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

[Prateek Gupta]

Hello Tazkera

As much I understood, you want to store values of Gx, Gy, Gz in array named
varlist. But your syntax
varlist = ['Gx', 'Gy', 'Gz']
will not store values of Gx, Gy, Gz. It stores Gx, Gy, Gz as strings in
varlist.
if you want to store values you have to write syntax as
varlist = [Gx, Gy, Gz]

But this will also not work for you. Because if you want to take the slice
plot then you have to define varlist as field I think.
I will try to solve it. And we will wait for Nathan reply also.


Regards
Prateek

On Sat, Jun 25, 2016 at 7:24 AM, tazkera haque <h.tazkera at gmail.com> wrote:

> Hi Prateek and Nathan,
>
> thanks for your suggestion, I did accordingly, and seems the field is not
> add up properly. I have attached the error.
>
> At this point I would ask you to kindly take a look at my code too, i am
> new in python and I want to make sure I am not doing other things wrong
> along with it.
>
> I am trying to make slice plots of gravitational acceleration through
> z-axis from the FLASH plotfiles  and make a folder with it.
> thanks in advance.
>
> Turhan
>
> On Fri, Jun 24, 2016 at 9:20 AM, Nathan Goldbaum <nathan12343 at gmail.com>
> wrote:
>
>>
>>
>> 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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