The Experts below are selected from a list of 2034 Experts worldwide ranked by ideXlab platform
Rajai Nasser - One of the best experts on this subject based on the ideXlab platform.
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on the convergence of the polarization process in the Noisiness weak topology
International Symposium on Information Theory, 2019Co-Authors: Rajai NasserAbstract:Let W be a channel where the input alphabet is endowed with an Abelian group operation, and let (Wn) n≥0 be Arikan’s channel-valued polarization process that is obtained from W using this operation. We prove that the process (Wn) n≥0 converges almost surely to deterministic homomorphism channels in the Noisiness/weak-∗ topology. This provides a simple proof of multilevel polarization for a large family of channels, containing among others, discrete memoryless channels (DMC), and channels with continuous output alphabets. This also shows that any continuous channel functional converges almost surely (even if it does not induce a submartingale or a supermartingale).
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on the convergence of the polarization process in the Noisiness weak ast topology
arXiv: Information Theory, 2018Co-Authors: Rajai NasserAbstract:Let $W$ be a channel where the input alphabet is endowed with an Abelian group operation, and let $(W_n)_{n\geq 0}$ be Ar{\i}kan's channel-valued polarization process that is obtained from $W$ using this operation. We prove that the process $(W_n)_{n\geq 0}$ converges almost surely to deterministic homomorphism channels in the Noisiness/weak-$\ast$ topology. This provides a simple proof of multilevel polarization for a large family of channels, containing among others, discrete memoryless channels (DMC), and channels with continuous output alphabets. This also shows that any continuous channel functional converges almost surely (even if the functional does not induce a submartingale or a supermartingale).
Carlos S Frenk - One of the best experts on this subject based on the ideXlab platform.
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momentum transfer across shear flows in smoothed particle hydrodynamic simulations of galaxy formation
Monthly Notices of the Royal Astronomical Society, 2003Co-Authors: Takashi Okamoto, Adrian Jenkins, V R Eke, Vicent Quilis, Carlos S FrenkAbstract:We investigate the evolution of angular momentum in smoothed particle hydrodynamic (SPH) simulations of galaxy formation, paying particular attention to artificial numerical effects. We find that a cold gas disc forming in an ambient hot gas halo receives a strong hydrodynamic torque from the hot gas. By splitting the hydrodynamic force into artificial viscosity and pressure gradients, we find that the angular momentum transport is caused not by the artificial viscosity but by the pressure gradients. Using simple test simulations of shear flows, we conclude that the pressure gradient-based viscosity can be divided into two components: one due to the Noisiness of SPH and the other due to ram pressure. The former is problematic even with very high resolution, because increasing the resolution does not reduce the Noisiness. On the other hand, the ram pressure effect appears only when a cold gas disc or sheet does not contain enough particles. In such a case, holes form in the disc or sheet, and then ram pressure from intra-hole hot gas causes significant deceleration. In simulations of galactic disc formation, star formation usually decreases the number of cold gas particles, and hole formation leads to the fragmentation of the disc. This fragmentation not only induces further angular momentum transport, but also affects star formation in the disc. To circumvent these problems, we modify the SPH algorithm, decoupling the cold gas phases from the hot ones, i.e. inhibiting the hydrodynamic interaction between cold and hot particles. This, a crude modelling of a multiphase fluid in SPH cosmological simulations, leads to the formation of smooth extended cold gas discs and to better numerical convergence. The decoupling is applicable in so far as the self-gravitating gas disc with negligible external pressure is a good approximation for a cold gas disc.
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momentum transfer across shear flows in smoothed particle hydrodynamic simulations of galaxy formation
arXiv: Astrophysics, 2003Co-Authors: Takashi Okamoto, Adrian Jenkins, V R Eke, Vicent Quilis, Carlos S FrenkAbstract:We investigate the evolution of angular momentum in SPH simulations of galaxy formation, paying particular attention to artificial numerical effects. We find that a cold gas disc forming in an ambient hot gas halo receives a strong hydrodynamic torque from the hot gas. By splitting the hydrodynamic force into artificial viscosity and pressure gradients, we find that the angular momentum transport is caused not by the artificial viscosity but by the pressure gradients. Using simple test simulations of shear flows, we conclude that the pressure gradient-based viscosity can be divided into two components: one due to the Noisiness of SPH and the other to ram pressure. The former is problematic even with very high resolution because increasing resolution does not reduce the Noisiness. On the other hand, the ram pressure effect appears only when a cold gas disc or sheet does not contain enough particles. In such a case, holes form in the disc or sheet, and then ram pressure from intra-hole hot gas, causes significant deceleration. In simulations of galactic disc formation, star formation usually decreases the number of cold gas particles, and hole formation leads to the fragmentation of the disc. To circumvent these problem, we modify the SPH algorithm, decoupling the cold from the hot gas phases, i.e. inhibiting the hydrodynamic interaction between cold and hot particles. This, a crude modelling of a multi-phase fluid in SPH cosmological simulations, leads to the formation of smooth extended cold gas discs and to better numerical convergence. The decoupling is applicable in so far as the self-gravitating gas disc with negligible external pressure is a good approximation for a cold gas disc. (abridged)
Takashi Okamoto - One of the best experts on this subject based on the ideXlab platform.
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momentum transfer across shear flows in smoothed particle hydrodynamic simulations of galaxy formation
Monthly Notices of the Royal Astronomical Society, 2003Co-Authors: Takashi Okamoto, Adrian Jenkins, V R Eke, Vicent Quilis, Carlos S FrenkAbstract:We investigate the evolution of angular momentum in smoothed particle hydrodynamic (SPH) simulations of galaxy formation, paying particular attention to artificial numerical effects. We find that a cold gas disc forming in an ambient hot gas halo receives a strong hydrodynamic torque from the hot gas. By splitting the hydrodynamic force into artificial viscosity and pressure gradients, we find that the angular momentum transport is caused not by the artificial viscosity but by the pressure gradients. Using simple test simulations of shear flows, we conclude that the pressure gradient-based viscosity can be divided into two components: one due to the Noisiness of SPH and the other due to ram pressure. The former is problematic even with very high resolution, because increasing the resolution does not reduce the Noisiness. On the other hand, the ram pressure effect appears only when a cold gas disc or sheet does not contain enough particles. In such a case, holes form in the disc or sheet, and then ram pressure from intra-hole hot gas causes significant deceleration. In simulations of galactic disc formation, star formation usually decreases the number of cold gas particles, and hole formation leads to the fragmentation of the disc. This fragmentation not only induces further angular momentum transport, but also affects star formation in the disc. To circumvent these problems, we modify the SPH algorithm, decoupling the cold gas phases from the hot ones, i.e. inhibiting the hydrodynamic interaction between cold and hot particles. This, a crude modelling of a multiphase fluid in SPH cosmological simulations, leads to the formation of smooth extended cold gas discs and to better numerical convergence. The decoupling is applicable in so far as the self-gravitating gas disc with negligible external pressure is a good approximation for a cold gas disc.
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momentum transfer across shear flows in smoothed particle hydrodynamic simulations of galaxy formation
arXiv: Astrophysics, 2003Co-Authors: Takashi Okamoto, Adrian Jenkins, V R Eke, Vicent Quilis, Carlos S FrenkAbstract:We investigate the evolution of angular momentum in SPH simulations of galaxy formation, paying particular attention to artificial numerical effects. We find that a cold gas disc forming in an ambient hot gas halo receives a strong hydrodynamic torque from the hot gas. By splitting the hydrodynamic force into artificial viscosity and pressure gradients, we find that the angular momentum transport is caused not by the artificial viscosity but by the pressure gradients. Using simple test simulations of shear flows, we conclude that the pressure gradient-based viscosity can be divided into two components: one due to the Noisiness of SPH and the other to ram pressure. The former is problematic even with very high resolution because increasing resolution does not reduce the Noisiness. On the other hand, the ram pressure effect appears only when a cold gas disc or sheet does not contain enough particles. In such a case, holes form in the disc or sheet, and then ram pressure from intra-hole hot gas, causes significant deceleration. In simulations of galactic disc formation, star formation usually decreases the number of cold gas particles, and hole formation leads to the fragmentation of the disc. To circumvent these problem, we modify the SPH algorithm, decoupling the cold from the hot gas phases, i.e. inhibiting the hydrodynamic interaction between cold and hot particles. This, a crude modelling of a multi-phase fluid in SPH cosmological simulations, leads to the formation of smooth extended cold gas discs and to better numerical convergence. The decoupling is applicable in so far as the self-gravitating gas disc with negligible external pressure is a good approximation for a cold gas disc. (abridged)
Radu Prodan - One of the best experts on this subject based on the ideXlab platform.
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simplified workflow simulation on clouds based on computation and communication Noisiness
IEEE Transactions on Parallel and Distributed Systems, 2020Co-Authors: Roland Matha, Sasko Ristov, Thomas Fahringer, Radu ProdanAbstract:Many researchers rely on simulations to analyze and validate their researched methods on Cloud infrastructures. However, determining relevant simulation parameters and correctly instantiating them to match the real Cloud performance is a difficult and costly operation, as minor configuration changes can easily generate an unreliable inaccurate simulation result. Using legacy values experimentally determined by other researchers can reduce the configuration costs, but is still inaccurate as the underlying public Clouds and the number of active tenants are highly different and dynamic in time. To overcome these deficiencies, we propose a novel model that simulates the dynamic Cloud performance by introducing noise in the computation and communication tasks, determined by a small set of runtime execution data. Although the estimating method is apparently costly, a comprehensive sensitivity analysis shows that the configuration parameters determined for a certain simulation setup can be used for other simulations too, thereby reducing the tuning cost by up to 82.46 percent, while declining the simulation accuracy by only 1.98 percent on average. Extensive evaluation also shows that our novel model outperforms other state-of-the-art dynamic Cloud simulation models, leading up to 22 percent lower makespan inaccuracy.
Vicent Quilis - One of the best experts on this subject based on the ideXlab platform.
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momentum transfer across shear flows in smoothed particle hydrodynamic simulations of galaxy formation
Monthly Notices of the Royal Astronomical Society, 2003Co-Authors: Takashi Okamoto, Adrian Jenkins, V R Eke, Vicent Quilis, Carlos S FrenkAbstract:We investigate the evolution of angular momentum in smoothed particle hydrodynamic (SPH) simulations of galaxy formation, paying particular attention to artificial numerical effects. We find that a cold gas disc forming in an ambient hot gas halo receives a strong hydrodynamic torque from the hot gas. By splitting the hydrodynamic force into artificial viscosity and pressure gradients, we find that the angular momentum transport is caused not by the artificial viscosity but by the pressure gradients. Using simple test simulations of shear flows, we conclude that the pressure gradient-based viscosity can be divided into two components: one due to the Noisiness of SPH and the other due to ram pressure. The former is problematic even with very high resolution, because increasing the resolution does not reduce the Noisiness. On the other hand, the ram pressure effect appears only when a cold gas disc or sheet does not contain enough particles. In such a case, holes form in the disc or sheet, and then ram pressure from intra-hole hot gas causes significant deceleration. In simulations of galactic disc formation, star formation usually decreases the number of cold gas particles, and hole formation leads to the fragmentation of the disc. This fragmentation not only induces further angular momentum transport, but also affects star formation in the disc. To circumvent these problems, we modify the SPH algorithm, decoupling the cold gas phases from the hot ones, i.e. inhibiting the hydrodynamic interaction between cold and hot particles. This, a crude modelling of a multiphase fluid in SPH cosmological simulations, leads to the formation of smooth extended cold gas discs and to better numerical convergence. The decoupling is applicable in so far as the self-gravitating gas disc with negligible external pressure is a good approximation for a cold gas disc.
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momentum transfer across shear flows in smoothed particle hydrodynamic simulations of galaxy formation
arXiv: Astrophysics, 2003Co-Authors: Takashi Okamoto, Adrian Jenkins, V R Eke, Vicent Quilis, Carlos S FrenkAbstract:We investigate the evolution of angular momentum in SPH simulations of galaxy formation, paying particular attention to artificial numerical effects. We find that a cold gas disc forming in an ambient hot gas halo receives a strong hydrodynamic torque from the hot gas. By splitting the hydrodynamic force into artificial viscosity and pressure gradients, we find that the angular momentum transport is caused not by the artificial viscosity but by the pressure gradients. Using simple test simulations of shear flows, we conclude that the pressure gradient-based viscosity can be divided into two components: one due to the Noisiness of SPH and the other to ram pressure. The former is problematic even with very high resolution because increasing resolution does not reduce the Noisiness. On the other hand, the ram pressure effect appears only when a cold gas disc or sheet does not contain enough particles. In such a case, holes form in the disc or sheet, and then ram pressure from intra-hole hot gas, causes significant deceleration. In simulations of galactic disc formation, star formation usually decreases the number of cold gas particles, and hole formation leads to the fragmentation of the disc. To circumvent these problem, we modify the SPH algorithm, decoupling the cold from the hot gas phases, i.e. inhibiting the hydrodynamic interaction between cold and hot particles. This, a crude modelling of a multi-phase fluid in SPH cosmological simulations, leads to the formation of smooth extended cold gas discs and to better numerical convergence. The decoupling is applicable in so far as the self-gravitating gas disc with negligible external pressure is a good approximation for a cold gas disc. (abridged)
