Prius Roof Design

My guess is that the humps keep the air flowing over the roof, instead of letting it spill over the sides. This would affect drag and probably lift, but we'll have to wait for an expert to give a more definitive explanation.

 

<div class='quotetop'>QUOTE(bee13 @ Sep 7 2006, 09:00 PM) [snapback]316043[/snapback]</div>

Hi Bee13,

I am hardly an aerodynamic expert, but here is my take on this.

Car aerodynamics are different than aircraft aerodynamics. First off, even the everyday airliner is tuned for optimum efficiency at about mach .85 or so, in thin air. So, its really really different aero environment than a car. At car speeds, socalled "bluff body" designs work best. Pointy noses on cars have lots of drag. Aerodynmicists will talk about the "Reynolds Number" when they are trying to make points about things like this. As much different as the Prius is from a Airliner, an Insect is as different than the Prius. All because the aero enviorment is different. What the Reynolds Number is dramatically effects what the best aerodynamic design is. The air is so effectly thick to an insect, that the insect is built like a swimmer.

Second off, the car is on the ground, but its also roughly wing shapped. So, what happens to the air that gets thrown downward off the back of the car? With airplanes that air just goes flying downward (and the reaction force lifts the plane). On a car it hits the road, and in doing so pushes back on the air still near the car.

When the air hits the road, and the upstream air feels that pressure, it might spill over the side of the car, and thus become a turbulent zone on the sides of the car. This effectively increasing the cross section area of disturbed air, which creates more drag. By scalloping out the roof of the Prius, I believe the designers were building resistance to the air going off side-ways, and this then helps to maintain laminar flow over the top of the car.

I had a ninja bike riding motorcylce guy get about 1 foot from my left rear bumper one day at 65 mph. I was in cruise control, and on level ground. Amazingly, the mileage indicated improved 2 to 3 mpg. I think that was due to the air coming down the side of the car being decoupled from the car, and flowing over the cycle, smoothly. Rather than tumbling into the air coming over the top of the car.

If you do some googling on Car Aerodynamics, eventually you will come across the mention of Vortex Generators. Mitubishi puts them on their EVO sports coupe, as the rear window styling has such a sharp drop off, that something is needed to help keep the flow from delaminating. If you look more into Vortex Generators, you will see that general aviation guys use them on the wings to keep the air flow attached at low speeds, and offset stall to a higher angle of attack. The higher the angle of attack, with attached air, the higher the lift (but also the drag). At low speeds, the planes have enough power to plow the wing through the air at the higher angle of attack, however. But, without these vortex generators, the flow would seperate from the wing, and lift would be lost. This is how a slower stall speed is achieved. Interstingly. the general aviation stall speeds we are talking about here are right around car urban highway speeds...like 55 to 65 mph. For the general aviation application, the vortex generators are placed on the lead edge of the wing, not the trailing edge (across the rear of the passenger compartment) like in the Evo.

For a car without the scalloped roof, vortex generators along the top of the windshield might be advantageous to help keep the air on the roof, rather than falling down the side of the car. The Evo paper showed that the air over the trunk lid was compressed down to the car better, reducing the effective disturbed air cross section. One thing I read on Car Aerodynamics said that reducing the disturbed air cross section, even by creating tubulence, will reduce drag. This apparently is the case with the Evo.

On the Prius, Vortex Generators placed where the nose rolls into the long mostly straight climb across the hood and windshield, might help keep air along this edge from spilling over the side of the car. Or in the middle of the hood, keep air from jumping up so high as it crests the peak of the passenger comparment. The trick would be to make the generators the right size, that they do not result in more drag than is saved. One thing probably not to do , is have the generator stick out beyond the car's cross section.

A vortex generator on the rear-view mirror might help there too.

Anyway...some things to think about.

 

that was intense!!

 

It's the same idea first used in cars like the 3rd gen RX-7 (double bubble roof).

A car will form a slight vacuum behind it. This increases drag. The air coming off the windshield rises above the roof, creating a bit of a vacuum as well causing a bit of lift at the rear wheels. The idea is to help the air moving over the roof come down and reduce the rear wheel lift and help to fill the vacuum created behind the car. In my RX-7 it was done to reduce rear wheel lift mostly, but it did contribute to lowering the overall CD number. In the Prius it's done to just lower the CD number, as rear wheel lift isn't a problem until you hit speeds well above those this car was designed to travel at.

 

Wow! I am impressed! I thought those bumps were there for more headroom!

 

<div class='quotetop'>QUOTE(marjam @ Sep 9 2006, 12:08 PM) [snapback]316986[/snapback]</div>

They also stiffen the roof without adding much weight. A thin piece of metal will "oilcan" in and out; adding an angle increases the radius of gyration, making the whole section less likely to move.

Tom

 

Further to that, it also looks cool.

 

A roof of this general shape was also used in the late-60's Merceded 230SL roadster.

 

<div class='quotetop'>QUOTE(tochatihu @ Sep 11 2006, 03:55 AM) [snapback]317419[/snapback]</div>

Same as 1967 Toyota 2000GT.

Ken@Japan

 

<div class='quotetop'>QUOTE(bee13 @ Sep 7 2006, 08:00 PM) [snapback]316043[/snapback]</div>

My uninformed guess was that the non-convex roof reduces air drag by reducing the frontal area while maintaing head room for the two front passengers, without necessarily reducing the drag coefficient. (Air resitance is proportional to the frontal area multiplied by the drag coefficient Cd.)

 

<div class='quotetop'>QUOTE(theorist @ Sep 12 2006, 11:40 AM) [snapback]318094[/snapback]</div>

If reducing the frontal area is the reason then why is there so much headroom inside? It seems that they could have lowered the roof line a few inches more thus reducing the overall frontal area and reducing the drag even more. This lowering of the roofline (while still maintaining the scalloping) might also improve the appearance of the car by taking away some of the 'bubble' look.

 

<div class='quotetop'>QUOTE(mcmurray @ Feb 13 2007, 10:39 AM) [snapback]389279[/snapback]</div>

Reducing the headroom in front while maintaining the overall shape would seriously intrude onto the headroom available to rear seat passengers. While there is more than adequate legroom in the rear seats, there isn't much headroom for tall folks. All engineering involves compromise.

 

<div class='quotetop'>QUOTE(mcmurray @ Feb 12 2007, 06:39 PM) [snapback]389279[/snapback]</div>

Because people have to fit inside.


Seriously, one of the primary reasons I bought this car was because it (just barely) has enough headroom for me. There are very few 4 door hatchbacks that have enough headroom. I might have bought a Mazda 3 if it had better headroom.