What are the vents for in front of the rear wheels of an Embraer 170/175? It seems like some sort of exhaust is exiting those vents but I can’t think what it would be.
I’d guess HVAC packs, with the NACA inlets just ahead being inlets, but it’s a guess and I can’t explain the black streaks.
black streak are probably from low pressure behind the vents (from turbulence) and dirt/dust sticking to fuselage.
Hmmm. Why are there streaks behind the aft vents but not the fore vents? Why are the port side streaks darker than the starboard side ones? If the issue was low pressure as you suggest, I’d expect to see more even staining.
If these are HVAC vents, does condensate drain out the vents? That would seem odd to me.