13,927 research outputs found
Implementation of boundary conditions for optimized high-order compact schemes
Full text linkThe optimized high-order compact (OHOC) finite difference schemes, proposed as central schemes are used for aeroacoustic computations on interior nodes. On near-boundary nodes, accurate non-central or one-sided compact schemes are formulated and developed in this paper for general computations in domains with non-periodic boundaries. The near-boundary non-central compact schemes are optimized in the wavenumber domain by using Fourier error analysis. Analytic optimization methods are devised to minimize the dispersion and dissipation errors, and to obtain maximum resolution characteristics of the near-boundary compact schemes. With the accurate near-boundary schemes, the feasibility of implementing physical boundary conditions for the OHOC schemes are investigated to provide high-quality wave solutions. Characteristics-based boundary conditions and the free-field impedance conditions are used as the physical boundary conditions for direct computations of linear and nonlinear wave propagation and radiation. It is shown that the OHOC schemes present accurate wave solutions by using the optimized near-boundary compact schemes and the physical boundary condition
Wavelength tunable 10-GHz 3-ps pulse source using a dispersion decreasing fiber-based nonlinear optical loop mirror
Full text linkWe experimentally demonstrate the use of a dispersion decreasing fiber (DDF)-based nonlinear optical loop mirror (NOLM) for the generation of wavelength tunable soliton-like pulses at a repetition rate of 10 GHz. We compress ~12 ps Gaussian pulses from an electro-absorption modulator (EAM) (followed by 125 m of DCF for preliminary linear dispersion compensation) into 3 ps pedestal-free pulses using both high-order soliton compression and nonlinear switching effects within an 8.5 km DDF-based loop mirror. The output pulses from the DDF-based NOLM show considerable pedestal reduction compared to those obtained by directly compressing the EAM seed pulses via a single passage through the DDF. Wavelength tuning of the compressed pulses over a ~15 nm bandwidth (from 1541 to 1556 nm) is demonstrated without a significant increase in pulse duration or degradation in pulse quality
Generalized characteristic boundary conditions for computational aeroacoustics
No full textAn extended conservative formalism of the characteristic boundary conditions is presented on the basis of the generalized coordinates for practical computational aeroacoustics. The formalism is derived for solving the entire conservative form of the compressible Euler or Navier–Stokes equations on the body-fitted grid mesh system by using the high-order and high-resolution numerical schemes. It includes the matrices of transformation between the conservative and the characteristic variables, which were already derived in the literature to analyze the eigenvalue–eigenvector modes in an arbitrary direction. The conservation-form governing equations with their full terms are solved at the boundaries, and no kind of extrapolation or simplification of the equations is included in this formalism. Additional correction terms are devised to preserve the conservative form of flux derivative terms in the generalized coordinates. Especially, the soft inflow conditions are presented to keep the nonreflecting features, as well as to maintain the mean value of inflow velocity at the inlet boundary. These boundary conditions are applied to the actual computation of two-dimensional viscous cylinder inflows with Reynolds number of 400 on the grid meshes clustered on the cylinder surface and the downstream region. The Strouhal number due to von Karman vortex streets, root-mean-square lift coefficient, andmean drag coefficient are evaluated correctly in comparison with experimental data. The far-field sound pressure levels are measured directly in this computation, and the accuracy is validated by an analytic formula derived in the literature
Characteristic interface conditions for multi-block high-order computation on singular structured grid
No full textA structured grid with a body usually has a certain point where an abrupt change in the slope of grid line exists. The grid metrics are discontinuous at the point because of the discrepancy between the left- and the right-hand limits of the gradients, which leads to grid singularity. It may cause serious numerical oscillations especially when high-order finite difference schemes are applied to solving conservation-form governing equations in generalized coordinates. In this paper, it is handled by decomposing a computational domain into blocks along the singular lines and imposing interface conditions at the block interfaces for communication between the blocks. A set of high-order finite difference schemes is used in each block: central differences on the interior nodes and one-sided differences on the near-interface nodes. The differencing stencils do not cross the block interfaces and each block is isolated without the singularity,
which results in no oscillations. For the communication between the isolated blocks, the interface conditions are newly derived from the characteristic relations of the compressible Euler or Navier-Stokes equations. The exactness and the feasibility of the interface conditions are investigated for the high-order multi-block computation on structured grid containing singular points
Adaptive nonlinear artificial dissipation model for computational aeroacoustics
No full textAn adaptive nonlinear artificial dissipation model is presented for performing aeroacoustic computations by high-order and high-resolution numerical schemes based on central finite differences. It consists of a selective background smoothing term and a well-established nonlinear shock-capturing term, which damps out spurious oscillations caused by the central differences in the presence of a shock wave and keeps the linear acoustic waves relatively unaffected. A conservative form of the selective background smoothing term is presented to calculate accurate propagation speed or location of the shock wave. The nonlinear shock-capturing term, which has been modeled by second-order derivative term, is combined with it to improve the resolution of discontinuity and enhance the numerical stability near the shock wave. An adaptive control constant for overall amplitude of the dissipation is automatically calculated according to given grid metrics and time-dependent flow conditions. It is shown that the improved artificial dissipation model reproduces the correct profile and speed of the shock wave, suppresses numerical oscillations near the discontinuity, and avoids unnecessary damping on the smooth linear acoustic waves. The feasibility and performance of the adaptive nonlinear artificial dissipation model for the computational aeroacoustics are investigated and validated by the applications to actual problems
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