Re: An undergraduate student's question after reading the manual

From: Leonard Romano <>
Date: Tue, 1 Jun 2021 22:06:55 +0200

Hi Zhengyang,

Let me try to answer your questions:

 1. The number of particles of each type is essentially given by your
    initial condition file. If you don't have such a file at hand and
    want to create initial conditions for cosmological simulations you
    can use NGENIC to do so. NGENIC, by default also uses an initial
    condition file, however, so if you want to really create initial
    conditions from scratch you will need to compile with the
    option**CREATE_GRID. In that case the parameter "GridSize"
    determines the number of DM particles to be (GridSize)³. NGENIC also
    allows you to create gas particles by splitting up the DM particles
    giving you 1 DM and 1 gas particle for each DM particle, so 2
    x**(GridSize)³ particles in total. In order to enable this, compile
    with the option GENERATE_GAS_IN_ICS*.*
 2. In principle all you need to know about the particle types is, that
    type 0 (i.e. gas) is an SPH particle, i.e. besides gravity also
    participates in hydrodynamics, and all other particle types are
    collisionless, i.e. only participate in the gravitational
    interaction. Of course if you implement your own subgrid model you
    can give the other particle types special meaning, and add some
    extra physics for them. There are more than two types, so one can
    make use of mass tables more efficiently for the different types.
 3. As described above "Bndry" is simply a name given to one of the
    collisionless particle types. In zoom-in simulations this type is
    typically reserved for the DM particles of the low resolution region.
 4. This really depends on your implementation of star formation. Some
    models transform gas particles directly into star particles, in this
    case the total number of particles and also the particles mass is
    unchanged. Other models spawn stars from gas particles without
    actually disposing of the gas. In that case the spawned star
    particles split up some fraction of the total mass of the gas among
    themselves. In that case both the number of particles and the mass
    of the particles changes.

I hope I could answer your questions!

Best regards,

On 01.06.21 18:59, 张正阳 wrote:
> Hi,all,
>   I am now an undergraduate student,and I have read the manual
> carefully,but there are still a few important questions that I don't
> understand.
>   1.What is the number of each particle in the simulation.For
> example,we choose Nsample=128,So how many gas particles are there, and
> how many other particles are there.
>   2.I now understand that the gas particle is the kind of SPH
> particle, but the other particles, such as disk, are it a
> collision-free gravitational particle or SPH plus collision-free
> particle.
>   3.What kind of particle is "Bndry", and does it have any practical
> effect.
>   4.In the simulation, will gas particles be transformed into stars
> particles, and if so, will the number or mass of particles be changed.
>   I would be most grateful if you could answer my questions.
> Best wishes,
> Zhengyang Zhang
Leonard Romano, B.Sc.(レオナルド・ロマノ)
Physics Department
Technical University of Munich (TUM), Germany
Theoretical Astrophysics Group
Department of Earth and Space Science
Graduate School of Science, Osaka University, Japan
he / him / his
Received on 2021-06-01 22:07:01

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