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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) Your first image is of the Ahmed body, which is on stilts and lacks wheel drag as we know it, so it's information is conditional from the get-go.
2) This Ahmed body's aft-body constitutes 21.5% of overall body length.
3) Cds associated for different rear slope angles on this model are accurate only for 21.5% aft-bodies, and without wheels.
4) The drag minimum, for a rear slope angle occurs at a 9-degree angle ( Cd 0.2298 )
5) The overall drag minimum ( depicted ), occurs at 25-degrees rear slope, 10-degree diffuser, and 10-degrees boat-tailing, with three counter-attacking vortices in three different planes.
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6) Your second image is from Hucho et al.'s SAE Paper 760185, Figure 24.
7) They're demonstrating how the VW can experience either a 'fastback' wake, or a 'squareback wake', based upon solely the rear slope angle.
8) The figure also depicts the 'vortex-drag' spike, between 26.5-degrees and 33.5-degrees, where 'bi-stable' flow ( both fastback and squareback can cycle back and forwards ) exists.
9) On this Volkswagen, the percentage of aft-body-to-total body length is not provided. ( the VW Golf/Rabbit was 16.4% )
10) So, as with the Ahmed body, drag values are only valid for the aft-body percentage shown.
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11) That said, if one made the mistake of doing a rear slope of, say, 28.5-degrees, you could add a rear spoiler, of which a straight-edge, laid across the top of the spoiler, to the rear of the car indicated below 26.5-degrees, to kill the vortex.
12) The spoiler would create the 'locked-vortex', with the swirling air captured in front of it, and laminar flow skipping over the vortex.
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13) If you add a spoiler to a rear slope which already has attached flow, you re-accelerate the flow to a higher velocity, which lowers the pressure at the very top of the spoiler, increasing the size of the wake, while lowering base pressure, increasing pressure drag, and overall drag.
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14) The 'template' is derived from a streamlined body of revolution of Cd 0.04. As a 'half-body', it's Cd 0.08. Adding modest wheels pushes it up to Cd 0.13. Full-coverage wheel fairings drops it back to Cd 0.10, with 'knife-edges.'
15) Since it's not uncommon for an aero modification involving compound curves to run to 800-man-hours ( for instance a bedcover ), fabrication time, I've limited the maximum rear slope angle on the 'template' to 21-22-degrees.( you don't want to do an 800-hour project 'twice' if you don't have to ).
At these 'angles' flow separation is impossible.
16) Volkswagen's lowest drag body reported so far, is the 1981 VW Flow-Body, long-tail, Cd 0.14, with a maximum rear slope of 21.5-degrees. Approximately Cd 0.105 with full wheel fairings. I don't have the plan-view of this car with me, but knowledge of it is important to the context of this shape's performance.
17) Volkswagen's lowest drag, running vehicle, the XL1, just happens to be a near-exact-match to the aft-body of the 'template, type-C.'
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17) There are lower drag bodies which have already been explored. Hucho shows half a dozen or so.
None of them that I've seen have the facility for forward, outward vision for the driver. They'd be okay for ( and have been used for ) a mileage marathon or solar car, but I can't see how one could operate them on public roads.