Isosurface, volume integrals, etc... Inside a Block instead of the whole Region
 

Hello,

I have a pretty wide water channel and I'd like to display some isosurfaces, streamlines, and to compute some values, but inside a small block within that channel, instead of in the whole domain.

When I say that the input part is a Block instead of my region, nothing gets computed, How can I do that ?

Thanks!