How to plot streamslice on velocity contour?
 

I have some simulations done with openfoam and have some wind tunnel data, which I would like to compare with velocity magnitude contour and streamlines. As I am writing a follow up thesis I would like to do this in the similar manner as my predecessor. I would like to achieve a images such as this:
Attachment 82106

I am unable so far to generate the streamline on a 2D plot so far. I think there is a python option streamslice, which should enable this. I however do not understand how I can apply this, even after quite a research... Could you give you explain to my the workflow for this?
I also would like to have the same horizontal length scales with x/H and y/H. How can I achieve this?

Sorry if this questions is to simply but I am quite stuck right now.

Many thanks already in advance.

Best Fidu13