12,468 research outputs found
Adjoint recovery of superconvergent functionals from PDE approximations
Motivated by applications in computational fluid dynamics, a method is presented for obtaining estimates of integral functionals, such as lift or drag, that have twice the order of accuracy of the computed flow solution on which they are based. This is achieved through error analysis that uses an adjoint PDE to relate the local errors in approximating the flow solution to the corresponding global errors in the functional of interest. Numerical evaluation of the local residual error together with an approximate solution to the adjoint equations may thus be combined to produce a correction for the computed functional value that yields the desired improvement in accuracy. Numerical results are presented for the Poisson equation in one and two dimensions and for the nonlinear quasi-one-dimensional Euler equations. The theory is equally applicable to nonlinear equations in complex multi-dimensional domains and holds great promise for use in a range of engineering disciplines in which a few integral quantities are a key output of numerical approximations
Analysis of Adjoint Error Correction for Superconvergent Functional Estimates
Earlier work introduced the notion of adjoint error correction for obtaining superconvergent estimates of functional outputs from approximate PDE solutions. This idea is based on a posteriori error analysis suggesting that the leading order error term in the functional estimate can be removed by using an adjoint PDE solution to reveal the sensitivity of the functional to the residual error in the original PDE solution. The present work provides a priori error analysis that correctly predicts the behaviour of the remaining leading order error term. Furthermore, the discussion is extended from the case of homogeneous boundary conditions and bulk functionals, to encompass the possibilities of inhomogeneous boundary conditions and boundary functionals. Numerical illustrations are provided for both linear and nonlinear problems.\ud
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This research was supported by EPSRC under grant GR/K91149, and by NASA/Ames Cooperative Agreement No. NCC 2-5431
An introduction to the adjoint approach to design
Optimal design methods involving the solution of an adjoint system of equations are an active area of research in computational fluid dynamics, particularly for aeronautical applications. This paper presents an introduction to the subject, emphasising the simplicity of the ideas when viewed in the context of linear algebra. Detailed discussions also include the extension to p.d.e.'s, the construction of the adjoint p.d.e. and its boundary conditions, and the physical significance of the adjoint solution. The paper concludes with examples of the use of adjoint methods for optimising the design of business jets.\ud
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This research was supported by funding from Rolls-Royce plc, BAe Systems plc and EPSRC grants GR/K91149 and GR/L95700
Analytic Adjoint Solutions for the Quasi-1D Euler Equations
The analytic properties of adjoint solutions are examined for the quasi-1D Euler equations. For shocked flow, the derivation of the adjoint problem reveals that the adjoint variables are continuous with zero gradient at the shock, and that an internal adjoint boundary condition is required at the shock. A Green's function approach is used to derive the analytic adjoint solutions corresponding to supersonic, subsonic, isentropic and shocked transonic flows in a converging-diverging duct of arbitrary shape. This analysis reveals a logarithmic singularity at the sonic throat and confirms the expected properties at the shock.\ud
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This research was supported by EPSRC under grant GR/K9114
The optical counterpart of SAX J1808.4-3658, the transient bursting millisecond X-ray pulsar
A set of CCD images have been obtained during the decline of the X-ray
transient SAX J1808.4-3658 during April-June 1998. The optical counterpart has
been confirmed by several pieces of evidence. The optical flux shows a
modulation on several nights which is consistent with the established X-ray
binary orbit period of 2 hours. This optical variability is roughly in
antiphase with the weak X-ray modulation. The source mean magnitude of V=16.7
on April 18 declined rapidly after April 22. From May 2 onwards the magnitude
was more constant at around V=18.45 but by June 27 was below our sensitivity
limit. The optical decline precedes the rapid second phase of the X-ray
decrease by 3 +/- 1 days. The source has been identified on a 1974 UK Schmidt
plate at an estimated magnitude of ~20. The nature of the optical companion is
discussed.Comment: 5 pages, 3 figures; published in MNRAS, March 15th 199
Observations of transients and pulsars with LOFAR international stations
The LOw FRequency ARray - LOFAR is a new radio telescope that is moving the
science of radio pulsars and transients into a new phase. Its design places
emphasis on digital hardware and flexible software instead of mechanical
solutions. LOFAR observes at radio frequencies between 10 and 240 MHz where
radio pulsars and many transients are expected to be brightest. Radio frequency
signals emitted from these objects allow us to study the intrinsic pulsar
emission and phenomena such as propagation effects through the interstellar
medium. The design of LOFAR allows independent use of its stations to conduct
observations of known bright objects, or wide field monitoring of transient
events. One such combined software/hardware solution is called the Advanced
Radio Transient Event Monitor and Identification System (ARTEMIS). It is a
backend for both targeted observations and real-time searches for millisecond
radio transients which uses Graphical Processing Unit (GPU) technology to
remove interstellar dispersion and detect millisecond radio bursts from
astronomical sources in real-time using a single LOFAR station.Comment: To appear in the proceedings of the Electromagnetic Radiation from
Pulsars and Magnetars conference, Zielona Gora, 2012. 4 pages, 1 figur
Spectral variation in the X-ray pulsar GX 1+4 during a low-flux episode
The X-ray pulsar GX 1+4 was observed with the RXTE satellite for a total of
51ks between 1996 July 19 - 21. During this period the flux decreased smoothly
from an initial mean level of ~ 6 X 10^36 erg/s to a minimum of ~ 4 X 10^35
erg/s (2-60 keV, assuming a source distance of 10 kpc) before partially
recovering towards the initial level at the end of the observation.
BATSE pulse timing measurements indicate that a torque reversal took place
approximately 10 d after this observation. Both the mean pulse profile and the
photon spectrum varied significantly. The observed variation in the source may
provide important clues as to the mechanism of torque reversals.
The single best-fitting spectral model was based on a component originating
from thermal photons with kT ~ 1 keV Comptonised by a plasma of temperature kT
\~ 7 keV. Both the flux modulation with phase during the brightest interval and
the evolution of the mean spectra over the course of the observation are
consistent with variations in this model component; with, in addition, a
doubling of the column density nH contributing to the mean spectral change.
A strong flare of duration 50 s was observed during the interval of minimum
flux, with the peak flux ~ 20 times the mean level. Although beaming effects
are likely to mask the true variation in Mdot thought to give rise to the
flare, the timing of a modest increase in flux prior to the flare is consistent
with dual episodes of accretion resulting from successive orbits of a locally
dense patch of matter in the accretion disc.Comment: 8 pages, 3 figures, submitted to MNRA
Algorithm Developments for Discrete Adjoint Methods
This paper presents a number of algorithm developments for adjoint methods using the 'discrete' approach in which the discretisation of the non-linear equations is linearised and the resulting matrix is then transposed. With a new iterative procedure for solving the adjoint equations, exact numerical equivalence is maintained between the linear and adjoint discretisations. The incorporation of strong boundary conditions within the discrete approach is discussed, as well as a new application of adjoint methods to linear unsteady flow in turbomachinery
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