View Single Post
Old 06-03-2020, 12:54 PM   #18 (permalink)
Master EcoModder
aerohead's Avatar
Join Date: Jan 2008
Location: Sanger,Texas,U.S.A.
Posts: 14,927
Thanks: 23,060
Thanked 6,815 Times in 4,337 Posts

Originally Posted by JulianEdgar View Post
It's very hard to argue when everything I write is twisted to suit your particular model of reality.

I've said it all before but I'll try again.

1. Yes, premature separation causes low pressures in the separated area. That's why I can write: "flow separation at the end of the roof of the older car is responsible for the low measured pressures on the rear of the car." It happened a lot on older cars.


2. Flow separation seldom occurs on cars of the last 30 odd years. Therefore, the low pressures do not come from separation. They come from, as I said in the book, the airflow "airflow generating low pressures over the rear half of the car as the air wraps over the long curve*".

You then write "All aerodynamicists would agree with this comment, however not with the implied source of the low pressure" completely ignoring that in fact none of the four professional aerodynamicists who read the draft raised any issue with that statement.

Basically, like with many of your statements with regard to car aero, they reflect:

1) You haven't kept up with the literature...

2) you apply models and understandings that were relevant 30-40 years ago but not to most cars now.

If you were to get out on the road and measure some pressures on recent cars, you would not have to rely on your theories about what you think is happening, and instead could actually see what is really happening.

(*I could have said instead "the lift comes from the camber of the body", but I try to write as simply as possible.)
1) Flow separation occurs on all cars, except streamlined cars. It's the reason they don't have as low as Cd 0.12. Hucho clearly explains this. And the rules haven't changed in the last 20-30-years.
2) The low base pressure of modern cars is a function of the local pressure at the separation line. It has absolutely nothing to do with 'airflow wrapping over the body.' That doesn't even happen on wings.
3) The only way your four professional aerodynamicists could be factually correct, with respect to low pressures over the aft-body is, that the intense,fastback, long-lived, attached, counter-rotating longitudinal vortices are inducing a downwash, which impells the 911's tufts to 'adhere' to the rear slope, which in no way demonstrates 'attached flow'. The low pressure originates from the low local pressure at the separation line at the base of the backlight, where the body contour exceeds 23-degrees off the horizon, which would show up in smoke flow visualization. Your premise is incorrect.

3) Until you study and master the Bernoulli Theorem you're never going to understand the ramification of flow separation as it relates to pressure at the separation point(s), how that impacts base pressure, pressure drag,and overall drag.
4) Current literature has not overturned fluid dynamic principles.
5) The models and understandings I present are founded in empirical research leading back to the 18th-century. Ram-tubes, static tubes, and manometers aren't a recent technology. Nor are moving-floor wind tunnels.
6) As to measuring pressures, the protocol would include 42-pressure taps and manometers, photo-recorded, simultaneously, in a laboratory, where atmospheric conditions could be closely monitored, and all results corrected for standard SAE atmosphere. If you're in an income bracket which can absorb the cost of 42-Dwyer Magnehelics, good for you!
7) Looking at your measurements for the 1979, Mercedes-Benz 230 sedan, and averaging the six pressure points listed, it looks like the top of the car produces 0.00625 atmospheres difference at 100-miles per hour. And it's quite odd that Jaguar placed the top of the XE's spoiler right on the 'template' line.
8) In 1969, one of the car magazines found that they could eliminate any front or rear lift with a couple dollars worth of sheet metal or plastic. Hucho later basically claimed the issue wasn't worth fretting over, unless people drove like you do. Which is completely illegal in the US, and considered 'homicide' in some states.
9) As to the 'template,' the streamline body of revolution (torpedo) from which it is derived, produces zero-lift. (Goro Tamai,MIT, The Leading Edge).
10) The streamline half-body from which the 'template' is further derived, 'could' generate lift, however this depends upon its inclination (angle -of-attack ) which,so far, you haven't researched.[ At 135-miles per hour, the Spirit of developed 30-pounds front downforce, and 22-pounds rear lift ( with a compromised belly and diffuser), on a 4,280-pound vehicle. Using the velocity-square law, you can see that driving even at 85-mph (the highest legal speed in the United States) would produce 'meaningless' 'lift.' Essentially, the 'template' produces a zero-lift body (measured in a laboratory under controlled conditions).
11) It is the roof 'camber' which helps decelerate and increase the pressure of the flow. The further the flow is from the suction peak ahead of the windscreen header, the higher the local pressure, and higher the base pressure once the flow does separate. On the 'template'(s) body side camber and boat-tailing, this is taken to the extreme, providing the entire deceleration ramp (twice the length of the acceleration ramp), which experiences zero separation, full pressure recovery (in light of the surface friction losses), producing the lowest possible drag.
None of this will mean anything to you until you get Bernoulli behind you. Ask your 'experts.'
Photobucket album:
  Reply With Quote