Orange!FD

Whether that really happens depends entirely on how much air actually goes in the scoop. If "all of it" goes in (i.e. the air doesn't slow down at all, it just turns and goes down through the intercooler), then there wouldn't be much change to the airflow over the rest of the hood, or over the windshield, relative to an NA hood.

Evil Aviator

The air doesn't really hit the top of the hood as much as it flows over the hood at a high velocity. The high velocity is what creates the low pressure. As a contrast, you can see that the air hitting the lower front of the car lowers its velocity as it bunches up like snow on a snow plow, which creates a high-pressure zone.

See this link. If you don't want your brain to explode trying to understand the text, just look at Figure 3.6 and see how it looks similar to the RX-7's profile with a high-pressure zone in front and a low-pressure zone on top.
http://www.av8n.com/how/htm/airfoils.html

The important thing to understand about all of this is that low pressure doesn't mean no air, or that the air reverses its course, it just means low pressure. For example, some of you may have seen the meteorologist on TV showing you that your region was experiencing a "high" or "low", but regardless of the pressure you could still breathe, your lungs didn't explode, and the air didn't flow away to Mars.

ShadowX

BLUE TII, could you elaborate on what the heck you were talking about?

Snrub

iTrader: (1)

Another myth cleared up by Evil.

If the conditions are right in the snow you can clearly see the TII scoop does work. The situation is like one of those flow simulations. If you come out of a car wash semi-wet you can sometimes see it too.

Edit: I'm pretty sure most cars of a somewhat 'sporty' shape will have the same thing happening on the rear. Changing air velocity changes the pressure. Think air dam or undertray. Bernoulli. (sp?)

88IntegraLS

I think Evil has an airplane to be knowing all this aerodynamic stuff. Notice that the air inlets for plane engines is on the blunt face of the nose, not on the side. So I would think that the best place for an air intake on and FC would be the blunt lower part of the bumper, and the pressure diagram shown in this thread shows a lot of positive pressure there. The best possible cooling duct setup for the radiator would probably be an opening in the blunt lower part of the bumper with an exit on the top front of the hood in that low pressure area.

Maybe I should modify my hood with a reverse scoop in that area . . but I don't have any cooling problems to justify the extra work. But it would look damn cool.

BLUE TII

iTrader: (9)

QUOTE-
BLUE TII, could you elaborate on what the heck you were talking about?

As that silhouette shows the hood provides downforce where it meets the windshield.

This affect can be exaggerated more if one adds strakes running the length of the hoods edges and up to the windshield (fastened by the fender bolts and extending out of the gap between the hood sides and fenders). It is like the ends of a race car's wing- it has spillways so airflow does not "fall off" the sides of the wing.

This straked hood idea for downforce has been applied on many race cars.

NZConvertible

You guys place spend far too much time and effort debating a drawing that was only ever published in a sales brochure! It's credibility is debatable at best. It has no units, explanation, nothing. From an engineering point of view it's totally meaningless.

Evil Aviator

While I agree that units and the conditions of the diagram would be good to know, I disagree that it is totally meaningless. The diagram is obviously a rendition based on test data rather than the actual data itself, but the diagram is in fact consistent with the actual performance of the car. I think the diagram is good for a general overview of the car's aerodynamics.

From an engineering point, everything is meaningful, even a simple photograph of the car.

BTW, is it just me, or does the flow pattern shown in that drawing look like it was taken from tests with the sunroof open, and then the data was transferred to a picture of the car with the sunroof closed?

LOL, I wish I had an airplane. For those who are willing to donate an airplane, I would really like a Caravan Amphibian.
http://www.wilsonaircraft.com/main/accessories/wipline

Orange!FD

Bingo. The units are going to be "pressure ratio", which is a dimensionless quantity. (thus making it a useless quantity, right NZConvertible? ) On the other hand, it'd obviously help if we knew what length of vector represented a P/R of say, 1.1

I agree, although I'm not sure where you're getting a correlation between this picture and the overall performance of the car. If there was some additional data from something other than the centerline of the car, that would make it even more useful, but, as NZC mentioned, this is a picture from a sales brochure.

Maybe, although the pressure-ratio discontinuity at the aft end of the open sunroof has been dramatically "smoothed" if that's what the data is from.

NZConvertible

Well that's twisting what I meant just a bit. The engineering term "dimensionless" is quite different to "not putting any dimensions on the drawing". The numbers don't have to be a dimensionless ratio either. They could be actual pressure measurements taken at a particular speed, but we don't know what speed do we?

You only have to read all the threads about that drawing to realise that it's very ambiguous and easily misinterpreted. You, me and Evil understand what it (generally) represents, but many obviously don't. It's a fair bet that Mazda customers reading that brochure in 1986 didn't really understand it either. It was put there to look impressive on the brochure.

Another important point is that the drawing is for a spoilerless NA, so it can't be used for any other model. The Turbo's hood scoop would have a significant effent on the shape of the pressure profile over the hood, and a rear spoiler would have the same effect at the rear. I don't think the drawing's necessarily wrong, I just think it's far too vague to deserve the amount of discussion it gets.

bingoboy

looks like we need cowl induction

NZConvertible

Yep, the cowl is a high-pressure area on most cars. That's why the inlet for the HVAC system is there. Unfortunately that location is a little impractical to use as an engine intake point on FC's. I'd love to see someone try though.

Evil Aviator

The high and low pressure points on the picture correlate to what independent racers have found; the lower front end is a high-pressure zone, the hood is a low-pressure zone, the cowl is a high-pressure zone, the rear deck is a high-pressure zone, etc. Most cars have these same qualities. Also, if you put a lot of wax on your car and drive it in light rain, you can see the water beads following a similar pattern along the boundary layer as would be expeceted based on that diagram.

Bambam7

I too am a little confused about the apparent negative pressure at the leading edge of the car. Maybe my interpretation of the chart is wrong.... but shouldn't that be the highest pressure point??

Orange!FD

Bambam, since this is incompressible flow (meaning that the diagram is valid for speeds of less than about 450 feet/second or so!), the pressure is going to directly follow the speed of the flow just outside the "boundary layer", which is a thin (like around 0.1 inches) coating of air that adheres to the painted surface of the car. What I mean by "directly follow" is that if the speed of the air at any point on the car is above the speed that the car in general is traveling, then the pressure will be below "atmospheric" pressure, and the arrow will point outward from the surface. Likewise, if the speed of the air is below the speed of the car, then the arrow will point in toward the surface, because the pressure will be above atmospheric pressure. So when you see a big area where the arrows are pointing outward, that simply means that the speed of the air across the surface in that region is greater, by some amount, than the speed of the car through the air. So, believe it or not, air that comes toward the front bumper and just grazes across it at the location of the ftp lenses will speed up significantly in order to follow that path, and then gradually slow down as it traverses the remainder of the hood.