302 research outputs found
Exact calculation of Fourier series in nonconforming spectral-element methods
In this note is presented a method, given nodal values on multidimensional
nonconforming spectral elements, for calculating global Fourier-series
coefficients. This method is ``exact'' in that given the approximation inherent
in the spectral-element method (SEM), no further approximation is introduced
that exceeds computer round-off error. The method is very useful when the SEM
has yielded an adaptive-mesh representation of a spatial function whose global
Fourier spectrum must be examined, e.g., in dynamically adaptive fluid-dynamics
simulations.Comment: 7 pages, 4 figures, submitted to J. Comp. Phys. 2005 June
Adaptive mesh refinement with spectral accuracy for magnetohydrodynamics in two space dimensions
We examine the effect of accuracy of high-order spectral element methods,
with or without adaptive mesh refinement (AMR), in the context of a classical
configuration of magnetic reconnection in two space dimensions, the so-called
Orszag-Tang vortex made up of a magnetic X-point centered on a stagnation point
of the velocity. A recently developed spectral-element adaptive refinement
incompressible magnetohydrodynamic (MHD) code is applied to simulate this
problem. The MHD solver is explicit, and uses the Elsasser formulation on
high-order elements. It automatically takes advantage of the adaptive grid
mechanics that have been described elsewhere in the fluid context [Rosenberg,
Fournier, Fischer, Pouquet, J. Comp. Phys. 215, 59-80 (2006)]; the code allows
both statically refined and dynamically refined grids. Tests of the algorithm
using analytic solutions are described, and comparisons of the Orszag-Tang
solutions with pseudo-spectral computations are performed. We demonstrate for
moderate Reynolds numbers that the algorithms using both static and refined
grids reproduce the pseudo--spectral solutions quite well. We show that
low-order truncation--even with a comparable number of global degrees of
freedom--fails to correctly model some strong (sup--norm) quantities in this
problem, even though it satisfies adequately the weak (integrated) balance
diagnostics.Comment: 19 pages, 10 figures, 1 table. Submitted to New Journal of Physic
Theoretical and numerical studies of chaotic mixing
Theoretical and numerical studies of chaotic mixing are performed to circumvent the difficulties
of efficient mixing, which come from the lack of turbulence in microfluidic devices. In order to
carry out efficient and accurate parametric studies and to identify a fully chaotic state, a spectral
element algorithm for solution of the incompressible Navier-Stokes and species transport
equations is developed. Using Taylor series expansions in time marching, the new algorithm
employs an algebraic factorization scheme on multi-dimensional staggered spectral element
grids, and extends classical conforming Galerkin formulations to nonconforming spectral
elements. Lagrangian particle tracking methods are utilized to study particle dispersion in the
mixing device using spectral element and fourth order Runge-Kutta discretizations in space and
time, respectively. Comparative studies of five different techniques commonly employed to
identify the chaotic strength and mixing efficiency in microfluidic systems are presented to
demonstrate the competitive advantages and shortcomings of each method. These are the stirring
index based on the box counting method, Poincare sections, finite time Lyapunov exponents, the
probability density function of the stretching field, and mixing index inverse, based on the
standard deviation of scalar species distribution. Series of numerical simulations are performed
by varying the Peclet number (Pe) at fixed kinematic conditions. The mixing length (lm) is characterized as function of the Pe number, and lm ∝ ln(Pe) scaling is demonstrated for fully
chaotic cases. Employing the aforementioned techniques, optimum kinematic conditions and the
actuation frequency of the stirrer that result in the highest mixing/stirring efficiency are
identified in a zeta potential patterned straight micro channel, where a continuous flow is
generated by superposition of a steady pressure driven flow and time periodic electroosmotic
flow induced by a stream-wise AC electric field. Finally, it is shown that the invariant manifold
of hyperbolic periodic point determines the geometry of fast mixing zones in oscillatory flows in
two-dimensional cavity
A comparison of spectral element and finite difference methods using statically refined nonconforming grids for the MHD island coalescence instability problem
A recently developed spectral-element adaptive refinement incompressible
magnetohydrodynamic (MHD) code [Rosenberg, Fournier, Fischer, Pouquet, J. Comp.
Phys. 215, 59-80 (2006)] is applied to simulate the problem of MHD island
coalescence instability (MICI) in two dimensions. MICI is a fundamental MHD
process that can produce sharp current layers and subsequent reconnection and
heating in a high-Lundquist number plasma such as the solar corona [Ng and
Bhattacharjee, Phys. Plasmas, 5, 4028 (1998)]. Due to the formation of thin
current layers, it is highly desirable to use adaptively or statically refined
grids to resolve them, and to maintain accuracy at the same time. The output of
the spectral-element static adaptive refinement simulations are compared with
simulations using a finite difference method on the same refinement grids, and
both methods are compared to pseudo-spectral simulations with uniform grids as
baselines. It is shown that with the statically refined grids roughly scaling
linearly with effective resolution, spectral element runs can maintain accuracy
significantly higher than that of the finite difference runs, in some cases
achieving close to full spectral accuracy.Comment: 19 pages, 17 figures, submitted to Astrophys. J. Supp
Spectral methods for CFD
One of the objectives of these notes is to provide a basic introduction to spectral methods with a particular emphasis on applications to computational fluid dynamics. Another objective is to summarize some of the most important developments in spectral methods in the last two years. The fundamentals of spectral methods for simple problems will be covered in depth, and the essential elements of several fluid dynamical applications will be sketched
Spectral collocation methods
This review covers the theory and application of spectral collocation methods. Section 1 describes the fundamentals, and summarizes results pertaining to spectral approximations of functions. Some stability and convergence results are presented for simple elliptic, parabolic, and hyperbolic equations. Applications of these methods to fluid dynamics problems are discussed in Section 2
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