[kach22i]

Originally Posted by Vman455

....With the spoiler in place, pressure increases on the window (and roof) ahead of it.

Any ideas on why the spoiler is able to slow down the air and increase pressure?

If not "downwash" as Aerohead mentioned years ago in first quote below, then what is going on?

Regarding 911 rear spoiler studies:
https://ecomodder.com/forum/showthre...e-36229-3.html

Originally Posted by aerohead

I'd be very leery of any flow obstruction leading to the cooling air inlet on the engine cover.
The whale tail type spoiler would capture the downwash,ensuring high pressure into the gill slits,and it's added length would help capture a vortex above the spoiler,like a half-tonneau does on a pickup.

Regarding 911 Ductail development:
https://ecomodder.com/forum/showthre...e-36229-4.html

The spoiler was incrementally increased in height until lift and drag reached a 'reversal point,' after which performance degraded again. ( Sweet-spot ).
Drag dropped from Cd 0.4239, to Cd 0.40.
Top speed improved from 149.2-mph, to 152-mph.
Separation-induced lift was killed off.
Engine cooling was improved.
Taillight back-soiling was mitigated.
As Wolf H. Hucho had commented in 1986, the spoiler performed almost as well as simply lofting the roofline up to where the 'tearing edge' of the ducktail ended.

A few pages of Ductail discussion on PP:
Ducktail vs standard lid - oil temp increase?
https://forums.pelicanparts.com/pors...-increase.html

Note: Reduction of "lift" is not the same as "increasing downforce", the Ductail reduces lift. Also as reported elsewhere the air flowing over the rear grille with Ducktail (2-points are, from top of rear window to top of Ducktail) is at a higher pressure and slower speed making the cooling fan's job of drawing air downward and over the cylinder heads more efficient.


https://www.google.com/url?sa=i&rct=...8&opi=89978449

Air is considered a fluid because it flows and can take on different shapes. Bernoulli asserted in “Hydrodynamica” that as a fluid moves faster, it produces less pressure, and conversely, slower moving fluids produce greater pressure.

Post #12
https://forums.pelicanparts.com/pors...-ducktail.html

If you want more cooling, leave the ducktail alone. It creates a high pressure area right where the engine decklid grill is and that should improve the amount of cool air getting to the engine intake and engine fan.

Walter_Middie
https://forums.pelicanparts.com/6152252-post8.html

I'm an aerodynamic engineer and I usually stay out of these discussions because they deteriorate quickly. I'll just say no - cutting slots in the spoiler will not help.

The spoiler works completely differently than a wing. The spoiler spoils and separates the airflow coming off the roof, creating a high pressure area on the back window and top of engine lid where the engine airflow intake is (as shown in your picture). Engine cooling air is increased because the intake is now in a high pressure region, and air is forced into the engine compartment. Cutting the hole shown in blue will decrease the effectiveness of the spoiler and decrease the amount of pressure pushing air into the engine compartment.

Post #31
https://forums.pelicanparts.com/pors...ucktail-2.html

The ducktail creates a high pressure on the leading face. This forces cool air into the engine bay and slightly changes to airflow across the top of the car. This later effect is usually at higher speeds.

EDIT:

I'd like to mention that maybe the Prius isn't the best example of a too steep of a fastback that needs a rear spoiler or Ductail to increase performance. Just saying, the Prius is very near the template contour.