[yt-users] Problem with gradient of a derived field

[José Utreras]

Thanks Nathan! Now I don't see that division in the field.
Still, I think that it does not have the correct values. In this case, with
the correct definition of the field, the derivative should be positive for
x>0 and negative for x<0.

2016-05-18 20:43 GMT-03:00 Nathan Goldbaum <nathan12343 at gmail.com>:

> Hi Jose,
>
> On Wed, May 18, 2016 at 1:17 PM, José Utreras <jutreras at das.uchile.cl>
> wrote:
>
>> Hi everyone,
>> I'm having the following problem: I want to calculate the gradient field
>> of the angular velocity which I have defined as follows:
>>
>> >def _Omega(field,data):
>> >        if data.has_field_parameter("bulk_velocity"):
>> >                bv =
>> data.get_field_parameter("bulk_velocity").in_units("cm/s")
>> >        else:
>> >                bv = data.ds.arr(np.zeros(3), "cm/s")
>> >        xv = data["gas","velocity_x"] - bv[0]
>> >        yv = data["gas","velocity_y"] - bv[1]
>> >        center = data.get_field_parameter('center')
>> >        x_hat = data["x"] - center[0]
>> >        y_hat = data["y"] - center[1]
>> >        r = np.sqrt(x_hat*x_hat+y_hat*y_hat)
>> >        x_hat /= r**2
>> >        y_hat /= r**2
>> >
>> >        return np.sqrt((xv*y_hat-yv*x_hat)**2)
>>
>
> The issue is this line ^
>
> You're taking the absolute value of z component of the angular velocity,
> which introduces a discontinuity in the first derivative - that's the
> artifact you're seeing in your plots.  I think you really want to have:
>
> > return (xv*y_hat-yv*x_hat)
>
> Here's what Omega and the gradient look like with that definition:
>
> http://i.imgur.com/7ElXqw6.png
> http://i.imgur.com/fjoLXX3.png
>
>
>> >yt.add_field("Omega", function=_Omega,take_log=False,
>> units=r"1/yr",validators=[ValidateParameter('bulk_velocity')])
>>
>> This is the profile I have got using the SlicePlot tool:
>>
>>
>> https://drive.google.com/file/d/0B43CkTTj8OeOcy16TkFaZlBhRE0/view?usp=sharing
>>
>> So, it is a function of radius. The density is also a function of radius.
>> The problem is that if I use
>>
>> >grad_fields = ds.add_gradient_fields(("gas","Omega"))
>> >slice = yt.SlicePlot(ds, 'z', "Omega_gradient_x")
>>
>> I get this
>>
>>
>> https://drive.google.com/file/d/0B43CkTTj8OeOLUxGVUJSQ253TE0/view?usp=sharing
>>
>> ​the sign should only change across x=0. I did the same for the density
>> field getting the correct behaviour.
>> I've also tried to define the gradient field
>>
>> >def _vorticity_2(field, data):
>> >    f  = (data[ftype, "Omega"][sl_right,sl_center,sl_center] -
>> >          data[ftype, "Omega"][sl_left,sl_center,sl_center]) \
>> >          / (div_fac*just_one(data["index", "dx"]))
>> >
>> >    new_field = data.ds.arr(np.zeros_like(data[ftype, "velocity_z"],
>> >                                          dtype=np.float64),
>> >                            f.units)
>> >    new_field[sl_center, sl_center, sl_center] = f
>> >    return new_field
>>
>> But I got the same results.
>>
>> If I'm doing something wrong please let me know.
>>
>> Cheers!
>>
>> Jose Utreras
>>
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>>
>
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