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

From: 张正阳 <>
Date: Wed, 2 Jun 2021 13:42:50 +0800 (GMT+08:00)

Hi Leonard Romano,

  Thank you for your reply. I think I understand better now.Thank you again for taking the time to answer my questions.

Best wishes,

Zhengyang Zhang

发件人:"Leonard Romano" <>
发送时间:2021-06-02 04:06:55 (星期三)
收件人: gadget-list_at_MPA-Garching.MPG.DE
主题: Re: [gadget-list] An undergraduate student's question after reading the manual

Hi Zhengyang,

Let me try to answer your questions:

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 optionCREATE_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.
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.

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.
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:


  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 2021-06-02 07:43:08

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