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

[José Utreras]

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)
>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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