63,297 research outputs found
Oblique Long Waves on Beach and Induced Longshore Current
This study considers the 3D runup of long waves on a uniform beach of constant or variable downward slope that is connected to an open ocean of uniform depth. An inviscid linear long-wave theory is applied to obtain the fundamental solution for a uniform train of sinusoidal waves obliquely incident upon a uniform beach of variable downward slope without wave breaking. For waves at nearly grazing incidence, runup is significant only for the waves in a set of eigenmodes being trapped within the beach at resonance with the exterior ocean waves. Fourier synthesis is employed to analyze a solitary wave and a train of cnoidal waves obliquely incident upon a sloping beach, with the nonlinear and dispersive effects neglected at this stage. Comparison is made between the present theory and the ray theory to ascertain a criterion of validity. The wave-induced longshore current is evaluated by finding the Stokes drift of the fluid particles carried by the momentum of the waves obliquely incident upon a sloping beach. Currents of significant velocities are produced by waves at incidence angles about 45 [degrees] and by grazing waves trapped on the beach. Also explored are the effects of the variable downward slope and curvature of a uniform beach on 3D runup and reflection of long waves
On obliquely magnetized and differentially rotating stars
We investigate the interaction of differential rotation and a misaligned
magnetic field. The incompressible magnetohydrodynamic equations are solved
numerically for a free-decay problem. In the kinematic limit, differential
rotation annihilates the non-axisymmetric field on a timescale proportional to
the cube root of magnetic Reynolds number (), as predicted by R\"adler.
Nonlinearly, the outcome depends upon the initial energy in the
non-axisymmetric part of the field. Sufficiently weak fields approach
axisymmetry as in the kinematic limit; some differential rotation survives
across magnetic surfaces, at least on intermediate timescales. Stronger fields
enforce uniform rotation and remain non-axisymmetric. The initial field
strength that divides these two regimes does not follow the scaling
predicted by quasi-kinematic arguments, perhaps because our is never
sufficiently large or because of reconnection. We discuss the possible
relevance of these results to tidal synchronization and tidal heating of close
binary stars, particularly double white dwarfs
Radiative Transfer in Obliquely Illuminated Accretion Disks
The illumination of an accretion disk around a black hole or neutron star by
the central compact object or the disk itself often determines its spectrum,
stability, and dynamics. The transport of radiation within the disk is in
general a multi-dimensional, non-axisymmetric problem, which is challenging to
solve. Here, I present a method of decomposing the radiative transfer equation
that describes absorption, emission, and Compton scattering in an obliquely
illuminated disk into a set of four one-dimensional transfer equations. I show
that the exact calculation of the ionization balance and radiation heating of
the accretion disk requires the solution of only one of the one-dimensional
equations, which can be solved using existing numerical methods. I present a
variant of the Feautrier method for solving the full set of equations, which
accounts for the fact that the scattering kernels in the individual transfer
equations are not forward-backward symmetric. I then apply this method in
calculating the albedo of a cold, geometrically thin accretion disk.Comment: 16 pages, 3 figures; to appear in The Astrophysical Journa
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