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Old 05-23-2012, 03:07 PM   #41
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if you look at the plot of the miata with colors, we can assume the green is areas of airflow into the car, relating to the arrows in the lower image (rx-7) pointing at the car. I'm looking at that miata image and thinking on the previous body style (like my 94), the inlet on the bumper of my car specifically is right at the point where green begins to turn to yellow so it should be right in the ideal to moderately ideal transition area, right?

So it would seem if that miata in the image had a hood line between the headlights, the transition from positive to less positive airflow looking to get into the bodywork would be a few inches below the front hoodline. that is approximately where my bumper intake is. not the ideal location mind you, but it should be functional.

then where the miata image is the most negative (flowing away from the car) is approximately where the hood exhaust vent is on my car, assuming i am correct in thinking air flowing away from the car is the ideal place to put an exhaust vent, which it would seem to be in my mind. it seems the exhaust vent on my hood is again in the transitional area from negative to positive airflow, so not ideal but at least in the usable vicinity. unless i've completely misread things, the intake and exhaust on my car is functional, although not ideally placed for maximum efficiency. Someone that actually knows what they are looking at, can you confirm what i am saying (or at least attempting to say). i feel rather like hustler's sig quote right now.

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Old 05-23-2012, 03:47 PM   #42
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The upper plot appears to be a cross section plot of total pressure. I dislike looking at total pressue because it is hard for me to visualize what behavior will result. It is a combination of static pressure and dynamic pressure so it has a normalized velocity squared term in it. You don’t know for sure if the color of an area is mostly the result of static pressure or velocity.

Bob
It actually looks like a plot of velocity magnitude. It is easy to tell from the fact that stagnation point is 0 (blue).
Generally, in CFD free stream pressure is set to 0 at the outflow boundary (the no-stress boundary condition). In the rainbow pictures blue = low, green = free stream, red = high (both for velocity and pressure)
What happened to Bernoulli in the wake? Pressure is low AND velocity is low?! Maybe not that simple...

The cross-section shown is not in the middle, so the effect of the little "rotary shaped bump" is not taken into account. It is very likely it accelerates the air and extends the low pressure region further back, so the vent discussed earlier should work fine. In addition, modeling the vehicle WITH the vents is the only way to assume that the pressure distribution is accurate. For example, look at all the 2012 DTM cars with their vents close to the windshield; maybe they know more than us...

Separate from all that, I am surprised people are using "rule of thumb" or pictures off the internet to argue that a TESTED design is garbage. As far as I know, people use computational analysis to reduce/prepare actual tests, not as a substitute.

Nik - Used CFD for my senior design project
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Old 05-24-2012, 01:56 PM   #43
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My 2 cents is pretty simple and comes from absolutly zero formal engineering training. It comes from 50 years of hot rodding and fooling around with race cars. Both graphs show pres. with the nose intact and air having to flow up and over the nose. It looks like Shanems setup should work like he says IF the upper opening is ducted in tight to the rad. and sealed with say foam weatherstripping, otherwise as Bob has stated the air is going to want to go out the upper opening as that would be the path of least resistance. My street learning would tell me that the much larger grille opening would create a much larger high presure area [ air volume rushing straight at a larger opening and in a tunnel, not being able to escape] and the radiator being a restriction area for the air flow to go through, that the path of least resistance would be out the upper opening IF NOT ducted and sealed. Add a exit vent bhind the rad. further forward then it is presently would relief the underhood pres and create a low air pres. area behind the rad. letting the airflow through from a high pres. zone to a low pres. zone, thus increasing the flow through the rad. if both inlets are sealed, otherwise alot of the air well want to go around the rad. and out the upper opening. It looks like the bottom of Shanem's opening is where the nose transistions from flat [vertical] to start the slope rearward so the path of the air is not being deflected upward, it is still entering straight at the opening, so if as stated above it is sealed to the rad. and a low pres. area created behind the rad. it should be a big improvement to cooling the upper part of the rad. where there also is no intercooler hindering air flow. So like I said on the first page and Shanem has said 2 post ago, the outlet is not in the preferred location, but should function if the inlet is ducted. I have seen alot of serious race cars back in the day and help create a few and the ducting was always sealed as good as practically could be done no matter if one or two openings were involved and in this case they have to be sealed from each other. To keep from screwing up the finished hood he could add a little slooped turn up piece in front and down each end of the opening, which would create more of a low pres. outlet area on the present location.
Just my 2 cents, no engineer, but seems like it makes sense to me.
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Old 05-24-2012, 02:41 PM   #44
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Here are some pics. Again for the money and the location of the vents I think it is a steal. Heck I paid $695 for a APR wing, and I love the wing, but this is alot more involved to make what with the hood latch and hinge locations and vents with rain gaurds. It could stand to be a hair wider and longer but still??

Last edited by jmann; 06-04-2012 at 02:18 AM.
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Old 05-24-2012, 03:19 PM   #45
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can't figure out how to post more then one pic at a time, keeps refusing my uploads except for one picture from the alblum.

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Old 05-24-2012, 03:26 PM   #46
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can't figure out how to post more then one pic at a time, keeps refusing my uploads except for one picture from the alblum.
upload to photobucket, then put the code here.
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Old 05-27-2012, 11:56 AM   #47
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try this again, oops! thought it had loaded, don't understand why it took 2 pics but won't load anymore.
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Old 05-27-2012, 12:06 PM   #48
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Quote:
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More details on the stock hood? The thin aluminum around the skeleton was cut out. It's not aerodynamically ideal, but the cutouts are placed in low pressure zones (the rear most one is borderline). This also helps a lot during stop and go traffic since I still DD this car and the MP62 sitting under there is quite hot.

There are more pictures on www.automotofoto.net under the coverage for Sigma Racing Time Attack #1 (which was on Sunday). Gallery is at the end of the article. I especially like the picture when my wheels were getting checked.
This one?



I didn't like this pic though... (EDIT - saw the discussion on the previous page)

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Old 05-29-2012, 11:25 AM   #49
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After looking at the images for pressure zones I wonder if I should rethink my use of the parking light holes for engine cold air intake and oil cooler feed.
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Old 05-29-2012, 12:09 PM   #50
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I haven't read this whole post but before you go buy a hood go to my home page and look how I solved my over heat. All this crazy coolant reroute, and hood modification and crazy biz about the engine being from a sideways car, forget it. I found that it comes down to your intercooler prevents air reaching the upper half of your radiator and the lower half gets heated air through the intercooler. Once you allow cool air to the top of the radiator you will eleminate a bunch of your heating issues. I run 12psi and make 290 at the wheels. I have run several HPDE with temps in the 90's and don't have any more problems.

Now once I go to 18psi.... Maybe the it will be a problem.

got to go.

greg
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Old 05-29-2012, 01:32 PM   #51
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K24 You have to remember that those air flows are with the lights in, if you remove them and make an opening the air well come in, the question is how much. If you relieve the pres. in the eng. compartment with a vent it should increase the air coming in.
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Old 05-30-2012, 12:05 AM   #52
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Originally Posted by k24madness View Post
After looking at the images for pressure zones I wonder if I should rethink my use of the parking light holes for engine cold air intake and oil cooler feed.
You drive down the highway with your side window up; air accelerates past the A-column and leaves a low pressure region behind it. You roll your window down: air is hitting your face now... flow with a hole is not the same as flow when there was no hole. Something to think about...
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Old 05-30-2012, 01:09 PM   #53
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Originally Posted by nikoror View Post
You drive down the highway with your side window up; air accelerates past the A-column and leaves a low pressure region behind it. You roll your window down: air is hitting your face now... flow with a hole is not the same as flow when there was no hole. Something to think about...
NACA duct shape inlets can also do amazing things.

My only experience with the upper opening in the bumper is watching somebody with water sprayers on an intercooler at the bottom of the main opening. Spray was coming out of the upper opening when the squinters sprayed when the car was full throttle down the front straight. It took putting a scoop shape around the opening to get it to work as an inlet rather than a hole in the surface that air was flowing rapidly past sucking air out of it.

Based on what I do know about the behavior of flow I can see why this would happen.

Bob
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Old 05-30-2012, 01:29 PM   #54
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Originally Posted by bbundy View Post
NACA duct shape inlets can also do amazing things.

My only experience with the upper opening in the bumper is watching somebody with water sprayers on an intercooler at the bottom of the main opening. Spray was coming out of the upper opening when the squinters sprayed when the car was full throttle down the front straight. It took putting a scoop shape around the opening to get it to work as an inlet rather than a hole in the surface that air was flowing rapidly past sucking air out of it.

Based on what I do know about the behavior of flow I can see why this would happen.

Bob
the venturi effect? ive been wondering about that. i've got a medical background, in mixing medical gasses if you pass a high speed flow through an opening, air will be sucked in through that opening (it's more complex than that, but simply put i think that explanation works). i have wondered if that same effect would occur at speed with a high speed stream of air passing over an exhaust port in the hood. if the air going over the hood is moving at higher speed than the air inside the hood, i woudl think that it woudl suck air out of the engine bay. however, i'm not sure if the venturi effect applies to airflow over and around a car.

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a modification of Bernoulli's principle, which states that the pressure of a gas is reduced just beyond an obstruction or restriction in the vessel through which the gas is flowing. The pressure drop can be nearly eliminated if dilation of the vessel does not exceed 15 degrees. The effect is a factor in the design of respiratory therapy equipment for mixing medical gases.
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Old 05-31-2012, 01:42 AM   #55
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Those diagrams give a rough indication of the air pressure outside the car, so if its ducted from one area to another we know which way the air will flow. But if its not ducted we wont know the air flow until someone sheds some light on the air passages and turbulance under the bonnet. Thats what Ive gathered anyway.
Very informative thread, I appreciate all the effort people like Bob are going to to share their knowwledge.
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Old 05-31-2012, 12:27 PM   #56
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Those diagrams give a rough indication of the air pressure outside the car, so if its ducted from one area to another we know which way the air will flow. But if its not ducted we wont know the air flow until someone sheds some light on the air passages and turbulance under the bonnet. Thats what Ive gathered anyway.
Very informative thread, I appreciate all the effort people like Bob are going to to share their knowwledge.
The flow under hood is highly turbulent and erratic. It is also all pretty high pressure because it is being force fed by the main opening in the bumper which is at the best location on the external surface of the car to force feed the engine compartment from.

You can almost think of the underhood as a pressurized box of fairly even pressure distribution flow is completely mixed up to the point it is sort of homogenous. Cut holes in it and air will flow out at those locations. Cut holes in some locations and air will flow out better than others because the pressure on the outside of the box is very much an uneven pressure distribution.

Bob
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Old 06-01-2012, 05:52 AM   #57
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I know at least two, maybe even three things about keeping track cars cool.

Hood vents go at the front, not at the back. The angle of the bumper drives air up and over the front of the hood, producing a big area of vacuum around the headlight lids. If you remove the lids and tape them over, then go for a drive, the tape is sucked UP, not pushed down. The ideal location is as close to the radiator as you can get without being in front of it. All of that air then comes crashing down at the base of the windshield, which is why the cowl area is a high-pressure zone.

You do not need more air in the front of the car. Period, end of story. BTDT. The bumper vents in the best case just add drag, and in the worst case they act as low-pressure vents in front of the radiator. You can feed a very high-horsepower Miata with less mouth opening than what came from the factory - you just need to use the opening efficiently. Heat exchangers work best when they are fed by an opening approximately 30% the size of their surface area - the airflow enters the opening, the pressure drops, the velocity increases, and the air more easily makes its way through the radiator. Expose a radiator to clean air, and you end up with very high pressure and very low velocity through the radiator. If you want a perfect example of this, look at F1 radiator ducting.

You must get fresh air to the radiator. You cannot cool a track car with the radiator fed only from what comes out of the back of the intercooler. BTDT. You do not need to get a lot of fresh air to it, but you must get some fresh air to it. It can be accidental (air flowing over the top of the intercooler to the top of the radiator) or highly deliberate (air ducted around the intercooler to the radiator).

Or, in other words:

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Old 06-01-2012, 09:37 AM   #58
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do want the black car. does it have a build thread floating around here somewhere?
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Old 06-01-2012, 01:12 PM   #59
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Air flow over the car changes as air sources change. The base of the windshield is a high pres area as has been stated by us all, but open up the seal of the hood to the cowl with say hood risers http://www.miataroadster.com/miataro.../i-404640.aspx, and release the pres. under the hood which is being fed from the front of the car which I suspect changes the base of the winshield from a high pres area to a LOWER pres area then UNDER the hood is thus letting air flow through. I have never seen an air tunnel result of this, but when we lived in Arkansas a few years ago where the humidity is the same as the temp., and that is high most all the spec miata cars had this done to them as they weren't allowed to cut vents in their hoods. Early chevelles and other cars used to run cowl inducted hoods with the hood scoop being reversed making the opening at the base of the winshield to feed more air into the carbs, BUT the hood was sealed to the cowl at the rear and the hood was not cut open except just the circle opeming around the air cleaner so as not to let any pres from the eng. compartment effect the effect of the induction of the air flow. Also nascar inducts air into their carbs, manifold efi, from the base of the winshield, but the air cleaner has a snorkel that goes to the back of the hood where the opening is at the windshield base and it is sealed so as not to let any air pres effect in. We have to remember that these posted air flow charts are with a body whole, start cutting holes in them and the FLOW and the LOW and High pres. areas greatly change. If as Shanem opening looks to be just above the vertical flat part of the nose for the bottom of hos opening the air is not being deflected upward nearly as much as if the bottom of the opening was in the sloping part of the nose and even then if the opening was big enough verticaly the air flow would eventually change from upward to going in as the straight line pres. over takes the upward air pres.flow the farther it gets from the deflection point. If the bottom of the opening was in the sloped part of the nose a hood could be installed on the opening to catch the defected air as Bob stated. Another change taking place is front down force when cutting openings in the nose. It lessens the down force on the nose and it must be substanchal, because if you have ever watched nascar they tape the upper grille opening up during qualifing to increase the down force as they are only running a few laps and aren't concerned with cooling. We need a wind tunnel.

Last edited by jmann; 06-01-2012 at 02:07 PM.
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Old 06-01-2012, 01:35 PM   #60
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or someone needs to drive behind the local mosquito fogger and see what happens
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