V. Berta, A. Mignone, M. Bugli, G. Mattia

“A 4th-order accurate finite volume method for ideal classical and special relativistic MHD based on pointwise reconstructions”,

Journal of Computational Physics 499 (2024) 112701

We present a novel implementation of a genuinely 4th-order accurate finite volume scheme for multidimensional classical and special relativistic magnetohydrodynamics (MHD) based on the constrained transport (CT) formalism.
The scheme introduces several novel aspects when compared to its predecessors yielding a more efficient computational tool. Among the most relevant ones, our scheme exploits pointwise to pointwise reconstructions (rather than onedimensional finite volume ones), employs the generic upwind constrained transport averaging and sophisticated limiting strategies that include both a discontinuity detector and an order reduction procedure. Selected numerical benchmarks demonstrate the accuracy and robustness of the method.

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Bernhard Baumschlager, Sijing Shen, and James W. Wadsley

“Spectral reconstruction for radiation hydrodynamic simulations of galaxy evolution”,

Astronomy & Astrophysics manuscript no. 10BandRT - October 27, 2023

Radiation from stars and active galactic nuclei (AGN) plays an important role in galaxy formation and evolution, and profoundly transforms the intergalactic, circumgalactic and interstellar medium (IGM, CGM & ISM). On-the-fly radiative transfer (RT) has started being incorporated in cosmological simulations, but the complex, evolving radiation spectra are often crudely approximated with a small number of broad bands with piece-wise constant intensity and a fixed photo-ionisation cross-section.
Such a treatment is unable to capture the changes to the spectrum as light is absorbed while it propagates through a medium with non-zero opacity. This can lead to large errors in photo-ionisation and heating rates. In this work, we present a novel approach of discretising the radiation field in narrow bands, located at the edges of the typically used bands, in order to capture the power-law slope of the radiation field.
In combination with power-law approximations for the photo-ionisation cross-sections, this model allows us to self-consistently combine radiation from sources with different spectra and accurately follow the ionisation states of primordial and metal species through time. The method is implemented in Gasoline2 in connection with Trevr2, a fast reverse RT algorithm with O(Nactive log2 N) scaling. We compare our new piece-wise power-law reconstruction to the piece-wise constant method in calculating the primordial chemistry photo-ionisation and heating rates under an evolving UV-background (UVB) and stellar spectrum, and find that our method reduces errors significantly, up to two orders of magnitude in the case of HeII ionisation.
We apply our new spectral reconstruction method in RT post-processing of a cosmological zoom-in simulation, MUGS2 g1536 (Keller et al. 2016), including radiation from stars and a live UVB, and find a significant increase in total neutral hydrogen (HI) mass in the ISM and the CGM due to shielding of the UVB and a low escape fraction of the stellar radiation. This demonstrates the importance of RT and an accurate spectral approximation in simulating the CGM-galaxy ecosystem.

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M. Zanotti, A. Mignone, and M. Marazakis

“SPACE: Space Parallel Astrophysical Codes for Exascale”,

ERCIM News No. 134, July 2023, pp. 43-44

In astrophysics and cosmology (A&C), high-performance computing (HPC)-based numerical simulations are invaluable instruments to support scientific discovery.
The efficient and effective exploitation of exascale (and beyond) computing capabilities will be key to paving the way for scientific discovery in this scientific domain, but requires a coordinated effort towards adapting A&C codes for space applications on exascale HPC systems.

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