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Old 12-02-2020, 01:31 PM   #172 (permalink)
aerohead
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Coanda Effect

Quote:
Originally Posted by JulianEdgar View Post
I use the term 'wrapping around curved surfaces' (and similar) to explain the low pressures that are generated when attached flow follows a convex surface*.

In an email to me last week, former head of Porsche aerodynamics, Dr Thomas Wolf, puts it this way:

"According to the Coanda-Effect the air follows the curved (upper) surface. According to Newton’s first law, there must be a force which pulls the air downwards. This force is exerted to the air by the car body and pulls the air downwards. According to Newton’s third law (action = reaction) there must be a force of the same magnitude in the opposite direction acting on the body."

But hey, what would Dr Wolf know?

Unfortunately, a lot of what Aerohead so steadfastly believes is simply wrong, as some simple on-car measurements very quickly shows. It's just a crying shame that Aerohead has so effectively mislead so many people here for so long.

(*The opposite happens with concave surfaces - high pressures are developed.)
1) The Coanda Effect has nothing to do with mass- produced passenger cars.
2) Nor Newton's 'Impact Theory', if that's where Dr. Wolf is going. Hucho addresses this specifically in his 2nd-Edition.
3) On the forebody of an automobile, the air is held against the body as the vehicle attacks the air.
4) Once past the maximum cross-section of the body, there's no reason for the air to remain attached, as it's now in a hostile pressure gradient.
5) If the this pressure gradient exceeds a particular, 'critical' ( in the words of P. W. Bearman ),adverse pressure gradient threshold, it will trigger turbulent boundary layer separation, in accord with Dr. Hermann Schlicting's boundary layer theory. ( W.H. Hucho was assistant to Dr. Schlicting at the Technical University of Braunschweig from 1961-to-1968 ).
6) Only a 'streamlined' aft-body contour can produce the sub-critical pressure gradient which will guarantee attached flow for the total length of the aft-body.
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As the flow continues rearwards it is simultaneously gaining pressure as it loses velocity. If the flow remains attached all the way, when it does finally detach at the trailing edge, it will be at its lowest velocity, highest pressure over the rear upper surface and sides.
By extending the length of the body, along the streamlined contour, at 'full' length ( lang-heck ) the pressure will be the highest attainable, drag the lowest, and minimum lift.
If one wishes to induce direct downforce by means of a rear spoiler, for high-speed operation, then this is recommended practice. Or one may simply build downforce into the car's sheetmetal, as with the new, 2021, Maserati MC20 supercar.
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A reading of U.S. Patent # 4,533,168 will disabuse you of blanket statements about the virtues of 'concave' geometry.
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