20,068 research outputs found
Numerical simulation of free shear flows: Towards a predictive computational aeroacoustics capability
Implicit and explicit spatial differencing techniques with fourth order accuracy have been developed. The implicit technique is based on the Pade compact scheme. A Dispersion Relation Preserving concept has been incorporated into both of the numerical schemes. Two dimensional Euler computation of a spatially-developing free shear flow, with and without external excitation, has been performed to demonstrate the capability of numerical schemes developed. Results are in good agreement with theory and experimental observation regarding the growth rate of fluctuating velocity, the convective velocity, and the vortex-pairing process
A fourier pseudospectral method for some computational aeroacoustics problems
A Fourier pseudospectral time-domain method is applied to wave propagation problems pertinent to computational aeroacoustics. The original algorithm of the Fourier pseudospectral time-domain method works for periodical problems without the interaction with physical boundaries. In this paper we develop a slip wall boundary condition, combined with buffer zone technique to solve some non-periodical problems. For a linear sound propagation problem whose governing equations could be transferred to ordinary differential equations in pseudospectral space, a new algorithm only requiring time stepping is developed and tested. For other wave propagation problems, the original algorithm has to be employed, and the developed slip wall boundary condition still works. The accuracy of the presented numerical algorithm is validated by benchmark problems, and the efficiency is assessed by comparing with high-order finite difference methods. It is indicated that the Fourier pseudospectral time-domain method, time stepping method, slip wall and absorbing boundary conditions combine together to form a fully-fledged computational algorithm
Hybrid Spectral Difference/Embedded Finite Volume Method for Conservation Laws
A novel hybrid spectral difference/embedded finite volume method is
introduced in order to apply a discontinuous high-order method for large scale
engineering applications involving discontinuities in the flows with complex
geometries. In the proposed hybrid approach, the finite volume (FV) element,
consisting of structured FV subcells, is embedded in the base hexahedral
element containing discontinuity, and an FV based high-order shock-capturing
scheme is employed to overcome the Gibbs phenomena. Thus, a discontinuity is
captured at the resolution of FV subcells within an embedded FV element. In the
smooth flow region, the SD element is used in the base hexahedral element.
Then, the governing equations are solved by the SD method. The SD method is
chosen for its low numerical dissipation and computational efficiency
preserving high-order accurate solutions. The coupling between the SD element
and the FV element is achieved by the globally conserved mortar method. In this
paper, the 5th-order WENO scheme with the characteristic decomposition is
employed as the shock-capturing scheme in the embedded FV element, and the
5th-order SD method is used in the smooth flow field.
The order of accuracy study and various 1D and 2D test cases are carried out,
which involve the discontinuities and vortex flows. Overall, it is shown that
the proposed hybrid method results in comparable or better simulation results
compared with the standalone WENO scheme when the same number of solution DOF
is considered in both SD and FV elements.Comment: 27 pages, 17 figures, 2 tables, Accepted for publication in the
Journal of Computational Physics, April 201
Impact of Locally Suppressed Wave sources on helioseismic travel times
Wave travel-time shifts in the vicinity of sunspots are typically interpreted
as arising predominantly from magnetic fields, flows, and local changes in
sound speed. We show here that the suppression of granulation related wave
sources in a sunspot can also contribute significantly to these travel-time
shifts, and in some cases, an asymmetry between in and outgoing wave travel
times. The tight connection between the physical interpretation of travel times
and source-distribution homogeneity is confirmed. Statistically significant
travel-time shifts are recovered upon numerically simulating wave propagation
in the presence of a localized decrease in source strength. We also demonstrate
that these time shifts are relatively sensitive to the modal damping rates;
thus we are only able to place bounds on the magnitude of this effect. We see a
systematic reduction of 10-15 seconds in -mode mean travel times at short
distances ( Mm) that could be misinterpreted as arising from a
shallow (thickness of 1.5 Mm) increase ( 4%) in the sound speed. At
larger travel distances ( Mm) a 6-13 s difference between the ingoing
and outgoing wave travel times is observed; this could mistakenly be
interpreted as being caused by flows.Comment: Revised version. Submitted to Ap
A Direct Method for Photoacoustic Tomography with Inhomogeneous Sound Speed
The standard approach for photoacoustic imaging with variable speed of sound
is time reversal, which consists in solving a well-posed final-boundary value
problem for the wave equation backwards in time. This paper investigates the
iterative Landweber regularization algorithm, where convergence is guaranteed
by standard regularization theory, notably also in cases of trapping sound
speed or for short measurement times. We formulate and solve the direct and
inverse problem on the whole Euclidean space, what is common in standard
photoacoustic imaging, but not for time-reversal algorithms, where the problems
are considered on a domain enclosed by the measurement devices. We formulate
both the direct and adjoint photoacoustic operator as the solution of an
interior and an exterior differential equation which are coupled by
transmission conditions. The prior is solved numerically using a Galerkin
scheme in space and finite difference discretization in time, while the latter
consists in solving a boundary integral equation. We therefore use a BEM-FEM
approach for numerical solution of the forward operators. We analyze this
method, prove convergence, and provide numerical tests. Moreover, we compare
the approach to time reversal.Comment: Revised Preprin
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