roof bubble on most cars

[Varn]

That is suprising, it may simplify some of the changes that I want to do. I assume that you mean on front edges rather that the rear. My vw has a lot of areas that have tighter front radii than that.

Originally Posted by PaleMelanesian

one of the aero experts, probably Phil Knox, quoted a surprisingly small radius required to maintain attached flow. I think it was 10 cm = 4 inches or something. If true, almost any car made in the last decade or more already has attached flow on the roof.

update: searching but can't find it yet.

[sid]

I see a lot of convertibles where the fabric top puffs up just behind the windshield when driven with the top up. This implies to me that the airflow is detaching at the start of the roof and the airflow is sucking the fabric top up. Anyone else notice this and have any thoughts about it?

[winkosmosis]

Originally Posted by sid

I see a lot of convertibles where the fabric top puffs up just behind the windshield when driven with the top up. This implies to me that the airflow is detaching at the start of the roof and the airflow is sucking the fabric top up. Anyone else notice this and have any thoughts about it?

Yeah the roof is low pressure once the airflow separates. Even if you don't see a bulge, you can feel it from the inside of a convertible.

[donee]

Hi All,

I am thinking that the difference between the 2nd and 3rd Gen Prius, besides the increased CdA of the 3rd, may be related to the lower rake of the 3rd Gen Prius windhield. You may know I am of the opinion the 3rd Gen is just not as good at higways speeds based on coasting performance at specific locations on my commute. Thus this discussion is interesting to me.

Rather than put the bubble on top of the roof, put it on the upper 1/4 of the windsheild and make it widthwise. Now, you keep the flow attached, and maybe the flow over the top balances with the flow around the sides?

[aerohead]

Originally Posted by mekanic

I have an idea that might help mostly on cars that have almost flat roof.
here is the idea: take a sheet of lexan or sheet metal and make a bubble top start at the top of the windshield go over the roof and stop at the back window. however in the migddle of the roof have it 4?6? 10? inches off the top of the factory roof. this would extend the angle of the windshield up onto the "new" roof and help the air flow better over that part of the roof?

or has this been tryed and I need to learn more'?

A flat roof is not an aerodynamic liability,nor flat sides.
As long as there is no flow separation then you're okay.If the front of your vehicle is as good as the 1st-gen VW Golf/Rabbit you're good.
Conventional wisdom is to do NOTHING which would add frontal area.Even a JEEP CJ-5,at Cd 0.8 can be 'fixed' ahead of the A-pillars with no increase in Af along or above the vehicle.
Remember,if the body is raked,with a nose-down configuration,the vehicle may be in an aerodynamic 'attack' mode with a positive pressure gradient against the body all the way back to the backlight header which will guarantee attached flow.That's all you're looking for!

[winkosmosis]

Originally Posted by aerohead

A flat roof is not an aerodynamic liability,nor flat sides.
As long as there is no flow separation then you're okay.If the front of your vehicle is as good as the 1st-gen VW Golf/Rabbit you're good.
Conventional wisdom is to do NOTHING which would add frontal area.Even a JEEP CJ-5,at Cd 0.8 can be 'fixed' ahead of the A-pillars with no increase in Af along or above the vehicle.
Remember,if the body is raked,with a nose-down configuration,the vehicle may be in an aerodynamic 'attack' mode with a positive pressure gradient against the body all the way back to the backlight header which will guarantee attached flow.That's all you're looking for!

Conventional wisdom is apparently wrong. Making a roof curved does reduce drag, even though frontal area increases. Obviously the best thing would be to keep roof height the same but curve the windshield but that's not usually practical. <warning! scribd link which slows down some browsers!> In This Chapter the Modifications That Were Carried Out To

Roof: Roofs are designed with a convex shape to ensure sufficient rigidity. For stylistic reasons an attempt is made to keep the convexity as small as possible. Fig 5.9 shows this for a medium sized notchback car. If the convex shape is designed so that the frontal area A of the vehicle increases, the aerodynamic drag of the vehicle decreases.



On the other hand, if the original roof height is kept constant the front and rear windows must be curved into the roof contour to eliminate obstruction of the view. This leads to expensive windows but results in lower drag.

The measurements plotted in Fig 5.10 (after Buchheim et al) show the same tendency for a car with a fastback. Here the chord length of the roof arch was used as the reference variable for the curvature.



Aerodynamic drag reduces with increased convexity for two reasons. First, the higher convexity allows for a larger radius at the transition from the windshield to the roof. This results in a less pronounced suction peak at this location. The momentum loss in the boundary layer during the following less steep adverse pressure gradient is therefore smaller and the boundary layer itself is less endangered by separation. Second, the convexity provides for gentle deflection of flow at the rear and the pressure rise at the rear end is therefore enhanced. The convexity of the roof and the rear end shape must be carefully matched.

[winkosmosis]

^all of which is common sense. We know that a "teardrop" can be more aerodynamic than a brick of smaller cross section. After all the coefficient of drag of the brick is about 1.0... Truisms like "never increase frontal area" get in the way of intuition-- like the intuition that both the OP and I had that you can reduce drag by adding an arch to a flat roof.

[Christ]

Did the decrease in drag, coupled with the increased A, result in a lower CdA?

If you've not increased frontal at all, it may produce a result... I feel, however, that increasing A to decrease Cd is probably going to get you less of a positive result than you might first think.

[winkosmosis]

Originally Posted by Christ

Did the decrease in drag, coupled with the increased A, result in a lower CdA?

If you've not increased frontal at all, it may produce a result... I feel, however, that increasing A to decrease Cd is probably going to get you less of a positive result than you might first think.

All it takes is for the improvement in Cd to be greater than the increase in A.

Frontal area is kind of an arbitrary number. With the air spilling off the windshield forcing surrounding air upward, it's basically creating an invisible increase in frontal area.


Look at how Jeep cuts off the image

[Christ]

You didn't answer the question, and I suspect that other than on a case by case basis, you won't be able to, either.