mark kibort

the point of this discussion between us right now, is about the naca ducts. they are in a low pressure region. the under hood pressure is going to be higher there is no purpose of them to feed flow over the 2 valve cam towers.. (as clearly seen, there is no exhaust components anywhere near the vent) and on the intake side, it is a air portal, at best. no flow will feed that ,unless it demands are greater that what are surrounding the air intake on the driver side. again, it is a low pressure zone, and naca ducts only work on a very slight pressure differential..... i just thought of a great test for you and we all have seen this. try and crack open your sunroof and put a naca duct up there. (known low pressure zone) see how much flow enters the car. the cabin is not even pressurized anywhere near the engine compartment.

Yes, we can look at the 968turbos made by porsche. yes, the ports are functional, as they are an aux air inlet , if the pressure differential is great enough to entice flow. at low speeds, yes... remember we are only talking about 0.1psi MAX at 80mph. a fraction of that lower speeds 0-60mph for example. it certainly can be a source of air flow with no low pressure on the hood area, and a very low pressure front at the inlet. thats what it is designed for . its an inlet vent, no more, no less. if you want power gains, the nose would be the source, not a hood naca vent. agreed????

again, there is not much flow below 60mph for example. there is not much pressure differential, so a naca duct just becomes an inlet that is functional for cool air as a source. however, as you can see by this RS design.....the vent for the intercooler is the same locaction of the naca ducts. which one is correct, they both cant be optimal and they are not. but , they are making the best of a non optimal situation point is if you are lookin gfor a high pressure zone for air inlet, the naca duct, lower front hood are NOT the best place the nose would be and anything where the stock 928 inlets would be

ptuomov

GT6ixer

So just a little bit on my background. I've spent my entire 20 year career thus far in applied aircraft aerodynamics and have two degrees in aeronautical engineering. I am just stating this to lend some credibility to the following statement which some may find incredulous based on his reputation here: Mark is 100% correct. Correct in so far as stating that a NACA duct, or any opening on a surface for that matter, will always flow in the direction from high to low pressure. A NACA duct does not push air into its inlet. Being flush with the surface it is under the boundary layer. Air will only flow into it if the pressure on the backside is less than the pressure flowing over top of it. And when placed on a curved surface like a hood of a car, the pressure will be less than ambient (i.e. suction). As I stated in an earlier post, NACA ducts were designed for high speed airflow on aircraft mitigate the drag that is created by adding a traditional ram air scoop that is outside of the boundary layer and thus either trips laminar flow into turbulent flow, or trips turbulent flow into separation. Adding a NACA duct on surfaces that are in low pressure regions requires that even lower pressure exists on the backside for air to flow into them. In the case of a car, this lower pressure could be created by the intake vacuum. But as Mark pointed out earlier, this is less efficient than letting the intake draw air from the higher pressure that exists under the hood, not the lower pressure that exists over it.

mark kibort

sorry i wasnt more clear.. i thought that i was, but as you might know, air flow is not simple, but can be , if you break it down to differential pressure. i cant answer in one word, because it is conditional depending on air speed.and if the engine is running and what throttle position its operating at. and if the fans are running. but generally, engine off and fans not running, , AT very slow speeds. yes... moderate speeds no, and high speeds no.. turn on the fans, there will be no flow...... with engine running , and depending on throttle position, this will determine whether there will be a lower pressure created, that is LOWER than the over hood pressure... this is not simple by any stretch, though you are making it out to be so.

so, lets talk about the 40mph . there could be more pressure differential at the NACA duct, than what builds up in the engine compartment. i cant confirm as all my tests were at speed 80 to 130mph.. But, lets make this real simple. the NACA function makes a small pressure differential that norallly would not be there but it is also dependent on where the air flow is routed. if it is higher pressure, it wont flow. if it is lower, it will flow. SO, to your question about naca ducts on the side of the car vs the hood............if you dont know the difference, this conversation will continue to be difficult.

the net net, is naca ducts in the low pressure zones on the hood are much less , (if at all) effective than taking inlet pressure from the nose or base of the windshield because of this, the front hood area is ideal for venting for downforce,and slight gains for radiator flow cooling.

there are lots of things that the car manufacturers do, that dont do anything or do anything like they are thought to do. (like many of the wings, spoilers, etc)

bureau13

So are you saying on that 968 (and presumably on my 931 as well) the NACA duct on the hood is actually acting as a vent? I could kinda see that on the 931, as it's a road car and maybe they just did it to look cool (and it does!) but I can't imagine Porsche mounting them on a race-oriented car if they're not actually functional. What am I missing?

mark kibort

again, you cannot make a differential pressure with a naca duct, IF the under hood pressure is greater. if you look closely at the driver side naca duct, it feeds an intake "area". under WOT that demand would make an even lower pressure zone than above the hood, so it is "functional", but its only because porsche with these only "4 factory RS turbos ever produced" didnt have a viable higher pressure intake for engine air. the passenger side is feeding the cam tower distributor cap and power steering reservoir. the exhaust is well below it. im thinking they just made one on that side to match the other side for aesthetics. it will in NO way be functinoal at speed, because we already know, the pressure on top of the hood in that region is going to be lower than under the hood .

GT6ixer

^ This. The only other possibility, and I have no knowledge if this is true or not, but if there is a belly pan under the entire car it could possibly create enough of a low pressure region (i.e. lower than that on the hood where the NACA ducts are located) that if it was vented under the engine would draw air through the NACA ducts. If so I would imagine this differential would be created at pretty high speeds only.

ptuomov

mark kibort

what is your point about the 100mph air flow? Where and why are you coming up with this. you are confusing the topic. That section of hood is actually low pressure and is pulling the hood and car UP. its not downforce, its lift. at 100mph, yes, you could be looking at near 0.17psi of frontal pressure ,and on the splitter, (in the nose area) its a paltry level of downforce, but where the downforce really comes from is reducing the air going under the car, and allowing it to go to the sides and guided to the frontal inlets , exiting the hood and raising the pressure there, also feeding the flow over the rear wing. so, there are No "ladies" equivilance sitting on the hood at 100mph. its not high pressure, its low pressure (lower than ambient) you seemed confused here, unless im misunderstanding your point about "pressure" and "ladies sitting on the hood". as far as the relative gains of 0.17psi, its about a 3hp on a 300hp car...... thats if you had a near 100% efficient inlet, at 100mph, which in most cases that is impossible. to do this, it wouldnt be at the naca duct, it would be at a nose or base of windshield air source. which i have on my car, both...........I've measured the pressures at the inlets and the inlet at the base of the windshield. at 120-130mph

and No, the NACA duct even going slow will vent radiator fan, regardless of the "direction " of the fans. its a pressure differential thing, not a direction thing.

ptuomov

Mongo

Old thread that still makes me want that NACA duct hood to the air intakes

mark kibort

Before you start correcting me , or claim to correct me, you better first understand what has been claimed or said.(posted). i never said kinetic pressure doesnt have a direction, what i said was that the under hood pressure would exceed the pressure in many cases.... even the cooling fans running can do this.

So far, as the other aero engineer has posted, what i have posted is 100% correct . air can easiy flow and i can push out and vent out a NACA duct ,even if it has some "kinetic pressure". that kenetic pressure will always be less than the under hood or incoming air pressure at the nose pressure unless. you need to study NACA ducts to see how they work to get your answer.

as far as your psi relating to Lbf, again, what are you taking about. you are using your example as downforce, and the hood pressure is lower than the nose or under the car pressure (ambient) so, you have lift, not downforce and it is about 1/5th the value vs the nose pressure, but below ambient. (opposite) so, in your example, if you had .17psi of frontal pressure, the pressure might be .04psi below ambient, which would cause about 100lbs of lift on the hood section. yes, people dont usually understand the force that air can make even at low pressure changes.

then you bring up cross sectional shape vs AOA? have you been drinking ? (using your own saying)............. what is your point there.if i had a large cross section i could get more lift than i could get with a smaller wing with a higher AOA.. if you are talking NACA numbers and are looking at air foil shapes, vs angle of attack, your point makes no sense there too. i can get 0 angle of attack wings of the same size (cross sectional area) , but different shape that produce more lift than another with a higher angle of attack... so that discounts your statement, unless you mean something else.

have you taken any aero classes? it doesnt seem like it!

to correct your incorrect claim, i can easily turn on my fans, make a scoop pointing forward and the fan will exit at the vent with air moving into the oncoming air, if it is slow enough as i mentioned. as long as the exit pressure is lower than the oncoming pressure, this will happen.

GT6ixer

Note sure what you mean by kinetic pressure always has a direction? The common aerodynamic term for what I assume you are calling "kinetic pressure" is "dynamic pressure", mathematically expressed as 0.5 x rho x V^2 and is technically measurement of the energy in the flow when stagnated on a surface.

The direction of airflow is driven by pressure differential. What Mark is trying to explain is that the radiator fan will lightly pressurize the engine bay by drawing static air from under the bumper then add energy to it by converting the rotational energy of the fan into kinetic energy in the form of increased mass flow. This increase of air mass increases the pressure in the engine bay beyond that of ambient. The air will then seek any opening exposed to ambient pressure, i.e. the NACA ducts, hood seems, bottom of the car, etc. Again a simple test as Mark suggested would be to put tufts on the lip of the NACA ducts and energize the fan.

ptuomov

mark kibort

you havent done testing or really understand what is happening, because you are getting confused on flow direction vs pressure differential. Look, i said , slow speed, The air will still vent out the naca ducts. I have put tufts on the outlet and they stand straight up. (with fans on) with the car moving, they still point straight up. at a certain speed, the air flow and pressure is quite a bit greater coming out of the engine compartment than the pressure and flow around the vents. a naca duct falls below surface flow and creates its own pressure differential (vs not having one) but, if the pressure below it, is greater than above, no flow will occur. your statement about the fans being "useless" at speed ,is also incorrect.. they add, near the same amount to the already existing flow as they did when moving 0 velocity air they obviously dont change the engine compartment pressure, as the incoming air is the dominant factor, but still add to the overall flow to the engine compartment through the radiator.. the equation for mass flow would be (in lbs of thrust )=
13( eff x diameter x hp )^2/3 this is the lbs of thrust that the fans, based on their diameter, efficiency at any flow rate, and HP input (watts for our fans).. as long as the motors are drawing current and are efficient to some level , they will contribute to the mass flow through the system.

the bottom line is this with a high pressure region under the hood, a naca duct, even with on coming flow will not create enough pressure at the entrance to over come the pressure in the engine compartment. this is why my Tufts point up at a 45 degree angle coming out of the vent at 120mph. there is LOT of air flowing out of the vent. if you think a naca duct has the ability to fight that pressure differential, you need to think again. like i said, i can mount a naca duct in that position and put some tufts on it to see which way the flow goes, but the flow will go forward until the pressure equilibrium is met, and then it will change directions and follow the low pressure flow over the car. the NACA duct provides a very small net pressure change, but its not meant to overcome higher pressure zones, by any stretch.

if you doubt anything ive said. give me a test to do, ill attach pressure sensors and tufts and you can see the results in a video.

I've spent a good deal of time in aerodynamic test environments and much of this is not very intuitive .

.