Quote:
Originally Posted by Frank Lee
Citations... or... anything?
|
Common sense? Take a cardboard folder, hold it at a 15 degree angle and move it through the air and note the resistance you feel. Then angle it to 45 degrees and fold it so that the frontal area is the same. Is the resistance more or less or the same??
Then look at the real world examples. Car windshields- you've never noticed that economy cars have very slanted windshields? Prius? Heck, look at your own picture of the Beetle in the wind/water tunnel. The fact that air piles up and forms a "bubble" proves that flow is not as easy as if the windshield were sloped back. Look at the fan and ask yourself why the blades are curved instead of flat.
I know you are into rear end aerodynamics. Did you ever ask yourself why airflow is better able to stay attached the shallower the rear surface angles are? It's because it takes less energy for the pressurized air to expand toward the surface and flow along it.
Just visualize-- use the spatial common sense that mother nature built into your brain which allows you predict where a ball will land depending on how hard and at what angle you throw it.
But if common sense and visualization isn't enough for you here is an article that talks about hood and windshield angles:
In This Chapter the Modifications That Were Carried Out To
Quote:
|
Windshield inclination angles of more than 60o are not practical because of light diffusion. In addition, large highly inclined windshields lead to increased solar heating of the passenger compartment. Two effects are responsible for the favorable, though moderate, influence of a highly inclined windshield on drag. First, the excessive speed in the area of the A-pillar is reduced so that the momentum loss occurring at the point is smaller. Second, the deflection of flow at the transition from the windshield to the roof is smaller. The low pressure peak occurring there is therefore smaller and the positive pressure gradient in the remaining flow is less steep. Hence the momentum loss in the boundary layer is lower, allowing greater pressure recovery in the area of rear end. Therefore even if a strongly inclined windshield does not contribute to a local drag reduction, it helps to improve the flow over the rear part of the car and thus to reduce the overall drag.
|
Note that even though the language suggests that a strongly inclined windshield doesn't "contribute to local drag reduction", their strong incline is less than 30 degrees from horizontal, much less than the OP's. I believe that's even a shallower angle than the cab forward Honda Civic's.
They say that a steeper hood angle doesn't help drag once it's good enough that flow stays attached, so it seems to me like the bubble that forms when the transition is too severe is the problem.
BTW, that article also has a section on arched roofs, where they explain that even though an arched roof increases frontal area, it can reduce total drag.