What causes stagnation pressure to change?

[Algren]

For a real mass-conserved(m dot is constant throughout the system) steady-state mono-substance(same substance throughout) flow , what are the different factors which can cause stagnation pressure and temperature to drop/rise?

 

[boneh3ad]

It basically comes down to viscosity. If viscous dissipation occurs, then the stagnation pressure drops. This happens whether the dissipation is due to the presence of a boundary layer ("friction") or a shock.