlations; the mean vortex diameter grows as ξ(t) ∝ √ ln t; there occur transitions to macroscopic shearing states, etc. The Cahn-Hilliard theory of spinodal decomposition offers a qualitative understanding and quantitative estimates of the observed phenomena. When intrinsic length scales are of the order of the system size, effects of physical boundaries and periodic boundaries (finite size effects in simulations) are important.
An initially homogeneous freely evolving fluid of inelastic hard spheres develops inhomogeneities... more An initially homogeneous freely evolving fluid of inelastic hard spheres develops inhomogeneities in the flow field u(r, t) (vortices) and in the density field n(r, t) (clusters), driven by unstable fluctuations, δa = {δn, δu}. Their spatial correlations, δa(r, t)δa(r ′ , t) , as measured in molecular dynamics simulations, exhibit long range correlations; the mean vortex diameter grows as ξ(t) ∝ √ ln t; there occur transitions to macroscopic shearing states, etc. The Cahn-Hilliard theory of spinodal decomposition offers a qualitative understanding and quantitative estimates of the observed phenomena. When intrinsic length scales are of the order of the system size, effects of physical boundaries and periodic boundaries (finite size effects in simulations) are important.
For a freely evolving granular fluid, the buildup of spatial correlations in density and flow fie... more For a freely evolving granular fluid, the buildup of spatial correlations in density and flow field is described using fluctuating hydrodynamics. The theory for incompressible flows is extended to the general, compressible case, including longitudinal velocity and density fluctuations, and yields qualitatively different results for long range correlations. The structure factor of density fluctuations shows a maximum at finite wavenumber, shifting in time to smaller wavenumbers and corresponding to a growing correlation length. It agrees well with two-dimensional molecular dynamics simulations.
For a freely evolving granular fluid, the buildup of spatial correlations in density and flow fie... more For a freely evolving granular fluid, the buildup of spatial correlations in density and flow field is described using fluctuating hydrodynamics. The theory for incompressible flows is extended to the general, compressible case, including longitudinal velocity and density fluctuations, and yields qualitatively different results for long range correlations. The structure factor of density fluctuations shows a maximum at finite wavenumber, shifting in time to smaller wavenumbers and corresponding to a growing correlation length. It agrees well with two-dimensional molecular dynamics simulations.
Replacement of cond-mat/0002383. Re-analysis of computer simulations, new figures and modificatio... more Replacement of cond-mat/0002383. Re-analysis of computer simulations, new figures and modification of conclusions.
Replacement of cond-mat/0002383. Re-analysis of computer simulations, new figures and modificatio... more Replacement of cond-mat/0002383. Re-analysis of computer simulations, new figures and modification of conclusions.
Spatial fluctuations in dissipative systems, such as rapid granular flows, behave very differentl... more Spatial fluctuations in dissipative systems, such as rapid granular flows, behave very differently from those in elastic fluids. Fluctuations in the flow field drive the linear and nonlinear instability in the density field (clustering), while vortex structures appear and grow through the mechanism of noise reduction. The dynamics of the flow field on the largest space and time scales is described by diffusion equations with different diffusivities for the transverse and longitudinal flow fields. The results are obtained from analytic and simulation methods.
Spatial fluctuations in dissipative systems, such as rapid granular flows, behave very differentl... more Spatial fluctuations in dissipative systems, such as rapid granular flows, behave very differently from those in elastic fluids. Fluctuations in the flow field drive the linear and nonlinear instability in the density field (clustering), while vortex structures appear and grow through the mechanism of noise reduction. The dynamics of the flow field on the largest space and time scales is described by diffusion equations with different diffusivities for the transverse and longitudinal flow fields. The results are obtained from analytic and simulation methods.
Revista Brasileira de Engenharia Agrícola e Ambiental, 2001
Go to AGRIS search. Revista Brasileira de Engenharia Agrícola e Ambiental (2001/05). Desempenho d... more Go to AGRIS search. Revista Brasileira de Engenharia Agrícola e Ambiental (2001/05). Desempenho do Distrito de Irrigação Senador Nilo Coelho. ...
Revista Brasileira de Engenharia Agrícola e Ambiental, 2001
Go to AGRIS search. Revista Brasileira de Engenharia Agrícola e Ambiental (2001/05). Desempenho d... more Go to AGRIS search. Revista Brasileira de Engenharia Agrícola e Ambiental (2001/05). Desempenho do Distrito de Irrigação Senador Nilo Coelho. ...
Revista Brasileira de Engenharia Agrícola e Ambiental, 2001
Go to AGRIS search. Revista Brasileira de Engenharia Agrícola e Ambiental (2001/05). Desempenho d... more Go to AGRIS search. Revista Brasileira de Engenharia Agrícola e Ambiental (2001/05). Desempenho do Distrito de Irrigação Senador Nilo Coelho. ...
International Journal of Modern Physics: Conference Series, 2012
Recently, a method based on stochastic quantization has been proposed to compute the Casimir forc... more Recently, a method based on stochastic quantization has been proposed to compute the Casimir force and its fluctuations in arbitrary geometries. It relies on the spectral decomposition of the Laplacian operator in the given geometry. Both quantum and thermal fluctuations are considered. Here we use such method to compute the Casimir force on the plates of a finite piston of arbitrary cross section. Asymptotic expressions valid at low and high temperatures and short and long distances are obtained. The case of a piston with triangular cross section is analyzed in detail. The regularization of the divergent stress tensor is described.
International Journal of Modern Physics: Conference Series, 2012
Recently, a method based on stochastic quantization has been proposed to compute the Casimir forc... more Recently, a method based on stochastic quantization has been proposed to compute the Casimir force and its fluctuations in arbitrary geometries. It relies on the spectral decomposition of the Laplacian operator in the given geometry. Both quantum and thermal fluctuations are considered. Here we use such method to compute the Casimir force on the plates of a finite piston of arbitrary cross section. Asymptotic expressions valid at low and high temperatures and short and long distances are obtained. The case of a piston with triangular cross section is analyzed in detail. The regularization of the divergent stress tensor is described.
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Papers by Ricardo Brito