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Alex E. Hay - One of the best experts on this subject based on the ideXlab platform.
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The Vertical Structure of the Wave Bottom Boundary Layer over a Sloping Bed: Theory and Field Measurements
Journal of Physical Oceanography, 2003Co-Authors: Qingping Zou, Alex E. HayAbstract:Abstract Theoretical solutions for the wave bottom boundary layer (WBL) over a sloping bed are compared with field measurements in the nearshore zone. The WBL theory is constructed using both viscoelastic–diffusion and conventional eddy viscosity turbulent closure models. The Velocity solutions are then matched with those of the interior flow, given by Chu and Mei potential theory for surface gravity waves over a sloping bottom. The field measurements were obtained with a coherent Doppler profiler over a 2° bed slope. Results are presented for both flat and rippled bed conditions, the latter being characterized by low steepness, linear transition ripples. Close to the bed, the Observed Velocity profiles change rapidly in amplitude and phase relative to potential flow theory, indicating the presence of a wave boundary layer with a thickness of 3–6 cm. The Observed Velocity and shear stress profiles are in good agreement with the theory. The sloping bottom has significant effects on the vertical Velocity, b...
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Velocity Profiles Above and Within the Wave Bottom Boundary Layer Over a Sloping Bottom
Coastal Engineering 2000, 2001Co-Authors: Qingping Zou, Alex E. HayAbstract:Theoretical solutions for the wave bottom boundary layer (WBL) are obtained using a viscoelastic turbulent closure model including sloping bed effects. The overlying free stream Velocity is given by nonlinear Stokes wave theory for a sloping bottom. The viscoelastic closure scheme extends conventional eddy viscosity models by incorporating the effects of eddy relaxation and diffusion on vertical momentum exchange in the WBL, and gives improved predictions of Observed Velocity profiles. Field measurements of nearbed Velocity profiles are then compared with theoretical solutions for a 2° bed slope. Field measurements were obtained in the lower 50 cm of the water column in 3.7 m depth with a coherent Doppler profiler. Results are presented from a storm event for flat bed. Within the region 8 cm above the bed, the Observed Velocity profiles change rapidly in amplitude and phase relative to potential flow theory, indicating the presence of the wave boundary layer. Furthermore, the Observed vertical Velocity profile in this region deviates significantly from the WBL theory for a horizontal flat bottom, but compares well with the sloping bottom theory presented here.
Qingping Zou - One of the best experts on this subject based on the ideXlab platform.
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The Vertical Structure of the Wave Bottom Boundary Layer over a Sloping Bed: Theory and Field Measurements
Journal of Physical Oceanography, 2003Co-Authors: Qingping Zou, Alex E. HayAbstract:Abstract Theoretical solutions for the wave bottom boundary layer (WBL) over a sloping bed are compared with field measurements in the nearshore zone. The WBL theory is constructed using both viscoelastic–diffusion and conventional eddy viscosity turbulent closure models. The Velocity solutions are then matched with those of the interior flow, given by Chu and Mei potential theory for surface gravity waves over a sloping bottom. The field measurements were obtained with a coherent Doppler profiler over a 2° bed slope. Results are presented for both flat and rippled bed conditions, the latter being characterized by low steepness, linear transition ripples. Close to the bed, the Observed Velocity profiles change rapidly in amplitude and phase relative to potential flow theory, indicating the presence of a wave boundary layer with a thickness of 3–6 cm. The Observed Velocity and shear stress profiles are in good agreement with the theory. The sloping bottom has significant effects on the vertical Velocity, b...
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Velocity Profiles Above and Within the Wave Bottom Boundary Layer Over a Sloping Bottom
Coastal Engineering 2000, 2001Co-Authors: Qingping Zou, Alex E. HayAbstract:Theoretical solutions for the wave bottom boundary layer (WBL) are obtained using a viscoelastic turbulent closure model including sloping bed effects. The overlying free stream Velocity is given by nonlinear Stokes wave theory for a sloping bottom. The viscoelastic closure scheme extends conventional eddy viscosity models by incorporating the effects of eddy relaxation and diffusion on vertical momentum exchange in the WBL, and gives improved predictions of Observed Velocity profiles. Field measurements of nearbed Velocity profiles are then compared with theoretical solutions for a 2° bed slope. Field measurements were obtained in the lower 50 cm of the water column in 3.7 m depth with a coherent Doppler profiler. Results are presented from a storm event for flat bed. Within the region 8 cm above the bed, the Observed Velocity profiles change rapidly in amplitude and phase relative to potential flow theory, indicating the presence of the wave boundary layer. Furthermore, the Observed vertical Velocity profile in this region deviates significantly from the WBL theory for a horizontal flat bottom, but compares well with the sloping bottom theory presented here.
James Wadsley - One of the best experts on this subject based on the ideXlab platform.
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How to Reconcile the Observed Velocity Function of Galaxies with Theory
The Astrophysical Journal, 2017Co-Authors: Alyson Brooks, Emmanouil Papastergis, Charlotte Christensen, Fabio Governato, Adrienne M. Stilp, Thomas R. Quinn, James WadsleyAbstract:Within a Lambda Cold Dark Matter (LCDM) scenario, we use high resolution cosmological simulations spanning over four orders of magnitude in galaxy mass to understand the deficit of dwarf galaxies in Observed Velocity functions. We measure velocities in as similar a way as possible to observations, including generating mock HI data cubes for our simulated galaxies. We demonstrate that this apples-to-apples comparison yields an "Observed" Velocity function in agreement with observations, reconciling the large number of low-mass halos expected in a LCDM cosmological model with the low number of Observed dwarfs at a given Velocity. We then explore the source of the discrepancy between observations and theory, and conclude that the dearth of Observed dwarf galaxies is primarily explained by two effects. The first effect is that galactic rotational velocities derived from the HI linewidth severely underestimate the maximum halo Velocity. The second effect is that a large fraction of halos at the lowest masses are too faint to be detected by current galaxy surveys. We find that cored dark matter density profiles can contribute to the lower Observed Velocity of galaxies, but only for galaxies in which the Velocity is measured interior to the size of the core (~3 kpc).
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how to reconcile the Observed Velocity function of galaxies with theory
The Astrophysical Journal, 2017Co-Authors: Alyson Brooks, Emmanouil Papastergis, Charlotte Christensen, Fabio Governato, Adrienne M. Stilp, Thomas R. Quinn, James WadsleyAbstract:Within a Λ cold dark matter (ΛCDM) scenario, we use high-resolution cosmological simulations spanning over four orders of magnitude in galaxy mass to understand the deficit of dwarf galaxies in Observed Velocity functions (VFs). We measure velocities in as similar a way as possible to observations, including generating mock H I data cubes for our simulated galaxies. We demonstrate that this apples-to-apples comparison yields an “Observed” VF in agreement with observations, reconciling the large number of low-mass halos expected in a ΛCDM cosmological model with the low number of Observed dwarfs at a given Velocity. We then explore the source of the discrepancy between observations and theory and conclude that the dearth of Observed dwarf galaxies is primarily explained by two effects. The first effect is that galactic rotational velocities derived from the H I linewidth severely underestimate the maximum halo Velocity. The second effect is that a large fraction of halos at the lowest masses are too faint to be detected by current galaxy surveys. We find that cored DM density profiles can contribute to the lower Observed Velocity of galaxies but only for galaxies in which the Velocity is measured interior to the size of the core (˜3 kpc).
Andrei P. Igoshev - One of the best experts on this subject based on the ideXlab platform.
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The Observed Velocity distribution of young pulsars II: analysis of complete PSRπ
Monthly Notices of the Royal Astronomical Society, 2020Co-Authors: Andrei P. IgoshevAbstract:Understanding the natal kicks, or birth velocities, of neutron stars are essential for understanding the evolution of massive binaries as well as double neutron star formation. We use maximum likelihood methods as published in Verbunt et al. to analyse a new large dataset of parallaxes and proper motions measured by Deller et al. This sample is roughly three times larger than number of measurements available before. For both the complete sample and its younger part (spin-down ages $\tau < 3$ Myr), we find that a bimodal Maxwellian distribution describes the measured parallaxes and proper motions better than a single Maxwellian with probability of 99.3 and 95.0 per cent respectively. The bimodal Maxwellian distribution has three parameters: fraction of low-Velocity pulsars and distribution parameters $\sigma_1$ and $\sigma_2$ for low and high-Velocity modes. For a complete sample, these parameters are as follows: $42_{-15}^{+17}$ per cent, $\sigma_1=128_{-18}^{+22}$ km s$^{-1}$ and $\sigma_2 = 298\pm 28$ km s$^{-1}$. For younger pulsars, which are assumed to represent the natal kick, these parameters are as follows: $20_{-10}^{+11}$ per cent, $\sigma_1=56_{-15}^{+25}$ km s$^{-1}$ and $\sigma_2=336\pm 45$ km s$^{-1}$. In the young population, $5\pm 3$ per cent of pulsars has velocities less than 60 km s$^{-1}$. We perform multiple Monte Carlo tests for the method taking into account realistic observational selection. We find that the method reliably estimates all parameters of the natal kick distribution. Results of the Velocity analysis are weakly sensitive to the exact values of scale-lengths of the Galactic pulsar distribution.
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The Observed Velocity distribution of young pulsars
Astronomy & Astrophysics, 2017Co-Authors: Frank Verbunt, Andrei P. Igoshev, Eric CatorAbstract:We argue that comparison with observations of theoretical models for the Velocity distribution of pulsars must be done directly with the Observed quantities, i.e. parallax and the two components of proper motion. We develop a formalism to do so, and apply it to pulsars with accurate VLBI measurements. We find that a distribution with two maxwellians improves significantly on a single maxwellian. The `mixed' model takes into account that pulsars move away from their place of birth, a narrow region around the galactic plane. The best model has 42% of the pulsars in a maxwellian with average Velocity sigma sqrt{8/pi}=120 km/s, and 58% in a maxwellian with average Velocity 540 km/s. About 5% of the pulsars has a Velocity at birth less than 60\,km/s. For the youngest pulsars (tau_c
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the Observed Velocity distribution of young pulsars
arXiv: High Energy Astrophysical Phenomena, 2017Co-Authors: Frank Verbunt, Andrei P. Igoshev, Eric CatorAbstract:We argue that comparison with observations of theoretical models for the Velocity distribution of pulsars must be done directly with the Observed quantities, i.e. parallax and the two components of proper motion. We develop a formalism to do so, and apply it to pulsars with accurate VLBI measurements. We find that a distribution with two maxwellians improves significantly on a single maxwellian. The `mixed' model takes into account that pulsars move away from their place of birth, a narrow region around the galactic plane. The best model has 42% of the pulsars in a maxwellian with average Velocity sigma sqrt{8/pi}=120 km/s, and 58% in a maxwellian with average Velocity 540 km/s. About 5% of the pulsars has a Velocity at birth less than 60\,km/s. For the youngest pulsars (tau_c<10 Myr), these numbers are 32% with 130 km/s, 68% with 520 km/s, and 3%, with appreciable uncertainties.
Alyson Brooks - One of the best experts on this subject based on the ideXlab platform.
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How to Reconcile the Observed Velocity Function of Galaxies with Theory
The Astrophysical Journal, 2017Co-Authors: Alyson Brooks, Emmanouil Papastergis, Charlotte Christensen, Fabio Governato, Adrienne M. Stilp, Thomas R. Quinn, James WadsleyAbstract:Within a Lambda Cold Dark Matter (LCDM) scenario, we use high resolution cosmological simulations spanning over four orders of magnitude in galaxy mass to understand the deficit of dwarf galaxies in Observed Velocity functions. We measure velocities in as similar a way as possible to observations, including generating mock HI data cubes for our simulated galaxies. We demonstrate that this apples-to-apples comparison yields an "Observed" Velocity function in agreement with observations, reconciling the large number of low-mass halos expected in a LCDM cosmological model with the low number of Observed dwarfs at a given Velocity. We then explore the source of the discrepancy between observations and theory, and conclude that the dearth of Observed dwarf galaxies is primarily explained by two effects. The first effect is that galactic rotational velocities derived from the HI linewidth severely underestimate the maximum halo Velocity. The second effect is that a large fraction of halos at the lowest masses are too faint to be detected by current galaxy surveys. We find that cored dark matter density profiles can contribute to the lower Observed Velocity of galaxies, but only for galaxies in which the Velocity is measured interior to the size of the core (~3 kpc).
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how to reconcile the Observed Velocity function of galaxies with theory
The Astrophysical Journal, 2017Co-Authors: Alyson Brooks, Emmanouil Papastergis, Charlotte Christensen, Fabio Governato, Adrienne M. Stilp, Thomas R. Quinn, James WadsleyAbstract:Within a Λ cold dark matter (ΛCDM) scenario, we use high-resolution cosmological simulations spanning over four orders of magnitude in galaxy mass to understand the deficit of dwarf galaxies in Observed Velocity functions (VFs). We measure velocities in as similar a way as possible to observations, including generating mock H I data cubes for our simulated galaxies. We demonstrate that this apples-to-apples comparison yields an “Observed” VF in agreement with observations, reconciling the large number of low-mass halos expected in a ΛCDM cosmological model with the low number of Observed dwarfs at a given Velocity. We then explore the source of the discrepancy between observations and theory and conclude that the dearth of Observed dwarf galaxies is primarily explained by two effects. The first effect is that galactic rotational velocities derived from the H I linewidth severely underestimate the maximum halo Velocity. The second effect is that a large fraction of halos at the lowest masses are too faint to be detected by current galaxy surveys. We find that cored DM density profiles can contribute to the lower Observed Velocity of galaxies but only for galaxies in which the Velocity is measured interior to the size of the core (˜3 kpc).
