On Mon, Jan 29, 2018 at 12:39 PM, Vadlamani Samhitha <vadlamani.samhitha@gmail.com> wrote:

Hi yt-users,

I am a newbie in using YT. I read through the documentation of field types but I still have some confusion in understanding when to use different field types, as in:

(1) 'io' vs 'all' (from what I understand, both are used for particles but I don't get the minute difference)

This distinction only matters if there are multiple types of particles in a single simulation. For example, Gadget data has 5 different particle types on disk, 'halo', 'disk', 'stars', 'bulge', and 'gas'. If I load a Gadget dataset with yt I can access the particles in each of these particle types separately by prefixing the particle field I want to access with the particle type I want to access.

"all" is a particle union (see http://yt-project.org/docs/dev/analyzing/filtering.html#particle-unions) that corresponds to the union between all on-disk particle types. If you access an "all" field, you will get back the field values for the union of all of the particles in the simulation. Only fields that are defined for all particle types are defined for the "all" particle union.

Since ramses only has one particle type, we call that type "io". For RAMSES (and Enzo and several other formats where all the particles are mixed together on disk) "io" and "all" are interchangeable.

(2) 'ramses' vs 'index' vs 'gas' (fluids described on the grid, I suppose?)

The "ramses" field type corresponds to the on-disk fields in the ramses outputs. Each of these fields has a corresponding "gas" field that it is aliased to. For example, ('gas', 'density') is an alias to ("ramses", "Density").

When you access a "gas" field, by default it will be returned in CGS units. When you access a "ramses" field, by default it will be returned in code units.

The 'index' fields are fields that depend on the AMR structure or the geometry of the data but not the on-disk fields. For example, ('index', 'x'), ('index', 'grid_level'), etc.

Hope that helps,

Nathan

It would be really helpful if someone could provide an elaborate explanation on these.

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