797 research outputs found
A steady separated viscous corner flow
An example is presented of a separated flow in an unbounded domain in which, as the Reynolds number becomes large, the separated region remains of size 0(1) and tends to a non-trivial Prandtl-Batchelor flow. The multigrid method is used to obtain rapid convergence to the solution of the discretized Navier-Stokes equations at Reynolds numbers of up to 5000. Extremely fine grids and tests of an integral property of the flow ensure accuracy. The flow exhibits the separation of a boundary layer with ensuing formation of a downstream eddy and reattachment of a free shear layer. The asymptotic (’triple deck’) theory of laminar separation from a leading edge, due to Sychev (1979), is clarified and compared to the numerical solutions. Much better qualitative agreement is obtained than has been reported previously. Together with a plausible choice of two free parameters, the data can be extrapolated to infinite Reynolds number, giving quantitative agreement with triple-deck theory with errors of 20% or less. The development of a region of constant vorticity is observed in the downstream eddy, and the global infinite-Reynolds-number limit is a Prandtl-Batchelor flow; however, when the plate is stationary, the occurrence of secondary separation suggests that the limiting flow contains an infinite sequence of eddies behind the separation point. Secondary separation can be averted by driving the plate, and in this case the limit is a single-vortex Prandtl-Batchelor flow of the type found by Moore, Saffman & Tanveer (1988); detailed, encouraging comparisons are made to the vortex-sheet strength and position. Altering the boundary condition on the plate gives viscous eddies that approximate different members of the family of inviscid solutions
Spatial discretization of partial differential equations with integrals
We consider the problem of constructing spatial finite difference
approximations on a fixed, arbitrary grid, which have analogues of any number
of integrals of the partial differential equation and of some of its
symmetries. A basis for the space of of such difference operators is
constructed; most cases of interest involve a single such basis element. (The
``Arakawa'' Jacobian is such an element.) We show how the topology of the grid
affects the complexity of the operators.Comment: 24 pages, LaTeX sourc
M\"obius Invariants of Shapes and Images
Identifying when different images are of the same object despite changes
caused by imaging technologies, or processes such as growth, has many
applications in fields such as computer vision and biological image analysis.
One approach to this problem is to identify the group of possible
transformations of the object and to find invariants to the action of that
group, meaning that the object has the same values of the invariants despite
the action of the group. In this paper we study the invariants of planar shapes
and images under the M\"obius group , which arises
in the conformal camera model of vision and may also correspond to neurological
aspects of vision, such as grouping of lines and circles. We survey properties
of invariants that are important in applications, and the known M\"obius
invariants, and then develop an algorithm by which shapes can be recognised
that is M\"obius- and reparametrization-invariant, numerically stable, and
robust to noise. We demonstrate the efficacy of this new invariant approach on
sets of curves, and then develop a M\"obius-invariant signature of grey-scale
images
Hamiltonian boundary value problems, conformal symplectic symmetries, and conjugate loci
In this paper we continue our study of bifurcations of solutions of
boundary-value problems for symplectic maps arising as Hamiltonian
diffeomorphisms. These have been shown to be connected to catastrophe theory
via generating functions and ordinary and reversal phase space symmetries have
been considered. Here we present a convenient, coordinate free framework to
analyse separated Lagrangian boundary value problems which include classical
Dirichlet, Neumann and Robin boundary value problems. The framework is then
used to {prove the existence of obstructions arising from} conformal symplectic
symmetries on the bifurcation behaviour of solutions to Hamiltonian boundary
value problems. Under non-degeneracy conditions, a group action by conformal
symplectic symmetries has the effect that the flow map cannot degenerate in a
direction which is tangential to the action. This imposes restrictions on which
singularities can occur in boundary value problems. Our results generalise
classical results about conjugate loci on Riemannian manifolds to a large class
of Hamiltonian boundary value problems with, for example, scaling symmetries
A note on the motion of surfaces
We study the motion of surfaces in an intrinsic formulation in which the
surface is described by its metric and curvature tensors. The evolution
equations for the six quantities contained in these tensors are reduced in
number in two cases: (i) for arbitrary surfaces, we use principal coordinates
to obtain two equations for the two principal curvatures, highlighting the
similarity with the equations of motion of a plane curve; and (ii) for surfaces
with spatially constant negative curvature, we use parameterization by
Tchebyshev nets to reduce to a single evolution equation. We also obtain
necessary and sufficient conditions for a surface to maintain spatially
constant negative curvature as it moves. One choice for the surface's normal
motion leads to the modified-Korteweg de Vries equation,the appearance of which
is explained by connections to the AKNS hierarchy and the motion of space
curves.Comment: 10 pages, compile with AMSTEX. Two figures available from the author
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