The Experts below are selected from a list of 105 Experts worldwide ranked by ideXlab platform
Tõnu Viik - One of the best experts on this subject based on the ideXlab platform.
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Source vector in Rayleigh-Cabannes scattering atmosphere: the nonconservative milne problem
Astrophysics and Space Science, 1993Co-Authors: Tõnu ViikAbstract:We consider the radiative transfer in a nonconservative homogeneous plane-parallel semi-infinite planetary atmosphere where the scattering processes are described by the Rayleigh-Cabannes phase matrix and where the primary sources are in infinitely deep layers. If we use the superposition principle we derive the Cauchy problem for the source vector. As a by-product the external field of radiation for the problem described is obtained using the principle of invariance by Chandrasekhar. The respective formulae for the radiation field in the deep layers and for the Extrapolation Distance are given. It is shown that the Rubenson degree of polarization even in the case of near-conservative atmospheres reaches the asymptotic regime at rather small values of the optical depth. The λ -plane reliefs of the characteristic equation, Extrapolation Distance and the normalized components of the source vector at the boundary are given along with a sample of zeros of the characteristic equation.
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Rayleigh-Cabannes scattering in planetary atmospheres III. The Milne problem in conservative atmospheres
Earth Moon and Planets, 1990Co-Authors: Tõnu ViikAbstract:The discrete ordinale method by Chandrasekhar is used to solve the conservative Milne problem in a homogeneous plane-parallel atmosphere which scatters the radiation according to the Rayleigh-Cabannes law. The approximate solution which is supposed to converge uniformly to an exact one when increasing the order of approximation is obtained explicitly. In addition to a tabulation of the Hopf vector for different factors of depolarization, the Extrapolation Distance, the values of c′ , q′ and the Rubenson degrees of polarization at the limb are given.
Rozenn Wagner - One of the best experts on this subject based on the ideXlab platform.
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Digitalization of scanning lidar measurement campaign planning
Wind Energy Science, 2020Co-Authors: Nikola Vasiljevic, Andrea Vignaroli, Andreas Bechmann, Rozenn WagnerAbstract:Abstract. By using multiple wind measurements when designing wind farms, it is possible to decrease the uncertainty of wind farm energy assessments since the Extrapolation Distance between measurements and wind turbine locations is reduced. A WindScanner system consisting of two synchronized scanning lidars potentially represents a cost-effective solution for multipoint measurements, especially in complex terrain. However, the system limitations and limitations imposed by the wind farm site are detrimental to the installation of scanning lidars and the number and location of the measurement points. To simplify the process of finding suitable measurement positions and associated installation locations for the WindScanner system, we have devised a campaign planning workflow. The workflow consists of four phases. In the first phase, based on a preliminary wind farm layout, we generate optimum measurement positions using a greedy algorithm and a measurement “representative radius”. In the second phase, we create several Geographical Information System (GIS) layers such as exclusion zones, line-of-sight (LOS) blockage and lidar range constraint maps. These GIS layers are then used in the third phase to find optimum positions of the WindScanner systems with respect to the measurement positions considering the WindScanner measurement uncertainty and logistical constraints. In the fourth phase, we optimize and generate a trajectory through the measurement positions by applying the traveling salesman problem (TSP) on these positions. The described workflow has been digitalized into a Python package named campaign-planning-tool, which gives users an effective way to design measurement campaigns with WindScanner systems. In this study, the Python package has been tested on three different sites characterized by different terrain complexity and wind farm dimensions and layouts. With minimal effort, the Python package can optimize measurement positions and suggest possible lidar installation locations for carrying out resource assessment campaigns.
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Digitizing scanning lidar measurement campaign planning
2019Co-Authors: Nikola Vasiljevic, Andrea Vignaroli, Andreas Bechmann, Rozenn WagnerAbstract:Abstract. Multiple wind measurements is a way to reduce the uncertainty of wind farm energy yield assessments by reducing the Extrapolation Distance between measurements and wind turbines locations. A WindScanner system consisting of two synchronized scanning lidar potentially represents a cost-effective solution for multi-point measurements, especially in complex terrain. However, the system limitations and limitations imposed by the wind farm site are detrimental to the installation of scanning lidars and the number and location of the measurement positions. To simplify the process of finding suitable measurement positions and associated installation locations for the WindScanner system we have devised a campaign planning workflow. The workflow consists of four phases. In the first phase, based on a preliminary wind farm layout, we generate optimum measurement positions using a greedy algorithm and a measurement 'representative radius'. In the second phase, we create several Geographical Information System (GIS) layers of information such as exclusion zones, line-of-sight (LOS) blockage, and lidar range maps. These GIS layers are then used in the third phase to find optimum positions of the WindScanners with respect to the measurement positions considering the WindScanner measurement uncertainty. In the fourth phase, we optimize and generate trajectory through the measurement positions by applying the traveling salesman problem (TSP) on these positions. The above-described workflow has been digitized into the so-called Campaign Planning Tool (CPT) currently provided as a Python library which allows users an effective way to plan measurement campaigns with WindScanner systems. In this study, the CPT has been tested on three different sites characterized by different terrain complexity and wind farm dimensions and layouts. The CPT has shown instantly whether the whole site can be covered by one system or not.
Raphael Aronson - One of the best experts on this subject based on the ideXlab platform.
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Diffusion boundary conditions for photon waves
Optical Tomography and Spectroscopy of Tissue: Theory Instrumentation Model and Human Studies II, 1997Co-Authors: Raphael AronsonAbstract:The use of diffusion theory in calculations on photon waves necessitates a new look at boundary conditions, since the standard boundary conditions have been derived under static conditions. When the underlain process satisfies the transport equation, the proper boundary conditions are obtained by solving the Milne problem. This paper presents benchmark- quality values for Extrapolation Distances calculated by transport theory, for various values of absorption and three models of the phase function -- isotropic, linearly anisotropic and Henyey-Greenstein scattering. The results show that the static boundary conditions are perfectly adequate up to photon wave frequencies of 1 GHz or even more. Specifically, the quantity (Sigma) trd, where (Sigma) tr' equals (Sigma) tr - ik, where (Sigma) tr is the macroscopic transport cross section and k the wave number in the medium and d the linear the linear Extrapolation Distance, is essentially independent of frequency over this range. We have also examined the ratio of the diffusion length as given by transport theory to that given by diffusion theory itself. This is extremely insensitive to frequency, but for substantial absorption, using the diffusion theory result can lead to substantial errors in thick media, especially for Henyey-Greenstein scattering.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.
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Boundary conditions for diffusion of light.
Journal of The Optical Society of America A-optics Image Science and Vision, 1995Co-Authors: Raphael AronsonAbstract:In connection with recent work on remote imaging of random media by light, a straightforward generalization of the proper diffusion boundary conditions is presented that takes into account Fresnel reflection. The Milne problem at exterior boundaries is solved for various values of index of refraction, absorption, and scattering anisotropy parameters to yield extrapolated end points and Extrapolation Distances. A generalized interface condition is derived to replace the usual condition of continuity of intensity. Benchmark-quality numerical results are given for the Extrapolation Distance and for the new index-dependent parameter in the interface conditions. Difficulties in using the extrapolated end point when the index is sufficiently large are discussed, and a new image procedure suitable for this case is presented.
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Extrapolation Distance for diffusion of light
Proceedings of SPIE, 1993Co-Authors: Raphael AronsonAbstract:Diffusion theory has been a useful and frequently applied analytical method to study the transport of light in random media. The diffusion equation requires unphysical boundary conditions. This is reflected in the fact that the diffusion solution must differ from the exact solution in a boundary region a few mean free paths thick. Exact transport theory indicates that for particle diffusion the true boundary is to be replaced by an extrapolated boundary 0.71 transport mean free paths outside of it. This is the number that has universally been used in treating light diffusion, although it is sometimes neglected because it is often a very short Distance. However, because there is reflection at the boundary due to mismatch in the index of refraction, the Extrapolation Distance for diffusion of light is longer than that for particles, and this must be taken into account. The correction is large, even for modest indices of refraction. We show here that the appropriate boundary condition is given in terms of an Extrapolation Distance and tabulate this quantity as a function of relative scattering probability and index of refraction of the medium.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.
U Woznicka - One of the best experts on this subject based on the ideXlab platform.
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Extrapolation Distance in critical and time dependent two region spherical systems
Kerntechnik, 1997Co-Authors: U WoznickaAbstract:Extrapolation Distances for the neutron flux distribution in bounded media are usually defined in such a way that agreement is obtained between diffusion theory and transport theory. A typical application is the interpretation of pulsed neutron experiments. In this work we extend the conventional treatment of Extrapolation Distances to two-region spherical bodies. Assuming neutrons of one speed, Extrapolation Distances are calculated for two-region spheres of different neutron properties. Anisotropic scattering of the neutrons is also taken into account and both critical and time-dependent cases are studied.
Nikola Vasiljevic - One of the best experts on this subject based on the ideXlab platform.
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Digitalization of scanning lidar measurement campaign planning
Wind Energy Science, 2020Co-Authors: Nikola Vasiljevic, Andrea Vignaroli, Andreas Bechmann, Rozenn WagnerAbstract:Abstract. By using multiple wind measurements when designing wind farms, it is possible to decrease the uncertainty of wind farm energy assessments since the Extrapolation Distance between measurements and wind turbine locations is reduced. A WindScanner system consisting of two synchronized scanning lidars potentially represents a cost-effective solution for multipoint measurements, especially in complex terrain. However, the system limitations and limitations imposed by the wind farm site are detrimental to the installation of scanning lidars and the number and location of the measurement points. To simplify the process of finding suitable measurement positions and associated installation locations for the WindScanner system, we have devised a campaign planning workflow. The workflow consists of four phases. In the first phase, based on a preliminary wind farm layout, we generate optimum measurement positions using a greedy algorithm and a measurement “representative radius”. In the second phase, we create several Geographical Information System (GIS) layers such as exclusion zones, line-of-sight (LOS) blockage and lidar range constraint maps. These GIS layers are then used in the third phase to find optimum positions of the WindScanner systems with respect to the measurement positions considering the WindScanner measurement uncertainty and logistical constraints. In the fourth phase, we optimize and generate a trajectory through the measurement positions by applying the traveling salesman problem (TSP) on these positions. The described workflow has been digitalized into a Python package named campaign-planning-tool, which gives users an effective way to design measurement campaigns with WindScanner systems. In this study, the Python package has been tested on three different sites characterized by different terrain complexity and wind farm dimensions and layouts. With minimal effort, the Python package can optimize measurement positions and suggest possible lidar installation locations for carrying out resource assessment campaigns.
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Digitizing scanning lidar measurement campaign planning
2019Co-Authors: Nikola Vasiljevic, Andrea Vignaroli, Andreas Bechmann, Rozenn WagnerAbstract:Abstract. Multiple wind measurements is a way to reduce the uncertainty of wind farm energy yield assessments by reducing the Extrapolation Distance between measurements and wind turbines locations. A WindScanner system consisting of two synchronized scanning lidar potentially represents a cost-effective solution for multi-point measurements, especially in complex terrain. However, the system limitations and limitations imposed by the wind farm site are detrimental to the installation of scanning lidars and the number and location of the measurement positions. To simplify the process of finding suitable measurement positions and associated installation locations for the WindScanner system we have devised a campaign planning workflow. The workflow consists of four phases. In the first phase, based on a preliminary wind farm layout, we generate optimum measurement positions using a greedy algorithm and a measurement 'representative radius'. In the second phase, we create several Geographical Information System (GIS) layers of information such as exclusion zones, line-of-sight (LOS) blockage, and lidar range maps. These GIS layers are then used in the third phase to find optimum positions of the WindScanners with respect to the measurement positions considering the WindScanner measurement uncertainty. In the fourth phase, we optimize and generate trajectory through the measurement positions by applying the traveling salesman problem (TSP) on these positions. The above-described workflow has been digitized into the so-called Campaign Planning Tool (CPT) currently provided as a Python library which allows users an effective way to plan measurement campaigns with WindScanner systems. In this study, the CPT has been tested on three different sites characterized by different terrain complexity and wind farm dimensions and layouts. The CPT has shown instantly whether the whole site can be covered by one system or not.
