properties at specified location in Expression

Atit

I'd like to set 'U2' as the average velocity at y=50 m, but y=50m is not the inlet or outlet of the domain, how can I refer to the average velocity at y=50 m in the domain?

I also want to refer my variable in Expression Editor to the velocity in 'theta' component, such as the angular velocity on the plane y=50 m at radius=4 m, how can I do this?

Thank you very much for your kind consideration.

Atit