The Experts below are selected from a list of 360 Experts worldwide ranked by ideXlab platform
Robin Stanley Johnson - One of the best experts on this subject based on the ideXlab platform.
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an exact steady purely Azimuthal flow as a model for the antarctic circumpolar current
Journal of Physical Oceanography, 2016Co-Authors: Adrian Constantin, Robin Stanley JohnsonAbstract:AbstractThe problem of flow moving purely in the Azimuthal Direction on a sphere is considered. An exact solution for an incompressible (constant density), inviscid fluid, which admits a velocity profile below the surface and along the surface, is constructed; this can be regarded as a model for the Antarctic Circumpolar Current (ACC). The new approach adopted here is to model the processes that produce the observed structure of the ACC by the introduction of a nonconservative body force. It is shown that if the body force is conservative, then the governing equations necessarily lead to profiles that are quite unrealistic. However, with a suitable choice of body force, which reverts to conservative outside the ACC, any velocity profile of any width can be constructed as an exact solution of the system. A fairly simple choice is made in this note in order to present some specific results: a profile on the surface that is zero outside the arc of the ACC, with a maximum at its center and decaying with depth...
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an exact steady purely Azimuthal equatorial flow with a free surface
Journal of Physical Oceanography, 2016Co-Authors: Adrian Constantin, Robin Stanley JohnsonAbstract:AbstractThe general problem of an ocean on a rotating sphere is considered. The governing equations for an inviscid, incompressible fluid, written in spherical coordinates that are fixed at a point on the rotating Earth, together with the free surface and rigid bottom boundary conditions, are introduced. An exact solution of this system is presented; this describes a steady flow that is moving only in the Azimuthal Direction, with no variation in this Direction. However, this Azimuthal velocity component has an arbitrary variation with depth (i.e., radius), and so, for example, an Equatorial Undercurrent (EUC) can be accommodated. The pressure boundary condition at the free surface relates this pressure to the shape of the surface via a Bernoulli relation; this provides the constraint on the existence of a solution, although the restrictions are somewhat involved in spherical coordinates. To examine this constraint in more detail, the corresponding problems in model cylindrical coordinates (with the equat...
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an exact steady purely Azimuthal equatorial flow with a free surface
Journal of Physical Oceanography, 2016Co-Authors: Adrian Constantin, Robin Stanley JohnsonAbstract:AbstractThe general problem of an ocean on a rotating sphere is considered. The governing equations for an inviscid, incompressible fluid, written in spherical coordinates that are fixed at a point on the rotating Earth, together with the free surface and rigid bottom boundary conditions, are introduced. An exact solution of this system is presented; this describes a steady flow that is moving only in the Azimuthal Direction, with no variation in this Direction. However, this Azimuthal velocity component has an arbitrary variation with depth (i.e., radius), and so, for example, an Equatorial Undercurrent (EUC) can be accommodated. The pressure boundary condition at the free surface relates this pressure to the shape of the surface via a Bernoulli relation; this provides the constraint on the existence of a solution, although the restrictions are somewhat involved in spherical coordinates. To examine this constraint in more detail, the corresponding problems in model cylindrical coordinates (with the equat...
Adrian Constantin - One of the best experts on this subject based on the ideXlab platform.
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a nonlinear three dimensional model for ocean flows motivated by some observations of the pacific equatorial undercurrent and thermocline
Physics of Fluids, 2017Co-Authors: Adrian Constantin, R S JohnsonAbstract:Using the salient properties of the flow observed in the equatorial Pacific as a guide, an asymptotic procedure is applied to the Euler equation written in a suitable rotating frame. Starting from the single overarching assumption of slow variations in the Azimuthal Direction in a two-layer, steady flow that is symmetric about the equator, a tractable, fully nonlinear, and three-dimensional system of model equations is derived, with the Coriolis terms consistent with the β-plane approximation retained. It is shown that this asymptotic system of equations can be solved exactly. The ability of this dynamical model to capture simultaneously fundamental oceanic phenomena, which are closely inter-related (such as upwelling/downwelling, zonal depth-dependent currents with flow reversal, and poleward divergence along the equator), is a novel and compelling feature that has hitherto been elusive. While details are presented for the equatorial flow in the Pacific, the analysis demonstrates that other flow configur...
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an exact steady purely Azimuthal flow as a model for the antarctic circumpolar current
Journal of Physical Oceanography, 2016Co-Authors: Adrian Constantin, Robin Stanley JohnsonAbstract:AbstractThe problem of flow moving purely in the Azimuthal Direction on a sphere is considered. An exact solution for an incompressible (constant density), inviscid fluid, which admits a velocity profile below the surface and along the surface, is constructed; this can be regarded as a model for the Antarctic Circumpolar Current (ACC). The new approach adopted here is to model the processes that produce the observed structure of the ACC by the introduction of a nonconservative body force. It is shown that if the body force is conservative, then the governing equations necessarily lead to profiles that are quite unrealistic. However, with a suitable choice of body force, which reverts to conservative outside the ACC, any velocity profile of any width can be constructed as an exact solution of the system. A fairly simple choice is made in this note in order to present some specific results: a profile on the surface that is zero outside the arc of the ACC, with a maximum at its center and decaying with depth...
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an exact steady purely Azimuthal equatorial flow with a free surface
Journal of Physical Oceanography, 2016Co-Authors: Adrian Constantin, Robin Stanley JohnsonAbstract:AbstractThe general problem of an ocean on a rotating sphere is considered. The governing equations for an inviscid, incompressible fluid, written in spherical coordinates that are fixed at a point on the rotating Earth, together with the free surface and rigid bottom boundary conditions, are introduced. An exact solution of this system is presented; this describes a steady flow that is moving only in the Azimuthal Direction, with no variation in this Direction. However, this Azimuthal velocity component has an arbitrary variation with depth (i.e., radius), and so, for example, an Equatorial Undercurrent (EUC) can be accommodated. The pressure boundary condition at the free surface relates this pressure to the shape of the surface via a Bernoulli relation; this provides the constraint on the existence of a solution, although the restrictions are somewhat involved in spherical coordinates. To examine this constraint in more detail, the corresponding problems in model cylindrical coordinates (with the equat...
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an exact steady purely Azimuthal equatorial flow with a free surface
Journal of Physical Oceanography, 2016Co-Authors: Adrian Constantin, Robin Stanley JohnsonAbstract:AbstractThe general problem of an ocean on a rotating sphere is considered. The governing equations for an inviscid, incompressible fluid, written in spherical coordinates that are fixed at a point on the rotating Earth, together with the free surface and rigid bottom boundary conditions, are introduced. An exact solution of this system is presented; this describes a steady flow that is moving only in the Azimuthal Direction, with no variation in this Direction. However, this Azimuthal velocity component has an arbitrary variation with depth (i.e., radius), and so, for example, an Equatorial Undercurrent (EUC) can be accommodated. The pressure boundary condition at the free surface relates this pressure to the shape of the surface via a Bernoulli relation; this provides the constraint on the existence of a solution, although the restrictions are somewhat involved in spherical coordinates. To examine this constraint in more detail, the corresponding problems in model cylindrical coordinates (with the equat...
Michio Yamada - One of the best experts on this subject based on the ideXlab platform.
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stability and bifurcation diagram of boussinesq thermal convection in a moderately rotating spherical shell allowing rotation of the inner sphere
Physics of Fluids, 2013Co-Authors: Keiji Kimura, Shinichi Takehiro, Michio YamadaAbstract:We investigate the stability and bifurcation of Boussinesq thermal convection in a moderately rotating spherical shell, with the inner sphere free to rotate as a solid body due to the viscous torque of the fluid. The ratio of the inner and outer radii of the spheres and the Prandtl number are fixed to 0.4 and 1, respectively. The Taylor number is varied from 522 to 5002 and the Rayleigh number from 1500 to 10 000. In this parameter range, the finite-amplitude traveling wave solutions, which have four-fold symmetry in the Azimuthal Direction, bifurcate supercritically at the critical points. The inner sphere rotates in the prograde Direction due to the viscous torque of the fluid when the rotation rate is small while it rotates in the retrograde Direction when the rotation rate is large. However, the stable region of these traveling wave solutions is quantitatively similar to that in the co-rotating system where the inner and outer spheres rotate with the same angular velocity. The structures of convective...
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stability and bifurcation diagram of boussinesq thermal convection in a moderately rotating spherical shell
Physics of Fluids, 2011Co-Authors: Keiji Kimura, Shinichi Takehiro, Michio YamadaAbstract:Stability and bifurcation of Boussinesq thermal convection in a moderately rotating spherical shell are investigated by obtaining finite-amplitude solutions with the Newton method instead of the numerical time integration. The ratio of the inner and outer radii of the shell and the Prandtl number are fixed to 0.4 and 1, respectively, while the Taylor number is varied from 522 to 5002 and the Rayleigh number is from about 1500 to 10 000. In this range of the Taylor number, the stable finite-amplitude solutions, which have four-fold symmetry in the longitudinal (Azimuthal) Direction, bifurcate supercritically at the critical points and become unstable when the Rayleigh number is increased up to about 1.2 to 2 times the critical values. When the Taylor number is larger than 3402, propagating Direction of the solutions changes from prograde to retrograde continuously as the Rayleigh number is increased. The associated transition of the convection structure is also continuous.
Trevor Lafleur - One of the best experts on this subject based on the ideXlab platform.
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characteristics and transport effects of the electron drift instability in hall effect thrusters
Plasma Sources Science and Technology, 2017Co-Authors: Trevor Lafleur, Scott D Baalrud, Pascal ChabertAbstract:The large electron drift (relative to the ions) in the Azimuthal Direction of Hall-effect thrusters is well known to excite a strong instability. In a recent paper (Lafleur et al 2016 Phys. Plasmas 23 053503) we demonstrated that this instability leads to an enhanced electron–ion friction force that increases the electron cross-field mobility to levels similar to those seen experimentally. Here we extend this work by considering in detail the onset criteria for the formation of this instability (both in xenon, and other propellants of interest), and identify a number of important characteristics that it displays within Hall-effect thrusters (HETs): including the appearance of an additional non-dimensionalized scaling parameter (the instability growth-to-convection ratio), which controls the instability evolution and amplitude. We also investigate the effect that the instability has on electron and ion heating in HETs, and show that it leads to an ion rotation in the Azimuthal Direction that is in agreement with that seen experimentally.
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theory for the anomalous electron transport in hall effect thrusters i insights from particle in cell simulations
Physics of Plasmas, 2016Co-Authors: Trevor Lafleur, Scott D Baalrud, Pascal ChabertAbstract:Using a 1D particle-in-cell simulation with perpendicular electric, E0, and magnetic, B0, fields, and modelling the Azimuthal Direction (i.e., the E0 × B0 Direction), we study the cross-field electron transport in Hall effect thrusters (HETs). For low plasma densities, the electron transport is found to be well described by classical electron-neutral collision theory, but at sufficiently high densities (representative of typical HETs), a strong instability is observed to significantly enhance the electron mobility, even in the absence of electron-neutral collisions. This instability is associated with correlated high-frequency (of the order of MHz) and short-wavelength (of the order of mm) fluctuations in both the electric field and the plasma density, which are shown to be the cause of the anomalous transport. Saturation of the instability is observed to occur due to a combination of ion-wave trapping in the E0 × B0 Direction, and convection in the E0 Direction.
Zhigang Yang - One of the best experts on this subject based on the ideXlab platform.
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the impact and freezing processes of a water droplet on different cold cylindrical surfaces
International Journal of Heat and Mass Transfer, 2017Co-Authors: Huanhuan Zhang, Zhigang YangAbstract:Abstract In the present study, we experimentally investigated the impact and freezing processes of a water droplet on different cold cylindrical surfaces. The results showed that, during the impact process of the water droplet, the maximum contact diameter along Azimuthal Direction was generally larger than that along axial Direction. Besides, during the water droplet recoiling process, the radius of the cylindrical surface was found to have an apparent effect on the spreading factor along Azimuthal Direction but a minor influence on that along axial Direction. During the freezing process of the water droplet, the change of the temperature as well as the radius of the cylindrical surface did not result in a significant variation of the ice bead shape.
