[yt-users] creating gradient of a field as a derived field

[Nathan Goldbaum]

On Fri, Jun 24, 2016 at 6:40 AM, Prateek Gupta <prateekgidolia at gmail.com>
wrote:

> Hello Turhan,
>
> I am not sure but I think this will work, you can try this...
>
>
> *ds = yt.load("data_file_Name")*
>
> *grad = ds.add_gradient_fields(("gas","gravitational_potential"))*
>
> *grad *list will have a list of new field names which representing the 3
> different components of the field and the magnitude of the gradient , e.g.,
> "*gravitational_potential_gradient_x*" , "
> *gravitational_potential_gradient_y*" , "
> *gravitational_potential_gradient_z*" and "
> *gravitational_potential_gradient_magnitude*"
>
> Now for getting the negative gradient of gravitational potential we can
> derive field as follows:
>
>
> *####...x-component of negative of gradient of gravitational potential...*
> *def _GradientX(field, data)*
> *     Gx = -1.0*data["gravitational_potential_gradient_x"]*
> *     return Gx*
>
> *yt.add_field("Gx", function=_GradientX, take_log=False, units="code
> length/ code time**2")*
>
> *####...y-component of negative of gradient of gravitational potential...*
> *def _GradientY(field, data)*
> *     Gy = -1.0*data["gravitational_potential_gradient_y"]*
> *     return Gx*
> *yt.add_field("Gy", function=_GradientY, take_log=False, units="**code
> length/ code time**2**")*
>
> *####...z-component of negative of gradient of gravitational potential...*
> *def _GradientZ(field, data)*
> *     Gz = -1.0*data["gravitational_potential_gradient_z"]*
> *     return Gz*
> *yt.add_field("Gz", function=_GradientZ, take_log=False, units="**code
> length/ code time**2**")*
>
>
>
> Otherwise you can do this also,
>
> *grad = -1.0*ds.add_gradient_fields(("gas","gravitational_potential"))*
>

This isn't quite right. The ds.add_gradient_fields function returns a set
of field names, not a field object itself. If you want to add the negative
signs to the field definition, you'll need to define some derived fields as
Prateek suggested above.


>
> I think this will work but this will also make the magnitude negative. But
> if you want to use only 3-components then this works and you can access it
> , e.g., *data["gravitational_potential_gradient_x"]* and similar for y
> and z component.
>
>
>
> Please let me know if this work.
>
>
>
> Regards
> Prateek Gupta
>
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>
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