Because if its diverting flow in any direction, there has to be a resultant force as a result of Newtons Laws Surely.
Precisely. And if you have a thin flat plate in the absence of viscosity, it can only exert forces at right angles to its surfaces.
As someone has mentioned, without viscocity, you cant get assymetric flow,
Why not? If you have a thin flat plate at an angle to the flow?
The airflow below the plate is compressed on meeting shock front, and then again on approaching the surface of the plate. The airflow above the plate meets no shock, but when the neighbouring air below is no longer there, it starts to expand against the upper surface of the plate. But the pressure remains below ambient. There is no way for the high-pressure air below the plate to affect the low-pressure area above in any way, because this would take upstream signals, which are forbidden.