arXiv subjects

P. Miocchi

Publications and source records attributed to P. Miocchi.

42 records · Page 3Linked to original sources

arXiv2001

An efficient parallel tree-code for the simulation of self-gravitating systems

We describe a parallel version of our tree-code for the simulation of self-gravitating systems in Astrophysics. It is based on a dynamic and adaptive method for the domain decomposition, which exploits the hierarchical data arrangement used by the tree-code. It shows low computational costs for the parallelization overhead -- less than 4% of the total CPU-time in the tests done -- because the domain decomposition is performed 'on the fly' during the tree setting and the portion of the tree that is local to each processor 'enriches' itself of remote data only when they are actually needed. The performances of an implementation of the parallel code on a Cray T3E are presented and discussed. They exhibit a very good behaviour of the speedup (=15 with 16 processors and 10^5 particles) and a rather low load unbalancing (< 10% using up to 16 processors), achieving a high computation speed in the forces evaluation (>10^4 particles/sec with 8 processors).

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arXiv1998

Parallelization of a Code for the Simulation of Self-gravitating Systems in Astrophysics. Preliminary Speed-up Results

We have preliminary results on the parallelization of a Tree-Code for evaluating gravitational forces in N-body astrophysical systems. For our Cray T3D/CRAFT implementation, we have obtained an encouraging speed-up behavior, which reaches a value of 37 with 64 processor elements (PEs). According to the Amdahl'law, this means that about 99% of the code is actually parallelized. The speed-up tests regarded the evaluation of the forces among N = 130,369 particles distributed scaling the actual distribution of a sample of galaxies seen in the Northern sky hemisphere. Parallelization of the time integration of the trajectories, which has not yet been taken into account, is both easier to implement and not as fundamental.

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arXiv1997

Gravitational Waves Generated by Globular Cluster Systems Collapse

According to the hypothesis that AGNs are massive black holes and the mass which feeds them is made of stars belonging to globular clusters frictionally decayed and tidally destroyed by the field of the central object, it is possible to give an estimate of the number and spectral characteristics of detectable gravitational wave impulses generated by the falling of such stars into the black hole. We find that the orbitating gravitational antenna LISA, should detect from 10 to 100 impulses per year.

astro-ph↗

arXiv1997

Galactic Nuclei Activity Sustained by Globular Cluster Mass Accretion

The decay of globular clusters to the center of their mother galaxy corresponds to carrying a quantity of mass sufficient to sustain the gravitational activity of a small pre-existing nucleus and to accrete it in a significant way. This is due to both dynamical friction of field stars and tidal disruption by the compact nucleus. The results of the simplified model presented here show that the active galactic nuclei luminosity and lifetime depend on the characteristics of the globular cluster system and are quite insensitive to the nucleus' initial mass.

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arXiv1997

A comparison between Fast Multipole Algorithm and Tree-Code to evaluate gravitational forces in 3-D

We present tests of comparison between our versions of the Fast Multipole Algorithm (FMA) and ``classic'' tree-code to evaluate gravitational forces in particle systems. We have optimized the Greengard's original version of FMA allowing for a more efficient criterion of well-separation between boxes, to improve the adaptivity of the method (which is very important in highly inhomogeneous situations) and to permit the smoothing of gravitational interactions. The results of our tests indicate that the tree-code is almost three times faster than FMA for both a homogeneous and a clumped distribution, at least in the interval of N (N< 10^5) here investigated and at the same level of accuracy (error ~ 10^{-3)). This order of accuracy is generally considered as the best compromise between CPU-time consumption and precision for astrophysical simulation. Moreover, the claimed linear dependence on N of the CPU-time of FMA is not confirmed and we give a ``theoretical'' explanation for that.

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arXiv1996

Force Evaluation in Particle Methods for Self-Gravitating Multi-Phase Systems

A modern approach to the evolution of a mixed (stars and gas) self-gravitating system is the fully Lagrangian particle approach. The gaseous (particle) phase differs from the compact because the mutual force is given by the sum of gravity and pressure gradient. In this note we report of some characteristics, advantages and limitations of this approach for what regards the evaluation of forces in the system. In particular, a comparison between classic tree-code and fast multipole algorithm to evaluate gravitational forces is discussed.

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