Quote:
Originally Posted by AeroMcAeroFace
But doesn't the amount of stuff ahead affect the boundary layer thickness? And doesn't that make the air less "sticky" and more likely to separate?
I would think that a longer vehicle that morphs into a semicircle compared to a shorter vehicle that morphs into a semicircle, would require different rear tapers before flow detaches.
A stretch limo has the same rear profile as the car that it is based on, but will, unless I am mistaken, have different separation characteristics.
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Great questions. Let's see if I can unpack them.
1) airflow at the front of a contemporary car will be 'held' in place, moving towards low pressure, essentially where the body is the thickest, and air velocity greatest ( Bernoulli Theorem [ a few key-strokes away on GOOGLE ]).
2) It's been this way since after Hucho published his drag optimization templates in 1976.
3) Aerodynamic streamlining, according to Hucho, and every other automotive aerodynamicist, has to do with reducing, or eliminating flow separation on the aft-body of the car. So it's all about the back of the car.
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4) Airflow beyond the largest cross-section of the body, and moving rearwards is in the 'lee' of the wind. The air is no longer 'attacking', as it was on the forward, inclined surfaces of the forebody.
5) Due to the 'size' of cars, their boundary layer transitions from laminar, to turbulent at around 20-mph ( approx. 33 Km/h ). Which is a 'good' thing!
6) A turbulent boundary layer has the ability to survive an adverse pressure gradient ( an increasing pressure ) in the flow direction.
7) The free-stream air which lies immediately outside this turbulent boundary layer ( TBL ) has the ability to transfer kinetic energy of momentum, towards the 'wall' or, 'boundary' of the body, because air is viscous, experiences, and can transfer shearing stresses when attacked by energetic fluid near it, IF, AND ONLY IF, the shape of the body converges only slightly from location to location, creating a very subtle deceleration ramp, on which the air can slow, while rebuilding static pressure ( static regain),as it possessed before the car came along and accelerated it to higher velocity/ lower pressure ( Bernoulli ).
8) This is where 'streamlining' comes in.
9) A streamlined body has such a gentle rear contour, as to never produce a high enough pressure rise, as would require the TBL to slow ( Bernoulli ) beyond a critical threshold velocity/ pressure.
10) The air immediately adjacent to the body's surface is already 'stopped' as a consequence of viscous effects. It is what it is. Nothing can be done about it. Like gravity.
10) A contour which is not 'streamlined' will create a slowing and pressure rise ( Bernoulli ) which is of such magnitude that, for conservation of momentum to survive ( which is an immutable law of physics ), the air adjacent to the 'boundary' would have to decelerate. Which is impossible, as the air there is already 'dead.'
11) A 'too-steep' contour will bring on this conundrum, and the consequence is that the the TBL will detach from the body and attempt to flow forwards towards the lowest pressure on the car, near the 'wide' part ( near the driver's head location on the sides, and in front of the windshield header on top.
12) As the flow reverses direction, it's shearing action triggers the formation of eddies, and whirls, growing in magnitude into full-blown turbulence.
13) A loose analogy:
* consider your car's upper body as a linear camshaft profile. An air molecule traversing the length of the car over the top, as a roller-cam lifter. And the atmosphere as a weak valve spring.
14) If the cam profile is too radical, the 'spring' will be unable to maintain lifter contact with the cam 'face' as it passes under, and the valve will 'float.'
15) Same with aero. If too steep ( like a Porsche 911) the air will 'float' off it as it passes ' under.'
16) A rear spoiler can lessen the injury.
17) Re-shaping the aft-body to a streamline contour eliminates the problem altogether..