Dirty rear end. Close those gaps!
 

Im planning to add some 'trip strips' to the back of my fiesta hatch. I see all newer hatches coming out with them and im not a fan of strapping some huge spoilers to the back.
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However since this mod seems so small I want to optimize the airflow over them! That led me to these links showing just how turbulent the rear of a car is. What stands out if you look closely is how the turbulence starts over the panel gap of the rear boot lid! in the first link u can see the turbulence along the Range Rover boot lid panel gap on the roof. The second link around page 50 it shows similar effects! definitely something to consider if you want to optimise this mod! Close those panel gaps!

https://www.exa.com/vehicle-handling-motorsport.html

http://publications.lib.chalmers.se/...ext/143193.pdf

strips
 

I think the strip may be a palliative for aerodynamic noise as much as drag reduction.The rounded C-pillars would have bi-stable flow which moves around,booming.The strip creates the 'burst'(Chrysler term) which fixes the Cd at a constant.
Don't expect any dramatic drag reduction.