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Originally Posted by tomi_k
Thanks, Aerohead & Freebeard ! Just trying to get my head around the topic what matters the most... and learning. And thanks for being patient to respond (probably basic and silly questions) but gotta start from somewhere....
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* Flow reattaches on top of the spoiler, and captures a locked-vortex against the body, as high as the spoiler.
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From drag perspective, the air flow before the spoiler what happens for the overall drag if
a) flow on front of the spoiler is attached? If flow is attached, is the spoiler directed air flow direction reducing the drag (i.e. surface pressure direction (at the end) -> without spoiler the surface pressure is against the direction of vehicle movement vs. with spoiler the surface pressure is in same direction of vehicle movement)?
b) flow on front of the spoiler is not attached -> and creates a "air bubble" (turbulent)... is this what causes overall drag increase if wake increase is not necessary the dominant factor?
According one of Hucho's document (see picture attached), if I understood correctly, regardless of attached or separated airflow, when the critical angle of rear portion of the shape is exceeded, overall drag will increase. Unfortunately, I found this information a bit too late but glad I found it.
The other attached picture is an illustration of what I am trying to understand... If Hucho's document info is correct and valid... is there a way to recover "penalty of overall drag" caused by exceeded rear slanted angle as it sounds like increased drag is evident regardless what type of flow over the rear shape one has and/or and if wake area is reduced?
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1) since the template is streamlined, the flow would be fully attached.
2) flow would be increasingly decelerating over the length of the entire roof, aft of the roof apex.
3) local static pressure would be increasingly rising, in proportion to the local velocity, as the flow moved rearwards.
4) when flow reached the end of the body and detached, it would be the 'slowest' and 'highest' pressure, having recovered pressure the entire length of the roof.
5) there would be no separated wake, just a thick turbulent boundary layer sloughing off the body.
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6) the addition of a rear spoiler would introduce an obstacle in the flow path, requiring the flow to re-accelerate to a higher velocity around the larger cross-section, and at lower local static pressure.
7) the local separation line would move to the periphery of the spoiler.
8) a turbulent wake would be introduced.
9) direct downforce would be introduced.
10) at the expense of lower base pressure
11) higher pressure drag.
12) higher overall drag.