[donee]

Hi All,

Hood/Windshield angle has to have some kinda optimum region. But its more complicated than a 2D model can determine. This discussion is with respect to primarily one plan or two intersecting plane front end designs.

Here is why. A steep angle results in the formation of vorticies at the upper corners. A shallow angle results in momentum effects across the whole top of the car. Or at least that is what I would look to verify in a CFD model if I was starting such an analysis.

The steep angle results in flow attachment in the center of the windshield/roof transition becuase the air slows as it reaches the top of the windshield. The slowed air at the edges starts to turn outboard pickus up speed and then has momentum to shoot beyond and outboard around the upper A-pillars. Then tumbles into the vortice.

The shallow angle results in fast air hitting the windshield/roof transition, and it cannot turn in time to stay attached. The result is a slight bubble at the central portion of the transition, which increases the effective cross section area. As the airspeed is great there, the flow never goes turbulant, just shoots up a little before turning. At the edges, the air never gets slow enough to start turning much, and then when it hits the A-pillar, it just wraps around. I saw this in the 3rd Gen Prius salt pattern on its upper 1/3 of the A-pillar. Paterns, I never saw on the 2nd Gen.

Now, I am writing like I know this for sure, but I do not. This is just a proposed hypothesis.

I think the angle is critical. And its somewhere between the 2nd Gen Prius, and the 3rd Gen Prius angles. Because in my 2nd Gen Prius, the turbulators on the upper 1/3 of the A pillars improved air flow. And in the 3rd Gen, they disturbed air-flow. But, there are other differences between the two - the inset of the windshield on the 3rd Gen. Its a 3D problem. And there is no getting around that.

Most cars in the steep angle regime. So, improvement really is not going to be made at the central zone of the transition, because the air stays close to the body there. The improvement to be made is in the upper 1/3 of the A-pillar wrap-around air, with the turbulators. Which prevent the vortice from forming, and keep the effective cross-section minimized.

An exception to this might be the older Saabs with the curved windshields. Which does not have an intesecting plane design. The curve probably prevents the vortice from forming, and being steeper, the air over the top is closely flowing to the roof. With this design, there might be improvement to be made at the lower 1/3 A-pillar, such as large fillet.

And an improvement in a new design using the planar features, would be to round over along the windshield/roof line, the sideglass/roof line, and A-pillar/sideglass lines. This requires more complicated metal forming and glass forming, all joining at that corner. The side door might actually have to be clipped on its forward upper corner, to accomadate the 3D curves. Which is probably why its not been done yet.