Hi Matthew, thank you for your answer! How would setting the bulk_velocity field parameter modify derived fields in the Dataset object? As I can see from the documentation, an example of use would be: ad = ds.all_data() ad.set_field_parameter("bulk_velocity", yt.YTArray([-100.,200.,300.], "km/s")) Can this be used to update deposited velocity fields in the original Dataset object?
I also suspect you might be able to speed up your calculation a fair bit if you got rid of the iteration:
ds.r['Gas','Velocities'] - ds.arr( ... )
You also probably want to assign this to another variable, as well.
By the way, I included that loop as to provide an example of how I was modifying the in-disk fields in the Dataset object, if that helps. Thanks again, Martin