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Old 05-27-2020, 07:37 AM   #9 (permalink)
California98Civic
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Quote:
Originally Posted by JulianEdgar View Post
Aerohead's theory is that lift is caused by separation, rather than attached flow. He also suggests that well-streamlined bodies have low lift.

However, in fact it is the airflow wrapping around the upper curves that is the primary cause of lift. Therefore, low drag shapes that have upper curves coming down to a small wake area have higher lift.

Aerodynamics of Road Vehicles, 1987, Hucho (ed), Page 281:



You can see that as the camber (upper curve) of the body becomes greater and longer (ie giving a smaller wake), Cd drops from 0.224 to 0.193 but the CLr (rear lift) rises from -0.1 to 0.056. Well streamlined bodies do not always have lower lift! In fact, because on road cars they need to have greater camber, lift (especially at the rear) is likely to be increased as wake area decreases.

Road Vehicle Aerodynamic Design, 2009, Barnard, Page 32.



You can see that lift is dependent on two aspects - degree of camber and angle of attack. The cambered shape gives more lift at all angles of attack.

Barnard immediately then goes on to this:



Note the reduced pressure over the cambered car shape. (The second and third diagrams show the changed undercar pressures possible with front/rear seals.) Note how premature separation is not even mentioned as a cause of lift.

Race Car Aerodynamics, 1995, Katz, Page 48.



Note the difference in lift between the theoretical 'low drag body of revolution' (Cl = 0) and the 'low drag vehicle near ground' (Cl = 0.18). Once camber is understood as a major cause of lift, of course such a shape will have high lift!

Road Vehicle Aerodynamics, 1975, Skibor-Rylski, Page 69 and 70



Note the direct parallel that is drawn between the lifting wing and the shape of the car. Also note attached flow on car until separation occurs at end of car (ie no premature separation). Skibor-Rylski then says on the next page that aero lift depends on the camber line of the vehicle shape.

Modifying the Aerodynamics of Your Road Car, 2018, Edgar, Pages 85 - 91.

Attached flow on upper surfaces of XE Jaguar:



Jaguar CFD showing low pressures where airflow wraps over upper curves (hotter colour = lower pressures):



On-road measured pressures showing low pressures where air wraps over upper curves:



The primary cause of lift on road cars is the attached airflow on upper curved surfaces. To argue otherwise is to show a grave misunderstanding of the mechanism of lift on road cars.

Lift depends on the difference in pressures on the underside and topside surfaces. Curved upper surfaces generate low pressures and so lift forces, however these can be offset by generating low pressures under the car through fast flow.
When I read this post the first time I appreciated the references, but I got stuck on a few things. Your example from Hucho (2cd Edition, p.281) seems to be an image of three differently streamlined bodies. Yet you seem to regard only the last as streamlined. Why? And isn't the difuser angle also relevant, not just the sweep of the top of the body?

A second point you made is contradicted by the evidence you show from Katz’s Race Car Aerodynamics. Katz’s images on p.48 show increases in drag and lift between body shapes 4, 5, and 6. The “low drag body of revolution” has zero lift and the lowest drag. The half-body has the second lowest lift and drag. And the generic contemporary production shape has higher lift and drag than either 4 or 5. Yet you claim that “low drag shapes that have upper curves coming down to a small wake area have higher lift.” But body shape #6 has the higher lift.

Lastly, you gloss aerohead's text this way: “Aerohead's theory is that lift is caused by separation, rather than attached flow.” And you conclude: “The primary cause of lift on road cars is the attached airflow on upper curved surfaces. To argue otherwise is to show a grave misunderstanding of the mechanism of lift on road cars.” But aerohead actually argued almost the same thing as you: “Lift is a function of low pressure acting 'over' a portion of the body.All separation should be at the back of the car. If it's not,it's in a lower pressure regime,as it's at a higher velocity.The closer you get to the windshield the lower the pressure and the higher the lift.”

I'll grant that aerohead sometimes writes haikus, or else with a lot of jargon, but I think his point about lift and pressure was clear enough.
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