What causes car drag?

[aerohead]

Originally Posted by JulianEdgar

Sorry, what has this got to do with your claimed ability to compare the drag of different cars based solely on measured centreline pressures? Are you retreating from this absurd claim?

1) forward stagnation pressure and base pressure gives us some insight into the overall delta-P across the body. Any information is way out ahead of nothing.
2) squarebacks have no facility for pressure regain, as they offer no cross-section contraction. And we'd anticipate low base pressure, and high drag.
3) notchbacks offer some pressure regain, higher base pressure, and lower drag.
4) a streamlined fastback offers the greatest pressure regain, highest base pressure, and lowest possible drag.
--------------------------------------------------------------------------------------
Especially if vehicles are normalized to identical frontal area, pressure profiles, even if only centerline, will give some insight into the macro-environment around the vehicle.
* Two water heaters of identical exterior dimensions, handling an identical volume of water, at equal inlet and outlet temperature, may exhibit different input energy requirements. Without X-Ray vision, one may correctly infer that, the unit with the lower energy requirement has a more efficient thermal insulation package.
* The inverse of the water heater's R-Factor, is it's thermal heat transfer coefficient, U. The greater the R-Factor, the smaller the U, and the more bang for the buck, with regards to hot water costs.
* Just as with the drag coefficient on the car.
* Delta-Ps can be a glimpse into efficiency.

[AeroMcAeroFace]

Originally Posted by aerohead

1) forward stagnation pressure and base pressure gives us some insight into the overall delta-P across the body. Any information is way out ahead of nothing.

Does it? Stagnation pressure is always related to speed, every car has a point at stagnation pressure.

You are deducing from speed that the pressure drop is higher. Doesn't that point say the faster you go the more drag increases?

"Delta-Ps can be a glimpse into efficiency" and speed, they tell you the speed more than anything.

"Seriously, this has gone from the sublime to the ridiculous." I completely agree Julian, this is the craziest aerodynamics deduction I have seen on this site (well second to the wind turbine thing you posted recently).

[aerohead]

Originally Posted by AeroMcAeroFace

Does it? Stagnation pressure is always related to speed, every car has a point at stagnation pressure.

You are deducing from speed that the pressure drop is higher. Doesn't that point say the faster you go the more drag increases?

"Delta-Ps can be a glimpse into efficiency" and speed, they tell you the speed more than anything.

"Seriously, this has gone from the sublime to the ridiculous." I completely agree Julian, this is the craziest aerodynamics deduction I have seen on this site (well second to the wind turbine thing you posted recently).

1) stagnation pressure and base pressure would trend together. They don't exist independent of one another.
2) drag would vary as the square of the velocity, by definition.
3) power to overcome aerodynamic drag varies as the cube of the velocity, by definition.
4) any vehicle which experiences a drag reduction, will demonstrate a lower delta-P at any given velocity. Streamlined, really-low-drag cars would exhibit the absolute lowest delta-Ps.
5) Julian Edgar appears not to yet possess enough educational background to make such comments.